Wet Enough For You?

Dire predictions of drought conditions this coming year, due to a “La Nina” condition in the Pacific Ocean, have given way to dire predictions of floods — on a scale that might scare Noah! See today’s article in the Los Angeles Times, who also provide today’s photo:

http://latimesblogs.latimes.com/lanow/2010/12/mudslides-flooding-close-roads-pch-huge-storm-set-for-monday-afternoon.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+lanowblog+%28L.A.+Now%29

The cure for drought is precipitation. No profound revelation there! But torrential rains alone won’t solve our water supply problems in southern California. Let’s look at a couple of other impacts on our supply today.

I have always liked the quip about the guy with his feet in the freezer and his head in the oven: on average, his temperature was just fine. With this record blast of storms pummelling California, we can expect at least an average rainfall year. But, as they say, “Timing is everything.” And the timing of this rainfall is far from optimum — just like our friend in the freezer and oven.

When rain hits the ground, one of two things can happen: percolation or runoff. Percolation is best, as this returns water to the soil, where we can access it later through our wells, or where it can slowly move downhill into natural streams and lakes. But once soil gets saturated, there is no more percolation, and runoff dominates. And this is what we will be seeing over the next few days here in California.

But this is nothing new to us. Our precipitation pattern has always been “feast or famine” — although maybe not to the extent we are experiencing right now. But we have developed tools to deal with this climate reality: reservoirs. With respect to water supply, their job is to save water on the “rainy day” for us to use during the not-so-rainy days that dominate our calendars. Our forebears were really good at this, and they left us with many reservoirs. However, we don’t have nearly enough today. Our population has continued to swell in California, but the number of reservoirs has not. So where we once had a surplus of storage capacity, we now are experiencing a shortfall. And good luck trying to build a new reservoir in California today.

But water supply is not the only mission for our reservoirs; flood control is another key function. Without these reservoirs in place, floods would devastate communities and farms. But a full reservoir can’t prevent flooding. So most of the reservoirs that are used for water supply in California are busy discharging water right now, to save room for flood flows. Our vital water supply is flowing into the ocean, because we don’t have a place to keep it! But we need the room in the reservoirs to protect us from floods — especially this early in the season. There is little risk of flooding in the arid California Summer and Fall, so reservoirs don’t “lose” water storage capacity late in the season — only in the Winter and early Spring months.

Nature has her own reservoir, and it is a huge factor in our water supply picture: the snow pack. Frozen water will not form runoff; it stays put up in our mountain ranges, and it slowly melts into liquid water for percolation and runoff later in the year — when we have less rain and thus a greater need for water. So the form of the precipitation is important to us: is it solid or liquid? This current storm is a pretty warm one, so it is not providing much snow, except at the highest elevations. (And it is providing quite a bit there!)

Our epic storm system is definitely a boon to our bleak water picture. However, there are three factors that will be lessening the supply benefit: insufficient storage reservoir capacity, storage capacity assigned to flood control instead of water supply, and a mostly rainy precipitation instead of a big snow maker.

But we will definitely take it, and be grateful for it!

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