Hysteresis of the solute concentration/discharge relationship in rivers during storms

Hysteresis of dissolved calcium, silicon, nitrate, nitrite, ammonium and phosphorus fractions, i.e. soluble reactive phosphorus, total dissolved phosphorus and total phosphorus, in river waters during a major storm event in the R. Swale catchment in Yorkshire (U.K.), are quantified using a semi-empirical model. The model separates point and base-flow contributions of the chemical determinants from diffuse inputs. The magnitude of the diffuse inputs is related to river water discharge so that the size and rotation of the hysteresis loop is characterized by a single parameter. The approach is applied to chemical and water discharge data collected at 2 h intervals from three river sites in the catchment over a complete storm hydrograph. The results illustrate hysteresis effects for all the determinants with the majority showing “clockwise” hysteresis, i.e. higher concentrations during the rising limb of the hydrograph compared with those measured during the falling limb. The model parameters, computed by optimizing agreement between the predicted and measured concentration–discharge relationships, are generally consistent with land-use patterns in the catchment. The method has potential for further development to enable comparisons of chemical and nutrient dynamics in river catchments and the assessment of the relative importance of diffuse and point sources during high flow conditions.