Exploration Of Appropriate Groundwater Depth For Water Saving,Salt Suppression And Optimization Of Maize Irrigation Schedule In The Hetao Irrigation District

Water shortage and soil salinization are important factors threatening the sustainable development of agriculture in arid and semi-arid areas.Groundwater contribution in arid areas is of great significance for the crop growth and prevention of soil secondary salinization.At present,there are few research to accurately determine the appropriate groundwater for water saving and salt suppression.Accurately estimating crop evapotranspiration during crop growth period and non-growth period to design appropriate irrigation schedules has become a top priority for research in arid and semi-arid areas.This study was conducted in the Hetao Irrigation District from 2017 to 2019 where six different groundwater depths（0.85 m,1.25 m,1.5 m,1.75 m,2.0 m and 2.25 m）were set in the water table lysimeters under the conventional irrigation and non-irrigation conditions.At the same time,several irrigation schedules were set up by using two large-scale weighing lysimeters,such as traditional irrigation schedule,water-saving irrigation schedule and simulated drip irrigation schedule with high frequency and small amount.Through field experiments,sensor monitoring,laboratory tests,statistics and theoretical analysis,combined with the SIMDual Kc model,the maize evapotranspiration during crop growth period and non-growth period were quantitatively analyzed.The appropriate groundwater depth for water saving and salt suppression in the middle and upper reaches of the Hetao irrigation District is determined,and the appropriate irrigation schedules for different hydrological years are also designed.The research can provide a theoretical basis for the efficient utilization of groundwater resources and the prevention of soil secondary salinization in the Hetao irrigation District.The main conclusions of this thesis are as follows:（1）The laws of groundwater contribution and deep percolation under the conditions of different irrigation measures and different groundwater depths（GWD）were quantitatively analyzed,and the quantitative supplement and discharge relationship between irrigation depth and groundwater depth was determined.The average daily groundwater contribution rate（G_a）in each maize growth period was significantly different,the minimum of G_a appeared in seedling stage(0.15～0.23 mm d^-1)and the peak of G_a appeared in tasseling stage(0.83～2.5 mm d^-1).Both irrigation measures and different groundwater depths have significant effects on daily groundwater contribution rate.For traditional irrigation schedule,the maximum daily groundwater contribution rate（G_m）was 4.08 mm d^-1with the GWD of 0.85 m.When the GWD is between 1.25～2.25 m,The G_m decreased by 16.4%～67%compared with the GWD of 0.85 m.Without irrigation,the G_m increased by 32.4%～121%.The groundwater contribution of traditional irrigation schedule and non-irrigation were 42.3～210.4 mm and 180.4～320.8 mm,respectively.The deep percolation was 13～150 mm and 0,respectively.（2）The laws of maize evapotranspiration during growth period and non-growth period with different irrigation schedules were clarified,the relationship between irrigation and evapotranspiration was revealed.Irrigation schedules had a significant effect on maize evapotranspiration.The cumulative evapotranspiration(ET_{c act})of traditional irrigation schedule was 461.3～477.9mm,and the daily average evapotranspiration rate was 3.1～3.4 mm d^-1;The ET_{c act} of water-saving irrigation schedule decreased by 15.8%～20.8%,and the daily average evapotranspiration rate decreased by 16.1%～20.6%.The simulated drip irrigation schedule had the highest ET_{c act}（596.9 mm）.Based on the simulations of SIMDual Kc model,the maize transpiration was 372～572 mm,and plastic film mulching significantly reduced soil evaporation（10～47 mm）.During the non-growth period,soil evaporation（85～102 mm）accounted for 84%～86%,and the residual diffusive evaporation component supplied by soil water below the dry surface and by soil water from beneath dense vegetation accounts for16%～22%.（3）The laws of salt distribution for plough layer under different groundwater depths and irrigation schedules were revealed,and the driving relationship between groundwater contribution and salt migration was clarified.The seasonal fluctuation of soil electrical conductivity（EC_e）in 0-60 cm main root layer with GWD of 0.85～1.5 m under traditional irrigation schedule was obvious after jointing,and the EC_e increased gradually from sowing to harvest under the condition of non-irrigation.The EC_e of root layer can be controlled within 2.8～3.2 d S m^-1with GWD between 2.0 m and2.25 m.Irrigation events before sowing and after harvest were effective for salt leaching.The EC_e for root layer before sowing decreased by 1.3%～21.9%compared with the EC_e was measured after the last harvest.The salt distribution was classified to surface aggregation type with GWD of 0.85 m,which changed to average type with GWD of 2.25 m.When there was no influence of groundwater,the simulated drip irrigation schedule can control the EC_efor root layer within 2.43～2.55 d S m^-1.（4）The influence mechanism of different groundwater depths and irrigation schedules on maize growth and yield was explored.According to different hydrological years,the optimized irrigation schedules with accurate consideration of groundwater contribution was formulated.The high salt content in irrigation water and groundwater(EC_iw=1.75～1.9 d S m^-1)was the main reason for the poor growth and yield reduction of maize with shallow groundwater.When the traditional irrigation schedule was adopted,for GWD of 0.85 m,compared with the maximum plant height(h_max),maximum leaf area index(LAI_max),relative chlorophyll content（SPAD）,aboveground biomass and yield（Y_a）decreased by 22.7%,37.6%,34.0%,26.0%and 29.7%respectively.When there is no irrigation,the yield was the highest with GWD of 1.75 m,and the yield reduction rate for other treatments was 3.9%～13.1%.The yield of irrigation schedule with high frequency and small irrigation depth increased by 22.9%～84.7%compared with the schedule of low frequency and excess irrigation depth.Yield response coefficients after modification of stewarts-S2 crop-water model（β_v,β_fandβ_m）are 1.2,3 and0.9,respectively.（5）The appropriate groundwater depth for water saving and salt suppression in the middle and upper reaches of the Hetao Irrigation District is recommended.Combined with maize growth,yield,evapotranspiration(ET_{c act}),rate of soil salt accumulation and total water use efficiency(WP_total),it is determined that the appropriate groundwater depth for water saving and salt suppression in the middle and upper reaches of the Hetao Irrigation District is 2.0～2.25 m,the value of ET_{c act} is 522～529 mm,the value of Y_a is 13858～14558 kg hm^-2,the value of WP_total is 1.61～1.75 kg m^-3,and the rate of soil salt accumulation for root layer was less than 7.4%.（6）According to different hydrological years,the optimal irrigation schedule with accurate consideration of groundwater contribution was formulated.Based on the irrigation time of the Yellow River Diversion and the groundwater contribution（72.3～95.9 mm）of dynamic groundwater depths in different hydrological years,it is recommended that the optimal irrigation quota of maize free from water stress in dry years,normal years and wet years are 263.3～281.8 mm,258.6～270.8 mm and 232.5～255.0 mm,respectively,and the yield reduction rate was 1.62%～4.24%.After the first and fourth irrigation quota were reduced by 10%,respectively,the yield reduction rate can still be guaranteed to be within5%.