2SAN FRANCISCO ESTUARY & WATERSHED SCIENCEVOLUME 21, ISSUE 4, ARTICLE 1wilder; dry out soils, vegetation, springs, lakes, and streams; and set the stage for notably larger and more intense droughts and wildfires. At the same time, the average projection of future precipitation amounts (over the past several generations of projections) has largely hovered near historical norms, except in the southern ramparts of California, Arizona, and New Mexico, where most projections agree it will be drier. One climate model projects more precipitation overall, another projects less; and this pattern repeats through the modern collections of projections (e.g., Dettinger 2005, 2016; Gershunov et al. 2019). Similarly, multi-model averages predict somewhat wetter conditions in one generation and drier conditions in the next generation (as found by comparisons between projections used in the Second through Fifth National Climate Assessments (Karl et al. 2009; Melillo et al 2014; Wuebbles et al. 2017; Crimmins et al. 2023). Among the growing number of models from which projections are available, the predicted precipitation changes are small—fractionally wetter overall, or drier overall, but not vastly so in either direction. This equivocation is a source of consternation for climatologists and for those who manage water—and land—across the region. How can they plan and manage if the overall water availability from precipitation cannot be pinned down more definitely than this?The tendency for precipitation projections to cluster with uncertainty around historical norms too often draws attention away from an important and common outcome in the projections. What the models agree on is that future precipitation will come in the form of reduced medium- to light-precipitation amounts (e.g., Dettinger 2016; Polade et al. 2017), and more dry days (Polade et al. 2014), all interrupted by occasional wetter-than-historical major storms and storm sequences (Dettinger et al. 2004; Pierce et al. 2013; Gershunov et al. 2019). Under these conditions—and together with trends away from snowfall and toward more rainfall—immediate surface runoff from those large storms is expected to increase, and the amount of water that makes its way down into slower, subsurface pathways will likely decline. In fact, climate models are nearly as unanimous in projecting increased large storms amid a background of more persistent and taxing dryness as they are in projecting a warmer future (Pierce et al. 2013; Polade et al. 2014; Dettinger 2016). The consistency in these predictions seems sufficient to guide policy. AN APPEAL TO CALIFORNIA WATER AGENCIES AND INSTITUTIONSThe lesson to be taken from modern projections of future precipitation and hydrology is that we should probably not fixate on the continuing uncertainty about precisely which direction precipitation totals will skew. Instead, we must focus on the high likelihood that amounts of precipitation will change far less than the way that precipitation arrives; thus, we must prepare for a future of a few, much larger storms and—if left unmitigated—enhanced spikes of runoff in the winter and spring, rather than the long-term moderate f lows in the warm seasons we have historically incorporated into water-resources planning. Considering this, the most pressing challenge posed by climate change in western
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