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Reference evapotranspiration (ET 0 ) is the key factor for hydrologic water balance, irrigation scheduling, and water resources planning. Based on Food and Agriculture Organization (FAO) Penman-Monteith method and the climate variables of 57 meteorological stations from 1960 to 2010 in southwest China, the spatial and temporal distributions of ET 0 were analyzed by using Mann-Kendall test and Sen's slope estimator. Sensitivity coefficient was used to analyze the sensitivities of ET 0 to four climate variables, and the key climate variables attributed to ET 0 change were determined. Result showed that there was a slight downward trend of ET 0 from 1960 to 2010 and spatially increasing trend from northeast to southwest in annual time scale. Results also showed that ET 0 had relatively higher sensitivity to wind speed and mean air temperature, and wind speed was the dominant variable for change of ET 0 in southwest China. The inverse relationship between increasing air temperature and decreasing evaporation, Bevaporation paradox,^existed in southwest China, and the negative contribution of wind speed to the changes of ET 0 offset the positive contribution of air temperature.

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Based on the laboratory study, the effects of sodium polyacrylate (SP) was investigated at 5 rates of 0, 0.08, 0.2, 0.5, and 1%, on water retention, saturated hydraulic conductivity(Ks), infiltration characteristic and water distribution profiles of a sandy soil. The results showed that water retention and available water capacity effectively increased with increasing SP rate. The Ks and the rate of wetting front advance and infiltration under certain pond infiltration was significantly reduced by increasing SP rate, which effectively reduced water in a sandy soil leaking to a deeper layer under the plough layer. The effect of SP on water distribution was obviously to the up layer and very little to the following deeper layers. Considering both the effects on water retention and infiltration capacity, it is suggested that SP be used to the sandy soil at concentrations ranging from 0.2 to 0.5%.

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