Estimation of potential evapotranspiration and its variability in the upper and middle reaches of the Heihe River basin

On the basis of the MODIS LAI remote sensing products, China Meteorological Forcing Dataset and other multi-source datasets, the Shuttleworth-Wallace (S-W) model, which considers the temporal variation of leaf area index (LAI), was applied to estimate potential evapotranspiration (ET_P) during the period of 2000—2010 in the upstream and midstream areas of the Heihe River basin. To investigate the temporal and spatial variability of ET_P, sensitivity of ET_P to meteorological parameters and LAI was calculated for each land cover type. To compare ET_P and ET₀ values for runoff simulation, a conceptual hydrologic model with precipitation and ET_P or ET₀ was developed in the upstream area of the Heihe River basin. Results showed that: ① S-W model, based on high-resolution remote sensing datasets, was able to accurately estimate annual and seasonal ET_P in large river basins with complex underlying surfaces as well as in ungauged basins. The values of ET_P in summer contributed more to the annual value than those of other seasons. ET_P in areas with different land cover types varied greatly. Generally speaking, ET_P decreased with the development of vegetation. ② In the study area, ET_P was most sensitive to relative humidity. The sensitivity of ET_P to LAI was as great as that to net solar radiation. The sensitivity of ET_P to meteorological parameters and LAI became greater with the decline of vegetation. ③ Compared with hydrologic model with ET₀ calculated by P-M model, the model with ET_P better simulated the depth of runoff at Yingluoxia hydrologic station. Hence, ET_P calculated by S-W model was more suitable for the simulation of runoff in the present study.