Less Snow and More Rain Means We're All Dried Up

Right about now is the most magical time of year in the mountains. Not just for the usual spring reasons of getting to play outside and saying farewell to lingering seasonal affective disorder, but because of water. This is when it all happens: the big melt. Mud season. The amount of water shed from high-country snowpack in the springtime will impact the whole rest of the year. The true amount of that water now choking valley rivers will determine which farmers actually get to farm this year, how much water Southern California can depend on even for basic things, and the severity of summer wildfires all over—to say little of tourism, wildlife habitat, and shipping navigation (remember the Missouri River crisis?).

Many regions in the especially snowpack-dependent states will even have a subsequent rainy season in July or August. The Colorado monsoons, for example, will dump water nearly every afternoon for several weeks straight on the state, but even in an epic monsoon year, it's nothing compared to the snowmelt. The rivers will turn muddy and rise up, but the water year will already have been spoken for. This is something westerners may already know well enough, but researchers are still fleshing out the precise hydrological relationships between precipitation as snowfall vs. rainfall, and what it might mean for a warming planet.

Part of the reason for that lack of information is the difficulty is determining projected runoff from snowpack measurements, or from obtaining good snowpack measurements at all. Typically, snow runoff and its impact on a river or stream is examined for a particular portion of a given year, rather than the overall year-by-year streamflow. The result is a picture that very conspicuously excludes snow's real effects. A study out today in the journal Nature Climate Change purports to be the very first to look at the effect snowfall has on yearly averages, and its conclusions aren't terribly surprising: if a river is composed of higher amounts of water from snowmelt (relative to other water sources, like rain) it is likely to discharge more total water.

The key to overcoming the snowmelt measurement difficulty, at least for this study, was looking big. The researchers took 420 different catchments (drainage basins) and looked at the fractions of total flow originating in snowpack vs. yearly streamflow anomolies. As the snowpack fraction increases, the anomolies both get more scattered and tend clearly in the direction of more water. It makes sense at the very least in terms of evaporation: surface water is subject to vaporization, while snow remains relatively impervious.

You can see the implication for the climate shifted future. While rainfall might increase over some areas, global warming will absolutely result in less snow and less snowpack. Even if precipitation in aggregate remains the same, the water we'll have available will go down. Of course, it's already down, and the American Southwest is already in various states of critical drought. California, in particular, is just blowing through the drought severity scale, with only a heat wave on the horizon. In light of the situation right now, contemplating a worse future is overwhelming.

In a statement, the authors note that, "With more than one-sixth of the Earth's population depending on meltwater for their water supply, and ecosystems that can be sensitive to streamflow alterations, the socio-economic consequences of a reduction in streamflow can be substantial." It's not just California or Colorado or Texas, it's Earth.