Strategy for Developing GIS-based Tools for Management of the Effects on Groundwater of Nitrate Leaching from Agricultural Land Use

EXTENDED ABSTRACT The end user requirement for this development is to be able to assess the impact of land use changes on groundwater quality, specifically that due to nitrate leaching from agriculture. Implementation will be for alluvial plains at scales up to about 2000 km. GIS databases provide information on land use and soils. This information, at individual field and farm scale, enables modelling of nitrate leaching by the companion FarmSim model that is run on a daily time basis in response to climate, soils, crop, and farm management options. The output leaching data are in the form of mean annual drainage and an associated value of average concentration. Upscaling of farm-scale nitrate leaching to the effects on groundwater quality is achieved by use of a steady-state, groundwater flow and contaminant transport models (AquiferSim). A 2-D horizontal groundwater flow model provides a piezometric surface along which land use changes and affected groundwater zones can be related by means of gradient searches that locate principal directions of groundwater flow. Along these principal directions, the horizontal and vertical dispersive transport of nitrate can be assessed within a 2-D vertical, steady-state, groundwater flow model based on stream function analysis. The stream function model allows simulation of steady-state contaminant transport for complex land use patterns and spatially varying aquifer properties, in the form of a mixing-cell model based on the computational grid. The cell size of this model is set to control numerical dispersion, as a surrogate for dispersion in the aquifer, caused by the horizontal and vertical components of the groundwater flux. The resulting distribution of nitrate concentration with depth in the aquifer can thus be determined for any location in relation to the upstream land uses. The transport model also simulates groundwater age, which is treated as a solute undergoing first-order growth. Spatial distribution of groundwater age and steady-state nitrate concentration addresses some of the end user questions that usually require a transient transport model. The groundwater models have been developed as prototypes with Microsoft Excel. These proven computational methods will be incorporated into an application to run on the Microsoft .NET framework. This application is being used as the software architecture that connects GIS databases, nitrogen leaching models, vadose zone and groundwater transport models, to user interfaces and information output displays. The lack of datasets that span the response time of many years between environmental effects and land use changes precludes total model calibration, and an alternative approach is proposed that tracks the predictive effects of contributing uncertainties.