Can plastic mulch save water at night in irrigated croplands?

Plastic mulching has been widely used in arid regions because it can decrease soil water evaporation and raise soil temperature. Previous studies, however, treated soil water evaporation under plastic mulch to be negligible, assuming that the plastic mulch can prevent water exchange between soil and atmosphere completely. In order to demonstrate validity of this assumption, experiments were conducted from 2014 to 2016 under irrigated maize (Zea mays) field in Northwest China, comparison experiments of soil water evaporation between mulched soil and the bare soil between mulches were carried out in two seed maize fields with different irrigation method, i.e. border irrigation under mulch (Site BM) and drip irrigation under mulch (Site DM), with micro-lysimeters placed under the plastic mulched soil and the bare soil between mulches to evaluate soil water evaporation of each experiment maize field. Our observations indicated that the soil water evaporation under plastic mulch (Ems) was about 4.04-7.07% of the total evapotranspiration, among which Ems in the daytime accounted for 3.58-5.37% of the total evapotranspiration and 0.99-2.10% of the total evapotranspiration in the nighttime. Thus Ems was considered not to be negligible. For two experiment sites, soil water evaporation under bare soil between mulches (Ebs) was obviously higher than the soil water evaporation under plastic mulch (Ems) in daytime, but the former was lower than the latter in the nighttime. At night, the mean soil temperature in the mulched soil was higher than that in the bare soil because of the warming effect of the plastic mulch. A soil-mulch-canopy-atmosphere model is used to consider the effects of the mulch, and the modeling results further support this finding. These results provide a new insight for understanding the effect of plastic mulch on water use efficiency.