Integrating modelling and field-based analysis for improving catchment-scale pollutant management: an application of the Catchmods Model in the Moruya River Catchment

Abstract

Water quality decline is a significant environmental issue in Australia and many other countries. Much emphasis has been placed on nutrient management especially for phosphorus (P) and nitrogen (N) in an attempt to reduce their loss to water bodies. Identifying nutrient sources and their transport potential can assist catchment managers to recognise critical nutrient sources and to devise management interventions for their control. High concentrations of ammonia and phosphate have been observed in the available water quality data for the Moruya River, indicating the potential risk of eutrophication (AMOG Consulting, 2003). Stream bank erosion has been identified as a major erosion problem for the Moruya Estuary and is a potentially significant source of nutrients. However, the impacts of stream bank erosion on sediment output and nutrient loss have not been well quantified and the costs of stream bank management have not been adequately assessed. This study demonstrates the use of an integrated modelling and field sampling approach to improve identification of nutrient sources. The Catchment-scale Management Of Diffuse Sources model (CatchMODS) was used to estimate suspended sediment yields under a variety of management change scenarios in the Mogendoura Creek subcatchment of the Moruya River, NSW. Nutrient source strengths were determined from field and laboratory analysis of soil nutrient concentrations (total N and total P). Soil samples were collected from different land uses (dairying, beef grazing and forest) and from actively eroding areas (gullies and stream banks). By incorporating the nutrient field data and modelled sediment yields, the annual particulate nutrient inputs under each of the management change scenarios were estimated. Scenarios were investigated representing the current (business-as-usual) situation, the effects of afforestation and the effects of various channel erosion management measures. Results show that stream bank erosion is the largest sediment and particulate nutrient input to the subcatchment. Currently, stream bank erosion is estimated to contribute over 2000 t/yr of suspended sediment yield, followed by gully erosion (336 t/yr) and hill slope erosion (2.3 t/yr). Stream bank, gully, and hill slope erosion contribute differently to nutrient loss. More than 85% of the N and P loss is estimated to come from stream bank erosion, mainly due to the high rate of suspended sediment delivery. Severe gullies contribute 12% of the N and P loss. The remainder of erosion types are estimated to contribute only 3% of total particulate nutrient loss. Accordingly, management practices applied to riparian areas, employing either engineering or revegetation techniques, are thought to be the most effective and also the most economical way to reduce nutrient-associated water quality impacts. Land use change, such as converting pasture to forest, was found to have a very limited effect on reducing nutrient transfer. The opportunity cost of losing income from grazing, however, is high. Nevertheless, for catchments which have lower levels of erosion and more intensive land uses, such as dairying, the recommended management interventions may be quite different. The paper demonstrates the advantages of combining modelling and field work for improving water quality management.