A hydrological and nutrient load balance for the Lake Clearwater catchment, Canterbury, New Zealand

Abstract

Nitrogen and phosphorus entering waterways from diffuse agricultural sources is a major problem in New Zealand and internationally. This problem is well documented for lowland areas but little is known about nutrient enrichment from farming in high country areas. The Lake Clearwater catchment, in the Canterbury high country of New Zealand, has a native ecosystem that has adapted to low-nutrient conditions. The Department of Conservation's Arawai Kakariki Wetland Restoration Programme identifies wetlands in the catchment as one of three key endemic wetland types. Uncertainty regarding diffuse nutrient load from agriculture into the lake and wetland is limiting effective management of this unique catchment. This study investigated the hydrological regimes and nitrogen and phosphorus concentrations and loads in five key surface waterways at ten surface water sites and three groundwater seeps for two years. It aims to improve knowledge of nutrient sources, characteristics and loads from agricultural land use in this 46 km2 high country catchment. Additionally, nutrient load predictions from the Catchment Land Use for Environmental Sustainability (CLUES) model were compared to measured nutrient loads to assess the applicability of the model in high country catchments. The CLUES model was developed, primarily for lowland areas, to predict changes in water quality and nutrient loads from land-use change. The total nitrogen concentrations downstream of farmland were typically above the Australia and New Zealand Environment and Conservation Council (ANZECC) water quality guideline and the median concentration for upland Canterbury waterways. Specifically, the nitrate concentration (0.19-0.29 g m-3) in farmland subsurface runoff was elevated, compared to streams in the Lake Clearwater catchment with unfarmed catchments, and was estimated to contribute 52% of total nitrogen yield from farmland. The total nitrogen yield (1.96-2.94 kg ha-1 year-1) for farmed land was comparable to minimum values for New Zealand pastoral land use reported in the literature. The total estimated nitrogen export from Lake Clearwater (2518 kg year-1) was 83% greater than the estimated diffuse input from all land in the catchment (1375 kg year-1). This indicated an additional source of nitrogen into the lake and seasonal nitrogen saturation. Total phosphorus yields (0.093-0.123 kg ha-1 year-1) downstream of farmland were well below yields for New Zealand pastoral land use reported in the literature. Total estimated phosphorus export from the lake (58 kg year-1) was 24% less than total estimated diffuse loads into the lake (76 kg year-1). The ratio of total nitrogen to total phosphorus in Lake Clearwater (49:1) indicated that phosphorus is the limiting nutrient and that nitrogen loads into the lake are above natural levels. Total nitrogen loads predicted by the CLUES model in the Lake Clearwater catchment were reasonable, providing land-use area inputs are accurate and nutrient loads exit catchments in surface water. However, CLUES greatly overestimated phosphorus loads from farmed and unfarmed land.