Review on agricultural non-point source pollution monitoring sections layout and pollutant loading estimation in small watershed

Watershed-scale non-point source pollution monitoring is an important basis for systematically understanding the occurrence, transportation and transformation processes of agricultural non-point source pollution and its effective control. Presently methods monitoring non-point source pollution at field scale are relatively mature, while methods applied at watershed scale, particularly the layout of monitoring section and sampling frequency setting, are rarely studied. This paper reviewed the main progress of sampling section layout, sampling frequency optimization and loading estimation of river sections. From the perspective of sampling layout in small watershed, there are three types of monitoring design including probabilistic design, targeted design and both. Sampling points were located with genetic algorithm, fuzzy logic method, entropy and modelling method, etc. Sample collection methods include random sampling, compound sampling, comprehensive sampling and continuous sampling. Compound sampling is widely used. Concerning the sampling frequency, once every 1-2 weeks is sufficient to obtain accurate pollution loading. If higher accuracy is necessary, the monitoring frequency and special sections can be increased when hydrology/water quality variation is large. For loading estimation, average method, interpolation method and regression or curve method were widely used. Concentration estimation algorithm by flow-weighted, interpolation and LOADEST are simple and exact. Selection of methods could be further optimized based on the characteristics of pollution sources in different periods of the basin. © 2022 by Journal of Lake Sciences.