Land-surface recharge and groundwater dynamics - Rakaia-Ashburton Plains

Abstract

The purpose of the report is to advance the technical understanding of the groundwater system and inform resource management decisions in the Rakaia-Ashburton Plains area. The report also aims to provide information for stakeholders about possible groundwater system responses to various irrigation development scenarios. The scenarios evaluated in the report include: converting borderstrip irrigation to spray irrigation across the Ashburton-Lyndhurst Irrigation Scheme (ALIS); and increasing groundwater sourced irrigation across the Rakaia-Ashburton Plains area. The report recommends resource management strategies for managing the risk of irrigation development in the area. The report has been written in the context of applications to take groundwater for irrigation beyond the current allocation limit. However, the report is not intended to provide an audit or assessment of effects of these applications. This study describes the occurrence of groundwater across the Rakaia-Ashburton Plains, and uses an eigenmodel to explore the relationship between climate, abstraction, and dynamic groundwater behaviour. Estimates of irrigated area and land-surface recharge (LSR) are provided for sub-areas of the Rakaia-Ashburton Plains. Descriptions of groundwater occurrence and dynamics are provided in the context of local recharge sources. These datasets are subsequently correlated by calibrating the eigenmodel, and predictions of future abstraction and LSR scenarios provided using the eigenmodel. LSR, river recharge, and groundwater abstraction are water budget components which influence the dynamic behaviour of the groundwater system. The dynamic responses of groundwater levels reflect influences such as sporadic and seasonal LSR, damped responses to LSR, steady river recharge effects and groundwater abstraction. Some groundwater level records in the Rakaia-Ashburton Plains area show long-term declining trends, others do not. A pattern of higher piezometric head nearer the rivers and decreasing piezometric head toward the centre of the Rakaia-Ashburton Plains area was found. The increasing piezometric head around the rivers reflects significant local river recharge sources compared with LSR. Down plains of State Highway 1 piezometric heads reflect relatively low vertical hydraulic gradients compared with those up plains. A soil moisture water balance model was used to estimate LSR under dryland and spray irrigation conditions. A modified approach was applied to estimate LSR occurring across the ALIS to represent border-strip irrigation and to take account of records of the volume of water delivered to the scheme from the Rangitata Diversion Race (RDR). Estimates of the proportions of border-strip and spray irrigation LSR over time were considered together with conveyance efficiency and by-wash flows. The additional LSR caused by the ALIS is significant when compared with other areas. Irrigated area is an important factor when estimating LSR as rainfall recharge through the soil increases under irrigation. Very little information is currently available about irrigation water use, or about areas actually irrigated. Three sources of information have been used to estimate irrigated area: 1) areas listed as irrigated in the Environment Canterbury RMA Database; 2) land parcels associated with consents from the Environment Canterbury RMA Database; 3) remote sensing. Considerable differences were found between these three sources. The eigenmodel method characterises an aquifer in terms of a set of conceptual groundwater reservoirs. This method quantifies the dynamic behaviour of groundwater storage and groundwater discharge in response to time-series of recharge. Recharge includes that from land surface, rivers and pumped abstraction. There is consistency between the eigenmodel method and the more conventional numerical groundwater models; however, the eigenmodel method enables significant model simplification and accessibility. Using the eigenmodel, the recharge component from the ALIS is shown to be significantly “propping up” groundwater levels in the vicinity of and down gradient of the ALIS command area. As more efficient irrigation practices develop within the ALIS, it is likely that some groundwater users will face reduced reliability or even dry bores depending on their proximity to the ALIS. To minimise piezometric head reductions arising from more efficient irrigation, surface water supply for irrigation should be used over the widest area possible to minimise groundwater pumping demand and maximise the additional recharge of rainfall via soil percolation. Managed aquifer recharge options could also be investigated to augment groundwater levels currently “propped up” by border-strip irrigation. Groundwater development scenarios were tested using the eigenmodel. The scenarios looked at the change between status quo and full irrigation development. The scenario testing showed that the effect on piezometric head due to fully irrigating areas down-plains of State Highway 1 would be less than full development above SH1. This is due to a combination of system dynamics and higher levels of current irrigation sourced from groundwater coastwards of SH1. Further groundwater development coastwards of SH1 is expected to have less of a cumulative effect on piezometric levels than development up-plains. Therefore, the preferred source for irrigation development up-plains of about SH1 should be surface water, not groundwater. Arranging irrigation supplies in this way will provide higher productivity yields whilst minimising the piezometric response in the groundwater system. All groundwater takes will contribute to a reduction in coastal discharge from the groundwater system. Therefore, if further groundwater is developed and/or if surface water irrigation is made more efficient, additional resource management measures are recommended. Such measures could include: developing a coastal monitoring and trigger level system; managed aquifer recharge; up plains trigger levels for deep wells; improving water use and irrigated area information.