Water And Salinity Management And Its Dynamic Of Wetland Under Controlled Drainage In Irrigated Area

Maintaining good salt and water balance through field drainage is the prerequisite of ensuring normal crop growth.But the conventional drainage sy stem speed up the soil moisture and nutrients losses,causing not only pollution of the receiving waters,but also waste of the valuable water resources.While retaining the function of drainage systems,controlled drainage can be used to adjust the drainage intensity to improve water use efficiency and reduce the negative impact of drainage on ecological environment.But reducing drainage intensity is contradictory with soil salinity control.With a case study in the Lubotan Reclamaton Area in Fuping county of Shaanxi province,China,this dissertation presents a comprehensive study on salt and water balance and key regulation method under controlled drainage;the research was conducted on the basis of field experiment and model simulation approaches,the beneficial effect of hydrometeorological factors on salt leaching under controlled drainage was analyzed accordingly.Major findings of this research are as below:(1)Field monitoring results showed that the depressional location of the study area allowed it receiving additional water from upstream areas,resulting in high water level in drainage ditches and subirrigation to crop fields;salt exchange between fields and ditches is important factor affecting salt balance under controlled drainage in the study area.DRAINMOD simulation results showed that,within 1m long drainage ditch,the total inflow was 9.3 m~3,outflow was 3.8 m~3,and the storage was 5.5 m~3 per annum.The calculated subsurface drainage was only 2.3 mm while the cumulative subirrigation was 49 mm during the study period.(2)Assuming that the difference in water table depth between cropland and bareland is caused by the crop uptake of shallow groundwater,a calculation model was established and verified with monitoring data of average soil water content.The results showed that the calculation model estimated the actual field condition well;in the calculated shallow groundwater use by cotton was 305.8 mm,which is equivalent to 0.64 mm/d,the estimated upward flux of groundwater is about 160 mm,accounting for about 24%of the total water demand.(3)An analytical model was developed for calculating rainfall leaching effect under controlled drainage.The model was tested on the basis of daily water balance predicted by the DRAINMOD.The results showed that for water table depth controlled at 1.2 m,with leaching irrigation of 150 mm,the rainfall leaching effect can keep soil salinity in the root zone below the crop tolerance level with probability of 87%in the study area.For the water table depth controlled at 1.5 m,with the average irrigation of 80 mm,no leaching irrigation is needed to maintain salt balance in the root zone.This can be considered as a rational field water management scheme in the study area;the direct water-saving is 140 mm when compared with the ideal conditions of supplementary irrigation.Therefore,rational use of rainfall leaching in semi-humid areas,the implementation of measures to control drainage,can effectively achieve the water-saving,salt control and environmental protection of the integrated objectives.(4)A simplified model for predicting salt leaching cycle was proposed on the basis of average condition assumptions;calculation results showed that for the existing conditions and water table depth at 2.1 m,1.5m and 1.2m,the predicted leaching cycle was>194days,140 days,and 119 days,respectively,requiring leaching water of 0mm,82.6mm and 94.1mm.Comparing with the conditions of no groundwater use,above results would save irrigation water use of 0.47 mm/day,1.53 mm/day,and 0.91mm/day for the controlled water table depth.Raising field water table depth from the present 2.1 m to 1.5 m through controlled drainage,total water saving in the whole growing season would be 85 mm.(5)A salt balance model was proposed for predicting salt accumulation in a wetland that receives drainage from the study area;for the present drainage condition with flow discharge ratio of 0.42,the predicted wetland water salinity would stablilize to 5.74 g/L in about 4 years.If the flow discharge ratio reduces 0.25,which is the optimal value under the current drainage system,the wetland salinity would stabilize in 18 years.Under the controlled drainage program(depth to water table at 1.5m with 80mm irrigation),there would be less drainage water discharge into the wetland,the wetland salinity,however,will not grow significantly according to the model predictions,indicating that controlled drainage will not adversely affect the wetland health.