Analysis of changing pan evaporation in the arid region of Northwest China

Decreases in pan evaporation (Ep) over the last decades have been reported in many regions of the world. In this study, we investigated Ep dynamics in the hyper-arid region of China during the period 19582010 using a generic physical model based on long-term meteorological data collected at 81 ground-based meteorological stations. We also quantified the contribution of climatic factors to the Ep change using partial derivatives. We found that Ep in the region exhibited an obvious decreasing trend until early 1990s (1993), at a rate of 6.0 mm yr2. However, the downward trend reversed in 1993, and the rate of increase after that was 10.7 mm yr2. We also assessed the sensitivity of rates of evaporative demand to changes in aerodynamic and radiative components, and found that pan evaporation could be mostly attributed to changes in the aerodynamic component, with some regional contributions from solar irradiance. Observed near-surface wind speed is the primary contributor to the decline of pan evaporation during 19581993, while wind speed (WS) and vapor pressure deficit (VPD) were both major contributors to the increase of pan evaporation after 1993.