How does film mulching modify available energy, evapotranspiration, and crop coefficient during the seed–maize growing season in northwest China?

Abstract

Plastic film mulching has been widely applied in farmland around the world, which could complicate the mass and energy exchange between soil surface and atmosphere. Although previous studies suggested that film mulching reduced available energy (Rn − G) and crop evapotranspiration (ET) in some crop growth stages, it was unclear how film mulching would influence energy partition, crop ET and crop coefficient (Kc) during the entire growing season. A two-year field experiment in a mulched (FM) and a non-mulched (NM) seed–maize field under drip irrigation was conducted in Shiyang River Basin of northwest China to quantify the effects of FM on energy balance components, ET and Kc, by measuring the net radiation (Rn), the soil heat flux (G), the evaporation (E) on ground surface, the transpiration (T), and the soil water content along the soil profile. In particular, ET was obtained by a water balance method and a three-temperature (3T) model combined with remote sensing. Results showed that FM significantly decreased daily Rn and daily G before jointing stage (i.e., the initial stage), which caused 19.0% reduction of the daily Rn − G. The effects of FM on Rn, G and Rn − G diminished after jointing stage (i.e., the crop development, middle and late stages). The reduction of Rn − G under FM treatment during the initial stage was accompanied with the decrease of ET during the initial stage, consequently 6.2% less total ET in the entire seed–maize growing season was recorded compared with NM treatment. FM had a significant effect on the partition of ET components, and E and T were 0.52 and 1.10 times of those under NM treatment, respectively. FM significantly reduced Kc during the initial stage (Kc-ini) by 55.3%, while in the middle (Kc-mid) and late (Kc-end) stages, it decreased by 5.7% and 9.7%, respectively, compared with NM treatment. The Kc varied with the leaf area index (LAI) following the parabolic relationship for both treatments. A structural equation model (SEM) showed that among the soil and crop factors, FM and soil temperature (ST) were the main factors affecting Rn − G, ET and Kc during the initial stage; while during the crop development, middle and late stages, ST and LAI were the main factors affecting Rn − G and ET, and Kc was mainly affected by LAI. SEM further confirmed that the FM impact was more significant during the initial stage than during the crop development, middle and late stages.