Shallow Groundwater Dynamics And Recharge Laws In Check Dams On The Loess Plateau

Check dam is one of the four major engineering measures for soil and water conservation in China.It serves as the last line of defense for soil and water loss in the Loess Plateau.It effectively controls soil and water loss in the Loess Plateau,reduces sediment input to the Yellow River,improves the efficiency of water resource utilization,reforms local Ecosystem structure and makes a virtuous cycle of the ecological environment.Check dams are important measures for the soil and water loss comprehensive management of in small watersheds.Therefore,exploring the shallow groundwater dynamics and recharge law in check dams has great significance for the scientific management and ecological restoration of local water resources.This study is located in the small watershed of Liudaogou,Shenmu City.A typical check dam was selected as study object.During 2016-2019,three water level monitoring wells,two Soil profile water content monitoring and the chemical tracer method were used to systematically study the ground water dynamic,recharge channels and recharge coefficients of the shallow groundwater in the check dam,in order to reveal the regular pattern of ground water dynamic changes and recharge methods.These will provide a scientific basis for the efficient and rational use of local water resources.The following conclusions are obtained:(1)The inter-annual change of the ground water level systematically shows a trend of alternating peaks and troughs.In winter,the ground water level is the shallowest,the average water level in the front of the dam is-3.02 m,the average water level in the middle is-3.28 m,and the average water level in the tail is-4.12 m The ground water surface will continues to decline after reaching the peak water level in winter.With the rise of temperature,due to the increase in evaporation intensity and the consumption of soil moisture by the growth of the plants,the rate of water level decline will accelerate.Until July-August The ground water water level will drop to the lowest within a year,and the average water level change reaches 1m.The concentrated rainfall in summer is an important source for ground water.After the rainy season,the water level continues to rise until the end of recharge in winter and reaches the peak Water level.(2)There is an obvious relationship between water level and the precipitation.The maximum precipitation determines the peak water level;the distribution of precipitation affects the duration of the recharge and appearance time of the peak water level.The water level dynamic method is suitable for the calculation of groundwater recharge in the loess area.The precipitation in 2017 was 553.2 mm,the average recharge coefficient was 0.25,and the recharge amount reached 155.0 mm;the precipitation in 2018 was 627 mm,the average recharge coefficient was 0.16,and the recharge amount was 100.4 mm.The results of the chemical tracer prove that in the Loess Plateau,precipitation infiltration can pass through the 4 m soil layer to recharge the shallow groundwater in the check dam;at the same time,it proves that the saturation zone in the check dam system has good connectivity,there is lateral movement of water,and the speed can reach 16 m / d.(3)Shallow groundwater in the check dam has multiple replenishment channels,and the sub-ditch plays an important role in the recharge of the entire shallow ground water system.The order of recharging the shallow groundwater is as follows: middle section,ditch head and tail section.Precipitation in the gully head section quickly enters the saturation zone through vertical infiltration to recharge the groundwater,then the water moves laterally under the hydraulic gradient to recharge the entire ground water system.The replenishment effect of the sub-ditch on the middle part of the dam is prior to the replenishment of the main ditch head,so the water level of the middle part of the dam rises fastest and earliest.Although the soil layer in the middle and tail section of the dam is thick,after the rainy season,a large amount of rainfall is converted into soil water.This part of the soil water can pass through the soil layer and enter the saturation zone to continuously recharge the ground water.