黄土高原坝地农田降水、土壤水、蒸散发与地下水补给的动态关系

Dynamic relationships among precipitation, soil moisture, evapotranspiration and groundwater recharge in a dammed valley farmland on the Chinese Loess Plateau

关键词:  地表水-地下水连续体  野外监测  涡度相关观测  碱性土壤  半干旱气候

Key words:  surface water-ground water continuum  field monitoring  eddy covariance measurement  alkaline soil  semiarid climate

摘要:

坝地农田是黄土高原宝贵的土地资源。然而，这些农田通常面临地下水位浅和由此导致的盐渍化问题。在浅层地下水系统中，降水、土壤水、蒸散发和地下水补给被认为是耦合联动的。然而，黄土高原坝地农田中这种地表水和地下水的动态联系研究存在不足，制约了对地下水位变化及其土壤盐渍化潜在影响的认识。文章假设黄土高原坝地农田的地下水补给主要受降水和蒸散发（ET）控制。为验证该假设，选取黄土高原北部近延安市宝塔区甘谷驿镇的一处坝地农田，于2020年利用涡度相关通量塔与土壤-地下水监测系统同步观测降水、土壤水分、蒸散发与地下水位的动态变化，并应用HYDRUS-1D模型模拟土壤水量平衡，分析了各水量的动态关系。结果表明：（1）在2020年旱季，1 m以下土壤水分未能得到降雨有效补给；而在2020年雨季，强降雨后3 m深度土壤含水量发生显著变化；（2）地下水补给主要由2020年8月强降水（267 mm）触发，一个月内地下水位上升了0.7 m；（3）降水、蒸散发与地下水位变化相互关联——蒸散发大于降水量时，地下水位下降，反之上升。研究区地下水位约为4 m，地下水位浅叠加黄土碱性特征，水位抬升将导致土壤盐渍化发生，而降水与蒸散发的平衡在调控研究区地下水位中起关键作用。论文建议，坝地农田种植高耗水玉米并结合合理的排水系统，将有效抑制地下水位，维持坝地的可持续利用。

Abstract:

Background, aim, and scope The dammed valley farmlands are valuable land resources for agriculture on the Chinese Loess Plateau (CLP). However, these farmlands often face the problems of shallow groundwater table and the resulting soil salinization. It is generally assumed that precipitation, soil moisture, evapotranspiration (ET), and groundwater recharge are coupled in a shallow aquifer system. However, the dynamic connections of surface water and groundwater are inadequately addressed in the dammed valley farmlands on the CLP, limiting our understanding of the changing patterns of the groundwater table and their potential impact on soil salinity. We hypothesize that groundwater recharge is mainly controlled by precipitation and ET in the dammed valley farmlands on the CLP. Materials and methods To test this hypothesis, a farmland in the dammed valley near the town of Ganguyi in Baota District of Yan'an City on the northern CLP, and the dynamic changes in precipitation, soil moisture, ET and groundwater table were synchronously monitored by an eddy covariance flux tower and soil-groundwater monitoring system in 2020. The dynamic relationships among precipitation, soil moisture, evapotranspiration and groundwater recharge were analyzed. Moreover, the HYDRUS-1D model was applied to simulate the soil water balance in this work. Results (1) in the dry season of 2020, soil water beyond a depth of 1 m was not recharged, whereas in the wet season, we measured significant changes in soil moisture content to a depth of 3 m after heavy precipitation; (2) groundwater recharge in the dammed valley farmland was mostly triggered by heavy precipitation occurring in August (267 mm), which resulted in the groundwater table increasing by 0.7 m in a month; and (3) precipitation, ET and the groundwater table were connected. If the cumulative ET is higher than the precipitation, the groundwater table declines, and vice versa. Discussion In 2011, a larger area of dammed valley farmlands was created in the hilly and gully regions on the CLP with the implementation of the Gully Land Consolidation (GLC) project. The results of this study showed that groundwater recharge in the dammed valley farmlands had a close relationship with the balance of precipitation and ET. If the cumulative precipitation was higher than the ET, the groundwater table would increase. For example, the cumulative precipitation in the rainy season of 2020 was much higher than the ET, the soil and groundwater were substantially recharged by a large amount of precipitation over a short time. Moreover, the groundwater was probably and mainly recharged by preferential flow induced by heavy rainfall events, e.g. a heavy rainfall (113.3 mm/d) occurred on August 4—5, 2020, and the lag time of groundwater table rise was only 42 hours after rainfall initiation. In this study, the dammed valley farmland had a groundwater table of about 4 m. Due to shallow groundwater table and alkaline loessial properties, soil salinization widely occurred in the dammed valley farmlands of the study area. If the valley farmland does not plant high-water consumption crops, the groundwater table in the farmland will rise more significantly. Thus, strengthening drainage and suppressing groundwater tables are extremely important for soil and water management in the dammed valley farmlands on the CLP. Conclusions We concluded that the balance of precipitation and ET played a crucial role in regulating the groundwater table in the dammed valley farmlands on the CLP. If the cumulative ET exceeds the cumulative precipitation amount, the groundwater table declines, and vice versa. Recommendations and perspectives To maintain the sustainability of the valley land resources on the CLP, we suggest that planting high-water consumption corn and building reasonable drainage systems should be combined to suppress the groundwater table.