AUTHORS COPY Journal of Hydrology 488 (2013) 1–16
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Influence of climate, fire severity and forest mortality on predictions of long term streamflow: Potential effect of the 2009 wildfire on Melbourne’s water supply catchments Paul M. Feikema ⇑, Christopher B. Sherwin, Patrick N.J. Lane Department of Forest and Ecosystem Science, The University of Melbourne, Parkville, Victoria 3010, Australia
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Article history: Received 2 November 2012 Received in revised form 29 January 2013 Accepted 4 February 2013 Available online 13 February 2013 This manuscript was handled by Geoff Syme, Editor-in-Chief Keywords: Process-based modelling Macaque Forest Wildfire Streamflow
s u m m a r y In February 2009, wildfire affected nine catchments, or approximately 28% of forested catchment area that supplies water to the city of Melbourne, Australia. This has potential to significantly affect the long term water use of these Eucalyptus forests and the consequential water yield because of the ecohydrologic response of some eucalypt species. Approximately 11% of the catchment area was severely burnt by intense fire, where vegetation mortality is higher. Catchment scale models using a physically-based approach were developed for the fire-affected water supply catchments. Different inputs of climate and forest mortality after fire were used to examine the relative contributions of rainfall, fire severity, forest type and forest age on post-fire streamflow. Simulations show the effect of fire on long term streamflow is likely to depend on a number of factors, the relative influence of which changes as rainfall becomes more limiting. Under average rainfall conditions, total reduction in post-fire streamflow after 100 years estimated to be between 1.4% (12 GL year 1) and 2.8% (24 GL year 1) are an order of magnitude lower than reductions in total catchment inflow during the period of low rainfall between 1997 and 2009, in which reservoir inflow was reduced by nearly 37%. The main reasons for the lower than expected changes in water yield are that a lower proportion of the catchments were affected by severe fire, and so mortality within the fire area was relatively low, and that the average age of the forest canopy (93 years) is younger than what is generally considered old growth forest. This means that the baseline (no-fire) streamflow used for reference is lower than would be expected with older, mature forest. The greatest post-fire affect on total water yield was predicted for the O’Shannassy catchment. This is due to the average forest age, which is the oldest of any of the catchments, that it has the highest average rainfall (1680 mm year 1), and that it contains the largest proportion of ash-type forest severely burnt (38.7%). Under wetter than average conditions, change in post-fire water yield is largely explained by changes in average age of the forest. The rates of ET are largely determined by the conductance and interception of the forest canopy. Under lower than average rainfall conditions, when water becomes limiting, annual rainfall is the best predictor of post-fire change in water yield. Under conditions of low rainfall and low soil water content that are conducive to larger wildfires, any initial increase in post-fire streamflow due to reduced canopy cover may not occur or be detected because a substantial soil water deficit must first be removed before appreciable changes in streamflow will occur. This partly explains the lack of increase in initial post-fire streamflow reported after wildfire compared to an increase in streamflow following forest harvesting experiments during wetter periods. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The water supply of Melbourne, Australia, is largely derived from a number of forested catchments to the north-east and east of the city. The amount of rainfall that is evaporated and transpired
⇑ Corresponding author. Current address: Department of Forest and Ecosystem Science, The University of Melbourne, 221 Bouverie St., Carlton, Victoria 3053, Australia. Tel.: +61 3 8344 0715. E-mail address:
[email protected] (P.M. Feikema). 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.02.001
from these forests largely depends on forest age and structure and so forest disturbance from wildfire and other events may lead to large changes in vegetation water use and consequential changes in streamflow and the amount of runoff into water supply reservoirs (Vertessy et al., 1998). Thirteen years (1997–2009) of below average rainfall in southeastern Australia (Timbal et al., 2010) has focused attention on the reliability of water resources in Melbourne’s water supply catchments. It has also increased the risk of wildfire, with three large series of wildfires occurring in south-eastern Australia in 2003, 2007 and in 2009.
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Under extreme weather conditions in February 2009, large wildfires spread across part of the state of Victoria, affecting more than 3000 km2 of forest and grassland (Teague et al., 2010). These severe wildfires affected approximately 363 km2 of forests in the Melbourne water supply catchments (representing 28% of the total water supply catchment area). This has potential to alter long term water use (>100 years) of the forests and subsequent water yield. With an increasing population and demand for water, the backdrop of a severe drought and the large extent of the fires and their locations within water supply catchments, many issues and concerns were raised in the general media about the long term effects of the fires on the quantity and quality of water resources (e.g. Kerr, 2009). Quantifying the potential effects of wildfire on water yield is of interest to Melbourne Water, the State-owned corporation responsible for Melbourne’s water supply. It is a challenge to secure sufficient water resources for future generations and for the environment in the context of climate change, population growth and resource scarcity. Estimates of likely changes in water yield are important to inform forward planning of water availability and catchment management with increased likelihood of drought and fire. Langford (1976) was the first to show that regenerating Eucalyptus regnans (Mountain Ash) forests burnt in severe 1939 wildfires used more water than the mature forests they replaced. These results have been supported by a large body of research (e.g. Kuczera, 1987; Vertessy et al., 1998, 2001; Watson et al., 1999b). The relationship between age and streamflow for E. regnans was generalised by Kuczera (1987) by the ‘Kuczera curve’, which predicts a decline in water yield after fire, leading to an average annual water yield reduction of up to 50% at between 20 and 30 years, followed by a gradual recovery in mean annual streamflow to an equilibrium water yield over the following 100–200 years (see Fig. 2 in Vertessy et al., 2001). This reduction in water yield is based on a 100% conversion of an average E. regnans forest catchment from oldgrowth to regrowth. Watson et al. (2001) revised the Kuczera (1987) relationship based on rainfall, transpiration and water yield data from smaller paired experimental catchments. Their revised relationship includes a sharp initial increase in water yield above long term values in the first 5 years after fire before it declines. Process-based studies have identified that changes in streamflow after forest disturbance are largely related to changes in evapotranspiration, which itself is mainly determined by age-related changes in transpiration (Dunn and Connor, 1993; Vertessy et al., 2001), canopy interception (Haydon et al., 1996) and evaporation from the forest floor (Vertessy et al., 2001). From a hydrologic perspective, the Eucalyptus forests in the water supply catchments may be classified as being either ashtype (less fire tolerant) or mixed eucalypt species (more fire tolerant). The ash-type forests include the canopy species E. regnans and E. delegatensis. They occur in the wetter regions within catchments, and tend to be killed by medium to high intensity fire (Benyon and Lane, 2013). Post-fire seedfall leads to vigorous seedling germination (obligate seeders) and growth resulting in dense regrowth (Ashton, 1976) and even-aged stands. Evapotranspiration increases rapidly to a peak at 15–30 years, before gradually declining as the forest stand thins out sufficiently to create large gaps and allow growth of understorey species with lower transpiration rates (Vertessy et al., 2001). In contrast, mixed (Eucalyptus) species occupy drier regions within the catchments, including sites at lower elevations (80%, unpublished data), and regrowth after clearfell timber harvesting where the proportion of retained trees is relative low (80%) of the vegetation in a catchment was killed by fire under relatively dry conditions, changes in streamflow due to differences in rainfall were generally greater than those from changes in vegetation mortality and structure. Referring back to the soil plant atmosphere continuum model by Cowan (1965), this coincides to a situation where the change in control on transpiration has moved away from the plant, and the rapid decline in hydraulic conductivity with decreasing water content means it now resides in the ability for the soil to provide water to plant roots. The variation in post-fire water yield for the different mortality scenarios was considerably lower than for different climate scenarios. With the certainty provided by other studies on mortality rates and associated variation of the forest types in our study
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Fig. 8. Change in predicted annual streamflow (GL) under an average climate (a–c) and a dry period (d–f) for three mortality scenarios. In each case, the average representative year (black solid line) is shown together with a wet (blue) and dry year representing the 99% percentile rainfall for the respective period. Grey dashed lines show 0% change. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(Benyon and Lane, 2013), we conclude that the variation in water yield predictions resulting from uncertainty in mortality rates is relatively low. Furthermore, the most uncertain of these rates – the mortality rate of mixed eucalypt species forests subjected to fire severity 1 – has little influence on our results, because very little area (generally less than 10%) of these forests was affected by the most severe fire.
Table 6 Multiple stepwise regression models of the form y = b0 + b1X1+ +bqXq for the change in total water yield (mm) over 100 years. Climatea
Step
Variableb
bc
Adj. R2
184% rainfall 100% rainfall 83% rainfall
1 1 1 2 3 1 2
Dage Dage % Severe fire and ash Rain Dccond Rain Dage
184 174 299 7.34 1114 11.2 45.3
0.974 0.974 0.693 0.960 0.992 0.309 0.678
55% rainfall a
Representative climate year used in simulations. Dage is change in average forest age in the catchment before and after the fire, % severe fire and ash is the percentage of ash-type forest in the catchment severely affected by fire, Dccond change in average canopy conductance the catchment before and after the fire, rain is average long term rainfall. c Standardised coefficients of predictors included in the models. b
Fig. 9. Correlation coefficients for the four most correlated variables (rainfall, change in forest age, change in canopy conductance and the percentage of ash-type forest severely burnt) with different rainfall years (184–55% of the long term average) for all catchments.
AUTHORS COPY P.M. Feikema et al. / Journal of Hydrology 488 (2013) 1–16
An important issue to consider in light of recent analysis of long term streamflow after timber harvesting by Webb et al. (2012) is the important influence that post-disturbance forest regeneration has on observed streamflow. Webb et al. (2012) showed that long term changes in streamflow were largely explained by changes in forest species composition, basal area and stand density. The observations by Webb et al. (2012) suggest that streamflow is inversely proportional to post-fire forest growth rate. This issue is of relevance to the current wildfire as Benyon and Lane (2013) have identified large areas where the understorey has been burnt and the overstorey has survived. This may result in a temporary increase in streamflow due to reduced understorey ET.
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For the O’Shannassy catchment, where the impact of the fire on long term streamflow is predicted to be greatest, the estimated immediate post-fire annual increase in streamflow (with 77% rainfall) was approximately 160 mm. The 95% prediction interval for the observed data for the O’Shannassy catchment was calculated to be approximately 240 mm year 1, and so the estimated impact of post-fire streamflow increase is unlikely to be detected. For the Wallaby Creek catchment, where the impact of fire on long term absolute streamflow is predicted to be lower, the estimated immediate post-fire annual increase in streamflow (with 77% rainfall) was approximately 120 mm. Given the 95% prediction interval for the Wallaby Creek of approximately 210 mm year 1, and so the estimated impact of post-fire streamflow increase as estimated above may be difficult to detect from observations.
3.6. Relationship of rainfall and change in water yield after fire 3.7. Influence of pre-fire soil water store on catchment response The stepwise regression analysis described in the previous section identified an increasing importance of rainfall, under low (
