Estimation of Regional Evapotranspiration in Alpine Area and Its Response to Land Use Change: A Case Study in Three-River
Headwaters Region of Qinghai-Tibet Plateau, China

Three-River Headwaters (TRH) region involved in this paper refers to the source region of the Changjiang (Yangtze) River, the
Huanghe (Yellow) River and the Lancang River in China. Taking the TRH region of the Qinghai-Tibet Plateau as a case, the annual
evapotranspiration (ET) model developed by Zhang et al. (2001) was applied to evaluate mean annual ET in the alpine area, and the
response of annual ET to land use change was analyzed. The plant-available water coefficient (w) of Zhang′s model was revised by using
vegetation-temperature condition index (VTCI) before annual ET was calculated in alpine area. The future land use scenario, an input of
ET model, was spatially simulated by using the conversion of land use and its effects at small regional extent (CLUE-S) to study the
response of ET to land use change. Results show that the relative errors between the simulated ET and that calculated by using water
balance equation were 3.81% and the index of agreement was 0.69. This indicates that Zhang′s ET model based on revised plant-
available water coefficient is a scientific and practical tool to estimate the annual ET in the alpine area. The annual ET in 2000 in the
study area was 221.2 mm, 11.6 mm more than that in 1980. Average annual ET decreased from southeast to northwest, but the change
of annual ET between 1980 and 2000 increased from southeast to northwest. As a vast and sparsely populated area, the population in
the TRH region was extremely unbalanced and land use change was concentrated in very small regions. Thus, land use change had little
effect on total annual ET in the study area but a great impact on its spatial distribution, and the effect of land use change on ET
decreased with increasing precipitation. ET was most sensitive to the interconversion between forest and unused land, and was least
sensitive to the interconversion between cropland and low-covered grassland.