A new approach for identifying catchment typologies based on phosphorus impact risks

Phosphorus (P) transfer indices (Mobilisation index and Delivery index) were recently introduced to facilitate a standardized, systematic and objective method to evaluate P transfer and impact risks at the catchment scale. The method was developed from high-frequency hydro-chemo-metric data using ratios of high and low percentiles of P concentrations and mass loads. Using a large dataset from 23 catchments in North-western Europe, we present a pooled catchment approach to establish a relationship between the Mobilisation and Delivery indices with the catchments’ baseflow and flashiness indices with the objective to identify catchment P impact risk typologies. While hydrology largely controls P transfer, the deviation from this hydrological relationship highlighted the presence of other influences, such as intrinsic P retention and point source or legacy P controls. The method distinguishes the type of dominating mobilisation and delivery risk (runoff, point source and/or legacy P) and of intrinsic retention (poor solubility and/or poor hydrological connectivity).

The P mobilisation in 12 of the catchments was dominated by hydrological controls. Five other catchments, with large flat areas, high water storage capacity and/or with a high P sorption capacity, had a potential to retain 39% - 68% of reactive P (RP) corresponding to an annual retention of 0.02 - 0.32 kg RP/ha. The highest intrinsic P retention was in a karstic limestone spring contribution zone rich in calcium. Finally, six of the catchments manifested a varying degree of point source influences, which elevated the RP mobilisation by 16% -77% and corresponded to an annual loss of 0.02 – 0.12 kg RP/ha. While hydrological controls dominated P delivery in all catchments, two catchments manifested a P delivery reduced by 72% and 76% due to poor hydrological connectivity (0.02 and 0.12 kg RP/ha per year). Eight catchments had a higher Delivery index in relation to the Mobilisation index, and these catchments were those with above average hydrological flashiness. We propose that these catchments are, to a varying degree, influenced by legacy P (river scouring and/or resuspension of P). This highlights that mobilisation risk could be independent from delivery risk owing to the hydrological connectivity of the landscape.

The proposed approach can guide P pollution management by identifying and quantifying the underlying dominant impact risks within catchments. Identifying catchment typologies based on P risk classes can be further useful for scaling up and for understanding the additional pressures caused by climate and land use changes.