1. Introduction
In the northwestern Loess Plateau, the mean annual precipitation
is between 250 and 350 mm (with large fluctuation from one
year to another), and over 70% of the precipitation occurs during
the monsoon months between June and September (Li et al., 2001).
Only 19–24% of rainfall occurs between May and June, when 60%
of the total crop water use happens (Li and Gong, 2002). In order to
solve the problem of crop water need shortage, many specific techniques
on soil and water conservation have been practiced. A sand
and gravel mixture mulch technique has been used for more than