Assessment of Water Use from Farm Dams in the Mount Lofty Ranges South Australia D McMurray Knowledge and Information Division Department of Water, Land and Biodiversity Conservation July 2003 Report DWLBC 2004/02 Knowledge and Information Division Department of Water, Land and Biodiversity Conservation L11, 25 Grenfell Street, Adelaide SA 5000 GPO Box 2834, Adelaide SA 5001 Telephone National (08) 8463 6990 International +61 8 8463 6990 Fax National (08) 8463 6999 International +61 8 8463 6999 Website www.dwlbc.sa.gov.au Disclaimer Department of Water, Land and Biodiversity Conservation and its employees do not warrant or make any representation regarding the use, or results of the use, of the information contained herein as regards to its correctness, accuracy, reliability, currency or otherwise. The Department of Water, Land and Biodiversity Conservation and its employees expressly disclaims all liability or responsibility to any person using the information or advice. © Department of Water, Land and Biodiversity Conservation 2003 This work is copyright. Apart from any use as permitted under the Copyright Act 1968 (Cwlth), no part may be reproduced by any process without prior written permission from the Department of Water, Land and Biodiversity Conservation. Requests and inquiries concerning reproduction and rights should be addressed to the Director, Knowledge and Information Division, Department of Water, Land and Biodiversity Conservation, GPO Box 2834, Adelaide SA 5001. ISBN 0 9751562 8 9 McMurray, D, 2003. Assessment of Water Use from Farm Dams in the Mount Lofty Ranges South Australia. Department of Water, Land and Biodiversity Conservation. Report, DWLBC 2004/02. Cover Photographs DWLBC photo-archive, Farm dams in the Mt Lofty Ranges FOREWORD South Australia’s natural resources are fundamental to the economic and social wellbeing of the State. One of the State’s most precious natural resources, water is a basic requirement of all living organisms and is one of the essential elements ensuring biological diversity of life at all levels. In pristine or undeveloped situations, the condition of water resources reflects the equilibrium between rainfall, vegetation and other physical parameters. Development of these resources changes the natural balance and may cause degradation. If degradation is small, and the resource retains its utility, the community may assess these changes as being acceptable. However, significant stress will impact on the ability of a resource to continue to meet the needs of users and the environment. Understanding the cause and effect relationship between the various stresses imposed on the natural resources is paramount to developing effective management strategies. Reports of investigations into the availability and quality of water supplies throughout the State aim to build upon the existing knowledge base enabling the community to make informed decisions concerning the future management of the natural resources thus ensuring conservation of biological diversity. Bryan Harris Director, Knowledge and Information Division Department of Water, Land and Biodiversity Conservation Assessment of Water Use from Farm Dams i Report DWLBC 2004/02 Executive Summary EXECUTIVE SUMMARY Surface water use is vital to the economics of the Mount Lofty Ranges region. However, the rapid development of farm dams over the last decade or two has raised considerable concerns over the sustainability of water resources and the impacts on the ecosystems dependant on them. Detailed hydrological studies are being conducted on the major catchments of the region in order to quantify the impact of farm dam development and to recommend water resource management actions. Knowledge of water extracted from farm dams is an important input for these hydrological studies. This technical report describes the methodology and outcomes of a study to determine by direct means the quantity of water extracted and evaporated from farm dams within the Mount Lofty Ranges, South Australia. This was achieved by comparing the quantity of water in a large sample of farm dams from pre-summer and post-summer aerial photography. For the study period (December 2001 to April 2002 inclusive), a total of 39% of farm dam storage was removed based on the average of all dams in the study over 1 ML. Around half (20%) of this was lost to evaporation and the other half (19%) was extracted (leakage was assumed to be negligible). When the results for only dams over 5 ML are averaged, slightly lower figures of total water lost (34%) were obtained, with 17% lost to evaporation and 17% extracted. These lower figures were attributed to a trend of lower extraction rates for larger dams. The study period was cooler than average (based on lower pan evaporation figures and subjective recollection). Thus, the figures for usage and loss to evaporation will be higher in a typical summer (say 25-30%) and even higher in a hotter than average summer. The study included an assessment of potential errors than became quite large for smaller farm dams, and dams from which the usage was low. This required all dams less than 1 ML to be excluded from the study, and dams with a low estimated usage to be assumed to be zero usage. Dams subject to apparent (and indeterminable) inflow or water transfers were also excluded. The main source of error was potential digitising errors in defining the water outline from the aerial photography. Uncertainties in evaporation data and pan factor also contributed to significant potential errors. When all sources of worst-case error are added, fairly wide error bands result. For example, the figure for average usage from dams over 1 ML could lie with in the range 8.5% to 27.5% (19+8.5/-10.5%). Although the error bands are quite large, the figure derived for average total water usage (19% of storage total volume for a less than typical summer), adds support to the figure (30% for a typical summer) currently being used in hydrological studies in the Mount Lofty Ranges. Assessment of Water Use from Farm Dams ii Report DWLBC 2004/02 Contents CONTENTS FOREWORD ........................................................................................................................I EXECUTIVE SUMMARY.....................................................................................................II CONTENTS........................................................................................................................III List of Tables................................................................................................................. iv List of Figures................................................................................................................ iv INTRODUCTION .................................................................................................................1 Aim.................................................................................................................................1 Study Areas....................................................................................................................1 METHOD .............................................................................................................................3 Farm Dam Data..............................................................................................................3 Evaporation Data ...........................................................................................................4 Rainfall & Stream Flow...................................................................................................4 Models............................................................................................................................5 Assessment of Errors.....................................................................................................5 RESULTS ............................................................................................................................7 Average of all Farm Dams .............................................................................................7 Individual Farm Dams ....................................................................................................8 Potential Errors ............................................................................................................11 DISCUSSION ....................................................................................................................12 Quantities of Water Used and Lost ..............................................................................12 Assessment of Errors...................................................................................................12 Digitising Accuracy.......................................................................................................12 Volume-Surface Area Relationships ............................................................................13 Climatic Data................................................................................................................13 Streamflow ...................................................................................................................14 CONCLUSIONS & RECOMMENDATIONS.......................................................................15 GLOSSARY.......................................................................................................................16 SI Units Commonly Used Within Text ..........................................................................17