The terrain of the tidal polder area of the river network is low,and the water level of the farmland drainage ditch is affected by the fluctuation of the water level of the outer river,which leads to the complicated process of non-point source pollution and river inflow.It is necessary to establish a method to study the process of dryland runoff generation and river inflow.In this study,typical dry land was selected in two different polders in Shanghai,and the water level of the triangular weir at the outlet of the dry land runoff was monitored by the float water level meter.Based on the Doppler principle,the flow velocity of the farmland drainage ditch into the estuary was monitored.The characteristics of phosphorus production in farmland runoff and the process of inflow into the river under two rainfall events were studied.The change of conductivity at the estuary of the ditch was used to assist in judging the rainfall-driven drainage process and verify the stability of the Doppler method for measuring the flow velocity.The results show that during the non-rainfall period,the farmland drainage ditch and river water are characterized by reciprocating motion.During the rainfall period,the farmland runoff of Qingpu and Chongming monitoring points accounted for 33.3% and 40.9% of the river inflow,respectively,and the process of entering the river was affected by the water level of the outer river.The highest speed of the drainage ditch into the river is 25 cm/s,and the highest water level difference is 20 cm.The flow velocity and conductivity of the ditch can better reflect the change of the flow velocity,but the measurement of the flow velocity of the ditch at Chongming point may be affected by the instability of the drainage flow in the ditch.The nitrogen and phosphorus in the farmland at the Qingpu point are smaller than those at the farmland drainage outlet,which is mainly related to the dilution of surface runoff around the farmland,and the concentration of nitrogen and phosphorus in the water quality of the Chongming soil drainage ditch into the river is significantly higher than that of the farmland runoff.This may be due to the fact that the flow rate of the drainage ditch exceeds the critical speed,resulting in the release of phosphorus in the sediment of the drainage ditch.In the tidal polder area,due to the low flow rate,the change of conductivity can be used to assist in identifying the hydrological process and judging the accuracy of the Doppler method to determine the flow rate.
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