1. Introduction
Gravel and sand mulch is a traditional water conservation technique that has been used for hundreds of years in the loess area of northwestern China. Recently, this technique has expanded its usage,due to the lack of water sources and high irrigation costs. In parts of this region, irrigating fields have almost been replaced by the use of gravel-sand mulch on dry land. Extensive research and practical experience have proved that gravel-sand mulch could markedly reduce soil evaporation and increase soil temperature at night as well as reduce soil temperature fluctuations ([1–5]). Nachtergaeleetal. (1998) determined that gravel mulch causes a significant increase in the soil temperature measured at two soil depths (mean increase of 1.0 °C and 1.5 °C at 3 and 10 cm soil depth, respectively) in vineyards of southern Switzerland. Chen et al. [6] and Xie et al. [5] studied the effect of gravel size on soil evaporation, and showed a positive linear relationship between gravel size and soil evaporation.