Modelling phosphorus transport in a surface irrigation drain

Understanding phosphorus (P) transport from agricultural land is essential to the development of effective management strategies that reduce the impact of agriculture on inland water quality. This paper describes the development and application of a process based model describing P transfer down a farm scale irrigation drain. The model uses a volume routing equation combined with the Kostiakov infiltration equation to route water down the drain. Using estimated infiltration parameters and literature values of Manning’s Roughness the flow-model predicted the total volume of water transfer down a 180 m long irrigation drain within 5%, with flow rates predicted within 10%. The flow was then used to provide input data for modelling P transfer down the drain. The P model combines a simple advective equation with rate equations to describe P release by plants and the uptake and release of P by bed sediments. Data from four field investigations were used to parameterise the model with concentrations and loads predicted within 5%. Applied over 14 months, the results from four modelled management scenarios suggest that a bare earth drain, or an intermittently cleared drain may reduce P export over 180 m by 9–19%. While the cost of current management strategies is likely to ultimately limit implementation on commercial farms, the model presented in this paper provides a useful basis for the investigation of further management scenarios, which may reduce P export from irrigated dairy farms.