Abstract: The estimation of pollutant loads in rivers and the analysis of their variations are the basis for non-point source pollution treatment. However, the low-frequency manual monitoring cannot reflect the daily continuous changes of water quality in rivers. Based on the monthly water quality monitoring data at the outlet section of Changle River in Zhejiang Province from 2003 to 2016, the LOADEST model for total nitrogen (TN) and total phosphorus (TP) load estimation in the river was calibrated and verified. Then, the daily TN and TP loads in the Changle River were estimated by using the continuous flow data. The results showed that, during 2003-2016, the average annual TN and TP loads in the river were (2 207.71±862.48) t·a^-1 and (71.43±31.56) t·a^-1, respectively, while their daily average loads were (6.04±11.81) t·d^-1 and (0.19±0.26) t·d^-1, respectively. Both the TN and TP loads had an increasing trend in the study period. Meanwhile, the variation patterns of TN and TP loads in the river during the research period were analyzed by using wavelet analysis method. It was found that the TN load in the Changle River had periodic changes with a main time span of 18 months. The variation of the TP load was similar to that of the TN load, but the varying intensity was relatively weak. The wavelet variance of TN and TP loads suggested that the periodic variations of TN and TP loads were mainly dependent on the periodic changes of river flow, yet less affected by TN and TP concentrations.

Key words: agricultural watershed, river pollution, total nitrogen, total phosphorus, wavelet analysis