DSpace at VNU: Potential evapotranspiration estimation and its effect on hydrological model response at the Nong Son Basin

DSpace at VNU: Potential evapotranspiration estimation and its effect on hydrological model response at the Nong Son Bas...

VNU Journal of Science, Earth Sciences 24 (2008) 213-223 Potential evapotranspiration estimation and its effect on hydrological model response at the Nong Son Basin Vu Van N ghi1’*, Do Duc Dung2, Dang Thanh Lam2 ' State Key Laboratory o f Hydrology, Water Resources and Hydrauỉic Engineering, Hohai ưniversity, China Southern Institute fo r Water Resources Planning, Ho Chi Minh City Received November 2008; received in revised fonn 28 November 2008 Abstract The potential evapotranspưation can be dứectly calculated by the Penman-Monteith equation, known as the one-step method The approach requừes data on the land cover and relatedvegetation parameters based on AVHRR and LDAS iníbrmation, which are available in recent years The Nong Son Basin, a sub-catchment of the Vu Gia - Thu Bon Basin m the Central Vietnam, is selected for this study To this end, NAM model was used; the obtained results show that the NAM model has a potential to reproduce the eíĩects of potential evapoữanspừation on hydrological response This is seemingly imnifesteđ in the good agreement between the model simulation o f discharge and the observed at the sứeam gauge Keywords: Potentíal evapotranspừation; Penman-Monteith method; Piche evaporation; Leaf area index (LAI); Normalized diíĩerence vegetation index (NDVI) Introduction methođ, but it is expensive and thus practical only in researches over a plot for a short time The pan or Piche evaporation has long records One o f the key inputs to hydrological with dense measurement sites However, to modeling is potential evapotranspiration, which apply it in hydrological models, fĩrst, a refers to the maximum meteorologically pan/Piche coeíĩicient Kp, and then a crop evaporative power on land surface Two ldnds coeíĩicient Kc must be multiplied as well Due o f potential evapoừanspiration are necessary to to the difference on sitting and weather be deíĩned: either from the interception or from conditions, Kp is often expressed as a íunction the root zone when thẻ interception is exhausted o f local environmental variables such as wind but soil water is freely available, speciíĩcally at field capacity [11, 32] speed, The huxnidity, actual upwind fetch, etc A global equation o f Kp is stíll unavailable The values of evapotranspiration is distinguished from the Kc from the literature are empirical, most for potential through the limitations imposed by the agricultural crops, and subjectìvely selected water dìcit Evapotranspiration can be dứectly Moreover, the observed Piche data show some measured by lysimeters or eddy correlation erroneous results which are diíTicult to explain [4], and the pan evaporameter is considered to CoiTesponđing author Tcl.: 0086-1585056977 be ừiaccurate [8, 10] On the other hand, a great E-mail: vuvannghi@yahoo.com v.v Nghi et a i / VNU Ịoumal o f Science, Earth Sàences 24 (2008) 213-223 214 number o f evaporation models has been developed and validated, from the single climatic variable driven equations [29] to the energy balance and aerodynamic principle combination meửiods [23] Among them, probably the Penman equation is the most physically sound and rigorous Monteith [20] generalized the Penman equation for waterstressed crops by introducing a canopy resistance Now the Penman-Monteith model is widely employed As a result, in this study the PenmanMonteith method is selected to compute directly potential evapotranspiration according to the vegetation dataset at 30s resolution based on AVHRR (Advanced Very High Resolution Rađiometer) and LDAS (Land Data Assimilation System) iníormation for the Nong Son catchment To assess the suitability o f this approach, the conceptual rainfall-runoff model kxiovvn as NAM [8] is used to examine its eíĩect on hydrological response Potential evap o tran sp iratio n description model 2.1 Penman-Monteith equation - G ) + P.c (e, ~ ea) ( 1) ẢE T = The Penman-Monteith approach as formulated above includes all parameters that govem energy exchange and corresponding latent heat flux (evapotranspiration) from uniíbrm expanses of vegetation Most o f the parameters are measured, or can be readily calculated from weather data The equation can be utilized for the direct calculation of any crop evapotranspiration as the surface and aerodynamic resistances are crop specific 2.2 Factors andparam eters determining E T 2.2 A +ỵ where ET is the evapotranspiration rate (mm.d' '), Á is the latent heat o f vaporization (= 2.45 MJ.kg '), R„ is the net radiation, G is the soil heat flux (with a relatively small value, in general, it may be ignored), e, is the saturated vapor pressure, e„ is the actual vapor pressure, (e, - ea) represents the vapour pressure deíĩcit o f the air, Pa is the mean air density at constant Ị Land surface resistance parameterừation a Aerodynamỉc resistance The rate o f water vapor transíer away from the ground by turbulent diffusion is conừolled by aerodynamic resistance ra, (s.m'1) which is inversely proportional to wind speed and changes with the height o f the vegetation covering ĩring the ground, as: _ Potential evapotranspiration can be calculated directly with the Penman-Monteith equation [3] as follows: AK pressure, Cp is the speciíic heat o f the air (= 1.01 ld.kg'1 K '1), A represents the slope o f the saturation vapour pressure temperature relationship, ỵ is the psychrometric constant, and r, and are the (bulk) suríầce and aerodynamic resistances lnKZ« ~ d )Ị 2o M { Ze (2) where zu is the height o f wind measurements (m); zt is the height o f humidity measurements; d is the zero plane displacement height (m); zom is the roughness length goveming momentum transfer (m); zoh is the roughness lengứi goveming ừansfer o f heat and vapour (m); U j is the wind speed; and K is the von-Karman constant (= 0.41) M any studies have explored the nature of the wind regime in plant canopies d and zom have to be considered when the suríace is covered by vegetation The factors depend upon tìie crop height and architecture Several empirical equations [6, 12, 21, 31] for estimating d, zom and z0)ị have been developed In this study, the V V Nghi et aỉ / V N U Ịoum al o f Science, Earth Sciences 24 (2008) 213-223 estimate can be made o f by assuming [5] that zom = 0.123 hc and zoh = 0.0123 hc, and [21] that d = 0.67 hc, where hc (m) is the mean height of the crop b Surface resistance The "bulk" surface resistance describes the resistance o f vapor flow through transpưing crop and evaporating soil suríace Where the vegetation does not completely cover the soil, the resistance íactor should indeed include the effects o f the evaporation from the soil surface If the crop is not transpinng at a potential rate, the resistance depends also on the water status o f the vegetation An acceptable approximation [1, 3] to a much more complex relation o f the surface resistance o f fully dense cover vegetation is: where r/ is the bulk stomatal resistance o f the well-illuminated (s.m '1), and LAIactìve is the active (sunlit) lea f area index (m2 leaf area over m2 soil suríace) A general equation for LAIactivt is [2,16, 30]: LAỈacll„ = S L Ả l (4) The bulk stomatal resistance r/ is the average resistance o f an individual leaf This resistance is crop specific and differs among crop varieties and crop management It usually increases as the crop ages and begins to ripen There is, however, a lack of Consolidated iníbrmation on changes in r, over the time for diíĩerent crops The iníbrmation available in the literature on stomatal resistance is often oriented towards physiological or ecophysiological stuđies The stomatal resistance is iníluenced by climate and by water availability However, the iníluences vary from one crop to another and diíĩerent varieties can be affected differently The resistance increases when the crop is water stressed and the soil water availability limits crop evapotranspưation Some studies [14, 15, 19, 33] indicate that stomatal resistance is 215 iníluenced to some extent by radiation intensity, temperature and vapor pressure deficit If the crop is amply supplied with water, the crop resistance rs reaches a minimum value, known as the basis canopy resistance The transpiration of the crop is then maximum and referređ to as potential ừanspiration The relation between r, and the pressure head in the root zone is crop dependent Minimum values o f rs range from 30 s.m '1 for arable crops to 150 s.m'1 for forest For grass a value o f 70 s.m '1 is often used [10] It should be noted that r, cannot be measured directly, but has to be derived from the Penman-Monteith formula where E T is obtained from, for example, the water balance o f a lysimeter The Leaf Area Index (LAI), a dimensionless quantity, is the leaf area (upper side only) per unit area of soil below it The active LAI is the inđex of the leaf area that actively contributes to the surĩace heat and vapor transfer It is generally the upper, sunlit portion o f a dense canopy The LA I values for various crops differ widely but values o f 3-5 are common for many mature crops For a given crop, the green LA I changes throughout the season and normally reaches its maximum beíore or at flowering L Ả I further depends on the plant density and the crop variety Several studieđ and empirical equations [19, 31] for the estimate of L A I have been developed If hc is the mean height o f the crop, then the L A Ỉcsn be estimated by [1]: L A l = 2Ahc L A I = 5.5 + 1.51n(/tc) (clippedgrassw ith0.05< hc ETp.M of about 27% Based on the climatic characteristics in Table 2, ETp.M shows a closer accord with the seasonal distribution Fig shows that ETpiche values are somewhat unrealistic, for example, potential evaporation in June 1985 has an average value of mm/day which is too high for any natural tropical humid area This result agrees with that of Nguyen [4] that the observed Piche data often give eưoneous outputs Fig Companson of monthly potentìal evapoữanspứation estìmated by the Penman-Monteith method and Piche tube data in the 1980-2004 period In order to assess íurther the suitability of ửie potential evapoứanspưation estimated dừectly by using the Penman-Monteith method and that derived from the Piche data, ửie NAM conceptual model was used to simulate the hydrology of the study area in the 1983-2003 period The NAM model períòrmance is evaluated with a set of two statìstical criteria: bias and NashSutcliffe eíĩiciency coeíĩicient [22] Table Performance measures of two potentìal evapotranspữation inputs during the simulatìon period (1983-2003) for the Nong Son catchment Perĩormance statistics ETpm 3.100 Bias (%) Nash-Sutcliíĩe efficiency, R2 0.880 ETpxkc -2.636 0.802 Discharge simulated by using the input data of ETpicte and ETp.si is shown as monửily averages in Fig Períbrmance measures are v.v Nghi et nì / V N U Ịourruứ o f Science, Earth Sciences 24 (2008) 213-223 222 given in Table While the overall simulated discharge with the input o f ETp.M is slightly smaller than the observed one, in the case of ETpiche it is the reverse However, the overall water balances (bias) in both cases are realistic (less than 5%) The good thing here is that ETp.M provides a better model performance in the term of the Nash-Sutcliíĩe efficiency (0.880) against that o f ETpiche (0.802) with respect to the model simulation of the discharge at the stream gauge Fig Observed vs simulated monthly discharges for the 1983-2003 period using the potential evapotranspứation inputs of ETpiche and ETp.M Conclusions Acknowledgcmcnts The Penman-Monteith method was used to compute directly the potential evapotranspiration for the Nong Son catchment The approach was assessed the suitability through the hydrological model response períormance The result o f this approach shows a close agreement between the simulated and observed discharges at the stream gauge in comparison with Piche observation The main conclusion here is that the PenmanMonteith evapotranspiration is more reliable than the Piche method as well as using pan data Although the approach requires the data on land cover and vegetation-related parameters, these data are available on internet in recent years Hence, due to the importance of evapotranspiration in water balance, the Penman-Monteith method is recommended as the sole Standard method to apply for similar catchments The authors would like to thank the Danish Hydraulic Institute (DHI) for providing the NAM software license, and the Southern Institute o f Water Resources for data support R eíerences [1] R.G Allen, A penm an for all seasons, Jour o f Irr & D rainage Engirteering 112(1987) 348 [2] R.G AUen, Irrigation enginecring principles, Utah State ưnivcrsity, Utah 12 (1995) 108 [3] R.G Allen, L s Pereira, D Raes, M Smith, Crop evapotranspiration-guidelines fo r computing crop water requirem ents, FAO Inigation and Dráinge Paper 56, Rom e, 1998 [4] N.N Anh, The evaluation o f w ater resources in the Eastem Nam Bo, Project K C 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(Land Data Assimilation System) iníbrmation AVHRR provides iníbrmation on globe land classiíìcation at 30 s resolution [13] Fig 2.b shows the vegetation classifícation at 30 s resolution for the. .. Net long wave radiation The exchange o f long wave radiation L„ (MJ.m'2.day ') between vegetation and soil on the one hand, and atmosphere and clouds on the other, can be represented by the following