Effects Of Soil Texture And Structure Of Vadose Zone On Precipitation Infiltration Recharge In Ordos Basin

Ordos Basin,a landform found in the arid and semi-arid regions of northwestern China,has characteristics of strong evaporation and rare precipitation.As the main water supply source in the region,groundwater affects the sustainability of social and economic development,as well as ecological environment protection.Previous studies have shown: in this area,vertical alternation is the main circulation pattern of groundwater,in which atmospheric precipitation is the main recharge source and evaporation is the main discharge mode.As the key part of atmospheric precipitation,surface water and groundwater circulation,the vadose zone has a particular type of lithological structure,which significantly impacts the water transport process.This structure also largely restricts the distribution of runoff and other water balance components.This paper is based on mathematical statistics and basic principles of soil water dynamics.The author combines in-situ testing with numerical simulation,and establishes the water transport numerical model of unsaturated using Hydrus-1d software.This model is simulated and verified by field-measured data in order to obtain different lithologic soil hydraulic parameters.Taking "Precipitation--vadose zone water--Groundwater" process as the main object of study,the author utilizes Hydrus-1d software to establish a saturated-unsaturated water transport numerical model.Combined with the typical lithology profile of the Ordos basin,the research simulates and explores the influences of various vadose zone lithologic structures on rainfall infiltration recharge.Because the permeability of lithology determines the distribution of lithologic structure,in order to describe conveniently,this paper divides the layered structure into two types according to the size of lithologic particles,namely " upper layer coarse and lower layer fine " and "upper layer fine and lower layer coarse".The paper concludes as follows:1.To establish the numerical model of unsaturated water transport in the unsaturated zone and to inverse the soil hydraulic parameters of different lithologies under field conditions.In order to further understand the transformation of unsaturated water,an important parameter basis was established to study the variation of rainfall infiltration flux in different lithologicl structures.2.Based on in-situ testing observations,the research found out that the lithologic structure of the vadose zone controls the depth of water transport and the lag time of water infiltration.Under the same rainfall condition,the homogeneous structure of aeolian sand and weathered sandstone shows that the coarser the particles are,the deeper the water infiltrates and the shorter the lag time will be.While the layered structure of Aeolian sand-weathered sandstone and Aeolian sand-silty sand(upper layer coarse and lower layer fine)shows that the transport depth of water is lesser than that of homogeneous structure of Aeolian sand.The layered structure works as resistance against the advancement of the infiltration peak.At the same time,the research finds out that the finer the lithological particles of lower layer in the “upper layer coarse and lower layer fine” are,the lesser the water infiltration depth and the longer the lag time will be.3.Using the numerical model of saturated-unsaturated water transport model,the research simulates the rainfall infiltration recharge of different lithologic structures and finds that the lithologic structure of the vadose zone greatly influences the rainfall infiltration recharge and lag time.Under the same annual rainfall condition,when the groundwater level is 5m and the upper layer has a thickness of 2.5m in the layered structure,the annual rainfall infiltration coefficients of aeolian sand,aeolian sand-sand,sand,aeolian sand-weathered sandstone,weathered sandstone-sand,weathered sandstone,silty sand-sand and silty sand are 0.16,0.121,0.097,0.088,0.035,0.018,0.014 and 0.008 respectively;the lag time for rainfall recharge groundwater in aeolian sand,aeolian sand-sand,sand,aeolian sand-weathered sandstone,weathered sandstone-sand and weathered sandstone are 315 d,317d,328 d,333d,345 d and 350 d respectively.At the same time,the rainfall infiltration recharge capacity of different lithologic structures has the following characteristics:(1)The coarser the lithological particles are,the shorter the lag time of rainfall infiltration and recharge of groundwater will be;(2)The “upper layer coarse and lower layer fine” lithologic structure is more conducive to the infiltration of precipitation than the “upper layer fine and lower layer coarse” lithologic structure.Fine soil controls the water infiltration process;(3)The rainfall infiltration recharge capacity of the homogeneous structure and the layered structure has the following relationship: homogeneous coarse granular lithology structure > "upper layer coarse and lower layer fine" lithologic structure > homogeneous fine granular lithologic structure,homogeneous coarse granular lithology structure> "upper layer fine and lower layer coarse" lithologic structure> homogeneous fine granular lithologic structure;(4)Texture with the multi-layer lithologic structure as vadose zone is the least conductive to water infiltration.4.As for “upper layer coarse and lower layer fine” structure of aeolian sand-weathered sandstone,under the same annual rainfall and the same groundwater depth,the thickness of the upper soil has a significant influence on the water infiltration in the layered structure.When the homogeneous structure of weathered sandstone is covered with the homogeneous structure of aeolian sand of 3cm~10cm thick,the annual rainfall infiltration recharge increases 6.07~7.34 times than that of the homogeneous structure of weathered sandstone,and the soil evaporation is reduced 46% ~ 59%.This method provides a new way to enhance rainfall infiltration recharge and reduce soil evaporation.