Wildfire impact on water yield within catchments

Transcription

1 Wildfire impact on water yield within Sydney s drinking water supply catchments Jessica Heath, Chris Chafer, Tom Bishop, Floris Van Ogtrop Agroecosytems Group Faculty of Agriculture, Food and Natural Resources

2 Background PhD project: Wildfire impacts on hydrology within the Sydney drinking water supply catchments Three components: 1) Assessing the impact of the 2001/2002 summer wildfires on water yield within the Sydney drinking water supply catchments; 2) The impacts of wildfire on soil water repellency, e via studying 2 sites burnt in late 2009; and 3) The role vegetation plays on the hydrological cycle post-wildfire 2

3 Background Wildfire impact on the hydrological cycle Victorian studies have demonstrated a change in water yield post-wildfire - Langford Vertessy et. al Victorian and Sydney catchments obtain very different vegetation - Victoria: Obligate seeders - Sydney: Obligate resprouters 3

4 Research Objectives Did the summer wildfires have had an impact on the postwildfire water yield? Catchment Average weekly Rainfall (mm) Average weekly Water yield (ML) Average weekly Max Temp (Degrees) Burnt Area (%) Pre Post Pre Post Pre Post Burke River Erskine Creek Glenbrook Creek Nattai River Grose River Kedumba River Kowmung River = burnt 4

5 Research Objectives 5

6 Study area Grose River Four burnt and three control catchments Kedumba River Kowmung River Glenbrook Creek Burke 83% fire affected Erskine Creek Sydney Erskine- 98% fire affected Lake Burragorang Glenbrook- 97 % fire affected Legend Nattai River Urban Reservoirs Burnt Control Burke River Kilometers Study area Nattai 57% fire affected 6

7 Methods Sub-catchment delineation Data processing Statistical Analysis 7

8 Methods Sub-catchment delineation Drainage area above each hydrometric site was defined - Depressionless DEM by filling in artificial sinks - Calculate flow direction and flow accumulation maps which are used to determine the watershed above each hydrometric station 8

9 Methods Data processing Data collected between the 1st January st January Hourly flow data: 7 hydrometric stations - Hourly rainfall data: Thiessen polygons were used to calculate total rainfall based of surrounding rainfall gauges - Maximum m daily temperature re data: Data gathered from closest temperature re gauge ge to hydrometric station All data was converted to weekly data through the use of R as daily intervals produced too much noise within the data First 12 months of data immediately post-wildfire ignored due to missing data- can be assumed that t an increase in water yield levels l occurred 9

10 Methods Statistical Analysis Distribution considered - Generalized Additive Models (gam) Guassian family Gamma family 10

11 Methods Statistical Analysis Predictor variables - Simple model: water yield, rainfall and temperature - Slightly more complex model: Extended simple model to include dummy seasonal variables (sine and cosine) - Most complex model: Lagged water yield were incorporated into the model Post-fire data was predicted based of the pre-fire model. Nash-Sutcliffe coefficient, 95% confidence interval, flow duration curves, residual plots 11

12 Results Gamma data Catchment NSE* pre-fire NSE* post-fire Burke River Erskine Creek Glenbrook Creek Nattai River Grose River Kedumba River Kowmung River * NSE ranges from - to 1 12

13 Results Confidence Intervals 13

14 Results Flow duration curves 14

15 Results Residuals- burnt 15

16 Results Residuals- unburnt 16

17 Summary Variation in water yield levels across both burnt and unburnt sites Pre-wildfire models predict the post-wildfire models well The FDC follow a similar trend across all sites No trend in residual plots= wildfire has no obvious effect on water yield 17

18 References Langford, K., Changes in yield of water following a bushfire in a forest of eucalyptus regnans. Journal of Hydrology 29, Vertessy, R.A., Watson, F.G.R., O Sullivan,S.K., Factors determining relations between stand age and catchment water balance in mountain ash forests. Forest Ecology and Management 143, Contact details: Jess Heath, jhea2850@uni.sydney.edu.au d 18