Hydrological consequences of land use change a riveiw ofthe stat of scicen

Hydrological Consequences of Land Use Change: A Review of the State-of-Science Keith N Eshleman University of Maryland Center for Environmental Science, Appalachian Frostburg, Maryland Laboratory, Rates of deforestation, agriculturalization, urbanization, wetland drainage, and several other types of land use change have accelerated as a function of the growth of human populations Hydrologists have recognized for nearly a half century that such land use changes can substantially affect hydrological processes at the scale of the research plot, the hillslope, and the small experimental catchment The hydrological consequences of land use change are of interest not only to the academic hydrologist and ecologist, but they are of critical importance to the practicing civil engineer At the beginning of the 21st century, these studies are increasingly being incorporated into multi-scale analyses used to address both scientific and manage­ ment questions at landscape, river basin, and regional scales Such efforts are being supported by major technological developments in collecting, analyzing, and mod­ eling hydrological data, as well as new capabilities for observing and quantifying land use and land cover changes using remote sensors In this chapter, ( ) summarize various scientific methods that have been used to determine the hydrological effects of land use change; (2) review the state-of-science with respect to understanding the effects of several different types of land use change on hydrological processes; and (3) identify key research issues related to uses of specific methodologies and to improved understanding, detection, quantification, and prediction of the hydrolog­ ical consequences of specific land use changes have accelerated as a function of the growth of human popu­ lations and economic development In recent decades, dra­ matic land use changes in Southeast Asia, South America, Africa, and elsewhere have been carefully documented by onthe-ground observations and through the use of modem remote sensing technologies In this century, it is likely that rates of expansion and intensification of agriculture, growth of urban areas, extraction of timber and other natural resources, and development of freshwater resources will all increase to meet the demands of expanding human populations for higher stan­ dards of living Hydrologists have recognized for nearly a half century that land use changes and a variety of types of land cover distur­ bances can substantially affect hydrological processes at the INTRODUCTION Land use change has been a ubiquitous component of human settlement of the earth's surface, particularly in Europe, North America, and Asia during the previous millenium The con­ version of land to support growing human populations is a major component of human modification of the environment Rates of deforestation, agriculturalization, urbanization, wet­ lands drainage, and several other types of land use change Ecosystems and Land Use Change Geophysical Monograph Series 153 Copyright 2004 by the American Geophysical Union 10.1029/153GM03 13 14 HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE scale of the research plot, the hillslope, and the small exper­ imental catchment Experimental manipulations of vegeta­ tive cover—in particular deforestation and reforestation—have demonstrated that water yields, interception losses, evapotranspiration rates, flood peaks, sediment transport rates, and concentrations of many water quality constituents can be dramatically modified through human activities on the land surface Not only are these and other consequences of land use change of interest to the academic hydrologist and ecologist, but they are of critical importance to the practicing civil engi­ neer The correct statistical interpretation of long time series of data from observational networks; the proper design and operation of flow conveyance devices, flood control struc­ tures, drainage networks, and water systems; and the appro­ priate use of mathematical models for forecasting future hydrological conditions are all critically dependent upon understanding systems that have been affected by human activities As the spatial extent and intensity of land use changes increase in systems for which we have observational records, our ability to casually assume stochastic stationarity becomes more and more questionable Development of the next generation of hydrological models will almost certainly include a new conceptualization and parameterization of land use change effects on land surface processes Such models will almost certainly need to go beyond our current "static" view of governing processes by considering the influence of both spatial extent and pattern of different land use changes on the watershed, as well as their temporal variation, on "dynamic" hydrological parameters At the beginning of the 21 st century, there is still much to be learned from small-scale experimental and observational studies of land use changes These studies can be made far more valuable, however, if they are part of a multi-scale analysis focused on how the smaller scale impacts are prop­ agated to larger river basin scales where management often occurs Within the last dozen years, at least three independ­ ent groups of hydrologists have issued concerted pleas for continuation, better coordination, and augmentation of basic hydrological observation networks to better understand human effects on the hydrological cycle One of these groups—the Eagleson committee—specifically identified "hydrologic effects of human activity" as one of its top five scientific priorities in the overall field of hydrological sci­ ence [NRC, 1991] Several important technological developments have recently converged to facilitate greater progress in understanding the hydrological consequences of land use change The first devel­ opment was obviously the extraordinary improvement in data collection, data archiving, data distribution, and computa­ tional (both hardware and software) capabilities to support such analyses The importance of the Internet as a tool for transmitting archived, as well as real-time data, to hydrolo­ gists cannot be overstated As recently as ten years ago, just the gathering of data to perform time series analysis may have taken ten times as long as it does presently, freeing up precious human resources that can now be focused on other aspects of the problem A second critical development is the unprecedented feasi­ bility of observing land cover changes using remote (e.g., satellite-based) sensors Landsat data are being acquired sys­ tematically around the world [Goward and Williams, 1997] and data from moderate resolution sensors such as MODIS are now widely available [Justice et al, 2002] Scientists are now able to efficiently store, manage, and process these images in ways not possible just a decade ago Satellite data, appro­ priately calibrated and validated with ground data, can now provide information on the spatial distribution of land cover types, as well as resolve changes over time [Hansen and DeFries, in press] Whereas previously such information could only be obtained for small areas using ground surveys or aer­ ial photography, satellite data can expand the spatial coverage to much larger areas and permit analysis at more frequent time intervals Remotely-sensed imagery can also be used to quantify the spatial distribution of many land cover parame­ ters, such as vegetation cover [Nemani et al, 1993], vegeta­ tion change [Nemani et al, 1996], and imperviousness [Slonecker et al, 2001] that are important elements of change Finally, satellite remote sensing is increasingly being used to provide extensive coverage of key hydrological variables, such as precipitation [Smith et al, 1996; Sturdevant-Rees et al, 2001], soil moisture [Sano et al, 1998], and river flooding [Portmann, 1997; Townsend and Foster, 2002] Eventually it may be feasible to subject these data to time series analysis as well Each of these types of data would have been virtually impossible to obtain through traditional field methods [Entekhabietal, 1999] The purposes of this chapter are to: (1) summarize various scientific methods that have been used to understand and quantify hydrological effects of land use change; (2) review the state-of-science with respect to understanding the effects of several different types of land use change on hydrological processes, particularly those processes that are observable at the land surface; and (3) identify key research issues related to uses of specific methodologies and to improved under­ standing, detection, quantification, and prediction of the hydrological consequences of specific land use changes SCIENTIFIC METHODS FOR ASSESSING HYDROLOGICAL EFFECTS OF LAND USE CHANGE Identifying, quantifying, and predicting the hydrological consequences of land use change have proven quite chal- ESHLEMAN lenging for several reasons The relatively short lengths of most hydrological records, superimposed on the relatively high natural variability of hydrological systems, make it dif­ ficult to isolate a land use signal from hydroclimatological "noise" This issue is most significant in "real" systems for which the timing and patterning of land use changes are not controlled, but it is even an issue for those small-scale stud­ ies for which land use modifications can be carefully imposed The problem is also complicated by the normal paucity of detailed hydrometric measurements that are typically used to characterize the hydrological consequences of land use change Stream discharge data from one location in a watershed are most often the only data available to assess the hydrological impacts of land use change The relatively small number of controlled small-scale experimental studies that have been performed has also limited progress in extrapolating or gen­ eralizing results from such studies to other systems Given the diversity and complexity of land use changes that are tak­ ing place around the world, satisfactory techniques for ana­ lyzing the hydrological consequences of land use must be considered to be in an early stage of development The devel­ opment of mathematical tools (i.e., models) for reliably pre­ dicting the hydrological effects of future land use changes is in its infancy [Beven, 2000] In this section, I classify and discuss the primary scientific methods that have been used to assess the hydrological con­ sequences of land use change It must be emphasized that for purposes of this chapter I not distinguish between land use change and land cover change (see Loveland and DeFries, this volume), since the two issues are closely related and essentially identical methods can be employed to address either issue The majority of research projects that have exam­ ined the hydrological consequences of land use change have employed scientific methods that can be described by one of the following six classes: • plot-scale experiments and observations • small watershed experiments and observations • empirical modeling • time series analysis • physically-based (lumped and spatially-distributed) modeling • landscape, river basin, and regional-scale analysis A discussion of these six types of methods is provided in the following sections Plot-Scale Experiments and Observations Experiments (and observations) made at the plot-scale are perhaps best exemplified by a large number of studies of the 15 evapotranspiration process Many of these studies were aimed at estimating consumptive use and consumptive use coeffi­ cients of different agricultural crops and natural vegetation In these studies, consumptive use (potential evapotranspira­ tion) can be directly measured using 0.6-3.0 m (2-10 ft.) diameter tanks known as lysimeters, in which inputs of water can be controlled and outputs measured by weighing [Blaney etal, 1930; Pruitt and Angus, 1960; Van Bavel, 1966] Two important advantages of plot-scale studies include (1) the ability to replicate measurements in both space and time, thus subjecting data to rigorous statistical tests of signifi­ cance; and (2) the ability to control environmental condi­ tions to some degree A major disadvantage of them is that they are best suited for measuring processes that can be (or are assumed to be) essentially occurring in the vertical dimen­ sion Direct extrapolation of plot-scale results to larger sys­ tems is often questionable, unless the full range of environmental and site conditions that are found in nature were encountered in the experimental study Interception losses by different plant species [Helvey, 1967], snow accu­ mulation and melting [Goodison et al, 1981; DeWalle and Meiman, 1971], and soil infiltration [Black, 1991] are other land surface processes—considerably influenced by land use—that are commonly addressed using plot-scale tech­ niques Many other hydrological studies have been conducted at somewhat larger (i.e., hillslope or field) scales, but few of these have dealt with the issue of land use change Small Watershed Experiments and Observations Understanding of the effects of land use changes on hydrological processes has also occurred through controlled, exper­ imental manipulations of land cover in small watersheds Studies of the effects of forest management practices (includ­ ing cutting, removal activities, and regrowth of forest vege­ tation) on annual and seasonal water yields, evapotranspirative losses, interception rates, and flood peaks have been con­ ducted in forests throughout the world [Bosch and Hewlett, 1982] The development and application of the small exper­ imental watershed technique beginning in the 1950s—exem­ plified by the careful, pioneering work conducted at Coweeta [Swank and Crossley, 1988] and Hubbard Brook [Hornbeck et al, 1970; Likens et al, 1977] in the eastern U.S.—clearly revolutionized the study of the hydrological effects of land use change and land management activities These studies typically involve instrumenting a pair of adjacent, small watersheds that (initially) have similar vegetation, soils, aspect, slope, and other geomorphic properties; precipita­ tion (Figure 1) and stream discharge (Figure 2) are meas­ ured using on-site instrumentation The watersheds are typically monitored for several years (the calibration period) 16 HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE to establish statistical relationships for key state variables, and one of the watersheds is then subjected to an experimental manipulation or land use change Subsequent monitoring of precipitation and discharge (in some cases both quantity and quality) is used to determine differences in hydrological (and ecosystem) responses that can be attributed to the experi­ mental manipulation The major advantage of the small exper­ imental watershed technique is that effects on important land surface processes such as stream discharge and evapotranspiration can be directly measured under somewhat "con­ trolled" conditions The major limitation of the technique is lack of replication: the resources required to properly instru­ ment two small watersheds and perform a single manipula­ tion normally preclude true experimental replication in a probabilistic sense The literature contains far fewer exam­ ples of controlled studies of the effects of permanent land con­ versions (e.g., forest to agriculture, agriculture to urban, etc.), however Paired-watershed studies of urbanization and agriculturalization are less common in the literature, but both of these types of land use change have generated an extensive liter­ ature from analysis of observational data from comparative or case studies of watersheds Similar to the small experi­ mental watershed technique, these studies involve the use of similar types of field instrumentation, but they suffer from the lack of a pre-manipulation calibration In some cases, these studies involved instrumenting multiple watersheds— an advantage of the strictly paired approach of the experi­ mental technique Empirical Modeling A variety of types of empirical models for representing watershed-scale runoff responses to rainfall have been devel­ oped over the years, including (1) the rational method, (2) the unit hydrograph method, (3) the USDA-SCS Curve Number method (hereafter the SCS method), and (4) the Modified Universal Soil Loss Equation (MUSLE) All four of these models have long been considered the state-of-the-art in hydrology and have been thoroughly summarized and reviewed by Chow [1964], among others Despite major improvements in computing and data availability, many of these empirical methods are widely used even today Beven [2000] recently dis­ cussed some of the well-known limitations and assumptions of empirical models, particularly the simplifying assumption that routing of water can be assumed to occur in a linear way, thus enabling straightforward solution by applying the principle of superposition The assumption of linearity imposes some severe constraints on the application of these models to fairly small watersheds, although models of this form have been extended to larger, more complex watersheds by treating them as if composed of a set of linear subsystems Such an approach thus assumes spatial linearity as well The lack of sufficient internal data for calibration, however, often calls into question the overall reliability of models of such complex systems The most widely used method for estimating infiltration (and storm runoff) from watersheds subjected to land use change is the SCS method developed by the U.S Soil Con­ servation Service [1973] using data from small experimen- Figure A shielded, weighing-type precipitation gage located on the Neff Run watershed near Frostburg, Maryland (photo courtesy of Timothy Negley) ESHLEMAN 17 Figure A truncated Parshall flume, stilling well, and digital water level recorder used to provide continuous measure­ ment of stream discharge on the Neff Run watershed near Frostburg, Maryland (photo courtesy of Timothy Negley) tal watersheds in the U.S This empirical method was later extended to urbanized watersheds as well The basic theory underlying the method is that direct (storm) runoff increases with increasing storm rainfall, according to a functional relationship that uses curve numbers that are assigned to different hydrologic soil groups for different land use sce­ narios: curve number (CN) values can theoretically range from to 100 The higher the value of CN, the greater the storm runoff (all else being equal) Empirically-derived CN values have been tabulated on the basis of land use, land treatment or practice, hydrologic condition, and hydrologic soil group; CN values can also be modified to account for dif­ ferences in antecedent moisture conditions A major advan­ tage of the SCS method is that hydrologic soil groups have been mapped for the U.S.; when combined with local data on land cover and land management practices, storm runoff can be easily estimated for either actual or design storms using the published CN values Chow et al [1988] provide an especially nice example of using the SCS method (using curve numbers tabulated from other studies) to address the problem of increased runoff from a watershed due to urbanization Fogel et al [1974] also used the SCS method to predict the likely effects of dif­ ferent degrees of urbanization on the return periods of storm runoff from desert shrub watersheds While flood frequency analysis is most commonly performed as a component of hydrological time series analysis, in this example flood fre­ quency curves were generated using the SCS method and estimated parameter values that corresponded to different land treatments The SCS method has been incorporated into a variety of other "event-based" runoff models (including TR-20 and HEC-HMS) and (with MUSLE) provides the con­ ceptual basis for several other empirically-based models that have been developed to address water quality problems asso­ ciated with land use changes Examples of such models include AGNPS [Young et al, 1989], SWRRB [Williams et al, 1984], SWAT [Arnoldetal, 1993; Bingner, 1996], and SWIM [Krysanova etal, 1998] Beven [2000] emphasized another modeling problem that is peculiar to land use change modeling: in predicting the effects of change using these models, at least some of the model parameters must also be expected to change Therefore, unless the effects of change can be independently accounted for through empirical study, one must expect that the uncertain­ ties associated with model parameters must increase relative to the case of stable land use Fortunately, extensive experience with some of these models (the SCS method and MUSLE, in particular) has resulted in detailed tables of data that can be used to parameterize the model for a specific application, ostensibly improving model predictability Time Series Analysis Time series of data for some important hydrological variables (e.g., river flow) have been collected using consistent methods for some watersheds for over a century by the United States Geological Survey; data collected prior to the 1960s were typ­ ically associated with very large U.S rivers and these data are most useful in addressing issues associated with water resources development, water resources management, and climate change in major river basins (e.g., Figure 3) Since the 1960s, however, larger numbers of relatively long time series have been col- 18 HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE lected for smaller watersheds that have experienced important land use changes, particularly urbanization in the U.S [McCuen, 2003] The improvements in data collection, data archiving, data distribution, and computational capabilities now make such time series analyses feasible To be useful in identifying and quantifying hydrological change statistically, time series methods must be able to dis­ tinguish between episodic and secular (i.e., gradual) effects in a long-term record [McCuen, 2003] Construction of a deten­ tion basin, diversion of streamflow to another channel, removal of a dam, or straightening of a natural river are all examples of episodic changes that could produce a sudden transition from one state to another Secular changes would be expected to dominate in watersheds that are, for example, gradually urbanized or agriculturalized One can easily imagine a water­ shed that is influenced by both types of land use change In all of these cases, land use change can be expected to induce nonhomogeneity or nonstationarity into the data records; a critical issue is the amount (both intensity and extensity) of change that can be detected, given a certain amount of random variability in a long-term record The flood frequency estimation problem for short (rela­ tive to the desired return period) records has been widely discussed in the literature, but the estimation problem for nonstationary time series attributable to land use change has not received as much attention McCuen [2003] discusses the use of graphical and statistical methods for detection of nonhomogeneity in hydrological time series, and Beighley and Moglen [2003] recently suggested a method for model­ ing changes in a flood frequency curve for a watershed in Maryland that experienced extensive urbanization during the gaging period Entekhabi et al [1999] described an important complication in statistical quantification of hydrological effects of land use change: hydrological variability can also be caused by long-term climate fluctuations and climate change In systems subjected to both types of change, it is nec­ essary to isolate the individual contributions of human activ­ ities at the land surface from those that are due to climate change, particularly in any analysis of extreme events (e.g., floods, droughts, etc.) In essence, identifying a linkage between hydrological change and land use change often involves finding a signal within a substantial amount of cli­ matic variability or "noise." Physically-Based Modeling (Lumped and Spatially-Distributed) Computational improvements in the last thirty years have enabled far faster processing of hydrologic data and thus more rigorous testing of hydrological paradigms Modeling of land use change has thus rapidly evolved from simple empir­ ical approaches that often involved "event" modeling [e.g., unit hydrographs, Jakeman et al, 1993] to more complex, phys­ ically-based models that could be applied to provide "con- 700 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 Figure Mean annual discharge in the Potomac River near Point of Rocks, Maryland (water years 1895 through 2000) Data from http://nwis.waterdata.usgs.gov/nwis/sw ESHLEMAN tinuous" outputs or simulations based on application of the water balance continuity equation Lumping or spatial aver­ aging is the most common simplification in watershed mod­ eling, which often limits application of the model to small watersheds with relatively homogeneous vegetation, soils, topography, and geologic strata [Blackie and Eeles, 1985] The first generation of physically-based models was com­ posed of lumped-parameter models such as the Stanford Watershed Model of Crawford and Linsley [1966], later known as Hydrologic Simulation Program—Fortran or HSPF [Bicknell et al, 1997] Lumped-parameter models such as HSPF include explicit soil moisture accounting, computation of actual evapotranspiration rates, and routing of flow through various compartments on a regular time step There are quite a few examples in the literature of the use of lumped-parameter models for evaluating and predicting hydrological response to land use change One of the earliest examples was the use of the USDAHL (U.S Department of Agriculture Hydrograph Laboratory) model by Langford and McGuiness [1976] to simulate runoff from a watershed that had experienced both reforestation and forest thinning operations Swift et al [1975] used a lumped-parameter model (PROS­ PER), based on the Penman-Monteith equation, to model changes in evapotranspiration due to clear-cutting and refor­ estation of research watersheds in the southern Appalachi­ ans Huff and Swank [1985] later employed PROSPER to evaluate effects of clear-cutting on water yields from six research watersheds located throughout the U.S Federer and Lash [1978] described the use of another lumped-parameter model (Brook) to evaluate the effects of different species of hardwood trees on the timing of transpiration and runoff from forests in the eastern U.S Eeles and Blackie [1993] used the Institute of Hydrology (IH) lumped model to evaluate the effects of afforestation on streamflow within the Balquidder catchments in the U.K An important improvement in this work was the use of a parameter optimization scheme [e.g., Nelder and Mead, 1965] to automatically fix 23 of the 30 parameters in the model Two important limitations of lumped-parameter models are (1) their inherent inability to represent the spatial variability of hydrologic processes and parameters [Moore et al, 1991] and (2) the effective parameters used by these models are not directly related to measurable watershed characteristics [Storck et al, 1998] Therefore, it has long been thought that greater progress could be made through the development and appli­ cation of spatially-distributed physically-based models that can ostensibly make use of high-resolution information on land use patterns and processes [e.g., Abbott et al, 1986; Dunn andMackay, 1995; Adams et al, 1995] As pointed out by Beven [1985; 2000], the primary rationale behind the move to such models is that land use change almost never occurs both 19 suddenly and uniformly over an entire watershed Spatiallydistributed models can, in theory, correctly implement any changes in parameter values and place them in their correct spatial context [Beven, 1985] Recent developments in geo­ graphic information system (GIS) technology have clearly expedited advances in spatially-distributed m o d e l i n g [DeVantier andFeldman, 1993; Maidment, 1993] The most important current limitations are the general lack of data with which to parameterize these models, the difficulties and expenses associated with gathering necessary data for para­ meterization, and the relatively large spatial variability in those data that are available Beven [1989] also questioned whether some of the "small-scale" physical parameters that are endemic to these models can actually be used to model phys­ ical processes at the larger grid scale, a problem common to both lumped and spatially-distributed models The literature includes descriptions of at least five spatiallydistributed watershed models that have been used to address the problem of land use impacts on hydrological processes, including (1) IHDM [Beven, 1985]; (2) CASC2D/GRASS [Doe et al, 1996]; (3) Macaque [Watson et al, 1999], based on the distribution function modeling concept common to RHESSys [Bandetal, 1993] andTOPMODEL [Beven and Kirkby, 1979]; (4) SHE/ SHETRAN/ARNO/NELUP [Abbott et al, 1986; Adams and Dunn, 1995; Dunn and Mackay, 1995; Dunn et al, 1996]; and (5) DHSVM [Storck et al, 1998; VanShaar et al, 2002] The issue of "overparameterization" of rainfall-runoff mod­ els has been brought up in the literature [e.g., Beven, 1989; Jakeman and Hornberger, 1993], but this issue may be some­ what less relevant to land use change modeling where the main objective is to predict how the intensity, extensity, tim­ ing, and patterning of land modifications alter hydrological variables Beven [2000], however, points out a more important modeling issue that must be reemphasized here: [T]here have been many studies that report predictions of the hydrological impacts of change but, as yet, I know of none where catchment-scale predictions made before a change have later been verified In other words, without more extensive field validation of model predictions, the overall reliability of physically-based models of hydrological response must be seriously questioned Landscape, River Basin, and Regional-Scale Analysis Most of the techniques that have already been described in the preceding paragraphs have been used to address hydrological consequences of land use change at larger (i.e., land­ scape, river basin, or regional) spatial scales While land use changes only occur uniformly in space and time under con- 20 HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE trolled experimental conditions, some approaches have been used to address the consequences of land use changes in real systems where temporal and spatial patterning is a compli­ cating factor This is a particularly important issue for river basins—both as a research focus and as a management ques­ tion A second topic relates to the quantification or predic­ tion of hydrological consequences at even larger regional scales where data may be particularly sparse News on and Colder [1989] provided an excellent review (and history) of some of the key problems in generalizing about the effects of forest management practices on the hydrological cycle at regional scales HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE In this section, I provide a review and discussion of the hydrological consequences of six major types of land use change—in each case a "conversion" from one type to another The six land use changes are: (1) deforestation/reforestation, (2) agriculturalization, (3) urbanization, (4) wetland drainage, (5) water resources development, and (6) surface mining/reclamation It must first be pointed out that these categories are not actually mutually exclusive; for example, in some cases deforestation may be considered one element of agriculturalization (i.e., conversion of previously forested land to agricultural land) The same holds true for wetland drainage: the process of draining a marsh or swamp may be one element of a larger process through which a wetland is converted into agricultural land (i.e., agriculturalization) or urban land (i.e., urbanization) Secondly, my goal in this sec­ tion is not to provide an exhaustive summary of the literature, since such a review would be impossible given the limited pages that are available to complete this task Moreover, the diversity within and among these land use changes, as well as the condition and inherent variability of natural systems onto which these changes are imposed, makes it virtually impossible to provide a truly "global" generalization—either in textual or mathematical form Rather, my primary goal in this section is to provide the reader with a reasonable summary of the present understanding of the hydrological consequences of each of the six major change types I have also included under each of the change categories a brief discussion of the effects of different management practices on hydrological processes, where such information can be gleaned from the scientific literature Deforestation/Reforestation Human conversions of forested lands—either to provide fuel and fiber or to merely provide land for agricultural pro­ duction—has received significant attention by both ecologists and hydrologists Forests cover roughly 30% of the land surface of the earth, less than half of their original areal cov­ erage prior to human settlement Deforestation is widespread, but the extent of forest clearing is most significant in south­ ern Europe, northern Africa, and western Asia where humans have been able to make use of fertile forest soils for agricul­ tural crops or as pastures for grazing animals [Dansmann, 1972] In recent decades, the rate of clearing of tropical forests in Africa, Asia, Central America, and South America has greatly accelerated, causing global concerns about effects on climate and local concerns about increased denudation of the landscape, flooding, and downstream sedimentation [Werth and Avissar, 2002] The most significant hydrological effects of permanent deforestation are at least qualitatively well known for many forests Since forests naturally occur in areas of relatively high precipitation (and the density of forest vegetation varies roughly as a function of precipitation), forest clearing causes an imme­ diate loss of interception capacity and subsequent reductions in interception and transpirational losses of water to the atmos­ phere [Swift et al, 1975] Forested lands subjected to clearing are also exposed to the direct impact of raindrops, inducing par­ ticle detachment from unprotected surfaces Forest soils— particularly those in temperate and boreal systems—are also characterized by relatively high infiltration capacities, owing to the presence of undecomposed organic matter at the surface and large quantities of organic matter within the upper mineral soil layers Under these conditions, infiltration-excess overland flow is minimized Forest clearing, and the subsequent loss of the surface organic layer and decline in soil organic matter, results in an increase in overland flow, raindrop detachment of soil particles, sheet erosion, rill erosion, gullying, and down­ stream sedimentation While the hydrological responses of lands following defor­ estation are highly variable, forest clearing at the scale of the experimental watershed generally results in a significant increase in annual water yield [Hibbert, 1967; Bosch and Hewlett, 1982; Whitehead and Robinson, 1993], depending upon a host of different factors These factors include the spe­ cific method of deforestation including cutting, wood removal, and road-building practices [Reinhart et al, 1963; Harr et al, 1975; Likens et al, 1978; Beschta, 1998]; the extent of forest cover removed within the watershed [Hewlett et al, 1969; Bosch and Hewlett, 1982]; the rate and type of revegetation or reforestation [Federer and Lash, 1978; Swank et al, 1988]; climatic conditions, especially the temporal distribution and magnitude of rainfall and snowmelt [Chow, 1964; Bosch and Hewlett, 1982; Whitehead and Robinson, 1993]; and chemical and physical properties of the soil, especially of the forest floor [Likens et al, 1978] Other small watershed stud- ESHLEMAN ies have examined in considerable detail changes in other measures of streamflow response, such as flow duration curves [Hornbecket al, 1970; Burt and Swank, 1992]; baseflow, low flow conditions, or flow minima [Hicks et al, 1991; Bowling et al, 2000]; peak flows [Hornbeck et al, 1970; Harr et al, 1975; Harr, 1981; Harr, 1986; Jones and Grant, 1996; Bur­ ton, 1997; Storck et al, 1998; Jones, 2000]; and the timing and magnitude of snowmelt runoff [Harr, 1986; Beschta, 1998; Storck et al, 1998] Despite the large numbers and types of analyses that have been conducted, it has been difficult to generalize across these different streamflow response measures Increases in streamflow can cause further denudation of the land surface through increased rill and channel erosion, as well as downstream sedimentation, but these responses are often episodic and thus not easily predicted [Grant and Wolff, 1991] On deforested slopes in particularly steep terrain that are subsequently subjected to intensive cropping practices or fire or both, rates of denudation of the landscape are maxi­ mized The "end result" of this process is exemplified by bar­ ren, unstable wastelands unable to support any vegetation, exemplified by the foothills north of Mexico City described by Starker Leopold in 1959 [see Dansmann, 1972] Globally, it has been estimated that the average erosion rate over the continents has doubled or tripled due to all human activities, but the most important factor has been forest clearing for agriculture and forest harvesting practices In better-managed systems where revegetation rapidly occurs, the effects of forest clearing on hydrological processes are transient and less extreme In these cases, hydrologists have focused on how the changes in vegetation type and den­ sity affect the rates and timing of runoff, evapotranspiration, sediment losses, and nutrient export from watersheds [Likens et al, 1978; Swank et al, 1988] Experimental conversions of natural forests to grasslands, to forest plantations, and back to native vegetation have been used to quantify the hydrological impacts of deforestation and to understand the transient con­ sequences of various forest management practices and natu­ ral disturbances [Swank et al, 1988] Reforestation (or afforestation) can be conceptualized to some extent as reversing the hydrological responses to defor­ estation Eeles and Blackie [1993] predicted that runoff would decline in an approximately linear proportion to the increase in the area of forest within the watershed, with a 50% increase in forest area producing a 12% reduction in runoff Flowduration analysis of the model results further suggested that these percentage reductions occurred uniformly across the entire range of flow conditions [Gustard and Wesselink, 1993] More recently, Colder [2003] proposed several modifications of the Penman-Monteith equation that allowed an assessment of the effects of afforestation on water resources In a com­ panion paper, Colder et al [2003] employed the Hydrologi­ 21 cal Land Use Change (HYLUC) model which predicted dra­ matic declines in average annual recharge plus runoff in response to conversion of grasslands to forests in the lowland areas of the U.K Complete reversal of the effects of defor­ estation may be dependent upon restoration of both vegetation and soil properties that are characteristic of native forests within a particular climate on a particular parent material Given the relatively slow speed of forest soil development, particularly in temperate climates, it is expected that hydrological processes under these conditions would be restored quite gradually At the river basin scale, the ability to predict integrated responses of hydrological variables over large areas has a fairly brief history Gentry and Lopez-Parodi [1980] presented empirical evidence that deforestation of the Upper Amazon River basin was causing increased flooding downstream, but Nordin and Meade [1982] refuted the study's conclusions on methodological grounds Shukla et al [1990] used a coupled atmosphere-biosphere model to predict that complete con­ version of the Amazon tropical rainforest to degraded pas­ ture would ultimately cause a 643 mm decrease (-26%) in mean annual precipitation, a 496 mm decrease (-30%) in mean annual evapotranspiration, and a 147 mm decrease (-18%) in mean annual runoff Other modeling studies have produced similar, if less dramatic, responses to Amazon deforestation [Werth and Avissar, 2002] Regional responses of river basins or large regions to forest management practices have been examined using models for the Tyne River basin (U.K.) by Dunn andMackay [1995], in the Columbia River basin (U.S.) by Matheussen et al [2000], and in the Central Sierra region of California (U.S.) by Huffet al [2002] Agriculturalization The conversion of grasslands and forested lands to agri­ cultural use is quantitatively the most dramatic land use change that has yet taken place Roughly 10-12 million k m of land are currently thought to be under cultivation (compared to the 18 million k m of land that are potentially suitable for agricultural use) Obviously, the term "potentially suitable" is subject to debate, since some agriculture can be practiced successfully for short periods of time on even marginallysuitable lands Subsidies of energy and materials (water, fer­ tilizers, pesticides, herbicides, etc.) can extend production in these systems for longer periods Western cultivation prac­ tices—involving deep plowing, liming, manuring, mulching, crop rotation, and fallowing—had their origin in the brown for­ est soils of Europe, and were later successfully practiced in the eastern U.S When properly used, these practices can protect the soil from erosion, maintaining soil structure, fertility, and productivity for long periods of time When improperly used, 2 22 HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE or practiced in regions that once supported tropical forests, soils are often quickly depleted of nutrients—causing the "shift­ ing agriculture" that is so common in these areas In the droughty grassland regions of the mid-western U.S., improper cultivation, combined with a period of extreme drought, caused the Dust Bowl of 1931, during which to 12 inches of topsoil were literally blown off of the land, together with thousands of farmers The call for soil conservation was a major result of the 1931 Dust Bowl, but water and wind erosion are still major problems that are largely attributable to poor agricultural practices [Dansmann, 1972] Arnold et al [1995] simulated the changes in streamflow and sediment yields for several watersheds in Texas that have been "recovering" from the Dust Bowl episode In the White Rock Creek watershed (area of 257 km ) north of Dallas, both the observational data and modeling results indicated a significant decline in erosion and reservoir sedimentation during the period 1910 to 1984 The decline in sediment yields was attrib­ uted to the combined effects of conversion from rural land to 77% urban during this period, changes in agricultural crops (replacement of cotton by wheat and grain sorghum), and implementation of soil and water conservation measures begin­ ning in the 1940s In general, it is thought that those agricultural practices that can retard soil erosion are practices that will also increase infiltration, thus reducing and delaying surface runoff Con­ tour plowing, terracing, strip cropping, and no-till cropping are four newer practices that attempt to reduce erosion of the land by water Applications of the SCS method indicate that storm runoff from agricultural lands is almost always greater than runoff from natural forested or range lands, all else being equal, while reductions in CN values are predicted through the use of better or best management practices There are few observational or experimental studies that have demonstrated the effects of agriculturalization or agricultural management practices on hydrological processes at any scale One excep­ tion is the Goodwin Creek Research Watershed, a 21.3 k m basin in north central Mississippi where the percentage of cultivated land decreased from 26% to 12% over an 8-year period from 1982 to 1990 Using a combined experimen­ tal/modeling approach, Kuhnle et al [1996] showed that 42% of the decline in total fine sediment concentrations could be explained by simulated reductions from agricultural lands The researchers were able to attribute the decline in sediment concentrations to reductions in peak discharge during the growing season, which was shown to vary as a function of percent cultivation Several studies of changes in hydrological processes have also been conducted in the Driftless Area of southwestern Wisconsin, an area dominated by agriculture Trimble [1981] showed that rates of upland sheet and rill erosion and down­ 2 stream sedimentation declined by nearly 30% in the Coon Creek watershed (area of 360 k m ) between the periods 1850-1940 and 1940-1975 Potter [1991] showed a signifi­ cant decrease in flood peaks and winter/spring flood volumes in the East Branch of the Pecatonica River (area of 572 km ) during the period 1940 to 1986 Gebert andKrug [1996] per­ formed a similar time series analysis of annual flood peaks and 7-day low flows for gaging stations in the Driftless Area, thus demonstrating significant reductions in flood peaks and increases in low flows during the 20th century (but no trends for forested basins in northern Wisconsin during the same period) All three studies attributed these effects to changes in agricultural management practices, particularly the wide­ spread use of soil conservation practices Hydrologists have also concerned themselves with how agricultural practices can increase rates of evapotranspiration from crops (i.e., consumptive losses) In much of the U.S., for example, irrigation is a major off-stream use of both sur­ face and groundwater resources Consumptive water use by irrigated crops is a major component of the water balance at many scales [Solley et al, 1998] Among the most signifi­ cant hydrological impacts of agricultural activities, ground­ water withdrawals for irrigation in the western U.S have contributed to dramatic increases in evapotranspiration, exces­ sive declines in water tables, surface subsidence, and soil salinization 2 Urbanization The conversion of forest or agricultural land to urban use has major ramifications for hydrological processes In a review published nearly a half century ago, Savini and Kammerer [1961] summarized the hydrological effects of urbanization and qualitatively described the effects based on an analysis of sev­ eral different stages of land use change: (1) transition from preurban to early-urban, (2) transition from early-urban to middle-urban, and (3) transition from middle-urban to lateurban Savini and Kammerer [1961] also tried to distinguish between the hydrological effects associated with the human uses of water from effects associated with human uses of the land, but they noted that relatively few studies had yet been conducted in order to quantify the effects of urbanization on hydrological systems Common to all three stages of urban­ ization are: (1) decreases in transpiration from loss of vege­ tation, (2) decreases in infiltration due to decreased perviousness, (3) increases in storm runoff volumes, (4) increases in flood peaks, (5) declines in water quality from dis­ charges of sanitary wastes to local streams and rivers; and (6) reductions in baseflow Design, installation, and maintenance of urban drainage systems for collection and disposal (i.e., "routing") of stormwater to reduce damage from floods is a ESHLEMAN primary goal of urban engineers In general, hydrologists have found that increasing imperviousness associated with urban development (streets, roofs, sidewalks, parking lots, etc.) reduces infiltration, thus increasing storm runoff volumes while reducing baseflow volumes The reduced roughness of the impervious surfaces—together with increased hydraulic efficiency associated with the use of gutters, curbing, sew­ ers, artificial channels, and other components of modern storm drainage collection systems—decreases the lag time and increases the peaks of urban watershed responses to rainfall [Hollis, 1975; Chow et al, 1988] As noted in the previous sec­ tion, the state-of-the-art in urban hydraulic engineering is still to use the SCS method for rainfall-runoff analysis to determine the increase in runoff caused by urbanization Suburbanization is a "modern" (post-1950s) form of urban­ ization that is characteristic of most U.S metropolitan areas Prior to 1950, urbanization resulted in increasing population densities, with imperviousness often approaching 100% Sub­ urbanization involves a lesser increase in imperviousness on an areal basis, but a greater increase on a per capita basis Therefore, with standard methods of sewering, suburbaniza­ tion can potentially exacerbate hydrological impacts because much more land is consumed On the other hand, suburban development has recently tended to occur with somewhat more attention given to upstream detention of water, usually by providing areas for infiltration (lawns, parks, forests, etc.) and by construction of detention areas or ponds for water stor­ age and flood attenuation Empirical analyses and modeling results have both sug­ gested increases in flood potential associated with urbaniza­ tion and suburbanization [Fogel et al, 1974; Hollis, 1975; Sauer et al, 1983; Wigmosta and Surges, 1997; Beighley and Moglen, 2002; Jennings and Jarnagin, 2002] Hollis [1975] showed that floods with short return periods are not affected by increases in imperviousness of less than 5%, but that urban­ ization otherwise increases the magnitude of flooding, in some cases by a factor of ten The effect of urbanization was shown to decrease in relative terms as the return period increases, however [Hollis, 1975; Sauer et al, 1983] This is due to the fact that as flooding becomes more intense, larger percent­ ages of the watershed are contributing to the flood rather than providing storage For a return period of five years, Fogel et al [1974] reported that a desert watershed with 40% imper­ viousness would be expected to yield roughly five times as much storm runoff as a desert shrub watershed, similar to the results obtained by Hollis [1975] The effect of urbanization on flood potential was also predicted to decline with an increase in return period Recent modeling efforts suggest that suburbanization can also cause increases in flood peaks relative to forested control watersheds, with some of the response attributable to signif­ 23 icant runoff from lawns and other vegetated surfaces [Burges et al, 1998] Jennings and Jarnagin [2002] found an urban­ ization signal in the mean daily discharge record for a small watershed in northern Virginia, while Beighley and Moglen [2002] suggested that the preferred measure of the effect of urbanization is a relative maximum discharge to precipita­ tion ratio that can be used to address nonstationarity of a long time series In the United States, Donner et al [2002] used the IBIS/HYDRA model to simulate nitrate-nitrogen runoff in the Mississippi River basin from 1955 to 1994 The model­ ing results indicated that about 25% of the increase in nitratenitrogen export from 1966 to 1994 could be explained by an increase in runoff across the basin, mostly due to increases in precipitation In a similar vein, Jordan et al [2003] analyzed nutrient and runoff data from multiple gaging stations in the Patuxent River watershed in Maryland and found statisti­ cally-significant linear relationships between annual runoff and the percentage of developed land for both a wet year and dry year In both years, runoff increased proportionally with development, with 50% development producing a roughly 75% increase in runoff in the dry year and a 10%) increase in runoff in the wet year The response of runoff to develop­ ment also appeared to vary as a function of geology, with a greater response evident in the Coastal Plain watersheds than in the Piedmont watersheds, the latter more dominated by baseflow contributions [Jordan et al, 0 ] Dow and DeWalle [2000] performed a statistical analysis of long-term U.S.G.S streamflow data (and precipitation data) for 51 watersheds in the eastern U.S and reported statistically sig­ nificant negative trends in annual evapotranspiration asso­ ciated with urbanization At 100% urbanization, the results suggested a decrease in annual evapotranspiration of 22 cm DeWalle et al [2000] performed a similar analysis for 60 stations throughout the U.S and found that urbanization increased mean annual streamflow roughly in proportion to the increase in population density; complete urbanization was found to produce an average runoff increase of 103% relative to a rural control watershed Wetland Drainage Wetland drainage—usually performed to provide aerated organic soils for agricultural production—has occurred exten­ sively in coastal areas of Europe (especially England and the Netherlands) and later in North America (especially on the Delmarva Peninsula, in southern Florida, and throughout much of the mid-western U.S.) In the U.S., the USDA [1955] estimated that more than 400,000 k m of the U.S had been subjected to drainage development by 1950, an area roughly the size of the state of California Drainage of wetlands is 24 HYDROLOGICAL CONSEQUENCES OF LAND USE CHANGE accomplished most efficiently through the use of clay tiles or plastic drainage pipes that promote "underdrainage" (i.e., sat­ urated groundwater flow through the peat) to open ditches The clay tile is sometimes cited alongside the plow as one of the major technological achievements of modern agriculture! The earliest examples of wetland drainage were actually in the Fens in England These efforts were begun by the Roman emperor Hadrian and were continued by the Church during the Middle Ages Wetland drainage peaked in the middle of the 17th century under the Stuart kings, who hired the famous Dutch engineer, Cornelius Vermuyden, to develop more effec­ tive drainage measures Purseglove [1988] gives an interest­ ing history of some of the key hydrological problems encountered in drainage of the Fens, especially the problems associated with peat shrinkage Dewatering of peatlands pro­ motes peat aeration and an increase in rates of peat decom­ position In coastal systems, particularly, where excessive peat decomposition can cause a lowering of the land surface relative to river and sea levels, wetland drainage can increase the susceptibility of the land to both inland flooding and coastal inundation In the case of the Fens, reclamation of the marshes caused a gradual reduction in the river gradient, leading to a reduction in scouring, siltation of the river out­ fall to the estuary, and an increase in inland flooding In addi­ tion, due to land level declines relative to sea levels, coastal inundation of the Fens in 1673 and again in 1713 provided some extreme examples of the consequences of wetland drainage Similar problems were encountered during drainage of the Everglades in South Florida during the 20th century [Snyder and Davidson, 1994], At least one paper has dis­ cussed the potential benefits of restoring wetland hydrolog­ ical functions through mitigation, particularly the relative changes in baseflow and stormflow volumes that might be expected [Potter, 1994] Water Resources Development Many of the world's large rivers are effectively regulated through the construction and operation of dams, reservoirs, diversions, levees, and artificial channels that effectively alter the storage and conveyance of water in ways that are consid­ ered beneficial to mankind While not normally considered part of land use change, these large hydraulic structures are sim­ ply too significant to omit from a chapter focused on human intervention in the water cycle As in the case of small deten­ tion ponds for urban stormwater management, large dams and reservoirs serve a similar purpose: to attenuate or reduce the magnitude of flood peaks, enabling a slower release and thus more efficient use of river discharge for specific or mul­ tiple purposes (hydroelectric generation, domestic water sup­ ply, irrigation, etc.) Construction of levees and artificial channels (or channelization of natural rivers) is performed to achieve the opposite purpose: to reduce storage and more efficiently convey surface water downstream Coe and Foley [2001] recently used a coupled biosphere/hydrological routing model (IBIS/HYDRA) to estimate changes in the water balance of the Lake Chad drainage basin in northern Africa, where rainfall has been declining since the 1960s The model predicted that long-term decreases in lake area, lake level, and river discharge were primarily attrib­ utable to climatic variations, but increases in water losses from rapidly-growing irrigation explained a large portion of the variation Ozdogan and Salvucci [2004] investigated a related aspect of a regional-scale water development project and attributed a decline in potential evaporation in arid, south­ eastern Turkey to increasing irrigated land area, decreasing wind speed, and increasing atmospheric humidity Surface Mining/Reclamation In some areas of the world, surface mining for coal or other minerals (and subsequent reclamation of the grossly altered land surface) represents a significant land use change While acid mine drainage is a common problem associated mostly with underground mining, excavation of previously consoli­ dated geological strata, followed by replacement of uncon­ solidated fill materials, approximate restoration of original slopes, and revegetation have the potential to alter hydrolog­ ical processes in watersheds where surface mining is a major activity Soil compaction by heavy machinery during the recla­ mation process has been shown to reduce soil bulk density, porosity, and infiltration capacities [Chong et al, 1986], but infiltration can be promoted by deep ripping or tillage of the soil [Chong and Cowsert, 1997] Infiltration-excess overland flow is usually considered the dominant flow pathway in mined/reclaimed watersheds, but in some cases this may not be the case [Harms and Chanasyk, 2000] Bonta et al [1997] showed that peak flow rates and SCS curve numbers increased during the coal mining and reclamation phases at three water­ sheds in eastern Ohio relative to the pre-mining period, but no forested watershed was maintained as a control Restoration of normal hydrological functioning of these systems was shown to take appreciably longer than the normal five-year period associated with active reclamation and land manage­ ment [Schuster and Hutnik, 1987; Holl and Cairns, 1994; Malik and Scullion, 1998] SUMMARY AND RECOMMENDATIONS Hydrologists have recognized for nearly a half century that land use and land cover changes can substantially affect hydrological processes at the scale of the small plot or experimen- ESHLEMAN tal watershed Extraordinary improvements in data collec­ tion, data archiving, data distribution, and computational capa­ bilities now make it feasible to advance our understanding of land use change impacts to larger scales, such as river basins, where they can be fully included in the land management decision-making process Satellite- and land-based sensors have added unprecedented capability in observing land cover changes from space, but can also provide information about the spatial variability of key hydrological variables such as precipitation, soil moisture, and flooding within a region Such information is critical in identifying key hydrological system changes and in finding the land use change signal among the confounding climatic trends and variability Notwithstanding these new capabilities, identifying, quan­ tifying, and predicting the hydrological consequences of land use change have proven quite challenging for several reasons The relatively short lengths of most hydrological records, superimposed on the relatively high natural variability of hydrological systems, make it difficult to isolate a land use sig­ nal from hydroclimatological "noise" This issue is most sig­ nificant in "real" systems for which the timing and patterning of land use changes are not controlled, but it is even an issue for those small-scale studies for which land use modifica­ tions can be carefully imposed The problem is also compli­ cated by the n o r m a l paucity of detailed hydrometric measurements that are typically used to characterize the hydrological consequences of land use change Stream discharge data from one location in a watershed are most often the only data available to assess the hydrological impacts of land use change The relatively small number of controlled small-scale experimental studies that have been performed has also lim­ ited progress in extrapolating or generalizing results from such studies to other systems Given the diversity and com­ plexity of land use changes that are taking place around the world, satisfactory techniques for analyzing the hydrological consequences of land use must be considered to be in an early stage of development The development of mathematical tools (i.e., models) for reliably predicting the hydrological effects of future land use changes is in its infancy [Beven, 0 ] While this review has discussed many different approaches that have been used to quantify and understand the hydrological consequences of land use change, it must be empha­ sized that each of the present methods offers specific advantages and disadvantages relative to the other methods Given this situation, it is somewhat surprising that few stud­ ies of specific land use change impacts have employed mul­ tiple approaches For example, a major limitation of paired watershed studies is the obvious lack of experimental repli­ cation across a full range of natural conditions Fortunately, paired studies usually provide very high quality experimental data with which to advance our mechanistic understanding 25 of the hydrologic response of watersheds to land use change and allow testing of mathematical models The coordination of field hydrometric measurements with modeling of paired watersheds can help hydrologists better understand the responses of watersheds of varying size, topography, and spa­ tial configuration, thus contributing to improved mechanistic understanding of hydrological processes Other combined approaches should also begin to occur more frequently As the length of our temporal records of satellite imagery increase, it should be possible to incorporate such imagery directly into mathematical models that can be used to simulate changes over many years or even decades Such modeling exercises could be used, for example, to test explicit hypotheses regarding whether the spatial patterning of land use changes and disturbances within a watershed are important factors in hydrological change Long records of satellite imagery could also provide data with which to cor­ relate temporal records of hydrological variables from either observational studies or small catchment experiments There are very few examples in the literature where long time series of hydrological data have been subjected to this type of analy­ sis, usually because the specific history of land use change in an area cannot be easily reconstructed Finally, collaboration among scientists from different dis­ ciplines, though still problematic, is finally gaining acceptance as a necessary research approach to addressing the full dimen­ sion of global and regional land use change Perspectives from other fields such as ecology are clearly needed in order to consider the biological consequences of hydrological change, such as responses to water quality changes that often occur con­ temporaneously Economists and other social scientists are now beginning to provide input in the form of various sce­ narios of land use change that can be generated from models that consider dominant economic and/or social drivers All of these fields will ultimately be needed in order to provide a complete understanding of the hydrological effects of land cover changes in a watershed, river basin, or region Acknowledgments The author gratefully recognizes the support and encouragement provided by Ruth DeFries during the writing of this chapter The author also thanks an anonymous reviewer for thought­ ful suggestions for improving the chapter This chapter is scientific contribution no 3786 from the University of Maryland Center for Environmental Science REFERENCES Abbott, M B., J C Bathurst, J A Cunge, P E O'Connell, and J Rasmussen, An introduction to 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