Daily Reference Evapotranspiration for Hyper-Arid to Moist Sub-Humid Climates in Inner Mongolia, China: II. Trends of ET0 and Weather Variables and Related Spatial Patterns

Ren X, Martins DS, Qu Z, Paredes P, Pereira LS
Water Resour Manage (2016) 30:37933814 DOI 10.1007/s11269-016-1385-8

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This study focuses on assessing trends of reference evapotranspiration (ET0) considering aridity. Weather data sets of 5462 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 ET0 computed with the FAO Penman-Monteith method (PM-ET0) using full data sets of maximum and minimum temperature (Tmax and Tmin), sunshine duration (SD) used to compute net radiation, relative humidity (RH) and wind speed (WS). Trends were also assessed for ET0 computed with the Hargreaves-Samani temperature eq. (ET0 HS) and the Penman-Monteith equation with temperature estimates of solar radiation and actual vapour pressure (ET0 PMT). In addition, trends relative to Tmax, Tmin, SD, RH and WS were assessed. Trends for PM-ET0 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 Tmax and Tmin and of decreasing trends for SD, RH and WS. Therefore, negative trends of ET0 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 ET0. Trends were coherent when considering seasonality influences. Contrarily, results for the temperature methods, ET0 PMT and ET0 HS, always identified increased trends for ET0 due to warming effects. These results show that it is inappropriate to assess ET0 trends when using simplified temperature methods.