Soil dispersion tests combined with topographical information can describe field‐scale sediment and phosphorus losses

Abstract

Abstract Methods for estimating sediment and phosphorus ( P ) transfer from agricultural land to surface waters are needed to mitigate their adverse impact on water quality. This study quantified sediment and P losses from five agricultural fields and evaluated the patterns observed based on the intrinsic risk of sediment and P mobilization from the soil, together with field topographical and hydrological data. The mobilization risk was estimated using the environmental soil dispersion test DESPRAL . High‐resolution L i DAR elevation data and crop management data were used to describe transport and delivery of the material mobilized. Annual flow‐weighted suspended solids ( SS ) concentration in drainage water ranged from 48 to 374 mg/L, total P ( TP ) from 0.12 to 0.39 mg/L and unreactive P ( UP ) from 0.08 to 0.33 mg/L. The mobilization risk, measured as turbidity in the aliquot recovered from the dispersion test, varied from 781 to 2310 nephelometric turbidity units ( NTU ). The method proved to be efficient in describing and differentiating sediment and P mobilization potential between fields. The topographical data also showed large differences between fields, with the length–slope ( LS ) parameter varying from 0.037 to 0.999. Based on assessments of source (mobilization risk) and transport factors ( LS , crop management data), it was possible to characterize fields as favoured or limited in terms of source and transport and to explain the long‐term SS and P losses observed at field scale.