Watershed-level analysis of exceedance frequencies for different management strategies

Exceedance of water-quality standards is important in assessing water quality. The effectiveness of soil conservation Beneficial Management Practices (BMPs) should be measured according to the BMPs’ impact on exceedance frequencies. However, estimating exceedance frequencies for different management scenarios with field measurements is practically impossible due to difficulties in obtaining adequate data for analysing different combinations of BMPs. The objective of this modeling research was to analyse exceedance frequencies for different management strategies applied in the Black Brook Watershed (BBW). Daily concentrations of total suspended sediments (TSS) and soluble phosphorous (sol-P) were predicted with the Soil and Water Assessment Tool (SWAT) and assessed against water-quality standards from the Canadian Council of Ministers of the Environment (CCME) and National Agri-Environmental Standards Initiative-Ideal Performance Standards (NAESI-IPS). The investigated BMPs included conservation tillage, reduced fertilizer application, crop rotation, flow diversion terraces (FDT) and the combination of all four BMPs. The results indicated that FDT was the most effective at reducing exceedance frequencies of TSS and sol-P. Under the different management scenarios, we calculated the annual exceedance frequencies of TSS and sol-P concentrations above the CCME (20–45% and 10–26%) and NAESI-IPS (32–55% and 20–38%).