Similarly, in our study, in the wet year 2007, WUE was significantly higher than in 2006 and 2008, resulting in higher fruit yield, although evapotranspiration did not significantly increase. In Experiment 1, the soil evaporation decreased with mulch thickness increased, but evapotranspiration did not change obviously for all treatments (Table 1). This shows that decrease in evaporation increased crop’s transpiration. Under arid and low transpiration conditions,the use of water for transpiration increased crop growth,so as to improve crop production and water use efficiency. But, for the 11 cm mulch thickness treatment, the soil temperature in daytime decreased due to very thick mulch, which consequently decreased watermelon yield and water use efficiency. In Experiment 2, yield and water use efficiency of 8 cm mulch thickness treatment were significantly greater than 3, 5 and 11 cm mulch thickness treatments in the three experimental years. It also indicates that a proper mulch thickness benefits to improve WUE and crop yield.