| There was a serious shortage of water resources in arid and semi-arid areas of Northwest China(hereinafter referred to as the arid area of Northwest China,the same below).The surface water and groundwater was excessively used in agricultural production.The shortage of water resources and soil salinization had seriously restricted the sustainable development of agricultural production and the improvement of ecological environment in arid area of Northwest China.As the background of the shortage of water resources and implementation status of water-saving irrigation technology in arid area of Northwest China,the Hetao Irrigation District of Inner Mongolia and Shiyang River Basin of Gansu province were chosen as typical study area.Dynamics law of groundwater,soil water-salt and main influencing factors were analyzed in cultivated land and saline wasteland(hereinafter referred to as cultivated-wasteland,the same below)based on the observation of soil water-salt and data colllection.Groundwater flow movement was simulated using MODFLOW numerical model in cultivated-wasteland.It estimated the groundwater salt movement amount between cultivated land and saline wasteland and analyzed the salt balance of cultivated-wasteland in the study area.The Soil-Water-Atmosphere-Plant(SWAP)model simulated soil water-salt dynamics and water-salt balance under saline water irrigation and Yellow River water irrigation based on data collection and field experiments in the typical study area.Crop irrigation schedule was simulated and optimized using SWAP model.The aim was to provide theoretical basis for the efficient utilization of water resources,agriculture sustainable development and the prevention and control of soil salinization in arid area of Northwest China.The main research contents and conclusions are as follows:(1)A typical study area was selected in Yongji Irrigation Area of Hetao Irrigation District.Soil water content,soil salt content,groundwater table depth and salinity of cultivated-wasteland were measured during 2018-2020 in the typical study area.Groundwater and soil water-salt dynamics and main influencing factors were analyzed in cultivated-wasteland.The results showed that soil water content of cultivated land changed more obvious compared with that of saline wasteland.The main influencing factors of soil water content were irrigation and rainfall,crop growth,groundwater depth and soil texture for cultivated land.The main influencing factors of soil water content were irrigation of cultivated land and groundwater depth for saline wasteland.The main influencing factors of soil salinity were irrigation,crop growth,groundwater table depth,and soil texture for cultivated land.The main influencing factors of soil salinity were topographic geomorphology and groundwater table depth for saline wasteland.The soil salinity of cultivated land was less than 3.5 g/kg,which was slightly salinization soil.The saline wasteland had high soil salinity and the soil salinity as 4.0-10.0 g/kg in the central area of saline wasteland,which was severely salinization soil.The main influencing factors of groundwater table depth were irrigation and crop growth for cultivated land.Groundwater table depth was the shallowest in spring and autumn irrigation,and the deepest before autumn irrigation.The main influencing factor of groundwater table depth was irrigation of cultivated land for saline wasteland.The main influencing factors of groundwater salinity were irrigation and topographic geomorphology for cultivated land and saline wasteland.The groundwater salinity was less than 3.0 g/L in cultivated land,which was brackish water.The groundwater salinity was about 7.5 g/L in the central area of saline wasteland,which was severely saline water.Groundwater of cultivated land flowed to saline wasteland in the study area.The groundwater salinity moved with the movement of groundwater flow.The saline wasteland became in the drainage and salt drainage area for cultivated land.(2)The groundwater flow numerical model was established based on MODFLOW under the observation of groundwater salt and data colllection in typical study area.The groundwater flow movement was simulated after model calibration and validation in cultivated-wasteland.The results showed that the ratio of cultivated land to wasteland was 1.14:1,the area of cultivated land was 80 hm2,and the average dry drainage amount was 3.03×104 m3 during crop growth period,the average dry drainage water ratio was 14.22%,the average dry drainage salt was 41.21t,and the average dry drainage salt ratio was 38.68%.The average salt accumulation in cultivated land was 65.35 t,the area of wasteland was 70 hm2,and the average salt accumulation in wasteland was 41.21 t under the conditions of current irrigation in the study area.Due to the increase of soil salt accumulation in cultivated land during crop growth period,autumn irrigation should be carried out in cultivated land to maintain soil salt balance after the end of crop period in Hetao Irrigation District.(3)The SWAP model parameters were calibrated and validated using field experiments observed data of sunflower and spring maize during 2019-2020 in the typical study area of Hetao Irrigation District.The SWAP model used to simulate soil water-salt flux and water-salt balance for sunflower and spring maize after calibration and validation.The results showed that soil water flux at the bottom of sunflower root layer mainly leaked downward in the irrigation stage,while groundwater supplied upward root layer soil of sunflower in the non-irrigation stage.Water flux cumulant of sunflower root layer were 52.5 mm and 60.6 mm during the two-year simulation period,respectively.Salt flux had a similar rule with water flux,but the cumulative amount of salt flux was-4.5 mg/cm2 and-4.9 mg/cm2 during the two-year simulation period,respectively.Soil salt accumulation in 0-100 cm soil layer was 7.5 mg/cm2 and 7.1 mg/cm2 in the growth period of sunflower,respectively.Water flux cumulant of spring maize had a similar rule with that of sunflower.Water flux cumulant were 111.6 mm and 63.1 mm during the two-year simulation period,respectively.The cumulative amount of salt flux was-10.3 mg/cm2 and-11.1 mg/cm2 during the two-year simulation period,respectively.Soil salt accumulation in 0-100 cm soil layer was 7.7 mg/cm2 and 6.9 mg/cm2 in the growth period of spring maize,respectively.Sunflower and spring maize could make full use of groundwater to meet their growth needs,and salt accumulated in soil under the conditions of shallow groundwater and the current irrigation in the study area.(4)The SWAP model parameters were calibrated and validated using saline water irrigation experiments and observed data for seed maize during 2013-2014 in Shiyang River Basin.Soil water-salt flux and water-salt balance were simulated after model calibration and validation.The results showed that soil water and salt fluxes at the bottom of the root layer for seed maize mainly leaked downward,and soil water and salt fluxes significantly affected by irrigation and rainfall during the simulation period of 2013 and 2014.Soil water flux,salt flux,cumulative amount of water flux and cumulative amount of salt flux gradually increased with the increase of irrigation water salinity.Soil salt in the root layer of seed maize was leached to the deep soil for irrigated water salinity with 3.0 g/L and 6.0 g/L under the condition of irrigation and rainfall.It would not cause a large amount of soil salt accumulation.(5)Saline water using mode and irrigation schedule of seed maize were simulated and optimized based on SWAP model.The results showed that irrigation water salinity with 0.71?2.0 g/L could be used for irrigation.Irrigation water salinity above 6.0 g/L was not suitable for irrigation.Irrigation water salinity with 3.0?5.0 g/L could be acceptable for short-term irrigation,but it was not suitable for long-term irrigation in the study area.The optimal rotation irrigation modes were the irrigation of two times fresh water and one time saline water,one time fresh water and two times saline water under brackish water irrigation of 3.0 g/L,and two times fresh water and one time saline water under saline water irrigation of 6.0 g/L in the study area.The optimal deficit irrigation with saline water mode of seed maize was to irrigate four times with total amount of 360 mm for brackish water irrigation of 3.0 g/L during the growing season of seed maize.The long-term simulation of soil water-salt dynamics indicated that it would not produce a large salt accumulation in soil with spring irrigation with 150 mm in the optimal irrigation mode. |
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