Groundwater Recharge Mechanism In Yinchuan Plain Under Different Irrigation Patterns

Yinchuan Plain is located in the arid and semi-arid region of northwest China,where rainfall is scarce and evaporation is strong,and the main source of groundwater recharge is irrigation water infiltration,and irrigation water recharges groundwater vertically through the vadose zone.Nowadays,traditional irrigation methods have been gradually replaced by intelligent water-saving irrigation.The changes in surface conditions and irrigation methods have caused changes in groundwater recharge and discharge relationships,so the study of water transport of vadose zone and groundwater vertical recharge mechanisms in Yinchuan Plain under different irrigation methods is beneficial to the rational formulation of irrigation systems and has important theoretical and practical significance for the rational development and utilization of groundwater resources in Yinchuan Plain.This paper selected Yinchuan Plain as the research area,and established the water transport model of the vadose zone by using the soil moisture content,evaporation and meteorological data observed in the field experiment and the soil particle analysis results obtained in the laboratory experiment.The model is used to simulate the influence of irrigation methods under different groundwater depth on groundwater recharge.The main conclusions are as follows:(1)According to the results of surface evaporation,it can be seen that the evaporation process of water surface exists area and depth effect,that is,when the depth is certain,the evaporation of water surface decreases with the increase of area,and the evaporation tends to be stable when the area increases to 12.17 m~2;when the area is certain,the evaporation of water surface decreases with the increase of depth,and the evaporation tends to be constant when the depth increases to 4.9 m.Water surface evaporation is positively correlated with net solar radiation and temperature,and negatively correlated with water vapor pressure and relative humidity.(2)According to the soil flux method,the infiltration of irrigation(rainfall)at the soil surface of each test sample was calculated,in which the infiltration recharge of irrigation water under the diffuse irrigation method was the largest,reaching 23 mm;the intelligent irrigation method was less effective than diffuse irrigation in recharging groundwater,recharging groundwater by 13.4 mm;grass recharged groundwater by 10.5 mm under a small amount of irrigation;bare ground recharged only under the condition of receiving only rainfall 1.9 mm.(3)The moisture content of the vadose zone showed obvious response characteristics to the irrigation process,and the water content of the vadose zone in the range of 10-100 cm increased rapidly after irrigation.The change of water content in the shallow part under smart irrigation was more obvious compared with that in the deep part,and the change of water content at 80 and 100 cm in the diffuse irrigation sample was greater than that in the shallow part,indicating that diffuse irrigation had a greater effect on the moisture in the deeper air-belt.The infiltration depth of irrigation water from single smart irrigation is limited and cannot recharge groundwater,while diffuse irrigation will have a greater impact along the depth direction.Under the influence of vegetation and irrigation,the variability characteristics of absolute water content were divided into a shallow strong variability layer(0-100 cm)and a slow variability stable layer(100-150 cm)for the smart irrigation sample,and a shallow slow variability layer(0-20 cm),a central strong variability layer(20-100 cm)and a deep stable layer(100-150 cm)for the diffuse irrigation sample.(4)Natural rainfall can only infiltrate to 50 cm for the corn field sample,and the limit infiltration depth is 30 cm for the barren grassland and bare ground,which cannot produce recharge to groundwater despite the presence of dominant flow.The temperature in the pocket shows S-shaped variation within the year and exhibits diurnal variation.The correlation between the temperature at 10 and 20 cm and the water content of the pocket is strong,while the correlation at 50 and 100 cm is weak.(5)Intelligent irrigation,diffuse irrigation,grassland and bare ground samples were constructed to establish a numerical model of water transport in the packet zone,and the model was identified and validated,and the model was evaluated comprehensively by combining Nash efficiency coefficient NSE,coefficient of determination R~2,relative error RE and other indicators,and the groundwater recharge of irrigation water under different irrigation methods and different surface conditions was derived based on the model calculation results after verifying that the model reached accuracy.(1)The best groundwater recharge effect is achieved by diffuse irrigation with an infiltration recharge coefficient of 0.73,while the infiltration recharge coefficient is 0.41 for smart irrigation and 0.47 and 0.19 for grass and bare ground samples,respectively.(2)The groundwater recharge situation between smart irrigation and diffuse irrigation is simulated in the groundwater depth range of 0.65-3.8 m.The results indicated that the infiltration recharge efficiency of smart irrigation was the highest at 1.3m of groundwater level,and the infiltration coefficient of irrigation was the highest at 1.8m of diffuse irrigation.