Soil water content of 0–180 cm layer at every 20 cm before sowing (A: on 12 April in 2009; B: on 18 March in 2010) and after harvesting (C: on 8 October in 2009; D: on 28 September in 2010) under different plastic film mulching patterns in 2009 and 2010 in Dingxi, northwest China.