Estimating potential evapotranspiration using Shuttleworth–Wallace model and NOAA-AVHRR NDVI data to feed a distributed hydrological model over the Mekong River basin

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 Shuttleworth–Wallace (S–W) model is developed for their estimation. In this parameterization, neither experimental measurement nor calibration is introduced. Based on IGBP land cover classification, the typical thresholds of vegetation parameters are drawn from the literature. The spatial and temporal variation of vegetation LAI is derived from the composite NOAAAVHRR NDVI using the SiB2 method. The CRU database supplies with the required meteorological data. They are all publicly available. The developed S–W model is applicable at the global scale, particularly to the data-poor or ungauged large basins. Using the century monthly time series of CRU TS 2.0 and the monthly composite NOAAAVHRR 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 changed significantly with LAI. By replacing the monthly with the 10-day composite NDVI and the albedo of 0.10 with 0.15 for substrate soil surface, annual PET relatively decreases less than 4% and 1.7%, respectively over the whole basin. The correlation with pan evaporation (Epan) is quite scattered but grouped with the vegetation types and consistent with a rough ratio as reported