Studies On Soil Moisture And Runoff Generation/ Infiltration Under Typical Grasslands In The Upper Reach Of The Heihe River

In recent years, the study of regional hydrology taking the Heihe River Basin as a typical case has been widely concerned, which has been promoted by the Heihe River Basin eco-hydrological process integrated research program. The Upper Reach of the Heihe River is located in the middle area of Qilian Mountain and is the main area of water generation.. Glassland is an important ecological type in the Upper Reach of the Heihe River. The soil moisture distribution,runoff generation and infiltration characteristics under grassland not only determine the ability of watershed runoff yield, but also is the key to understanding of regional surface hydrological processes and eco-hydrological response. This study selected different types of grasslands in the Upper Reach of the Heihe River, including agro-pastoral transitional zone, grass-forest transitional zone, savannah and meadow. Through studying field observations, artificial rainfall experiments combined with laboratory experimental analysis, the different grassland meteorological factors, soil moisture, infiltration inflows process are analysed by using statistical methods. The regulatity of soil moisture distribution with its influencing factors and the process and mechanism of runoff and infiltration under different grasslands are discussed to get the conclusions as follows:(1) Soil moisture content(SMC) of grassland in agro-pastoral transitional zone, grass-forest transitional zone and meadow decrease with depth, and in the steppe grassland, the soil moisture increases with depth. The performances of soil volumetric water content(VWC) in different grasslands are: meadow grassland > grass-forest transitional zone > agro-pastoral transitional zone > savannah grassland.(2) There is no significant corresponding relationship between variation coefficient and depth in agro-pastoral transitional zone and grass-forest transitional zone. For savannah and meadow, the deeper the soil is, the weaker the temporal variability of soil moisture is. The soil moisture content will not affect the variability. The principle factors that caused the heterogeneity of soil moisture are the short-term high-intensity rainfall, and continued high temperatures without rain.(3) According to the study on driving factors of temporal variability of soil moisture, the main driving factors that affect the four types of grassland in the Upper Reach of the Heihe River are temperature, rainfall and soil properties. Temperature is the principal factor to study layers of soil moisture in this region; the impact of rainfall on soil moisture concentrated in the surface soil with the depth of 10cm; although soil property is not a decisive factor, it can still affect soil moisture by influencing the rainfall infiltration and evaporation process.(4) Under different simulated rainfall conditions, all the typical grasslands showed runoff yield. The runoff characteristics were mainly influenced by rainfall intensity, rainfall volume and underlying surface conditions. There was a logarithmic relationship between runoff time and rainfall intensity in different grasslands, and the runoff yield could be expressed by the exponential function of rainfall. The rainfall-runoff depth of the grassland in the upriver area of Heihe was related to rainfall intensity, rainfall, initial soil water content and runoff time. By regression analysis, the fitting empirical formula was a=0.054i-1.374W-0.074P-0.014T+0.756.(5) When the rainfall intensity was less than 30mm/h, the runoff yield increased stably, when the rainfall intensity is more than 30cm/h, the runoff yield change process was increase-decreasestability or increase-decrease-increase-stability. The main reason for this phenomenon was the gas within the soil, meanwhile the appearance also need other conditions, including continuous heavy rainfall and the closed boundary formation caused by the rapid accumulation of water on the slope as the external conditions, and the special soil structure and physical properties as the internal conditions.(6) The soil water content of each layer increased with the effect of the high frequency rainfall, and the soil water was mainly concentrated and stored after the first rainfall. The study of the runoff infiltration characteristics by artificial rainfall in the field is an important way to understand the pattern, functional diversity and the evolution regulation of grassland ecosystem.