app47Estimating potential evapotranspiration using Shuttleworth-Wallace model and NOAA-AVHRR NDVI data to feed a distributed hydrological model over the Mekong River basinIntroductionEvapotranspiration modelShuttleworth-Wallace (S-W) modelParameterization of S-W modelClimate-related parametersAerodynamic resistancesBulk stomatal and boundary layer resistancesSurface resistance of substrate soilNet radiations over vegetation canopy, substrate soil surface and soil heat fluxNet radiation over vegetation canopyRadiation flux over substrate soil surfaceSoil heat fluxVegetation parametersLeaf area index (LAI)Vegetation heightLeaf widthPrescribed parametersHydrological modelSoil water balance and runoff generation in BTOPMC modelRoot zoneUnsaturated zoneSaturation zoneModification of the evapotranspiration module of BTOPMC modelInterception and its evaporationSoil moisture in root zone and actual evapotranspirationStudy region and input dataMekong River basinData collection and processingTopographic dataLand coverSoil typeNDVI dataDischarge dataMeteorological dataPrecipitationTemperatureCloud cover and actual vapour pressureResults and discussionYearly and seasonal change of potential evapotranspiration and LAIEffect of albedo of substrate soil surface on potential evapotranspirationComparison among potential evapotranspiration, reference evapotranspiration and pan evaporationPotential evapotranspiration for vegetation interceptionHydrological modeling with the S-W estimated PET0 and PET as inputSummary and conclusionsAcknowledgementsSolar radiationReferencesEstimating potential evapotranspiration usingShuttleworth–Wallace model and NOAA-AVHRRNDVI data to feed a distributed hydrologicalmodel over the Mekong River basinM.C. Zhou*, H. Ishidaira, H.P. Hapuarachchi, J. Magome,A.S. Kiem, K. TakeuchiDepartment of Civil and Environmental Engineering, Interdisciplinary Graduate School of Medicine and Engineering,University of Yamanashi, Takeda 4-3-11, Kofu 400-8511, JapanReceived 10 February 2005; received in revised form 14 November 2005; accepted 15 November 2005Summary One of key inputs to hydrological modeling is the potential evapotranspiration,either from the interception (PET0) or from the soil water of root zone (PET). The Shuttle-worth–Wallace (S–W) model is developed for their estimation. In this parameterization, nei-ther experimental measurement nor calibration is introduced. Based on IGBP land coverclassification, the typical thresholds of vegetation parameters are drawn from the literature.The spatial and temporal variation of vegetation LAI is derived from the composite NOAA-AVHRR NDVI using the SiB2 method. The CRU database supplies with the required meteorolog-ical data. They are all publicly available. The developed S–W model is applicable at the globalscale, particularly to the data-poor or ungauged large basins.Using the century monthly time series of CRU TS 2.0 and the monthly composite NOAA-AVHRR NDVI from 1981 to 2000, annual PET is estimated 1354 mm over the Mekong River basin,spatially distributed strikingly non-uniformly from 300 to 2040 mm, and seasonally changedsignificantly with LAI. By replacing the monthly with the 10-day composite NDVI and the albedoof 0.10 with 0.15 for substrate soil surface, annual PET relatively decreases less than 4% and1.7%, respectively over the whole basin. The correlation with pan evaporation (Epan) is quitescattered but grouped with the vegetation types and consistent with a rough ratio as reportedKEYWORDSPotentialevapotranspiration;Physically-baseddistributed models;Land cover;NOAA-AVHRR NDVI;Mekong river0022-1694/$ - see front matter ª 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jhydrol.2005.11.013*Corresponding author. Tel.: +81 55 220 8727; fax: +81 55 253 4915.E-mail addresses: [email protected] (M.C. Zhou), [email protected] (H. Ishidaira), [email protected](H.P. Hapuarachchi), [email protected] (J. Magome), [email protected] (A.S. Kiem), [email protected](K. Takeuchi).Journal of Hydrology (2006) 327, 151– 173available at www.sciencedirect.comjournal homepage: www.elsevier.com/locate/jhydrolin the literature. In contrast, the PET and the reference evapotranspiration (RET) are vegeta-tion-type-dependently correlated very well. The PET0is estimated 1.63 times of PET in averageover the whole basin. The application of BTOPMC model shows that the derived LAI, PET0andPET behave very well in the distributed hydrological modeling.ª 2005 Elsevier B.V. All rights reserved.IntroductionOne of key inputs to hydrological modeling is the potentialevapotranspiration, which refers to maximum meteorologi-cally evaporative power on land surface. Two kinds of po-tential evapotranspiration are necessary to be defined:either from the interception, denoted by PET0, or fromthe root zone when the interception is exhausted but soilwater is freely available, specifically at field capacity (Fed-erer et al., 1996; Vorosmarty et al., 1998), denoted by PET.The actual evapotranspiration is distinguished from the po-tential through the limitations imposed by the water deficit.Evapotranspiration can be directly measured by lysime-ters or eddy correlation method but expensively and practi-cally only in research over a plot for a short time. The panevaporation has long records with dense measurement sites.To apply it in hydrological models, however, first, a pan coef-ficient, Kp, then a crop coefficient, Kc, must be multiplied.Due to the difference on sitting and weather conditions, Kpis often expressed as a function of local environmental vari-ables such as wind speed, humidity, upwind fetch, etc. A glo-bal equation of Kpis still lack. The values of Kcfrom theliterature are empirical, most for agricultural crops, and sub-jectively selected. On the other hand, a great number ofevaporation models have been developed and validated,from the single climatic variable driven equations (e.g.Thornthwaite, 1948) to the energy balance and aerodynamicprinciple combination methods (e.g. Penman, 1948). Amongthem, probably the Penman equation is physically soundestand most rigorous. Monteith (1965) generalized the Penmanequation for water-stressed crops by introducing a canopyresistance. Shuttleworth and Wallace (1985) extended thePenman-Monteith method to the sparse vegetation to con-sider two coupled sources in a resistance network: the tran-spiration from vegetation and the evaporation fromsubstrate soil. Now