Climatic controls on watershed reference evapotranspiration vary dramatically during the past 50 years in southern China

Abstract. Reference evapotranspiration (ETo) is an important hydrometeorological term widely used in water resource management, hydrological modeling, and understanding and projecting the hydrological effects of future climate change and land use change. Identifying the individual climatic controls on ETo helps better understand the processes of global climatic change impacts on local water resources and also simplify modeling efforts to predict actual evapotranspiration. We conducted a case study on the Qinhuai River Basin (QRB), a watershed dominated by a humid subtropical climate and mixed land uses in southern China. Long term (1961–2012) daily meteorological data at six weather stations across the watershed were used to estimate ETo by the FAO-56 Penman−Monteith model. The seasonal and annual trends of ETo were examined using the Mann−Kendall nonparametric test. The individual contributions from each meteorological variable were quantified by a detrending method. The results showed that basin-wide annual ETo had a decreasing trend during 1961–1987 due to decreased wind speed (WS), solar radiation (R_s), vapor pressure deficit (VPD), and increased relative humidity (RH). These variables had different magnitudes of contribution to the ETo trend in different seasons examined during 1961−1987. However, during 1988–2012, both seasonal and annual ETo showed an increasing trend, mainly due to increased VPD and decreased RH and, to lesser extent, to decreased absolute humidity (AH) and a rising air temperature. We show that the key climatic controls on ETo have dramatically shifted as a result of global climate change during the past five decades. Now the atmospheric demand, instead of air temperature alone, is a major control on ETo. Thus, we conclude that accurately predicting current and future ETo and hydrological change under a changing climate must consider changes in VPD (i.e., air humidity and temperature) in the study region. Water resource management in the study basin must consider the increasing trend of ETo to meet the associated increasing water demand for irrigation agriculture and domestic water uses.