黄土塬区深剖面土壤水分平衡和地下水补给对土地利用/覆被变化的响应

Since the 1980s, the widespread land use/land cover change (LUCC) has significantly affected the hydrological processes in the Loess tableland of China, such as the enhancement of small water cycle, the weakness of large water cycle, the intensification of dry soil layer, and the decrease of groundwater recharge. The changes in hydrological process have aggravated the shortage of water resource for the Loess tableland, and have seriously threatened water resource security, as well as the sustainable agriculture developmentin the local area. In this project, we plan to investigate the indices to water chemistries and isotopic compositions of rainwater, well water, spring water, and water storages in both low-lying land and deep soil profiles under different scenarios of land uses/land covers (LU/LC)in the Changwu Loess tableland. Meanwhile, we will monitor the precipitation, groundwater level, spring flow and water potential of the deep soil profiles under these scenarios. Finally, based on tracer techniques, zero flux planemethod and chloride mass balance method, we try to quantify the characteristics of water balance in deep soil profiles under these LU/LC scenarios, and to indicate the transformation processes of precipitation-soil water-groundwater, and to reveal LUCC effects on water cycle and groundwater recharge for the Loess Tableland. The outcomes of this project is significant in further understanding the underlying mechanisms of water cycle and the processes of water balance in deep soil profiles under different LU/LC scenarios for the Loess Tableland. Additionally, it is of significant importance to make reasonable regulations to the land use structures, and to maintain appropriate levels of land productivities for the sustainable utilization of water resources.

上世纪80年代以来，黄土塬区显著的土地利用/覆被变化（LUCC）对该区水文过程造成重要影响，如水分小循环加强与大循环削弱、土壤干层强化、地下水补给量减少等，加重了黄土塬区水资源短缺的状况，严重威胁该区水资源安全和农业可持续发展。申请项目拟通过对长武黄土塬区降水、井水、泉水、涝池等洼地积水、不同土地利用/覆被（LU/LC）下深剖面土壤水水化学性质、同位素组成的测定，以及降水量、地下水位、泉水流量和不同LU/LC下深剖面土壤水势的监测，综合示踪技术、零通量面法、氯离子平衡法等，深入定量研究不同LU/LC下深剖面土壤水分平衡特征以及降水-土壤水-地下水的转换过程，揭示LUCC对黄土塬区地下水补给及水循环过程的影响。该研究有助于深化对黄土塬区水循环机理和不同LU/LC条件下深剖面土壤水量平衡过程的认识；对于合理调控土地利用结构和保持适度土地生产力，从而实现水资源可持续利用具有重要意义。

土地利用/覆被变化（LUCC）深刻影响黄土塬区水文循环过程。本项目按照计划书开展工作，定量研究不同土地利用/覆被（LU/LC）下深剖面土壤水分平衡特征以及地下水补给过程，揭示LUCC对黄土塬区地下水补给及水循环过程的影响。黄土深剖面粘粒(<0.002 mm)、粉粒(0.002–0.02 mm)和砂粒(0.02–2 mm)平均含量分别为30.6±2.9%、34.5±2.4%和34.9±4.6%，黄土和古土壤层土壤容重分别为1.31 g/cm3和1.50 g/cm3，田间持水量分别为21.21%和24.68%。农田深剖面土壤水分长期稳定，但是生产力提升将会降低降水深层渗漏发生的几率；农田转换为苹果林地后，深剖面土壤水分由浅及深逐年降低，深层土壤储水量呈倒“S”曲线趋势下降，27龄之后进入稳定期。降水直接入渗补给（DR）和集中入渗补给（FR）共存于黄土塬区地下水补给过程，DR发生于农田、草地等土地利用方式，FR则发生于村庄涝池等地势低洼处。两种补给方式中，活塞流形式通过包气带补给地下水。农田条件下，DR只有丰水年份才能形成，多年平均下渗速度10.3 cm/yr，平均潜在补给量为30.2 mm/yr，占年均降水量的5.2%。农田转换为苹果林地后形成的深厚土壤干层将阻断降水对地下水的补给。雨季涝池下FR下渗速度高达26 cm/d，涝池集水区内地下水平均潜在补给量为134 mm/yr，达年均降水量的23.1%。DR贡献了地下水总补给量的55%到68%，FR则贡献了32% 到45%。农田转换为苹果林地将减少DR发生面积，而村庄涝池的破坏将使FR补给量减少，两者共同导致黄土塬区地下水资源减少。因此，为了实现黄土塬区潜水资源的有效补给与可持续利用，提高村庄涝池下FR的贡献比例，实施农果轮作、构建合理农果地面积比例和农果镶嵌布局具有重要作用。