Soil water and salt dynamics and its coupling model at cropland-treebelt-desert compound system

Abstract: Water shortage and soil salinization are two of the most concerned problems in the arid inland river basins of Northwest China with very low precipitation and extremely high evaporative demand. Water-saving irrigation has been popularized within the arid regions of China to enhance the efficient use of water and fertilizer and improve economic efficiency. However, this has triggered a series of evident/potential eco-environment issues due to regional water and salt imbalance, including land deterioration, groundwater level descending, and natural vegetation degradation, etc. Thus, identifying the spatial-temporal dynamic, variability, and distribution characteristics of soil water and salt in arid ecosystem is of great significance to regional water resources management and vegetation conservation. This study aimed to investigate the dynamics of soil water and salt and the variation of water and salt flux within and between different landscape units in arid areas under water-saving irrigation conditions. To achieve the objective, the soil water content (SWC), soil electrical conductivity (SEC), groundwater and vegetation dynamics were continuously monitored along a cropland-treebelt-desert compound system at the oasis-desert ecotone in the Sangong River Basin, Northwest China, from April to September 2018. A coupling model of soil water and salt in the cropland-treebelt-desert compound system based on BP neural network algorithm (BPNNA) was proposed, and the sensitivity of parameters of the coupling model and its application feasibility were discussed. The results showed that: 1) SWC and SEC of each landscape unit had obvious vertical stratification, horizontal progressive and seasonal fluctuation characteristics during the growth period (from April 1 to June 28) and non-growth period (from June 29 to September 15). Based on the coefficients of variation of soil water and salt, a 0-220/300 cm soil profile could be divided into three typical layers: active layer (0-40 cm), sub-active layer (>40-140 cm) and relatively steady layer (>140 cm). With increasing proximity to the treebelt, the SWC and SEC of the cropland showed a decreasing and increasing trend respectively, while the desert showed an increasing trend; the time-varying process in SWC and SEC of each typical soil layer of each landscape unit after precipitation and irrigation event could be well described by exponential decay function and three quadratic function, respectively. 2) Within the active or sub-active layer (140 cm in depth), a downward water flux was observed at the cropland and treebelt during the growth period, and a upward water flux was observed during the non-growth period, while at the desert a downward water flux was observed at the two periods. Soil water storage and accumulated soil salt of cropland and treebelt showed a decrease trend with the decrease of water table depth and/or the increase of evapotranspiration. Soil water and salt flux of desert had weak response to various influencing factors and their interaction effects, the leaching effect of the last adequate irrigation at the growth period could make the accumulated soil salt among cropland-treebelt-desert tend to be in a relatively balanced state. 3) The coupling model of soil water-salt using BPNNA with the 32-36-6 structure had a high simulation accuracy and applicability in the cropland-treebelt-desert compound system. Irrigation and water table depth were the main factors affecting soil water and salt dynamics in this system. The results could provide an insight for seeking a balance mechanism between production demand and ecological protection in oasis-desert systems under water-saving irrigation conditions.