Mass Mixing Approach For Assessing Effects Of
Irrigation On Water Quality
Author(s)
I. Fraser, S. Douglass, B. Ellwood & N. Norton
Abstract
Predicting the magnitude of any increase in nutrient loads on surface water and
groundwater resources under future irrigation scenarios is a complex matter that
necessarily relies on modelling approaches to simplify and approximate the key
elements of the catchment(s).
Our water quality assessment for the proposed Hunter Downs Irrigation
Scheme (HDIS) in New Zealand utilised a mass mixing model to predict nitrate
concentrations within groundwater (and surface water) individual catchments.
This approach was selected due to the hydrogeology of the scheme area, which is
characterised by poorly draining downland hill country intersected by permeable
alluvial gravel aquifers which are interconnected with intermittent or perennial
streams. This approach requires a robust understanding of the current hydrology,
hydrogeology, existing and proposed landuse distribution and associated nutrient
concentrations in soil drainage water. Our modelling was calibrated by
comparing the output of the existing landuse simulations with the available
current day groundwater and surface water quality data, providing further
confidence in the modelling output. The approach also allowed the effects of
different landuse scenarios and management processes to be considered.
The modelled effects on water quality varied across the scheme area due to
differences in existing landuse and interactions between groundwater and surface
water that affected the dilution of irrigation drainage. The predicted changes in
nutrient concentrations for each catchment were used to assess the risk of
increased algae blooms in rivers and coastal lagoon that is of significant
ecological and cultural value to the local indigenous Maori people.
In this paper we show how the use of a mass mixing model has provided a
realistic basis for quantifying the potential adverse effects of irrigation on
groundwater and surface water quality, and evaluating the potential effectiveness
of mitigation measures such as nitrification inhibitors.
Keywords: nutrients, assessment of environmental effects, surface water,
groundwater, modelling, New Zealand.
Keywords
nutrients, assessment of environmental effects, surface water,
groundwater, modelling, New Zealand.
Related Book
Other papers in this volume
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