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.

Lakes—quintessential features of Earth's surface prized from perspectives of water security, aquatic ecosystems, and recreation alike—are shrinking in water‐limited regions of all of Earth's inhabited continents. Here we assessed Landsat‐derived long‐term decrease in global lake area relative to historical lake extent aiming to determine the role of recent Anthropocene levels of irrigated agriculture in the global phenomenon of lake desiccation. As of 2015, 11% (1.8 · 10⁵km²) of global lake area has already been lost, primarily due to increased water consumption in support of irrigated agriculture in endorheic basins within water‐limited regions. However, current levels of irrigated agriculture portend substantial additional shrinkage of global lakes before reaching new equilibria with present‐day inflows, with an additional 60–130% increase in endorheic lake loss anticipated. The time required for shrinking lakes to attain new equilibria ranges from decades to centuries depending on lake hyposometry. Even a small decrease in lake area can portend lake transition from exorheic to endorheic and dramatic reductions in water quality. Thus, lake area changes severely understate the perilous condition of global lakes. The watershed area contributing to shrinking (endorheic and exorheic) lakes accounts for 18% of Earth's land area, far too large for the irrigated agriculture therein to be transferred elsewhere in order to save these lakes, though continued developments in the efficiency of water consumption in agriculture and urban areas can save significant quantities of water. This suggests that global lake shrinkage may be a harbinger signaling mankind having exceeded Earth's sustainable carrying capacity.

On all inhabited continents, wildfires are important ecological disturbances that influence water resources. As an exemplar of this global phenomenon, wildfire activity in the western United states increased in the 1980s and has continued to trend upward in subsequent decades. However, the hydrologic impact of wildfire has been difficult to ascertain at a continental scale due to the complexity of processes that influence critical zone dynamics. We calibrated Fuh's equation to wildfire‐impacted gauged watersheds and used the validated model to retrospectively quantify the proportion of streamflow attributed to wildfire hydrologic impacts from 1986 to 2015 across ungauged watersheds, accounting for interwatershed variability in water retention capacity and climate as well as how these factors interact with burned area and postwildfire vegetation recovery. Here we show that in parts of the western United States with high proportion of area burned by fire, wildfire‐linked streamflow accounts for as much as 20% of streamflow, rivaling or exceeding predicted climate change impacts. This analytic framework demonstrates that over a multidecadal period, the present trajectory of rising temperatures and increases in large wildfires may enhance the regional‐scale importance of wildfires in streamflow generation, thereby partially offsetting projected streamflow reductions due to rising temperatures and evaporative demand.