Risks to Shared Water Resources
Total Page：16
File Type：pdf, Size：1020Kb
0 100 200 300 400 500 600 700 MURRAY-DARLING BASIN COMMISSION Risks to Shared Water Resources Impact of the 2003 Alpine Bushfires on Streamflow Summary report December 2007 Prepared by SKM and the University of Melbourne for the Victorian Department of Sustainability and Environment, and the Murray-Darling Basin Commission. Impact of the 2003 Alpine Bushfires on Streamflow – Summary Report Published by Murray-Darling Basin Commission Postal Address GPO Box 409, Canberra ACT 2601 Office location Level 4, 51 Allara Street, Canberra City Australian Capital Territory Telephone (02) 6279 0100 international + 61 2 6279 0100 Facsimile (02) 6248 8053 international + 61 2 6248 8053 E-Mail [email protected] Internet http://www.mdbc.gov.au For further information contact the Murray-Darling Basin Commission office on (02) 6279 0100 This report may be cited as: Impact of the 2003 Alpine Bushfires on Streamflow – Summary report MDBC Publication No. 23/08 ISBN 978 1 921257 63 6 This project was funded by the Department of Sustainability and Environment, the Murray-Darling Basin Commission and the Gippsland Lakes Taskforce. This work is copyright. Graphical and textual information in the work (with the exception of photographs and logos) may be stored, retrieved and reproduced in whole or in part, provided the information is not sold or used for commercial benefit and its source (Impact of the 2003 Alpine Bushfires on Streamflow – Summary report) is acknowledged. Such reproduction includes fair dealing for the purpose of private study, research, criticism or review as permitted under the Copyright Act 1968. Reproduction for other purposes is prohibited without prior permission of the copyright owners through the Murray-Darling Basin Commission or the individual photographers and artists with whom copyright applies. To the extent permitted by law, the copyright holders (including their employees and consultants) exclude all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this report (in part or in whole) and any information or material contained in it. The contents of this publication do not purport to represent the position of the Murray-Darling Basin Commission or the copyright holders. They are presented to inform discussion for improvement of the Basin's natural resources. Front cover images copyright Department of Sustainability and Environment, Victoria. Used with permission Impact of the 2003 Alpine Bushfires on Streamflow – Summary Report 1 Introduction Following the 2003 bushfires, which burnt over a million hectares of forest and grazing land (Figure 1), a major study was commissioned to assess the likely impact of the fires on water quality and quantity. This document provides a summary of the background to, and results from, the Program. The study was managed by the Cooperative Research Centre for Catchment Hydrology and involved Sinclair Knight Merz; CSIRO Land and Water; and the Department of Civil and Environmental Engineering and School of Forest and Ecosystem Science (formerly the Forest Science Centre) of the University of Melbourne. The work was commissioned by the Victorian Department of Sustainability and Environment and the Murray Darling Basin Commission with additional financial support from the North Eastern, West and East Gippsland Catchment Management Authorities and Gippsland Lakes Task Force. Motivation for the work came from two obvious concerns. Firstly, fire is known to have a significant impact on water yield when a forest is killed. The initial loss of vegetation results in increased runoff (for 5-10 years), while subsequent regeneration consumes significantly more water per unit area as the new growth flourishes, thereby reducing runoff over an extended period. The maximum impact is for around 15-25 years with some impact continuing for up to 100 years. Eventually, as the forest approaches the age of the pre-fire forest, runoff returns to pre-fire levels. The impact on streamflow is most severe for Mountain Ash forests, but occurs in all native species. As will be shown, the impacts on water yield of the 2003 fires are considerable and will have to be accounted for in future water resource planning. Secondly, a significant deterioration in water quality, and increases in nutrient loads and greatly increased sediment loads are expected following a fire. Recovery occurs as surface cover regenerates, but the timescales for this recovery are highly variable, being dependent on the severity of burn, terrain and soils, forest type, and the subsequent climatic conditions. These time scales will range from a couple of years to a decade or more. In addition, coarser sediment may be temporarily stored and potentially remobilised by large events in the future. As will be shown, the 2003 fires resulted in load increases above pre-fire conditions of up to several hundred times. Recovery is generally occurring but loads are still between two and ten times pre-fire conditions in most catchments, with some being higher. Impact of the 2003 Alpine Bushfires on Streamflow – Summary Report Figure 1. Area affected by the 2003 bushfires, along with catchment boundaries used in water yield analyses. The Water Yield Assessment component of the Bushfires Recovery Program was designed to quantify the impacts on water yield and water quality for the major catchments flowing to the Murray River including Dartmouth dam (on the Mitta Mitta River), the Gippsland Lakes (where the Tambo and Dargo/Mitchell Rivers were affected) and the Snowy River. Seven tasks were undertaken in the Program and are linked as shown in Figure 2. The fundamental tasks in relation to water yield are Tasks 1 and 4. These deal with the overall impacts on annual and seasonal water yield expected for each of the 12 major catchments (identified in Figure 1). An extensive network of water quality monitoring sites was set up immediately following the fires to assess impacts and any possible recovery. Monitoring ceased in 2005. Analysis of these data was undertaken in Task 5, while Task 6 used the results along with catchment modelling to extrapolate the likely water quality impacts to the relevant catchment scales. Task 2 was to enable the Macaque model, a detailed hydrological model designed to assess the impacts on yield of land-use change over relatively small catchments, to be generally available to agencies, with appropriate documentation and support. Task 3 applied Macaque to smaller subcatchments to assess in more detail, likely yield responses. Macaque was also used to undertake numerical experiments, i.e. modelling studies, in Task 4 to test what refinements could be made to the broad- scale assessment, in particular to determine seasonal effects and any differences resulting from different catchment characteristics. Task 7 was a detailed analysis of the East Kiewa catchment. Separate, detailed reports are available for each of the tasks (see figure 2 and bibliography). This document provides an overview of the key outcomes, focusing on implications for management. Impact of the 2003 Alpine Bushfires on Streamflow – Summary Report Figure 2. The seven major tasks in the Bushfires Recovery Program indicating the lead group and assisting groups (in parentheses). (SKM is Sinclair Knight Merz. FSC is the Forest Science Centre, now the School of Forest and Ecosystem Science at the University of Melbourne. CRCCH refers to the combination of CSIRO Land and Water and the Department of Civil and Environmental Engineering, University of Melbourne.) 2 Impacts on water yield 2.1 Broadscale Impacts Water yield impacts of the 2003 fires are significant with reductions of up to 50% in annual flow over the average conditions in the twenty years prior to 2003. Table 1 shows the expected maximum impact for the 12 catchments and the summed impacts in the Murray River and Gippsland Lakes. In general these maximum impacts would be expected to occur between 15-25 years after the fire. Results are shown in ML/year, mm of runoff per year and as a percentage of Mean Annual Flow. Impact of the 2003 Alpine Bushfires on Streamflow – Summary Report The results presented in this summary report are all relative to the average conditions in the twenty years prior to 2003. An alternative way of interpreting the results is to compare the conditions that would have been expected into the future had no fire occurred. This latter interpretation is included in the detailed report on Task 1 (MDBC, 2007a). In general, the relative impacts of the fires appear larger when compared to the “no-fire future”. This is because the age profile of the 2003 pre-fire vegetation would have resulted in increased runoff over time compared to the pre-fire average conditions. Catchment Maximum reduction in streamflow ML/year mm/year % MAF Northern Catchments Buffalo -26,000 -22 -6 Corryong -26,000 -53 -16 Dartmouth -320,000 -89 -20 Kiewa -20,000 -49 -3 Mitta Mitta (d/s of Dartmouth) -29,000 -44 -1 Ovens -65,000 -52 -12 Upper Murray -170,000 -71 -26 Other Northern -36,000 -45 River Murray (d/s of -692,000 -10 confluence with Ovens) Southern Catchments Dargo -51,000 -96 -29 Tambo -31,000 -35 -27 Wongungurra -58,000 -80 -20 Other Southern -15,000 -59 Gippsland Lakes -155,000 -6 Buchan -74,000 -87 -53 Snowy -230,000 -24 -31 Table 1. Estimated maximum impact on annual water yield for each catchment outlet (boundaries are shown in Figure 1) with respect to the average annual streamflow in the twenty years prior to the 2003 fires. Maximum yield reductions expected approximately 20 years after the fire. Results are in ML/year, mm runoff per year, and percentage of mean annual flow. Figures 3 and 4 illustrate, for the Murray River (downstream of the confluence with the Ovens River) and the Gippsland Lakes respectively, the expected impacts and flow change over time, including the initial increase, maximum decline, and period for recovery.