Daily Reference Evapotranspiration for Hyper-Arid to Moist Sub-Humid Climates in Inner Mongolia, China: II. Trends of ETo and Weather Variables and Related Spatial Patterns

This study focuses on assessing trends of reference evapotranspiration (ETo) considering aridity. Weather data sets of 54-62 years of Inner Mongolia, a Chinese Province where climate varies from hyper-arid in the West to wet sub-humid in the East, were used. Trends were analyzed for ETo computed with the FAO Penman-Monteith method (PM-ETo) using full data sets of maximum and minimum temperature (T-max and T-min), sunshine duration (SD) used to compute net radiation, relative humidity (RH) and wind speed (WS). Trends were also assessed for ETo computed with the Hargreaves-Samani temperature eq. (ETo HS) and the Penman-Monteith equation with temperature estimates of solar radiation and actual vapour pressure (ETo PMT). In addition, trends relative to T-max, T-min, SD, RH and WS were assessed. Trends for PM-ETo show to vary with aridity, with decreasing trends in the areas marked by aridity in the West and increased trends in less arid and sub-humid areas in the East. The detected trends are well explained by the trends in weather variables which consist of large increasing trends of T-max and T-min and of decreasing trends for SD, RH and WS. Therefore, negative trends of ETo occur where impacts of increases in temperature and decreases in RH are smaller than impacts of declining SD and WS; otherwise, when warming influences are larger it results a positive trend for ETo. Trends were coherent when considering seasonality influences. Contrarily, results for the temperature methods, ETo PMT and ETo HS, always identified increased trends for ETo due to warming effects. These results show that it is inappropriate to assess ETo trends when using simplified temperature methods.