Study On The Eco-hydrological Mechanism In Semi-arid Regions Of China

After years of research,hydrology,water resources,ecological protection research has achieved relatively consistent consensus for arid and semi-humid and semi-arid region in China.The transition region between them,with rainfall in the 300?400 mm was defined as semi-arid areas in this paper.Research for this region is lagging behind.It is very urgent to develop water resources,eco-hydrology research for this region.West Liao River Basin,with an area of 135000 km2,belongs to this region and has a strong typical significance.Researches on water resources and eco-hydrology for West Liao River and even Eastern Inner Mongolia grassland are scattered and lack of systematicness.Studies on water resources planning and management and related environmental issues in the Song Liao River Basin have taken West Liao River Basin as a same unit.Water resources utility and ecological protection for agricultural and pastoral areas are rarely and superficial.Rainfall in Arid plains is too small to form runoff.Groundwater comes from the mountain pass runoff and forms the oasis ecological circling structures.Rainfall in semi-humid and semi-arid regions produces surface and underground runoff.River and groundwater has the characteristic of adjusting each other and forming a watershed ecological system.Semi-arid region defined in this paper has unique characteristics in the hydrologic and eco-hydrology relationships:Annual rainfall is 300?400 mm and does not produce the surface runoff,but vertical infiltration is active.The formation of groundwater recharge relies on rainfall,and it also supplies the grassland vegetation.Therefore,groundwater is the foundation for rational development and utilization of water resources and ecological protection in semi-arid regions.This paper takes West Liao River Plain as an example.The hydrologic cycle and eco-hydrological characteristics were expounded.The eco-hydrological effect mechanism based on the groundwater runoff formation and dissipation was studied.Critical depth of groundwater supporting vegetation ecosystem and protecting groundwater were defined respectively.Based on theoretical analysis and field observation,analyzed the physical process of groundwater critical level and built calculation model.According to agricultural and pastoral pattern,groundwater control index were decided respectively to realize a scientific management with the groundwater as the core,considering the social economic development and ecological protective.The main research results are as follows:(1)Reveals the semiarid areas eco-hydrological effect mechanism.Combined with hydrologic cycle characteristics,water resource utilization,vegetation composition and distribution eco-hydrological effect mechanism in the West Liao River Plain was analyzed,showing that rainfall support typical grassland vegetation forming by herb,through the groundwater evaporation,forming grasslands with meadow,shrubs and a few trees.Groundwater is the foundation of exploitation and utilization of water resources and ecological protection.It put forward the critical hydrological conditions to support vegetation system and protect groundwater aquifer system.(2)According to capillary action principle,the paper described the physical process of groundwater evaporation and vegetation diving,and the groundwater recharge vegetation critical depth was defined;Using soil physics theory,taking the soil porosity and the effective size as conditions,effective aperture calculation model was constructed to the description of soil structure,and with the field observation,calculated capillary water higher level for different types of soil;And the rationality by using HYDRUS-1 D model to simulate the height of the capillary water rise was analyzed;using plant physiology theory,analyzed the principle of the roots of the vegetation;Combined with vegetation root survey,the critical depth was defined scientifically.The critical depth was given for different soil types with different vegetation:in sand and meadow soil,the depth is 6.5-7m for tree,2?2.5m for shrub and 1?1.5m for herb;in chestnut soil,the depth is 7.5m,3m and 1?1.5m respectively.(3)For the purpose of protecting groundwater aquifer,the paper expounded the physical mechanism of infiltration for groundwater supply critical depth,and built the rainfall infiltration depth calculation model based on the theory of saturated and unsaturated infiltration theory.Infiltration depths in different hydrological year with different soil moisture were calculated,the calculation results were analyzed and showing that:benefit infiltration depth is 8.8 m to 16.1 m,disadvantage infiltration depth is 2.9m to 4.6 m,general infiltration depth is 5.5m to 8.5 m.Consider agriculture irrigation water for groundwater supplies,general infiltration depth is 7m to 10 m.The HYDRUS-1 D model was used to simulate the same conditions infiltration depth to verify the calculation results,the error between them is 4.5%and proved the rationality of the model built in the paper.(4)Management advice for ecological stability and reproducibility of groundwater was put forward in West Liao River Plain.This paper analyzes the groundwater supply and drainage dissipative role;Combined with ecological stability objective in pastoral areas,ecological water level was suggested as 2m to 2.5 m(meadows and sand soils)or 3 m(chestnut soil);Combined with groundwater aquifer protection objective,considering irrigation return for groundwater,agricultural water level was suggested as 7m to 10 m.considering underground aquifer thickness,pumping well distance was suggested as as twice times of affecting radius.