Small scale spatial variability of water infiltration and its influencing factors

Contents Acknowledgements Abstract Introduction Review of literature 2.1 Variability of hydraulic conductivity (Ks) 2.2 Role of vegetation cover Methods 3.1 Site conditions 3.2 Soil samples 3.2.1 Soil dry bulk density 3.2.2 Experimental design and infiltration measurements 10 Results 12 4.1 Infiltration rates 12 4.1.1 Infiltration rate of soil under shrub site 12 4.1.2 Infiltration rate of soil under plantation forest site 13 4.2 Asymptotic infiltration 14 4.2.1 Asymptotic infiltration of soil under shrub area 14 Table 4.21: Asymptotic infiltration of soil under shrub area 14 4.2.2 Asymptotic infiltration of soil under forest plantation 15 Discussion 18 Limitations of using ring infiltrometer 19 Conclusions 20 References 21 Acknowledgements First of all, I would like to express my deepest gratitude with special thanks to MSc Phi Dang Son for his guidance and encouragement Similarly, I am so grateful and thankful to Prof Lee McDonald for his guidance To complete the research I also received a lots of helps from Dr Le, Dr Bui Xuan Dung and Ms Huyen who work in the Laboratory of Vietnam Forestry University, therefore I would like to say thank you to all of them for lending me the equipment to measure the parameters of my thesis Finally, I would thank all friends and everybody who contributed to this thesis Abstract This research was conducted to examine the small scale variability of infiltration and the effect of the lower vegetation layer (under plantation forest and shrub) on the water infiltration characteristics using the single ring infiltrometer method on Luot Mountain Infiltration rate was measured under plantation forest and shrub with replicates for each type of vegetation Findings of this study showed the asymptotic infiltration rate of soil under shrub area ranges from 1.6 to 2.5 higher than that under plantation forest Soil porosity and infiltration rate have a positive relationship Introduction The role of water as a driver of erosion has received considerable scientific study The loss soil due to erosion, much stronger than the creation of soil in a natural process, a few centimeter of soil can be lost only after some rains, thunderstorm or cyclone while to get a few inches of soil That should take hundreds of years, even thousands of years to create amount of soil Infiltration is the process by which water on the soil surface penetrates the soil The infiltration capacity of soil determines the amount of water which will go into the soil and the amount of water which will become runoff (Hillel, 1982) Therefore, the water infiltration rate can be considered as an important soil property which significantly influences the amount of surface runoff and subsequently soil erosion and water quality Quantifying the soil infiltration capacity is of great importance to understand and predict the hydrologic behavior of a system Water infiltration and movement in soil are controlled by the characteristics of pore space in soil, which are determined by the soil physical properties such as soil texture, aggregate stability, cracks and crusts at the soil surface The presence of macro pores and the consequently high soil porosity promote the infiltration process Other soil properties, such as the soil organic matter content, soil biological activities have strong effects on the soil structure and therefore they have impacts on the water infiltration Plant roots are also proved to enhance the water infiltration rate by creating channels within the soil Until now, soil conservation researches were aiming at erosion control However, soil protection also needs more knowledge about the impacts of vegetation type on other indicators, such as water infiltration into the soil profile Therefore, more concentration is required on the strong relationship between the vegetation cover types and their influences on water infiltration into soil The land cover (forest, shrubs) have different impacts on the infiltration capacity and water storage in the soils The knowledge about these relationships is very important to prevent or minimize soil water erosion and to guarantee high infiltration rates that will be beneficial under different climatic conditions In the case of humid region facing excessive rainfalls, increased soil infiltration capacity results in the storage of a great proportion of precipitation, reducing overland flows and flooding occurrence Whereas, in the arid areas where the rainfall is limited, high soil infiltration capacity keeps most of rainwater inside the soil preventing water loss by surface runoff and evaporation The presented study is a contribution to the investigations on factors influencing the water infiltration capacity of forest soil In Vietnam, the research on infiltration capacity of the soil is often accompanied by research forest hydrology, soil erosion, and surface runoff So far, there is few researches worked detail in infiltration capacity of the soil The majority of studies address only the flow velocity in one aspect is a factor influencing erosion and surface runoff Therefore, research on infiltration is necessary to provide good data for future researches in hydrology at the site, to estimate spatial variability of infiltration rates and identify the effects of vegetation types on the infiltration capacity of the soil Originating from that problem I have chosen the topic” Small-scale spatial variability of soil water infiltration and its influencing factors” It is assumed that the water infiltration characteristics would be varying significantly even within a relatively small scale as a result of those controlling factors evenly under the same vegetation cover Objectives - To examine the variability of infiltration rate at a small scale - To identify the effects of vegetation types on the infiltration capacity of the soil - To provide field data for future researches on hydrology at the site Kết thúc trò chuyện Review of literature 2.1 Variability of hydraulic conductivity (Ks) Saturated hydraulic conductivity is one of the most important parameters for soilwater-plant interactions, water and solute movement and retention through the soil profile It is a critically important parameter for estimation of various other soil hydrological parameters necessary for modeling the flow through the naturally unsaturated vadose zone Among different soil hydrological properties, the Ks is reported to have the greatest statistical variability by several authors (Biggar and Nielsen, 1981; Hern and Melancon, 1986; Webb et al., 2000) The variability of Ks is associated with soil types, land uses, positions on landscape, depths, instruments and methods of measurement and experimental errors (Stockton and Warrick, 1971) It has been suggested that more studies are required on the variability of Ks across different landscapes The variability of Ks has a profound influence on the overall hydrology of the soil system Therefore, focus of this review is centered on the variability of saturated /unsaturated hydraulic conductivity due to a large number of factors 2.2 Role of vegetation cover Many studies on streamflow in the world have shown the result that it often accounts for 1-3% of the total rainfall This is a low rate compared to other water balance components Many forest hydrologists observed that, in many cases amount of streamflow of bigger diameter trees is less than the small ones This may be due to differences in way branching by water in the amount falling down from the canopy of trees on the top floor to the lower layer Soil hydrologic condition is the result of interactions between soil and vegetation Infiltration rate and sediment yield integrate these factors and are good indicators of hydrologic condition (Thurow, 1986) Vegetation succession is the results of interactions between soil and vegetation, which induced changes in soil hydrology One consequence of this change is the amelioration of soil (Fisher 1990) resulting in an alteration of the hydrologic characteristics of the site (Thurow 1991) Under the cover of the vegetation, the accumulation of organic matter and the moderation of soil microclimate (Kittredge 1948) The enhanced soil structure that results from these factors improves infiltration The type and extent of vegetation are of primary importance in determining the amount and timing of infiltration and surface runoff (Dabrowolski et al 1990) Vegetation contributes to determine interception, infiltration, runoff, and erosion processes (Gifford 1985, Wood 1988, Calder 1993) Plants disperse raindrop energy, increase infiltration, reduce surface runoff velocity, filter sediment from runoff, and bind soil particles (Wischmeier and Smith 1978) The effect of plant cover in processes like interception, infiltration, and runoff can be expressed as a simple function of percentage of water that is intercepted by plants and the percentage of plant cover (Armstrong and Mitchell 1988) At the soil surface, plants and litter reduce the impact of rainfall energy and then increase infiltration and reduce surface runoff (Thompson and James 1985) Vegetation influences infiltration through the reduction of raindrop impact and the subsequent maintenance of favorable soil conditions for infiltration Soil texture, structure, and pore volume directly affect infiltration Maintenance of good soil structure, especially at the soil surface, will greatly increase the infiltration rate of water Temperature, seasons of the year and slope also have some minor influence on infiltration Plant cover probably has the greatest impact on infiltration which in turn defines other hydrologic processes such as surface runoff, percolation, and detention, as well as the erosion processes (Gifford 1985, Wood 1988, Wood and Eldridge 1993) Plant cover influences some of the rainfall properties as drop size and spatial distribution, which in turn are important factors for infiltration, surface runoff, and soil detachment (Armstrong and Mitchell 1988) Plant cover is an important parameter in water distribution within a watershed because: plants intercept raindrops and then reduce surface sealing and soil detachment by raindrops; plant stems and litter increase surface roughness and hydraulic resistance, and decrease surface runoff velocity; and plant roots bind soil and diminish soil erodability (Wischmeier and Smith 1978, Branson et al 1981, Abrahams et al 1988, Johnson and Gordon 1988, Brooks et al 1991) Consequently, infiltration rates are often observed to vary under different life forms (Blackbum 1975; Wood 1981; Knight 1984; Thurow, 1986) Recently, number of studies demonstrated the effect of increasing cover of ground-storey plants, particularly grasses, on reducing runoff and erosion (Pressland, 1982; Eldridge, 1993) Perennial plants are generally more effective than annual or ephemeral plants (Eldridge, 1992) Vegetation or plant cover is the percentage of ground surface covered with plant material, and must be referred to only as the vertical projection of the vegetation parts onto the ground, such vegetation is the one in which a person is interested (Barbour et al 1987, Causton 1988, Bonham 1989) Plant canopy depth is the distance from the top of the canopy to the bottom of the canopy (Armstrong and Mitchell 1988) Plant cover is normally expressed as a percentage (Causton 1988) Vegetation cover is of special importance in studies of competition for radiation, nutrients, and soil moisture Also, plant cover is very important in studying the water balance of a watershed and the soil water redistribution Scientists just focused on the interception role of lower vegetation cover as a second or third layer reduces the rainfall intensity They ignore the role in influencing water infiltration rate Therefore, I decided to investigate the role of lower vegetation on infiltration rate Methods 3.1 Site conditions The research was conducted in Luot Mountain, which is a part of Vietnam Forestry University campus and has large area of Pinus massoniana plantation, Imperata cylindrica grass, and shrubs The mountain has moderate mountainous terrain with two small mountains, the upper mountain is 133 meters above sea level and the lower mountain is 99 meters above sea level The average slope is relative high with about 15 20 º Luot Mountain has tropical monsoon climate The average temperature is 23.9◦C, the lowest temperature is 17.1◦C in January and the highest temperature is 28.5 ºC in June and July Average relative humidity is 81.5%, the highest humidity is 85.5% in March and the lowest humidity is 78 % in December Annual precipitation is 1647 mm/year The highest precipitation is in July and August above 300 mm and the lowest is 22 mm in December Soil is clay loam Discussion I will now discuss (a) why infiltration rate under forest layer with shrubs is higher than under forest layer without shrubs, (b) why infiltration rate at S1 has a strange trend, (c) limitations and recommendations Some studies mentioned effects of vegetation types on infiltration rates such as Pho (1992), Dzung (1993) They affirmed that forest trees can consume large amount of water In addition, they also affirmed forest soil also was a factor that affected permeable velocity The higher infiltration rate under forest layer with shrubs is due to the loosening of surface soil arising from lateral spread of roots Tree roots aid in improving soil structure in several ways One of the most significant plant-induced changes in soil structure is the formation of continuous macro-pores (i.e., channels) by penetrating roots (Angers and Caron 1998) A large proportion of pores formed by roots fall into the macropore range (>30 μm) (Gibbs and Reid 1988) These macro-pores facilitate soil aeration and water percolation and storage as well as create zones of failure, which help fragment the soil, form aggregates, and decrease resistance for further root growth Roots form macropores by creating compressive and shear stresses when growing through the soil matrix Radial pressure exerted by growing roots compresses adjacent soil (Dexter 1987), which enlarges existing pores and creates new ones Bartens et al (2008) demonstrated that live roots can create channels through compacted soils and vastly increase water infiltration, although flow may be greater once roots die and decay (Mitchell et al 1995) As root decay occurs, tissue remnants and associated micro-flora coat pore walls, which may enhance water transport efficiency (Barley 1954; Yunusa et al 2002) Point S1 in under forest layer with shrubs did not follow Horton’s law because at this point there a dead root under soil that made an about 0.3cm diameter hole that 18 enhanced water go through soil faster There also was a big stone at 10 cm under soil surface That leaded to a strange curve as we saw at (Figure 4.11) Limitations of using ring infiltrometer Although this method has the advantage of being easy to use and easy to interpret, it does have its disadvantages For example, the water inside the infiltration ring often flows horizontally through the soil as well as vertically, thus giving results greater than they would be achieved if the flow was confined only to downward movement through the soil profile There are ways for correction this but for comparative purposes they are not necessary There assumption has to be made that the proportion of the water which moves sideways is always similar As only a small area is used, this technique is very sensitive to worm and root holes and other cracks in the soil Any crack in the soil surface will result in much faster flow than would otherwise be achieved As these cracks are often not visible at the surface it is not always possible to avoid them when choosing a site 19 Conclusions I investigated the variability of infiltration and the effects of vegetation types on the soil infiltration capacity by measuring infiltration rates and soil porosity in two different vegetation cover types (under plantation forest and shrubs) The vegetation cover types contribute different role in protecting soil especially forest soil The research data provided us a relationship between vegetation cover types and infiltration rates; infiltration rates and soil porosity After conducted this project, our findings were that the The asymptotic infiltration under forest plantation ranges between 17and 19 cm/hour and between 27 and 48 cm/hour with shrubs present Findings of this study showed (1) the asymptotic infiltration rate of soil under shrub area ranges between 1.6 and 2.5 higher than that under plantation forest; (2) soil porosity and infiltration rate have a positive relationship; (3) the average initial infiltration rate of soil under shrub is more than times higher than the one under plantation forest; (4) the variability of infiltration within shrub cover area ranges between 1.7 and 1.8 times and 1.1 times change in plantation 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