Changes in reference evapotranspiration over an agricultural region in the Qinghai-Tibetan plateau, China

Reference evapotranspiration (ET₀), as an estimate of the evaporative demand of the atmosphere, has been receiving extensive attention in researches on hydrological cycle. Sensitivity of ET₀ to major climatic variables has significant applications in climatology, hydrology, and agrometeorology and is also important to improve our understanding of the connections between climatic conditions and ET₀ variability. In this study, we used the Penman-Monteith equation to calculate ET₀ and adopted a nondimensional sensitivity coefficient formula to analyze sensitivities of ET₀ to four climatic variables based on daily meteorological data from eight meteorological sites in the Huangshui River basin and surrounding areas during 1961–2010. The results indicated that (1) strong correlations with R ² up to 0.76 exist between observed E ₚₐₙ and calculated annual ET₀; (2) ET₀ had a decreasing trend in the Huangshui River basin (HRB) during 1961–2010; (3) Spatially, distribution of ET₀ was largely correlated with altitude, for instance, the average annual ET₀ was larger in low-altitude areas than in high-altitude areas; (4) ET₀ was more sensitive to actual vapor pressure in high-altitude areas while it was more sensitive to temperature in low-altitude areas; and (5) ET₀ showed a decreasing trend and was consistent with the decreases in net radiation and wind speed at seasonal and annual time scales in HRB during 1961–2010. Sensitivity analysis of ET₀ to major climatic variables revealed that temperature was primarily responsible for changes in ET₀ in the growing season while actual vapor pressure was the dominating factor causing changes in ET₀ in the nongrowing season. However, annual averaged ET₀ was more sensitive to actual vapor pressure (R ² = 0.63), indicating that actual vapor pressure was possibly the primary climatic variable that causes changes in annual ET₀.