Land use and water quality

The issue of land use and land-use intensity has long been linked to water quality globally. Changing land use in New Zealand, especially in pastoral systems, over the last 50 years have caused nitrate-nitrogen (N) concentrations to increase within streams, rivers, lakes and groundwaters (Julian et al. 2017). However, good management practices have also either offset or decreased the concentrations of some other contaminants like phosphorus (P) and sediment (Snelder 2018; McDowell et al. 2019). Nevertheless, the current state of water quality is perceived to be poor by the New Zealand public (Hughey et al. 2019). As a result, policy to improve water quality is becoming more stringent. In this special issue are several papers that address a range of topics related to land use and water quality. I have arranged these topics into two broad categories: (1) Processes that characterise the loss and transport of water quality contaminants from land to water and within aquatic systems, and (2) Assessment of what has and could be done within primary production systems to reduce losses from land to water. Papers within the first category cover underpinning data on the transport of P in subsurface flow – revealing new knowledge about a transport system for P that is often neglected in favour of studying surface runoff losses (Gray et al. 2021), the refinement of the Catchment Land Use and Environmental Sustainability model used to estimate concentrations and loads of contaminants in surface waters (Semadeni-Davies et al. 2021), and the development of a new framework to link the land, surface and groundwater systems together at a national scale (Srinivasan et al. 2021). Papers within the second category examine new actions to mitigate the loss of N from sheep and dairy farm systems by manipulating the timing of feeding sheep, thereby reducing urinary N (Thomson et al. 2021) and adding a flocculating agent to farm dairy effluent to reduce contaminant loads in effluent irrigation (Chisholm et al. 2021). The agent causes nutrients, sediment and E. coli to settle to the bottom of the pond. The liquid is far more dilute thereby avoiding surplus N being applied to land and lost either as leachate or via denitrification to the atmosphere. Other papers examine the eco-efficiency of dairy systems in New Zealand and Uruguay (Darré et al. 2021; Soliman and Djanibelkov 2021), and examine the use of actions to mitigate nutrients and sediment loss from land to water. This work asks what has been achieved over the last 20 years (Monaghan et al. 2021; Monaghan et al. 2021), informing what water quality would have looked look like if no action was taken. Additional work looks at what can still be done if we were to implement all established and developing actions to improve water quality (McDowell et al. 2021). The final paper examines the history and provides a commentary on how these actions could be achieved via a farm planning process (Stokes et al. 2021). The use of farm plans has recently been mandated in New Zealand, a world first, hence learning from the past on how these would be best developed and implemented is essential to achieving water quality objectives. Collectively, these papers are designed to inform landowners about the current state of science on modelling and mitigating the effects of land use on water quality. They may also be useful for informing policy options and implementation of policy at both central and local government levels.