陕北水蚀风蚀区切沟对土壤水分过程影响的试验研究

The water cycle on slopes and water resources utilization in catchment of Loess Plateau are deeply affected by the widely-distributed gullies. Understanding of soil water effects in gullies is helpful to regional water resources regulation and ecological restoration. At present, much attention has been paid to Grain for Green Project as well as land creation projects including check dams building across the region and campaign to bulldoze mountains to build cities in Yan’an and Lanzhou. It is necessary to improve our understanding of the gully hydrological environment and its ecological role. Gully is a common geographical unit in the catchment and is an important component of the slope-gully system. Water loss processes around gullies are complicated by the gully wall evaporation combined with vegetation consumption on the bank. This program selects typical gullies in Liudaogou catchment of Shenmu county, northern Shaanxi Province, and studies the spatio-temporal distribution of soil water resources inside and outside of the gullies. Methods of positional observation and indoor tests are employed. The program will study gully effects on soil water resources of the banks, the interrelation of soil water processes on gully bank, slope and bottom. What’s more, relation of dried layers, a special hydrological phenomenon of loessial soil, with the gullies will be explored. The results are expected to clearly recognize the soil water effects of gullies, promote the understanding of gully role in ecosystem, and supplement the formation reason of dried soil layers. In practice, the control of gully erosion and the water resources management are expected to benefit from the research results.

黄土高原沟道纵横影响坡面水循环和流域水资源利用，沟道土壤水分效应研究对该区域土壤水资源管理和植被合理建造具有重要意义。近期国内和国际上对退耕还林还草、治沟造田和填沟造地等工程的关注显示，有关沟道水文环境及其生态功能的认识亟需提高。切沟是黄土高原坡沟系统重要的组成部分，在小流域各级支沟分布十分普遍。沟壁蒸发与植被耗水共同作用决定了沟岸地土壤水分散失的复杂性，使得切沟与沟岸地干层可能具有一定联系。本项目拟在陕北水蚀风蚀区六道沟流域选择典型切沟，采用野外定位观测与室内分析测定相结合的方法，研究切沟内外土壤水资源时空分布特征，认识沟壁蒸发和植被耗水对沟岸地干层贡献作用的大小，探索切沟沟底、沟坡和沟岸地土壤水分变化的相互关系。该研究在理论上有助于明确沟道的土壤水分效应和丰富干层形成的认识，为沟道生态功能的提升提供依据，在实践上可为黄土高原丘陵沟壑区侵蚀沟治理和利用、流域水资源合理调控提供参考。

黄土高原是沟蚀引发环境威胁与地貌演变的代表性区域。沟蚀在吞噬土地和大量产沙的同时，塑造了地形破碎和沟谷密布的地表形态。近年来国内和国际上对该地区退耕还林还草、治沟造田和填沟造地等工程的关注显示，有关坡沟系统土壤水分变化及其生态水文功能的认识亟需提高。本项目以“陕北水蚀风蚀区切沟对土壤水分过程影响的试验研究”为题，研究工作历时四年，主要明确了切沟形态复杂性及沟内陷穴的发生发育特点，揭示了坡沟系统土壤水分的时空分布格局及破碎地形区干层分布特点，探索了土壤干层的发生机制和恢复可能性，阐明切沟在坡沟系统土壤水分空间格局的塑造中起主导作用。主要发现如下：（1）切沟的计盒维数介于1.011–1.293，较高的计盒维数对应分支和边缘破碎更加明显的切沟。支沟是划分切沟类型（线型、沟头扩展型、沟头分支型和沟底分支型）和深入认识流域破碎地貌不可忽视的重要组成部分。沟内陷穴能够促进切沟的进一步发展，建议通过坡面植被措施抑制陷穴的发生和发育；（2）流域分水线具有土壤水分垂直分层和土壤干层沿坡面不连续分布的特点。坡沟系统土壤储水量主要受质地影响，距沟缘远近的作用大于土层深度，其逐步回归模型为：SWS=4.42+0.975*Clay+0.004*Distance+0.003*Depth (R2=0.727,p<0.001)。沟缘效应加剧沟岸地南侧土壤干燥，引起切沟两侧横断面土壤水分的不对称分布格局；（3）依托坡面小区连续14a的土壤水分观测数据，描述了干层从无到有的发展动态，提出了柠条林地和苜蓿草地干层恢复的3个直接证据：干层起始深度的动态变化、其水分含量的波动及“夹心式”干层现象，并提出湿润气候对干层发展具有双向影响和植被在干层的发生过程中可能起决定性作用的观点；（4）切沟不仅引起不同地形位置之间土壤基本物理性质和土壤大孔隙特征产生差异，而且增强了坡沟系统土壤持水能力的空间异质性。通过全面深入研究坡沟系统土壤水分时空变化，本项目相关结果可促进黄土高原丘陵沟壑区土壤水分生态环境健康发展，对流域水资源管理和植被合理建造亦具有重要参考价值。