River water quality changes in New Zealand over 26 years: response to land use intensity

Abstract

Relationships between land use and water quality are complex with interdependencies, feedbacks, and legacy effects. Most river water quality studies have assessed catchment land use as areal coverage, but here, we hypothesize and test whether land use intensity -the inputs (fertilizer, livestock) and activities (vegetation removal) of land useis a better predictor of environmental impact. We use New Zealand (NZ) as a case study because it has had one of the highest rates of agricultural land intensification globally over recent decades. We interpreted water quality state and trends for the 26 years from 1989 to 2014 in the National Rivers Water Quality Network (NRWQN) -consisting of 77 sites on 35 mostly large river systems. To characterize land use intensity, we analyzed spatial and temporal changes in livestock density and land disturbance (i.e., bare soil resulting from vegetation loss by either grazing or forest harvesting) at the catchment scale, as well as fertilizer inputs at the national scale. Using simple multivariate statistical analyses across the 77 catchments, we found that median visual water clarity was best predicted inversely by areal coverage of intensively managed pastures. The primary predictor for all four nutrient variables (TN, NO x , TP, DRP), however, was cattle density, with plantation forest coverage as the secondary predictor variable. While land disturbance was not itself a strong predictor of water quality, it did help explain outliers of land use-water quality relationships. From 1990 to 2014, visual clarity significantly improved in 35 out of 77 (34/77) catchments, which we attribute mainly to increased dairy cattle exclusion from rivers (despite dairy expansion) and the considerable decrease in sheep numbers across the NZ landscape, from 58 million sheep in 1990 to 31 million in 2012. Nutrient concentrations increased in many of NZ's rivers with dissolved oxidized nitrogen significantly increasing in 27/77 catchments, which we largely attribute to increased cattle density and legacy nutrients that have built up on intensively managed grasslands and plantation forests since the 1950s and are slowly leaking to the rivers. Despite recent improvements in water quality for some NZ rivers, these legacy nutrients and continued agricultural intensification are expected to pose broad-scale environmental problems for decades to come.

Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures

“…In Australasia (i.e., Australia and New Zealand), intensive pasture-based grazing is often the primary land use in catchments with impaired water quality (Dairy Australia, 2019;McDowell et al, 2009a;Meat and Livestock Australia, 2017;PricewaterhouseCoopers, 2011;Sinclair Knight Merz, 2011). Although some aspects of water quality may be improving in New Zealand (Larned et al, 2016), P exports from pastures remain a concern (Ballantine and Davies-Colley, 2014;Julian et al, 2017;McDowell et al, 2009a). The New Zealand government has responded to concerns about water quality and use with its National Policy Statement for Freshwater Management (New Zealand Government, 2017).…”

Effects of Landscape Development Intensity on River Water Quality in Urbanized Areas

“…Environmental factors (EF) include forest coverage rate (FCR) and percentage of cultivated land (CLP). FCR refers to the proportion of forest area in the watershed to the total area of it (%), which measures the degree of landscape changes made by human beings in the watershed through the activities such as vegetation removal and planting [40]. CLP is the percentage of cultivated land area in the watershed occupies a total area of it (%).…”

“…Therefore, some scholars introduced the concept of "intensity" into the study of the relationship between human activities interference and water quality, defining "land use intensity" as the levels of socio-economic inputs and human activities of the land use by unit area and time [37,39,40]. Land use intensity indicators such as land inputs (e.g., fertilizer use and livestock feeding) and land disturbance (e.g., vegetation removal or planting) are believed to be better predictors of human impacts than land use proportion alone [40], and can be directly applied to the study of the correlation with water quality [37,41].…”

Applying Place-Based Social-Ecological Research to Address Water Scarcity: Insights for Future Research

“…Global demand for freshwater sources combined with a declining water supply and quality translates in an issue for approximately two-thirds of the global population and nearly every regional-scale watershed experiences severe water scarcity for at least one month of the year [1][2][3][4][5]. Developing equitable and effective governance solutions for water scarcity is a global priority [6][7][8] to achieve global sustainability [9][10][11].…”