Research On Bushfire Impact On Catchment Evapotranspiration And Streamflow

Forest fires show an increase due to the increase in forest fire weather (low precipitation and relative humidity, high temperature and wind speed) caused by gobal warming. Forest fires affect catchment hydrological cycle, including reduced infiltration rates caused by changes in soil properties, reduced evapotranspiration rates caused by loss of vegetation and surface litter and destroyed ecological environment. The hydrological effects of forest fires have been recived wide attentions from scholarsathome and abroad. In the context of climate change, researches on hydrological response to forest fires help to reveal the water cycle mechanism, to rationally allocate forest and water resources and to promote the sustainable development of terrestrial ecosystems in forest catchments. Taken two forest fires (1983and2003bushfire) in the state of Victoria, Australia as examples, statistical method and hydrological modeling are used to evaluate forest fire impact on evapotranspiration and streamflow. In order to improve runoff estimates, this paper incorporates soil moisture and vegetation dynamic into rainfall runoff model. The main focus and conclusions of this study are as follows:(1) Forest fire weather is greatly affected by climate and weather conditions. Four meteorological indexes are selected to analyze climate change impact on the chances of a fire starting. They are temperature (maximum temperation, minimum temperature and mean temperature), precipitation, relative humidity and potential evapotranspiration, respectly. The5-year moving average method and anomaly analysis are used to test changing trend of the four indexes in the test period. The results show that the regional climate becomes warmer and drier. It shows an increase chance of a fire starting. Preventing work should be ready for forest fires.(2) Forest fires show impacts on evapotranspiration through destroying vegetion cover. Forest fires result in a noticeably reduction of mean leaf area index (LAI) and surface albedo in the year of fires. Then, LAI and surface albedo gradually recover along with vegetation recovering. Changes in suface albedo reflect changes in the surface radiation balance, thus affecting evapotranspiration rates at catchment scale. The impact of forest fires on evapotranspiration is related to fire severity. The more severe the fires are, the more evapotranspiration reduces. Conversly, the less evapotranspiration reduces. Subsequently, the evapotranspiration increases along with vegetation recovering. (3) This paper presents the impact of forest fires on streamflow using hydrological modeling approach. The hydrological models are used including AWRA-L, Xinanjiang and GR4J model from Australia, China and France, respectively. Firstly, the applicability of hydrological modelling to quantify bushfire impact on streamflow mainly depends on how the model parameters are calibrated and how they are transferred from calibration period to simulation period. It is important to investigate the transposability of model parameters in time (i.e., to make sure that their estimation is not dependent on climate characteristics of the calibration periods). This can provide us with a better understanding of uncertainty associated with using hydrological models for quantifying bushfire impacts on streamflow. Four median-size catchments close to the three study catchments are selected to investigate the model transposability. Validation results provide confidence in the bushfire impact assessments based on hydrological modelling. Secondly, the three hydrological models are used to quantify the bushfire impact on streamflow. The results show that there is a substantial increase in streamflow after the immediate1983bushfires that is not attributable to climate. Then, the increment of streamflow is reduced due to plantation recovering.(4) Soil moisture and remotely sensed vegetion dynamic information are incorporated into Penman-Monteith (PM) equation. Xinanjiang model is modified by replacing the original three-layer with one-layer evapotranspiration sub model (PM equation) to improve runoff estimates for four south-east Australian catchments which experienced severe bushfire impacts. The three modeling experiments are designed for the modified Xinanjiang model (XAJ-ET) to investigate the simulation capability of the modified model in four forested catchments which were affected by extensive bushfires in January2003. The improvement is indicated by a slight increase (1%-7%) in the Nash-Sutcliffe efficiency of daily runoff and noticeable decrease (3%-11%) in volumetric errors. It indicates that incorporation of vegetation dynamic data into Xinanjiang model can improve runoff simulation efficiency.