Estimating nitrate-nitrogen leaching rates under rural land uses in Canterbury

Abstract

Over the last two decades, agricultural production in the region has grown as a result of the increasing use of inputs, such as fertilisers, supplementary feeds and irrigation water, accompanied by the conversion of plantation forests and areas of extensive sheep and beef grazing into dairy farms. At the same time, there is increasing evidence that Canterbury’s freshwater resources are becoming degraded as a result of increasing inputs of nutrients, bacteria and sediment from these changing land uses (ECan 2008). If these land use changes continue under current management practices, modelling studies suggest that nitrate-N concentrations in shallow groundwater are likely to continue increasing in the future (Di & Cameron 2002; Bidwell et. al. 2009). Faced with this pressure on the region’s water resources, Environment Canterbury is reviewing its approach to managing the cumulative effects of land use, especially diffuse nutrient inputs, on water quality. Initially, Environment Canterbury undertook a preliminary study to examine the effects of agricultural land uses on water quality between the Rakaia and Waimakariri rivers (Di & Cameron 2004). More recently, the Canterbury Mayoral Forum (2009) commissioned modelling at a regional scale to assess the potential changes to water quality as a result of concern over the consequences of intensifying agricultural land uses in the region (Bidwell et. al. 2009 ). The Proposed Natural Resources Regional Plan set measurable water quality objectives for surface waters and groundwaters addresses point source discharges and sets limits for nutrient losses from irrigated properties in inland areas of Canterbury. However, the plan did not include provisions to adequately address the cumulative effects of nutrient loads from intensifying land uses and multiple point-source discharges. To remedy this problem, Bidwell (2008 & 2009) proposed an allocation approach, based on a “first in first served” basis to address the effects of nitrate-N discharges on shallow groundwater in relation to drinking water quality. A consent application to use water for irrigation would be assessed against existing land uses within a predetermined distance from the property where the proposed activity was going to take place. The discharge of nitrate-N from the proposed activity would be assessed in combination with the estimated nitrate-N leaching from land uses within the “area of interest.”