Mataura Catchment Strategic Water Study Liquid Earth Aqualinc Research Harris Consulting May 2011 i Mataura Catchment Strategic Water Study Mataura Catchment Strategic Water Study Report Prepared for Environment Southland May 2011 Printed: 12 September 2011 Last saved: 31 May 2011 13:52:12 File name: D:\Mataura Catchment Study\Reporting\Mataura Catchment SWS_Final.doc Author: Brydon Hughes/Simon Harris/Peter Brown Project manager: Brydon Hughes Name of organisation: Environment Southland Name of document: Mataura Catchment Strategic Water Study Document version: Final ii Mataura Catchment Strategic Water Study Executive Summary Study background Recent years have seen a significant increase in the volume of water allocated for consumptive use in the Mataura catchment, primarily associated with the expansion of pasture irrigation. Over this period significant changes have also occurred in terms of land use and land use intensity. Combined, these factors have increased pressure on the overall quality and quantity of water resources in the catchment. The Mataura Catchment Strategic Water Study was initiated by Environment Southland to identify potential options for future water resource management which could potentially provide for future water demand while enhancing the social, cultural and environmental values associated with the Mataura River. Water resources of the Mataura catchment The Mataura catchment extends over an area of approximately 5,400 square kilometres from headwaters south and east of Lake Wakatipu to the south coast at Fortrose. The catchment experiences a range of climate conditions reflecting the transition from a marine-dominated climate near the south coast to more sub-alpine conditions in the upper catchment. The Mataura River carries a median discharge of approximately 70 m3/s in its lower reaches. River flows exhibit significant seasonality with highest discharge typically occurring in spring and lowest flows during late summer. Major tributaries include Roberts Creek and Eyre Creek in the upper catchment, the Waikaia River, Waimea Stream and Waikaka Stream in the middle catchment and the Mimihau Stream and Mokoreta River in the lower catchment. The Mataura catchment also contains a significant groundwater resource hosted in alluvial gravel deposits along the riparian margins of the Mataura and Waikaia Rivers. Extensive interaction occurs between the river and these aquifer systems with alternate reaches gaining or losing flow depending on the local hydrogeological setting. Baseflow (groundwater) discharge to the river helps maintain river flows during periods of low rainfall and exerts a significant influence on surface water quality (particularly in terms of nutrients) during periods of low flow. Current Water Resource Management The Water Conservation (Mataura River) Order 1977 (referred to in this report as the MCO) establishes the ‗nationally outstanding‘ character of the fisheries and angling amenity within portions of the Mataura River system1 and provides a basic framework for management of water quality and quantity in the catchment. Key provisions of the MCO include: . A prohibition on damming of the main stems of the Mataura and Waikaia Rivers; . A simple proportional allocation for consumptive use of 5 percent of the naturalised flow above the Mataura Island Bridge and 10 percent downstream of this point; . Three water quality standards that must be met by point source discharges after reasonable mixing in different parts of the ‗protected waters‘. 1 Referred to as the ‗protected waters‘ iii Mataura Catchment Strategic Water Study Provisions of the MCO are essentially complemented by the Regional Water Plan (RWP) which establishes objectives, policies and rules covering activities outside the direct scope of the MCO. Current Water Allocation The volume of water allocated for consumptive use in the Mataura catchment has increased significantly over the past 10 years from approximately 100,000 m3/day in 2000 to around 300,000 m3/day in late 2010. A significant proportion of this increase is associated with the expansion of pasture irrigation from approximately 200 ha to 5,400 ha over the same period. Allocation for other uses including industrial and municipal supplies also increased in recent years, but to a lesser degree than irrigation. The increase in water allocation from 2000 to 2010 has been almost exclusively from groundwater which currently comprises approximately 85 percent of all allocation. However, when potential effects of groundwater abstraction on surface water2 are taken into account, approximately 40 percent of the total allocation is attributed to surface water. Based on this calculation, the Mataura River is currently considered to be fully allocated under the MCO provisions (in terms of direct surface water and hydraulically connected groundwater takes) at flows below mean annual low flow (MALF) across a majority of the catchment. This means that further allocation for consumptive use is only available at moderate to high river flows. Current Water Use Water use compliance information indicates that current water use is significantly lower than allocated rates and volumes. On a seasonal basis, few consents utilise anywhere near their full allocated volumes, with typical use in the range of 30 to 50 percent of seasonal allocation. The available data also suggest that short term (i.e. instantaneous and/or daily) abstraction, although proportionally higher than seasonal use, is again significantly less than allocated rates/volumes. Future water use Potential future water demands were estimated over a nominal 20-year planning horizon based on ‗conservative‘ and ‗accelerated‘ estimates of future irrigation, municipal and industrial demand growth. These scenarios are intended to provide upper and lower bound estimates of potential growth in water demand in the absence of regulatory constraints on water use. In reality, the extent to which these demands can be met largely depends on the regulatory regime in place. Given the current level of allocation under the MCO regime, these estimates are best viewed in terms of potential future shortfalls in supply. Results of the assessment suggest irrigation is likely to be the primary driver of future water demand in the Mataura catchment. However, lignite mining and secondary processing may also make a significant contribution to future water demand. Based on estimates of future irrigation, industrial and municipal demand growth, potential supply shortfalls in 2030 are estimated to range between 400,000 and 800,000 m3/day. 2 Referred to as stream depletion iv Mataura Catchment Strategic Water Study Factors influencing future water demand and availability Analysis of historical climate data suggests natural climate variability, particularly in terms of rainfall, has a significant influence on water demand and availability in the Mataura catchment. During El Niño conditions westerly airflows typically increase and rainfall is above average over southern New Zealand whereas during La Niña conditions westerly airflows decrease and rainfall is generally below average. Historical data illustrate that the occurrence of historical ‗drought‘ events in Southland are primarily associated with La Niña conditions. However, possibly of greater significance in terms of potential future water demand and availability than individual El Niño/La Niña events are decadal-scale climate variations which are observed in historical climate (particularly rainfall) data from the Southland Region. These variations, associated with a phenomenon termed the Interdecadal Pacific Oscillation (IPO), influence sea surface temperatures and atmospheric circulation patterns across the Pacific region on a timescale of the order of 20 to 30 years. Warm (positive) phases of the IPO tend to associated with an increase in the frequency of El Niño events, while cool phases typically result in more frequent La Niña conditions. Climate indices suggest a return to the cool phase of the IPO since 2000 with a corresponding increase in the frequency of summer dry conditions in Southland compared to the two preceding decades. However, conditions over this period still remain appreciably wetter than those experienced over the last negative IPO phase from the early 1950‘s to late 1970‘s. Projected impacts of climate change indicate that the Southland Region will experience warmer temperatures over the next 30 years accompanied by an increase in westerly airflows and higher rainfall. However, in all except the most extreme modelled scenarios, changes in water demand and availability resulting from climate change are likely to be significantly less than natural variability resulting from short to medium-term variations in atmospheric circulation Costs and benefits of future water use A number of scenarios were modelled to investigate the effect of supply reliability (essentially an outcome minimum flows and total allocation) on the economics of irrigation under different allocation scenarios. Results of this assessment suggest that, under the current MCO flow regime, existing allocation is approaching the point where additional abstraction for irrigation (from the river or hydraulically connected groundwater) is unlikely to be economically viable. Therefore, while the MCO does not prescribe a maximum allocation limit, this analysis suggests that the catchment is close to the point where the water resource can be considered fully allocated with respect to future run-of-river irrigation