放牧强度对典型草原生态水文过程的影响机制

Typical steppe is the important livestock husbandry base and ecological defense of Northern China, but it shows degradation of grassland vegetation and worsening of desertification because of long-time overgrazing, along with eco-hydrological function imbalance and ecological service functions decline. The critical junctures that affect eco-hydrological process are soil water infiltrates, runoff and evapotranspiration, which conjunctly determine the pattern and evolutionary trend of soil-vegetation community, and influence its’ ecological function and hydrological cycle in the typical steppe, meanwhile they response to the changes of soil-vegetation pattern. Grazing induces the differentiation of the vegetation distribution pattern, soil properties and micro topography, affects all junctures from infiltrate and evapotranspiration to runoff. So the effect and influence mechanism of grazing intensity on the every juncture became urgent issues. The research object is Xilamuren typical steppe, Inner Mongolia. Using field survey, located monitoring, combining rainfall simulation, the study focus on the dynamic changes of the process in the soil infiltration, surface runoff and evapotranspiration under different grazing intensities. The aims are to clarify the quantitative relation of infiltration- runoff- evapotranspiration- process (IRE process) among the different grazing intensities, and to reveal the response caused by grazing. The study will find out the effect and influence mechanism of grazing intensity on the hydrological process and set up quantitative threshold model of eco-hydrological process under grazing intensities. The research has important theory meanings in the promotion of eco-hydrological functions in typical steppe. Therefore, it will provide scientific basis for repairing of degraded ecosystem and formulating reasonable grazing system in order to keep ecosystem productivity of the grazing grassland.

典型草原是北方重要的畜牧业生产基地和生态屏障，过度放牧导致草原植被退化、生态水文功能失调、荒漠化加剧和生态服务功能下降。土壤水分入渗、径流及蒸散是影响生态水文过程的关键环节，三者共同决定着典型草原土壤－植被格局与演化动态、生态功能和水分循环，并响应于土壤－植被格局的变化。放牧诱发地表植被分布格局、土壤特性与微地形的分异，影响入渗、蒸散及径流等各个环节，但放牧强度对各环节的作用及影响机理亟待深入研究。.本研究以内蒙古希拉穆仁典型草原为对象，采用野外调查、降雨模拟与定位监测相结合的方法，研究放牧强度下典型草原区土壤入渗、地表径流产生、地表蒸散过程的动态变化，阐明不同放牧强度下入渗－径流－蒸散过程间的数量关系及其对放牧强度的响应，探明放牧强度变化对水文过程的作用及影响机制，构建量化放牧强度生态水文过程的阈值模型，为典型草原生态水文功能提升、生态修复和合理制定放牧制度提供科学依据。

本研究以希拉穆仁典型草原为对象，研究了不放牧、轻度、中度和重度放牧强度（CK、LG、MG和HG）下的入渗、径流、蒸散及截留过程及作用机制。结果表明：（1）随放牧强度增加，植被高度、盖度、生物量下降；土壤颗粒变粗，质地变差，紧实度增加，有机质和养分含量减少，LG可避免土壤与植被劣变，较好权衡草地生态与生产功能。（2）土壤入渗以LG的初始入渗速率（5.12 mm/min）、稳渗速率（1.93 mm/min）和累积入渗量（213.42 mm）最高，MG或HG最小；总孔隙度、容重与放牧强度显著影响入渗过程，Horton模型最适描述入渗过程。（3）植物与群落的截留量（率）与株高、叶面积、生物量及群落盖度呈线性相关，随放牧强度的增加而降低，随雨强的增加而增加。（4）不同放牧强度Ⅰ型降雨的径流量均为0.03L/m2，Ⅳ型降雨的径流量为HG（0.41L/m2）>MG>LG>CK；冲刷条件下，不同放牧强度产流时间表现为LG>CK>MG>HG，径流率与减流效益为LG<CK<MG<HG，三者的差异均随冲刷强度增大而减小。径流率（RR）与放牧强度（G）、冲刷强度（Q）符合方程RR=0.048G+1.045Q-1.27。放牧强度下的水力学参数差异明显，均以LG最小。6）不同放牧强度下，各植物蒸腾速率为LG>MG>HG，土壤含水率为CK>MG>HG>LG；日蒸散量在放牧强度间为LG>HG>MG>CK，时间上为8月上旬>7月>8月下旬>6月>9月，日变化上表现为单峰曲线。7）放牧强度直接影响植被与土壤因子的变化，进而影响植物-大气界面水文过程（截留与蒸腾）、植物-土壤界面水文过程（入渗与径流）与土壤水文过程（入渗与水分贮存），各影响因子与水文过程互相作用，机制复杂。研究对解析放牧强度条件下生态水文过程变化及影响机制具有重要理论意义，可为典型草原生态水文功能提升、生态修复和合理制定放牧制度提供科学依据。