Rainfall Recharge Assessment for Otago Groundwater Basins
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Rainfall recharge assessment for Otago groundwater basins Otago Regional Council Private Bag 1954, 70 Stafford St, Dunedin 9054 Phone 03 474 0827 Fax 03 479 0015 Freephone 0800 474 082 www.orc.govt.nz © Copyright for this publication is held by the Otago Regional Council. This publication may be reproduced in whole or in part provided the source is fully and clearly acknowledged. ISBN 978-0-478-37621-0 Prepared by Scott Wilson and Xiaofeng Lu Published December 2011 Rainfall recharge assessment for Otago groundwater basins i Foreword Groundwater in Otago is frequently the sole or major source of water to supply basic water needs to communities and stock watering. Currently groundwater only supplies a small proportion of irrigation needs, however there is increasing pressure for people to turn to groundwater because surface water supplies are heavily allocated. Over abstraction can result in loss of supply to other users and therefore careful management is required to keep abstraction rates sustainable. Groundwater resources have varying rates of recharge and often form a complex dependency with adjacent water courses, wetlands and stream networks. The effects of inappropriate land and water use and development on groundwater quantity and quality are often long term, and in some cases permanent. It is therefore important that particular consideration be given to the protection of aquifers for the continuing benefit of present and future generations. Through the Regional Plan: Water we ensure linkage with the community to deliver the efficient use and protection of our groundwater aquifers. This report provides an assessment of the amount of rainfall recharge to some of the regions main aquifers. The information has been calculated using the best available information and will assist the understanding of the status of this valuable resource. ii Rainfall recharge assessment for Otago groundwater basins Executive summary Groundwater recharge due to rainfall is one of the primary sources of replenishment for Otago aquifers. This report estimates rainfall recharge for 13 groundwater basins in Otago. The rainfall recharge totals will assist in informing groundwater allocation assessment within the Council’s Regional Plan: Water with respect to Maximum Allocation Volumes. A soil moisture balance model was used to calculate a daily soil water budget. Input data for this model included rainfall and potential evapotranspiration data, which was mostly sourced from the National Institute of Water and Atmospheric Research (NIWA) database. The hydraulic properties of soils and their spatial distribution were supplied as a GIS layer by Landcare Research. The input data and modelled recharge values have been archived in ORC’s Hilltop database. The model results are summarised in the following table. Mean annual recharge volumes range from 1.2 to 64.6 million m3/year. To compare values between regions it is more informative to study recharge depths, expressed as mm/year, or as a percentage of median annual rainfall rather than volumes. Groundwater Mean recharge Median annual (mm/yr) Percentage basin (Mm3/year) Rainfall PET (mm/d) Recharge recharge Clydevale-Wairuna 64.6 781 2.1 152 20 Cromwell 1.2 408 2.2 53 12 Ida Valley 18.8 499 2.2 61 12 Inch Clutha 10.4 726 1.9 76 10 Kuriwao 24.2 836 1.9 205 25 Lower Waitaki 18.5 521 2.0 75 17 Maniototo 31.6 483 2.2 29 5 Manuherikia 45.1 531 2.2 64 13 Pomahaka 52.1 944 2.1 212 22 Roxburgh 1.8 599 2.2 88 17 Strath Taieri 8.3 486 2.3 25 5 Tokomairiro 10.3 771 2.1 137 19 Wakatipu 12.5 746 2.6 184 23 Mean annual recharge volumes were found to range from 5 to 25% of median annual rainfall. Higher recharge percentages were found in South Otago and Wakatipu. The Wakatipu basin also showed the least annual recharge variability in the region. The lowest recharge percentages were found in the Strath Taieri and Maniototo basins, where recharge was most vulnerable to dry conditions. In these two areas, along with the Lower Waitaki, recharge showed the most annual variation from the mean. Mean recharge values were found to be 5 to 30% higher than the median, indicating that mean values were strongly biased towards wetter years. Wakatipu was the exception, with its mean recharge values being biased towards drier years. This is because the Wakatipu basin is more prone to extreme dry events than extreme wet events. Rainfall recharge assessment for Otago groundwater basins iii Table of contents Foreword ..................................................................................................................................... i Executive summary .................................................................................................................... ii 1. Introduction .................................................................................................................. 1 2. Methodology ................................................................................................................ 2 2.1 Rainfall and evapotranspiration data ............................................................. 2 Rainfall sites................................................................................................... 3 Potential evapotranspiration ........................................................................... 5 2.2 Soil classes ..................................................................................................... 7 2.3 Soil moisture balance model .......................................................................... 8 3. Rainfall recharge assessments .................................................................................... 11 3.1 Introduction .................................................................................................. 11 3.2 Assumptions ................................................................................................. 12 3.3 Inch Clutha ................................................................................................... 12 3.4 Clydevale-Wairuna ...................................................................................... 13 3.5 Cromwell...................................................................................................... 15 3.6 Ida Valley ..................................................................................................... 16 3.7 Lower Waitaki Plain .................................................................................... 18 3.8 Kuriwao........................................................................................................ 19 3.9 Maniototo ..................................................................................................... 20 3.10 Manuherikia ................................................................................................. 22 3.11 Pomahaka ..................................................................................................... 23 3.12 Roxburgh...................................................................................................... 24 3.13 Strath Taieri ................................................................................................. 26 3.14 Tokomairiro ................................................................................................. 27 3.15 Wakatipu ...................................................................................................... 28 4. Regional summary ..................................................................................................... 31 4.1 Recharge comparisons ................................................................................. 31 4.2 Climate and recharge variability .................................................................. 32 4.3 Comparison with previous studies ............................................................... 34 5. Conclusions ................................................................................................................ 36 6. References .................................................................................................................. 37 Appendix A – Soil hydraulic data ............................................................................................ 38 References .................................................................................................... 39 iv Rainfall recharge assessment for Otago groundwater basins Appendix B – Model verification ............................................................................................. 40 References .................................................................................................... 42 List of figures Figure 1 Monthly values of the Southern Oscillation Index plotted with a six month moving average .................................................................................................................... 3 Figure 2 Relationship between PET at two sites (Gore and Balclutha) ............................................. 6 Figure 3 Inch Clutha basin boundary and sites used for the recharge assessment ........................... 13 Figure 4 Clydevale-Wairuna basin boundary and sites used for the recharge assessment .......................................................................................................................... 14 Figure 5 Cromwell aquifer boundary and sites used for the recharge assessment ........................... 15 Figure 6 Ida Valley basin boundary and sites used for the recharge assessment ............................