Estimating the water characteristic curve for soil containing residual plastic film based on an improved pore-size distribution

Abstract

Although plastic-film mulching has played an important role in agriculture production in arid areas, the practice has resulted in the accumulation of large amounts of residual plastic film (RPF) in the soil. This intensely impedes the movement of the water and solutes in soil by seriously affecting the physical and hydraulic properties of soil, in particular the soil water characteristic curve (SWCC). Despite the severity of the problem, studies on how RPF affects soil properties and the SWCC remain scarce. This paper thus aims to achieve a better understanding of the influences of RPF on soil pores and SWCC. In this study, SWCCs were measured under five concentrations of RPF (0 kg·hm−2, 50 kg·hm−2, 100 kg·hm−2, 200 kg·hm−2 and 400 kg·hm−2) and three soil textures (sandy soil, sandy loam and loam soil) based on the pressure plate extractor test. A model for estimating SWCC for soil with residual plastic film (RPF-SWCC) based on an improved pore-size distribution is proposed, and its performance was evaluated and compared with those for existing models, including the van Genuchten (VG), Brooks-Corey (BC) and log normal distribution (LND) models. The results showed that there is a clear influence by the RPF in soil on the shape of the SWCC, and the order of the different soil types in regards to soil water holding capacity was loam soil > sandy loam > sandy soil, with or without RPF. The RPF-SWCC model accounted for the effects of water blocking by RPF, thereby achieving improved fitting accuracy compared to that by the VG, BC and LND models, especially for soil with high RPF concentration. The average Nash-Sutcliffe efficiency coefficient (NSE), mean absolute percentage error (MAPE) and symmetric mean absolute percentage error (SMAPE) for achieved for the RPF-SWCC model were 0.9943, 3.37% and 3.43%, respectively, which was an improvement on the 0.9938, 5.94% and 5.06% achieved for the VG model, 0.9926, 4.43% and 4.25% for the BC model and 0.9923, 15.00% and 7.53% for the LND model, respectively. In addition, the correlation between the parameters of the RPF-SWCC model and RPF concentration showed that the saturated soil water content and residual soil water content decreased and increased with increasing RPF concentration, respectively. In addition, soil RPF increased and decreased the proportion of macropores and soil water holding capacity, respectively. This study can provide a theoretical and technical basis for research into soil water movement and the efficient utilization of water and fertilizer in areas with high agricultural film residue.