Estimating catchment vulnerability to diffuse herbicide losses from hydrograph statistics

The prediction of diffuse herbicide losses to surface waters using process-based models is data and time demanding. There is a need for simpler and more efficient catchment screening tools. We developed and tested a new proxy for screening catchments for their vulnerability to diffuse herbicide losses relying on widely available river flow data only. The proxy combines the fast flow index (FFI) (i.e., the long-term average proportion of fast flow to total discharge) and the fast flow volume (FFVs) (i.e., the stormflow component of the hydrograph during the spring flush period). We tested the proxy and the underlying hypotheses by regression analyses of existing high-frequency, multiannual monitoring datasets of atrazine, metolachlor, alachlor, and acetochlor concentrations from six US and European catchments. The percentages of applied amounts of three of the four herbicides lost to surface water were positively correlated with the FFV, and the slope of the correlation was positively correlated with the FFI. Based on the empirical data, we derived quantitative proxies to estimate atrazine and metolachlor losses based on the measured FFI and FFV values. The application to 65 European catchments revealed that many of them had a lower vulnerability than the six test catchments. The observed FFI dependency of the slope seems reasonable because with increasing FFI less rain is needed to trigger fast flow and more herbicide is available in the topsoil. The proposed FFI-FFVs proxy can guide researchers and authorities in selecting monitoring areas, setting monitoring results into context, and prioritizing mitigation strategies according to catchment vulnerability.