Water And Salt Characteristics Of Brackish Water Irrigation And Its Migration Simulation In Saline Alkali Soil Of The Yellow River Delta

The amount of surface freshwater resources in the Yellow River Delta is small due to its natural conditions.Freshwater resources become one of the bottlenecks restricting the development of efficient ecological economic zone in the Yellow River Delta.Facing with the shortage of fresh water resources,it is significant to develop the brackish groundwater resources scientifically and rationally to reduce the excessive reliance on the Yellow River water and to alleviate the pressure of regional fresh water resources.Based on the investigation of the current characteristics of water and salt,the infiltration rules of brackish water irrigation in the saline soil of the Yellow River Delta,and the distribution of soil water and salt under brackish water irrigation were studied and discussed.The effects of brackish water irrigation on soil physical and chemical properties,physiological and ecological characteristics of winter wheat and photosynthetic characteristics were also studied in the typical salinization area of the Yellow River Delta.The brackish water irrigation model and soil water and salt control method were put forward.The results will provide scientific reference and management basis for the reasonable brackish water irrigation and soil environmental protection in the Yellow River Delta.The following results in details were obtained:(1)Based on field investigation,sampling and data collection,the mathematical statistics method is used to understand the characteristics of regional groundwater,soil salt and their influencing factors.In general,the groundwater level in the region is low and the depth is shallow.The mean value of groundwater level was 4.57m and the mean depth was 2.59m.In the study area,the salinity of groundwater is different(5-36g/L),the average reference crop evapotranspiration in the past 50 years was 1155mm,and the annual average evapotranspiration ratio was 2.1.Strong evaporation can easily cause soil salinization.The soil soluble salt of the Yellow River Delta was mainly Cl,SO4 and Na ions,which was consistent with the water components of shallow groundwater.Salt surface aggregation phenomenon was obvious.(2)For the simulated experiment of water infilration,the infiltration characteristics of brackish water showed obvious difference compared with freshwater infiltration.Infiltration rate,cumulative infiltration and wet front depth all increased with the increasing of the degree of mineralization.The depth of the infiltration wetting front and the cumulative infiltration for brackish-freshwater infilration were higher than those of freshwater-brackish infilration.As the initial salt content of soil is highe(5g/kg),deionized water and brackish water all produced leaching salt(surface desalination),and the whole soil moisturizing body was divided into two parts:desalination area and salt area.At the same depth in the desalination area,the soil salinity increased with the increase of the mineralization degree.In the continuous infiltration,the last infiltration(irrigation)water salinity will have greater influence on the surface soil salinity.Under the same conditions of mineralization degree of infiltration water,the increase of sodium adsorption ratio will lead to the difficulty of water infiltration,and the wetting front was low at the same time.The simulation results show that the one-dimensional algebraic infiltration model,the Green-Ampt model and the Philip infiltration model can describe the brackish water and salt water infiltration process more accurately.The results provided the basis for soil water and salt dynamic characteristics of the field experiments and simulation under the brackish water irrigation.(3)Through the pot experiment of winter wheat with different salinity irrigation,the brackish water treatment(3g/L)did not significantly affect the growth of winter wheat,while 5g/L treatment significantly affected the growth of winter wheat.The net photosynthesis,plant height and leaf area index decreased significantly,and part of them appeared to wither and dead seedlings.The results of pot experiments provided the basis for selecting the brackish water irrigation mode for subsequent field experiments.(4)Through the irrigation experiment of brackish water in field,brackish water irrigation resulted in an increase in salt content during the winter wheat growth period.The salt content in the soil was basically consistent with the background value of the experiment area,after leaching by rainfall in rainy season.Brackish water irrigation did not affect the physical properties such as soil texture,soil bulk density and soil porosity by sampling comparative analysis before and after irrigation in brackish water field.There was no significant difference(P>0.05)in plant height,panicle length,leaf area index,chlorophyll content and grain size of winter wheat between fresh water irrigation and brackish water irrigation(p>0.05).Although the measured yield of winter wheat was slightly decreased,the difference was not significant(p>0.05).There was no significant difference in plant height,yield and 100-grain weight of summer maize grown in the farmland after irrigation with brackish water(p>0.05).The stomatal conductance decreased slightly,but the difference was not significant(p>0.05).The net photosynthetic rate did not decrease significantly(p<0.05),while the transpiration rate of winter wheat was significantly decreased(p<0.05).Brackish water irrigation maintained a high net photosynthetic rate while decreasing the transpiration rate,and the water use efficiency of brackish water irrigated significantly improved.(5)During the irrigation period of Winter Wheat(March-May),the mineralization degree of the Yellow River water in the studying area was 0.65g/L,and the groundwater salinity in the experimental area was 5.0-6.0g/L.The mixture of brackish water and fresh water(the ratio of 1:1-1:1.5,the mineralization degree 3g/L)can be used for irrigation.Through the field experiment,fresh water(f)-brackish water(b)-brackish water(b)irrigation mode will not cause excessive accumulation of soil salinity.The results provided a basic basis for the utilization of underground saline water in the experimental area.The control measures for soil salt content such as surface cover and drainage and salt drainage were put forward.(6)Based on the classical theory on water and salt transport and field experiments,the soil water and salt migration of different irrigation modes were numerically simulated by Hydrus 1-D model.Based on the identified field water and salt transport model,the characteristics of soil salinity change under eight different irrigation patterns were analyzed.For fresh water(f)-brackish water(b)-brackish water(b)irrigation mode,the mineralization degree of brackish water should be not higher than that of 3g/L.For f-b-f irrigation,the mineralization degree of brackish water should be not higher than that of 5g/L.In the winter wheat growth period the soil salt accumulation within 100 cm depth using brackish water(3g/L)was in the order:f-f-f<b-f-f<f-b-f<f-f-b<b-b-f<b-f-b<f-b-b<b-b-b.When Irrigation the same number of brackish water,the order of brackish irrigation affected the soil salinity,and the salinity degree of the last irrigation water had a greater effect on the salinity of the surface soil.This is consistent with the experimental results of indoor simulated infiltration.Irrigation program should take into account the salt tolerance characteristics of soil salt for crop growth stage.Winter wheat has the lowest resistance to salt tolerance during seedling-to-greening period,and it is not recommended to use brackish water.After jointing period stage brackish water can be used irrigation.In the case of less precipitation,it is advisable to use the f-b-f irrigation model(the salinity of the brackish water no more than 5g/L).In the median or high flow year,brackish water can be used to irrigate two times in winter wheat growth period(brackish water salinity no more than 3g/L).The experimental results can provide scientific basis and reference for the safe use of brackish water and the sustainable development of agriculture in the studying area.