Prediction of dissolved reactive phosphorus losses from small agricultural catchments: calibration and validation of a parsimonious model

C Hahn,R Schulin,P Lazzarotto,V Prasuhn,M W H Evangelou,C Stamm

doi:10.5194/hess-17-3679-2013

Abstract

Abstract. Eutrophication of surface waters due to diffuse phosphorus (P) losses continues to be a severe water quality problem worldwide, causing the loss of ecosystem functions of the respective water bodies. Phosphorus in runoff often originates from a small fraction of a catchment only. Targeting mitigation measures to these critical source areas (CSAs) is expected to be most efficient and cost-effective, but requires suitable tools. Here we investigated the capability of the parsimonious Rainfall-Runoff-Phosphorus (RRP) model to identify CSAs in grassland-dominated catchments based on readily available soil and topographic data. After simultaneous calibration on runoff data from four small hilly catchments on the Swiss Plateau, the model was validated on a different catchment in the same region without further calibration. The RRP model adequately simulated the discharge and dissolved reactive P (DRP) export from the validation catchment. Sensitivity analysis showed that the model predictions were robust with respect to the classification of soils into "poorly drained" and "well drained", based on the available soil map. Comparing spatial hydrological model predictions with field data from the validation catchment provided further evidence that the assumptions underlying the model are valid and that the model adequately accounts for the dominant P export processes in the target region. Thus, the parsimonious RRP model is a valuable tool that can be used to determine CSAs. Despite the considerable predictive uncertainty regarding the spatial extent of CSAs, the RRP can provide guidance for the implementation of mitigation measures. The model helps to identify those parts of a catchment where high DRP losses are expected or can be excluded with high confidence. Legacy P was predicted to be the dominant source for DRP losses and thus, in combination with hydrologic active areas, a high risk for water quality.

Highlights

Eutrophication of surface waters due to diffuse phosphorus (P) inputs continues to be a severe water quality problem worldwide (Carpenter et al, 1998; Kleinman et al, 2011b), causing, for example, algal blooms, oxygen shortage, fish death and loss of water bodies for recreation and drinking
Our results demonstrate that the RRP model is able to make useful predictions of discharge and dissolved reactive P (DRP) losses from grassland-dominated catchments
The validity of the underlying concept is further supported by the agreement between spatial predictions of runoff generation risks with ground measurements of soil moisture, surface runoff and groundwater levels

Summary

Introduction

Eutrophication of surface waters due to diffuse phosphorus (P) inputs continues to be a severe water quality problem worldwide (Carpenter et al, 1998; Kleinman et al, 2011b), causing, for example, algal blooms, oxygen shortage, fish death and loss of water bodies for recreation and drinking. Critical source areas are characterized by a direct transport connection of available P sources to a receiving water body (Gburek and Sharpley, 1998). Erosion and surface runoff were assumed to be the only relevant transport mechanisms, but it is recognized that subsurface flow can significantly contribute to P export (Doody et al, 2012; Kleinman et al, 2007, 2011b; Stamm et al, 2002; Watson and Matthews, 2008).

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Conclusion