Response Of Soil And Water Environment To Well And Canal Double Irrigation In Yongji Irrigation Area

Hetao Irrigation area is the largest irrigation area in the Yellow River Basin.With the increasing shortage of water resources,the contradiction between agricultural irrigation water and domestic and industrial water becomes more and more prominent,which seriously restricts the sustainable development of agriculture in the irrigated area.Groundwater is an important part of water resources,and brackish underground water resources are abundant in the irrigation area.Combined irrigation of groundwater(brackish water)and surface water(yellow diversion water)has become an effective way and irrigation measure to alleviate the contradiction between supply and demand of agricultural water in Hetao irrigation area.Combined well and channel irrigation can not only make full use of water resources,but also have the dual function of open source and throttle.In order to explore the influence of surface water-groundwater-surface water(QJQ)irrigation mode on farmland soil and water environment in the well-canal and double-irrigation area of Hetao irrigation district,field tests were conducted in 2018 and2019 to monitor the variation of groundwater depth,salinity and soil water salinity.HYDURS model was used to simulate the soil water recharge,deep seepage and salt transfer characteristics under QJQ irrigation mode,and determine the most suitable groundwater depth.The main conclusions are as follows:(1)Under the QJQ irrigation mode,the areas with electrical conductivity greater than3.5 ms/cm in the 0～20 cm and 20～40 cm soil layers decreased by 82.68% and 42.76%,respectively,compared with the conventional irrigation mode.There was a certain risk of salt accumulation in the 0～20 cm soil layers after 2 water irrigation in summer.The soil moisture content showed an increasing trend.The proportion of 25%～30% moisture content in 0～20 cm and 80～100 cm soil layer increased by 16.14% and 27.30%,respectively.Soil total nitrogen decreased from 0.5～0.7 g/kg to 0.2～0.5 g/kg.The total phosphorus content of soil in the test area was relatively small on the whole,and the total phosphorus content of 0～20 cm soil layer was 0.3～0.7 mg/g.(2)After the QJQ irrigation mode,the average groundwater depth decreased by 0.15 m,the average amplitude increased by 0.26 m,and the coefficient of variation increased by1.13%,which had the most obvious effect on the groundwater depth in the upper reaches.The area of groundwater with high total salt content decreased in the test area,and the total salt content of groundwater decreased significantly in the southeast of the upper reaches and the northwest of the lower reaches,and the total salt content fluctuated from 500-5000mg/L to 500-3500 mg/L.The nitrogen and phosphorus nutrients in groundwater decreased slightly,and the area with total nitrogen content greater than 2 mg/L decreased significantly,by 14.38% on average.The total nitrogen content in groundwater varied between 1 and 2mg/L.The area with total phosphorus content less than 0.09 mg/L increased by 29.04% on average,and the area with total phosphorus content greater than 13 mg/L decreased by15.58% on average.(3)The groundwater depth was significantly negatively correlated with soil moisture and salinity,and the correlation coefficients were both greater than 0.50.When the groundwater depth was 2～3 m,the moisture content of the 0-20 cm soil layer was 5.54%and 75.43% higher than that when the groundwater depth was 3～4 m and 4～5 m.When the groundwater depth is 3～4 m,the soil conductivity is small and decreases with the increase of the groundwater depth.Groundwater depth is significantly correlated with soil total nitrogen and phosphorus.When groundwater depth is greater than 3 m,soil total nitrogen reaches the maximum value of 1.96 g/kg;when groundwater depth is 3.5 m,soil total phosphorus reaches the critical value of 1.16 mg/g.(4)The Hydraus-1D simulation showed that the average cumulative recharge from0-20 cm layer to the upper layer during the two-year experiment period was 32.19 mm and33.66 mm,respectively,and the seepage from 100 cm layer was 157.64 mm and 184.42 mm.Under the QJQ irrigation mode,the average moisture content of the 20 cm soil layer is 12.67%,25.18% and 49.56% larger than that of the groundwater depth of 2.5,3 and 3.5m when the groundwater depth is 2 m,and the salt deposit in the 20 cm soil layer is more serious than that of the groundwater depth of 1.5 and 2 m.The desalting rate of 40,60 and80 cm soil layer was higher than that of 1.5,2,3 and 3.5 m groundwater depth,which were15.89%,25.07% and 43.80%,respectively.