Abstract
Continued growth of the human population on Earth will increase pressure on already stressed terrestrial water resources required for drinking water, agriculture, and industry. This stress demands improved understanding of critical controls on water resource availability, particularly in water-limited regions. Mechanistic predictions of future water resource availability are needed because non-stationary conditions exist in the form of changing climatic conditions, land management paradigms, and ecological disturbance regimes. While historically ecological disturbances have been small and could be neglected relative to climatic effects, evidence is accumulating that ecological disturbances, particularly wildfire, can increase regional water availability. However, wildfire hydrologic impacts are typically estimated locally and at small spatial scales, via disparate measurement methods and analysis techniques, and outside the context of climate change projections. Consequently, the relative importance of climate change driven versus wildfire driven impacts on streamflow remains unknown across the western USA. Here we show that considering wildfire in modeling streamflow significantly improves model predictions. Mixed effects modeling attributed 2%−14% of long-term annual streamflow to wildfire effects. The importance of this wildfire-linked streamflow relative to predicted climate change-induced streamflow reductions ranged from 20%−370% of the streamflow decrease predicted to occur by 2050. The rate of post-wildfire vegetation recovery and the proportion of watershed area burned controlled the wildfire effect. Our results demonstrate that in large areas of the western USA affected by wildfire, regional predictions of future water availability are subject to greater structural uncertainty than previously thought. These results suggest that future streamflows may be underestimated in areas affected by increased prevalence of hydrologically relevant ecological disturbances such as wildfire.
Highlights
IntroductionSince the 1950s accelerated human activities have altered key Earth cycles and systems in ways that have profound, often indirect, future effects on renewable water resources (Karl and Trenberth 2003) and have made the assumption of hydrologic cycle stationarity untenable (Milly et al 2008)
Our results demonstrate that in large areas of the western USA affected by wildfire, regional predictions of future water availability are subject to greater structural uncertainty than previously thought
We developed a single framework to directly interrogate the importance of wildfires as a structural uncertainty affecting the accuracy of future water resource availability projections at the scale of the western USA
Summary
Since the 1950s accelerated human activities have altered key Earth cycles and systems in ways that have profound, often indirect, future effects on renewable water resources (Karl and Trenberth 2003) and have made the assumption of hydrologic cycle stationarity untenable (Milly et al 2008). Acknowledging that our climate is non-stationary, future water resource availability relative to the present is typically projected on the basis of future greenhouse gas emissions scenarios and their consequent impacts on precipitation and evaporative demand, as determined by hydrometeorologic models (Milly et al 2005) Implicit in these highly impactful projections of climate change effects on water resources is an assumption that anthropogenic climate change is essentially the only component of global-scale human-induced changes relevant to future water resource availability. Widespread upland global change-induced ecological disturbances include woody encroachment (Van Auken 2000, Wine et al 2015), tree mortality due to pest-pathogen complexes (Kurz et al 2008), and wildfire (Westerling et al 2006) Assuming that these ecological disturbances can substantially influence Earth’s future water balance, they represent structural uncertainties in the current approach to projecting future water resource availability. The extent of these ecological disturbances in the western USA is not yet sufficiently widespread to estimate their future effects on regional water availability from retrospective environmental data
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