Study On Subsurface Drainage Layout In Coastal Reclamation Area To Meet Different Soil Desalination Goals And Crop Harvesting Conditions

The soil salt content of newly reclaimed farmland in Jiangsu coastal beach is high,which needs drainage and salt washing measures to achieve different soil improvement goals and meet the requirements of agricultural production;the improved farmland generally implements rice wheat rotation system,and the construction of farmland drainage system is an important guarantee to achieve high grain yield.Considering the meteorological and soil characteristics of coastal reclamation area in Jiangsu Province and the development requirements of agricultural mechanization,based on the meteorological data of Dongtai mayor sequence,this paper studies the effect of subsurface drainage intensity on desalination process of initial reclaimed soil in coastal reclamation area by using farmland drainage model DRAINMOD,and the reasonable layout of subsurface drainage system under rice wheat rotation to meet the requirements of mechanized harvesting.This paper first verified the accuracy of DRAINMOD model for groundwater table depth prediction,and then built a soil salt dynamic model,analyzed the soil salt change process under different subsurface drainage conditions,and established the relationship between desalination years and drainage intensity of saline soil in the study area.Then,under different drainage conditions,the subsurface drainage layout of rice wheat rotation farmland was simulated to meet the requirement of a certain mechanical harvest guarantee rate,and the subsurface drainage scheme to achieve the maximum economic benefits was verified.The main research results are as follows:(1)The validation results of DRAINMOD with the measured data of groundwater level in the study area show that the model can accurately predict the dynamic change process of groundwater level in the study area.(2)Based on the analysis of long-term meteorological data in the study area,the results show that under the condition of artificial drainage,the soil desalination can be achieved by using the rainfall of times and inter annual rainfall;in general,the soil desalination can be achieved only by using the rainfall of rainy season(June to September).(3)DRAINMOD simulation results show that the subsurface drainage system can achieve different soil desalination goals under the effect of rainfall in the study area:the local surface roughness is higher(Sm=1cm),the soil salinity can be reduced to less than 1.5g/kg after 10 years when the depth of subsurface pipes is 100cm and the spacing is 10m;while the soil salinity can be achieved only after 34 years when the spacing is 50m;when the drainage interval is 10m,the soil salinity can be reduced to less than 1.5g/kg when the distance is more than 100m,the goal of soil desalination cannot be achieved.The functional relationship between soil desalination year(n/a)and depth(d/cm)and spacing(s/m)was n=33.24-0.272d+0.57s.(4)Considering the drainage requirements of rice wheat rotation farmland mechanical harvesting in coastal reclamation area,the simulation results show that when the groundwater level of rice and wheat harvest period falls below 80cm,the depth of subsurface pipe is 90-120cm,and the surface roughness is high,the maximum distance between subsurface pipes is 22.5-33.5m in 95%years.In 98%of the years,the maximum distance between subsurface pipes is 14.8-18.6m.(5)The incremental benefit cost ratio method is used to compare four schemes of subsurface pipe drainage system.The results show that the four schemes have good economic benefits.The best economic benefits can be obtained when the local surface roughness is higher(Sm=1cm,Si=0.5cm),the depth of subsurface pipe is 110cm and the spacing is 15m.In 98%of the years,the depth of the pipes is 110cm,and the spacing is 15.3m,which can ensure that the large harvester can operate within 2 days.This arrangement scheme is consistent with the drainage intensity of the pipes under the salt drainage condition of the coastal newly reclaimed farmland,and is the best scheme considering the salt drainage rate of the newly reclaimed farmland and the mechanical harvest guarantee rate of the existing farmland.