Increasing nitrate-N (N) concentrations in fresh waters is impacting adversely on New Zealand’s environmental and economic resources. We developed a simple catchment model, CLUES-GW, that provides mean-annual contaminant load and concentration estimates for surface water. CLUES-GW represents an extension of the existing CLUES, the Catchment Land Use and Environmental Sustainability model (Elliott et al. 2016)*. CLUES-GW includes up to two groundwater reservoirs under each surface water sub-catchment. The groundwater reservoirs exchange water with the adjacent (groundwater) reservoirs. The shallow reservoir also exchanges water with the surface-water streams. The runtime of the model is approximately 4 seconds, and so the quick runtime allows extensive model calibration, uncertainty and scenario analyses. The model was applied to two case study catchments to investigate N transport. The first catchment, Hauraki is located within the Waikato region of New Zealand and consists of 7,295 sub-catchments. Monte Carlo parameter conditioning from 10,000 parameter realisations was used to identify the best 200 ‘posterior’ parameter sets. The posterior parameter distributions were used to examine the N loads and concentrations at key locations under the current N loading dynamics and a hypothetical future scenario of 20% reduction in N loading. The second catchment, Mid-Mataura in the Southland region of New Zealand, is relatively smaller and has 1,535 sub-catchments. The model was calibrated using PEST software against measured river flows, and N loads and concentrations. An approach was developed to disaggregate model outputs, of mean-annual contaminant loads and concentrations, to daily timesteps using a rating-curve relationship between measured flows and concentrations. The calibrated model output along with the relationships developed for disaggregation were used to develop daily N concentration timeseries using flow data. The daily N concentration timeseries will be used as a management tool to assist with land and water management within the catchment.
- Elliott, A.H., Semadeni-Davies, A.F., Shankar, U., Zeldis, J.R., Wheeler, D.M., Plew, D.R., Rys, G.J., Harris, S.R. (2016) A national-scale GIS-based system for modelling impacts of land use on water quality. Environmental Modelling & Software, 86: 131-144. http://dx.doi.org/10.1016/j.envsoft.2016.09.011