Study On Output Characteristics Of Non-point Source Pollution From Typical Planting Systems In Plain Polder Areas Based On High-frequency Monitoring

In order to clarify the characteristics of rainfall and irrigation runoff and the discharge characteristics of non-point source pollutants such as nitrogen（N）and phosphorus（P）in paddy and dryland（open field vegetable）of polder areas,dryland and paddy in polder areas of Shanghai suburb was taken as the study objects.With the application of online automatic hydrology monitoring and water quality monitoring based on auto sampler,collaborative monitoring high-frequency monitoring of both hydrology and water quality was carried out for farmland runoff.The high-resolution electrical conductivity（EC）and turbidity（TURB）data were used to characterize the concentration variation of dissolved and particulate pollutants.,the discharge characteristics of dissolved and particulate pollutants were studied from the perspective of hysteresis effect by analyzing the concentration-flow（C-Q）relationship.The migration pathways of different forms of pollutants were thus clarified.This dissertation was aimed at providing basic support for the prevention and control of farmland non-point source pollution in the plain river-network polder areas.The main conclusions were as follows:（1）The water quality analysis results of paddy and dryland runoff indicated that turbidity could better reflect the total suspended solids（TSS）content（r=0.869,P<0.01）and total phosphorus（TP）content（r=0.635,P<0.01）.EC could well reflect the soluble salt concentration in runoff（r=0.975,P<0.01）.The correlation between EC and dissolved nitrogen and phosphorus were weak at low nitrogen conditions,while EC and nitrogen exhibited good correlation（r=0.905,P<0.01 for dryland,r=0.707,P<0.01 for paddy）at high nitrogen conditions post fertilization.（2）The hysteresis analysis results of high-frequency monitoring data in paddy and dryland indicated that EC-Q exhibited counter-clockwise hysteresis in dryland and clockwise hysteresis in paddy,and the dilution is the main output characteristic of dissolved pollutants.TURB-Q of paddy and dryland exhibited a clockwise hysteresis,indicating that the export of particulate pollutants were mainly influenced by flushing effect in both paddy and dryland.The export of TP in dryland mainly exhibited flushing effect,while the export of other forms of N and P varied in different runoff events（with both flushing and dilution effects）.The export of ammonium nitrogen（NH₄⁺-N）in the paddy field exhibited both flushing and dilution effects,and the export of other forms of N and P generally exhibited dilution effect.（3）The monitoring and analysis results of dryland runoff and soil solution index indicated that TP in runoff was significantly from the loss of particulate phosphorus in soil,while TN in runoff was mainly from soil solution.The maximum TP concentration of runoff had no correlation with the TP concentration of soil solution.TP was principally from the surface soil.The short migration pathway was the main reason why the export of particulate pollutants mostly showed a clockwise hysteresis mode.There is a good correlation between maximum TN concentration of runoff and TN concentration of soil solution（r=0.969,P<0.05）.TN mainly came from soil solution,and the variable migration pathway was the reason why the export of dissolved substances mostly showed a counterclockwise hysteresis pattern.（4）The results of hydrology monitoring of paddy and dryland showed that the average runoff coefficient of paddy was higher than that of dryland.The runoff of paddy was more stable,while the runoff coefficient of dryland varied greatly with different rainfall events.There was a good correlation between runoff coefficient and rainfall in both dryland and paddy field（r=0.876,P<0.01;r=0.744,P<0.01）.The runoff production of dryland was greatly affected by the Antecedent Moisture Condition（AMC）,and the runoff curve number（CN）representing the soil runoff production capacity of dryland was the best relationship with the Antecedent Moisture Condition of 10 cm soil layer(AMC₁₀)（r=0.878,P<0.01）.Rainfall,irrigation and artificial drainage were the main reasons for the significant variation of surface water level in paddy.（5）The results of collaborative monitoring of hydrology and water quality showed that the TN export load of paddy was twice that of dryland and TP export load of paddy was less than that of dryland.TN load of paddy during growth period was 24.76 kg·hm^-2and the main loss form was NH₄⁺-N.The TP discharge load of paddy was 0.93 kg·hm^-2,which was less than that of the dryland.The main loss form of phosphorus was dissolved total phosphorus（DTP）.The N and P export load of paddy was affected by rainfall and fertilization,and the N loss load had low relationship with runoff under high N application rate.The annual load of TN in dryland was13.39 kg·hm^-2,and the main loss form was NO₃^--N.The annual load of TP was 2.05 kg·hm^-2,and the main loss form was particulate P.N and P loss load in dryland was affected by runoff,and NH₄⁺-N had the best correlation with runoff（r=0.956,P<0.01）.