Abstract
Irrigated agriculture in arid and semi-arid regions is seriously threatened by water shortage and soil salinization. The well-canal conjunctive irrigation scheme provides a stable groundwater resource for irrigation and can reduce surface salt accumulation by decreasing the groundwater levels, which makes it more suitable to alleviate the problems of irrigated agriculture in arid and semi-arid regions. However, the soil salinization process requires assessment on regional spatial and decadal time scales, as it is a continuous but slow change. Therefore, a water and salt balance model (WSBM) for well-canal conjunctive irrigation is developed herein to obtain long-term predictions of regional root zone salinity dynamics in canal- and well-irrigated areas. In the developed model, the characteristic length of the well-canal conjunctive irrigated area (Lc) is used to couple the canal- and well-irrigated areas. The performance of the WSBM as well as a sensitivity analysis and the value rule of the special parameter Lc are evaluated by comparing the simulation results with those derived from the MODFLOW. The results demonstrate the validity of the developed model, and the special parameter Lc is found to be insensitive, with a value approximately two-thirds of the center distance when the canal and well irrigation districts are regularly adjacent or centrosymmetric. Moreover, when a real-world application is adopted, the water table depth and root-zone soil salinity are simulated in the Longsheng well-canal irrigation area in the Hetao Irrigation District, Inner Mongolia, China. Water table depth and soil salinity collected from 2002–2005 and from 2006–2020 are used to calibrate and validate the model. The calibrated model is subsequently used to predict soil salinity dynamics in the next 100 years under current and future water-saving conditions. The predictions indicate that the soil salinity is basically stable at a relatively low level (<0.2 kg/100 kg) under current irrigation practices. The study could support planning making before implementation of well-canal conjunctive irrigation.
Highlights
More than 100 countries face soil salinization [1,2], especially in arid and semi-arid regions [3–6], which occupy more than one-third of the world’s irrigated land. This situation is further aggravated when excessive canal irrigation is used without proper drainage systems, which reduces the water usage efficiency and raises the groundwater level, significantly increasing phreatic evaporation and resulting in secondary soil salinization in the root zone [7–10]
Well-canal conjunctive irrigation is among the most promising schemes supporting the rational use of water resources and has been adopted in many arid agricultural areas, especially in arid areas with high groundwater levels, such as the South Platte system in Colorado in the United States, and the Yinchuan Plain in China [11–13]
Investigating the groundwater level and root zone soil salinity dynamics in well-canal conjunctive irrigation areas on regional spatial and decadal time scales is of great significance for maintaining the ecological balance in these regions
Summary
More than 100 countries face soil salinization [1,2], especially in arid and semi-arid regions [3–6], which occupy more than one-third of the world’s irrigated land. This situation is further aggravated when excessive canal irrigation is used without proper drainage systems, which reduces the water usage efficiency and raises the groundwater level, significantly increasing phreatic evaporation and resulting in secondary soil salinization in the root zone [7–10]. Groundwater provides a stable water resource for irrigation [14], guaranteeing the sustainable development of agriculture in arid areas; at the same time, the use of well-canal conjunctive irrigation decreases the groundwater level and reduces phreatic evaporation [15,16], achieving the purpose of controlling soil salinization. Investigating the groundwater level and root zone soil salinity dynamics in well-canal conjunctive irrigation areas on regional spatial and decadal time scales is of great significance for maintaining the ecological balance in these regions
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