River water quality changes in New Zealand over 26 years (1989–2014): Response to land use and land disturbance

Abstract

Abstract. River water quality reflects land use in the catchment (mobilizing diffuse pollution) as well as point source discharges. In New Zealand (NZ) diffuse pollution vastly outweighs point sources which have largely cleaned up over many decades. Because NZ has good geospatial data on physiographic variables, land cover and agricultural statistics, and time series on water quality at the national scale over several decades, the country is a natural laboratory for investigating water quality response to land use/disturbance and associated diffuse pollution "pressures". We interpreted water quality state and trends for the 26 years from 1989 and 2014 in the National Rivers Water Quality Network (NRWQN), consisting of 77 sites on 35 mostly large river systems with an aggregate catchment amounting to half of NZ's land area. To characterize water quality pressures, we used multiple land use datasets spanning 1990–2012, plus recently-developed 8-day land-disturbance datasets using MODIS imagery. Current state and directions of change in visual clarity and nitrate-nitrite-nitrogen provide a particularly valuable summary of impact, respectively from mobilization of fine particulate matter and soluble nutrients. We show that the greatest impact on river water quality in NZ over the 1989–2014 period is high-producing pastures with their high nutrient inputs to support high densities of livestock. While land disturbance was not itself a strong predictor of water quality, it did help explain outliers of land use-water quality relationships, especially those with large areas of plantation forest. Plantation forestry was strongly associated with water quality impacts, particularly on visual clarity and particulate nutrients when land disturbed for harvesting generated sediment runoff and nutrient mobilization. In all, our study demonstrates how interdisciplinary combinations of expertise including geospatial analysis, land management, remote-sensing, and water quality can advance understanding of broad-scale and long-term impacts of land use change on river water quality.

Nonlinear Changes in Land Cover and Sediment Runoff in a New Zealand Catchment Dominated by Plantation Forestry and Livestock Grazing

“…While we were not able to thoroughly assess all of these management practices across the entire catchment, numerous field observations and discussions with local farmers indicate that some of these practices were occurring at various times in our study period. However, management intensity in this region is relatively minor compared to other parts of New Zealand [63], and thus most of the land-water relationships we observed were likely from biophysical processes. The main exception was plantation forest harvests.…”

“…From several years of observation by us and others [42], there are large stores of sediment along the mainstem Hoteo River, which change from flood to flood. While it is this in-channel sediment that makes the Hoteo River one of the most turbid rivers in all of New Zealand [63], this sediment has been derived from major landscape disturbances over the past 150 years [42]. These legacy effects also contribute to nonlinear changes in water quality and a level of complexity that was beyond the temporal scope of this study.…”