REG75 — A tool to estimate mean annual flow for the South West of Western Australia Department of Water Surface Water Hydrology October 2007 REG75 Surface Water Hydrology, no. 25 Department of Water 168 St Georges Terrace Perth Western Australia 6000 <www.water.wa.gov.au > Telephone +61 8 6364 7600 Facsimile +61 8 6364 7601 For more information about this report, contact Water Resource Management Division, Department of Water. ISBN 978-1-921094-68-2 October 2007 ii Department of Water Surface Water Hydrology, no. 25 REG75 Contents Contents ..................................................................................................................... iii Summary .....................................................................................................................1 1 Introduction..............................................................................................................2 2 REG6 ......................................................................................................................5 3 A simple water balance model.................................................................................7 4 Data requirements.................................................................................................10 4.1 Catchment areas and surface water management areas and sub-areas........................10 4.2 Rainfall..............................................................................................................................10 4.3 Evaporation.......................................................................................................................12 4.4 Clearing.............................................................................................................................12 4.5 Streamflow........................................................................................................................13 5 Application to WA..................................................................................................16 6 Modified water balance model...............................................................................20 6.1 Why don’t the Zhang curves work for WA?......................................................................20 6.2 Groupings/regions ............................................................................................................21 6.3 Calibration and application ...............................................................................................23 6.3.1 Modified Zhang — one group .......................................................................................................28 6.4 Model prediction and accuracy.........................................................................................29 6.4.1 Errors............................................................................................................................................29 6.4.2 Factors..........................................................................................................................................31 6.4.3 Model validation............................................................................................................................32 6.4.4 Sustainable yields and allocation limits.........................................................................................33 6.5 Comparison to REG6 .......................................................................................................35 7 Conclusions...........................................................................................................36 Appendices................................................................................................................37 References ................................................................................................................50 Contributors...............................................................................................................52 Appendices Appendix A — ANUSPLIN fitting characteristics .......................................................38 Appendix B — Streamflow gauges ............................................................................39 Appendix C — Spatial distribution of streamflow gauges ..........................................48 Appendix D — Statistics ............................................................................................49 Department of Water iii REG75 Surface Water Hydrology, no. 25 Figures Figure 1 Estimation of sustainable yield with increasing use/demand..................2 Figure 2 Runoff-rainfall relationship with respect to clearing in the REG6 model .6 Figure 3 Runoff-rainfall with respect to clearing for the period 1975 to 2003........6 Figure 4 Ratio of mean annual evapotranspiration to rainfall as a function of the index of dryness (E o/P) and Zhang curves for forest and pasture ..........8 Figure 5 Mean annual rainfall isohyets (1975 to 2003).......................................11 Figure 6 Extent of native vegetation (NLWRA, 2001).........................................13 Figure 7 Comparison of modelled and observed mean annual flow for application of the Zhang model to south-west WA..................................................17 Figure 8 Comparison of modelled and calculated evapotranspiration for application of the Zhang model to south-west WA ...............................17 Figure 9 Relationship between annual evapotranspiration and rainfall for south- west WA with Zhang curves for forested and grassed catchments overlain.................................................................................................18 Figure 10 Ratio of mean annual actual evapotranspiration to rainfall as a function of the index of dryness (E o/P) with Zhang curves overlain ...................20 Figure 11 Modified Zhang curves for forest and pasture ......................................21 Figure 12 Pan evaporation E o versus mean annual actual evaporation grouped by hydrographic basins .............................................................................22 Figure 13 Group 1 and Group 2 based on hydrographic basins...........................22 Figure 14 Comparison of observed and modelled mean annual flow (Group 1)...24 Figure 15 Comparison of observed and modelled mean annual runoff (Group 1) 24 Figure 16 Comparison of observed and modelled mean annual flow (Group 2)...25 Figure 17 Comparison of observed and modelled mean annual runoff (Group 2) 25 Figure 18 Residuals plot of the mean annual flow data against catchment area..26 Figure 19 Spatial distribution of difference in observed and modelled values ......29 Figure 20 Residuals plot of the mean annual flow data for the gauging station locations across the south west of WA.................................................30 Figure 21 Example of the development of factors for surface water management sub-areas using the difference in observed to modelled mean annual flow at each gauging station.................................................................31 Figure 22 Allocation limits (AL) corresponding to level of use, mean annual flow and sustainable yield............................................................................33 Figure 23 Sites where the REG75 allocation limit exceeds the observed sustainable yield and mean annual flow...............................................34 iv Department of Water Surface Water Hydrology, no. 25 REG75 Tables Table 1 Statistics comparing the observed and modelled mean annual runoff and mean annual flow results of the Zhang application to south west WA........................................................................................................18 Table 2 Statistics comparing the observed and modelled mean annual runoff and mean annual flow results of the modified Zhang application to Group 1 ................................................................................................27 Table 3 Statistics comparing the observed and modelled mean annual runoff and mean annual flow results of the modified Zhang application to Group 2 ................................................................................................27 Table 4 Statistics comparing the observed and modelled mean annual runoff and mean annual flow results of the modified Zhang application as one group ....................................................................................................28 Table 5 Percent difference of REG75 estimates and observed mean annual flow for validation catchments......................................................................32 Table 6 Error margins between observed and modelled mean annual flows for REG75 and REG6................................................................................35 Table 7 Comparison between REG75 and REG6 approaches..........................35 Department of Water v Surface Water Hydrology, no. 25 REG75 Summary The sustainable yield is the maximum quantity of water available for abstraction from a surface water resource after environmental water requirements have been satisfied. In areas where the level of surface water use is low, estimates of the notional sustainable yield are based on regional models and have typically been estimated as a function of the mean annual flow. The Department of Water has developed a simple regional model to determine the mean annual flow, and hence the notional sustainable yields, for systems in the south west of Western Australia with low