While any precipitation is better than none, the type of precipitation that falls can have a dramatic effect on how the precipitation is absorbed by the soil and later recovered in the aquifers and reservoirs and how it will ultimately ease the record-breaking and extensive drought across the state of California.
Over the past several weeks, lots of rain has moved across the western coast of the United States. California has been on the receiving end of quite a bit of it and at times it’s been falling inches per day, day after day. Though flooding, mud slides, sink holes, mandatory evacuations and death have occurred, you would not be alone in assuming that despite the tremendous damage and tragedy, the rain has got to be doing tremendous good towards easing the drought. To some degree it is, but unfortunately most of the rainfall is hitting storm drains and estuaries and rushing out to sea almost as fast as it’s falling from the sky. In terms of the drought, it’s only a tantalizing tease.
Rushing out to sea rather than being absorbed by the soil? After years of drought in which rain is not only not falling from the sky, moisture has been evaporating from the soil. Soil without moisture compacts, making it difficult to absorb new moisture quickly. A downpour from the sky or a downhill rush of rainfall from higher elevations will have difficulty penetrating such soil, so it will tend instead to take the path of least resistance and continue downhill to the sea.
The type of soil also has a dramatic effect on whether rainfall is absorbed or not. Mark McFarland with Texas Agrilife Extension, “Typically, sands are going to have larger pore spaces and so the movement of water into the soil is much more rapid, and it can move down into the soil more quickly,” he says. “Clays on the other hand are fine-textured soils and in other words the particles are much smaller and typically the pore space is considerably smaller. The rate of infiltration and the conductivity of water down into the soil are going to be slower.” (Link.)
To best ease the California drought, a series of slow and steady, as well as prolonged rainfall events are needed. With soil properly hydrated, additional moisture can be absorbed, slowing what flows into rivers, drains, and spillways.
Rain however, is not the only form of precipitation to fall from the sky. What may be the best form of long-term drought relief is what is now falling in the higher elevations of California, even though its benefit may still be months from now: snow. As the snowpack slowly melts in the spring and summer of 2015, it will have tremendous effect on the drought, not only re-hydrating the soil but also filling reservoirs as the steady and predictable run-off is pulled downhill.
ImpactWeather’s long-range forecast specialist, Senior Meteorologist Fred Schmude, attributes two ongoing and developing features to the predicted amount of snowfall: El Niño and the Pacific blocking ridge. “I would favor the central and southern areas with the best risk for heavier than normal snowpack, especially from the central and southern Sierra of California eastward across Utah, Arizona, Colorado and most of New Mexico, thanks in large part to a stronger than normal southern storm track and a higher frequency of disturbances. Over the Northern Rockies I would expect the snowpack to be closer to normal, except over the Pacific Northwest from northern California northward across Oregon, Washington, northern Idaho and western Montana where below normal snow pack is expected.”
Fred’s forecast relies on what does or does not happen with El Niño. “Even though we’re highly confident a weak to moderate El Niño will develop this year which will provide an extra punch to the southern storm track, there’s still some uncertainty on the exact position of this feature. If the southern storm track winds up farther to the east, then we could see most of the precipitation-bearing systems remaining east of the Rockies towards the East Coast and Atlantic Seaboard. For now though, we favor the southern third to half of the Lower 48 seeing the most precipitation.”
Looking ahead to the last week of December and the early weeks of January and February, Fred expects the Pacific blocking ridge to drive storm tracks farther south and east than is typical. “Over the Rockies, the best chance for heavier than normal snowfall during this time period will be centered over the southern third of the area, especially from California eastward across Utah and Arizona to Colorado and New Mexico,” he said.
If all goes well, there should be plenty of water flowing into the reservoirs that feed the major population and agricultural centers of California in just a few months’ time.
In a study released today, NOAA researchers offer insight into how understanding the relationship between ocean and atmospheric patterns can lead to better understanding and prediction of the ongoing drought, as well as future drought events. View the full report here: http://cpo.noaa.gov/MAPP/californiadroughtreport.