CLUES model calibration and its implications for estimating contaminant attenuation

Catchment water quality models are essential for fresh water management; however, their value is dependent on their performance and level of uncertainty and for this information to be communicated in a transparent and understandable way to key stakeholders. Here we present the latest calibration of the Catchment Landuse for Environmental Sustainability model (CLUES) and examine the implications of the calibration for estimating in-stream contaminant attenuation. CLUES estimates catchment mean annual loads of Total Nitrogen, Total Phosphorus and E. coli and is widely used in New Zealand for both catchment planning and policy development. CLUES contains three freshwater model components derived from existing water quality models. Two are based on the New Zealand Overseer and SPASMO models and have been pre-calibrated for use in CLUES. The third component is based on the USGS SPARROW model and its parameters have been calibrated against annual loads estimated from monthly water quality data from catchments across the country. We found that CLUES gives reasonable load estimates at the catchment scale (Nash-Sutcliff efficiencies > 0.8 for all the contaminants). However, there was significant uncertainty in the SPARROW parameterisation. We conclude that while CLUES can be used to estimate catchment contaminant loads, it cannot adequately estimate instream attenuation. Improved representation of lower order streams in the calibration data would allow us to evaluate the source yields from each of the model components to better estimate attenuation.