Among the treatments, WUE was lowest for conventional ridge
planting (treatment 7). The second lowest WUE was for the flat
gravel–sand mulch field or RFRHS with no-plastic-covered. In
general, WUE was relatively high for all RFRHS treatments with
plastic-film mulching. The WUE was the highest for the RFRHS
with sand mulch furrow plus plastic film whole mulch and the
ratio 5:3 of ridge: furrow (treatment 4), and it was significantly
higher than the conventional ridge planting and flat gravel–sand
mulched field and RFRHS with no plastic covered ridge (treatment
5). In general, WUE was greatly improved when ridges were plastic
mulch. For the gravel–sand mulch field (treatment 1 vs.
8), irrigation remarkably increased WUE in the dry year of 2009.
However, WUE was decreased in the wet year of 2007 and had no
effect on the WUE value in 2008 with a medium rainfall. This indicated
that precipitation had a strong effect on WUE of gravel–sand
mulch field.