Study On Monitoring And Simulation Of Farmland Drainage Process In Rice-wheat Rotation Field In Along Grand Canal Irrigation District

In the river plain area of South China,it is rich in water resources,but farmlands use it excessively.Especially in the process of paddy field drainage,pollutant emissions could not be controlled effectively,which brought enormous pressure to aquatic ecological environment in local areas.This paper,from the angle of agricultural water management and control in the Along Grand Canal Irrigation District,aiming at reducing farmland drainage to reduce the output of agricultural non-point source pollution,chooses DRAINMOD model as the research tool to simulate hydrological process under different irrigation conditions,distribution of nitrogen and phosphorus,and their losses in paddy field.It also analyzes an optimal layout of farmland drainage system for optimum economic benefit of wheat.Findings from it can provide theoretical basis and technical support for the local agricultural production.Major findings are as following:(1)The water table depth during the period of rice growth was higher,and the drainage volume was relatively large.All of them present periodicity when irrigation appearing.Calibrating parameters of DRAINMOD found that the simulation results are consistent with the measured values,it showed that DRAINMOD can be used to predict the hydrological process of rice-wheat rotation field accurately.Sensitivity analysis of the main input parameters showed that the soil lateral saturated hydraulic conductivity and the drainage coefficient should be selected cautiously when using DRAINMOD to simulate the processes in studying region.(2)Using DRAINMOD model to simulate and analyze the three irrigation regimes of conventional irrigation,deep water irrigation and shallow water irrigation,found that,the latter two can not only improve crop yields but also save agriculture water and reduce emissions,especially in shallow water irrigation.Under this regime irrigation amount decreased by 108mm,cutting the rate of 11.8%.The drainage volume was reduced by 25%,and the crop yield added 5.5%,which is consistent with measured yield increasing 532.5 kilograms per hectare.Therefore,shallow water irrigation is a more scientific choice of rice irrigation system.(3)Through the study area farming records and monitoring samples from different groundwater and drainage ditch at regular intervals,we found different forms of nitrogen in farmland were higher during rice planting,and ammonium nitrogen changed obviously.Its concentration is 2.67mg per litre during this period,besides it's only 0.33 mg per litre.The content of total phosphorus was smaller in the whole monitoring period.With the increase of soil depth,the concentration of pollutants showed a trend of decreasing.(4)Through the study on the loss of nitrogen and phosphorus in farmland,we found the loss of phosphorus in farmland was less than that of nitrogen fertilizer during rice growing period.Amount of ammonium nitrogen loss in July alone accounted for 38.7%of the total nitrogen loss.The results showed that a large part of the nitrogen in the form of ammonium nitrogen was drained from the field after the fertilization period,especially after urea.The reduction of nitrogen and phosphorus loss was more obvious in shallow water irrigation.Compared with conventional irrigation,nitrate nitrogen and ammonium nitrogen were reduced by 21.1%and 36.5%,and total phosphorus decreased 0.30kg per hectare.Rational application of chemical fertilizer not only increases the yield of rice,but also increases the economic income.Even more it can reduce environmental negative effects of agricultural water.(5)Under the existing drainage conditions in the study area,it could not meet the requirements of wheat drainage and waterlogging.A simulation study of the drainage system with maximum wheat yield found that,the higher the drainage intensity,the higher the relative yield of wheat.Relative yield also increased with the improvement of surface drainage,and it strongly affected by waterlogging stress.The change was more clearly after the drainage spacing is more than 20 meters.According to the simulative results,to ensure the yield of crops and considering the factor of cost,study area has well surface drainage and drainage system with design of which depth of drainage pipe is 1.2m and spacing is 20m,that can obtain optimum benefit.