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How Wet Is It?

Mar 20, 2017

California Water Commission

“In just a 6-week period beginning in mid-January through the end of February, we passed through Lake Oroville the entire volume of the lake essentially, says SWP Principal Engineer John Leahigh

At the March meeting of the California Water Commission, John Leahigh, Principal Engineer for the State Water Project, gave this on the current hydrological conditions and operations of the State Water Project.

John Leahigh began by acknowledging the unprecedented conditions this year, noting that it has been a big challenge for water managers and reservoir operators up and down the state, and continues to be a challenge for folks in the San Joaquin Basin in particular.

So how wet has it been?  Mr. Leahigh presented a graphic of the Northern Sierra 8-station precipitation index, which shows the precipitation accumulated for the water year since October 1st.  This index shows the precipitation in the Shasta, Feather, American, and Yuba River watersheds.  The wettest year on record was the 1982-83 year, where 88.5” was accumulated in the Northern Sierra.

This year to date, we’ve been running at a record pace for the majority of the winter in terms of the amount of precipitation that we’ve seen in the Northern Sierra,” he said.  “We’re currently at just below 70” for the year, that’s 211% of average to date.  The average for the northern Sierra is 50” for the water year.  By contrast, you can see the driest years.  So California has the most varied annual precipitation of any state in the entire country, and so this is pretty much par for the course for us in terms of the extremes that we’ll expect to see.”

So how wet has it been?

Precipitation for the Northern Sierra 8-station index in January and February was 46”, which is pretty much the average annual precipitation for the Northern Sierra.  The previous record was 40” for that same January-February period.

Inflow into Lake Oroville for February alone was over 5 times the average volume of runoff for February.  The January-February inflow was 4.4 MAF which is equal to the average annual inflow into Lake Oroville.  The previous high was the 3.48, which was set back in 1909.  It’s well above the 3.19 MAF in 1986 and 3.07 MAF in 1997, which was the year with the New Year’s Day big event that caused some major flooding.

The big thing this year has just been the volume of inflow that we’ve had to manage,” he said. “In just a 6-week period beginning in mid-January through the end of February, we passed through the lake the entire volume of the lake essentially.  So 3.5 MAF is the maximum storage for Lake Oroville; we passed that complete volume through the combination of the spillway and the power plant over a 6 week period.”

Mr. Leahigh then displayed the precipitation for the San Joaquin River basin, which is referred to as the 5-station index, which includes everything from the Stanislaus River watershed down through the San Joaquin main stem watershed, including the Tuolumne and Merced.  The basin is running a little over 200% of average to date, and further south, the Tulare Basin is not quite as wet but still on par with some of the wettest years on record.

The snowpack is doing quite well, some of the largest on record, although 1982-83 was a little bit higher.  We’re running about 166% of average to date on the snowpack,” Mr. Leahigh said.  “If you recall, we’re well over 200% of average on precipitation and about 166% on snowpack, so what that tells us essentially is we’ve been seeing the volume of precip has been coming in a warmer form than would be average, so that’s why the snowpack is not keeping up with the volume in terms of precipitation.”

He noted that the snowpack number is a little bit closer on the Central and Southern Sierra, and there is still a concern moving into the spring of being able to manage the snowmelt that’s going to occur on the San Joaquin basin.  “In the San Joaquin basin, a lot of their flood control diagrams are snowmelt based, so they are going to have a real challenge as we get later in to the spring as far as managing those flows as they come into the system.”

Mr. Leahigh then presented a graphic showing the unimpaired runoff forecast.  “It includes not only the runoff that we’ve seen to date, but the anticipated runoff for the rest of the water year, so that would include the melting of the snow that’s up there now, plus any additional precipitation that would be expected to occur from this point forward,” he said.  “Really this runoff forecast is a result of the measurements done for the March 1st snow surveys.  That’s when they are plunging their metal tubes into the snow, once a month collectively between 100 and 120 survey locations up and down the Sierra Nevada.  One of the principal results of that snow survey is the production of this runoff forecast, which all of the reservoir managers are reliant upon in order to manage the resource as we go through the year.”

Mr.Leahigh noted that the exceedances shown on the bottom of the slide have to do with the amount of historical precipitation from this date forward.  “So the 50% exceedance would include all of the snowpack melt that’s occurred as of March 1st plus precipitation that would occur on average from that point forward.  90% exceedance just means those conditions that precipitation was exceeded 90% of the time historically, which historically is a drier case; 10% would be a wetter assumption on precipitation for the remainder of the year, but that shows us that our expectation on that 50% would be about twice the annual runoff for the Sacramento River basin.”

A Commissioner asks where that is measured at.

Mr. Leahigh noted that this is the four river index that is measured at Benbridge on the Sacramento River, inflow into Oroville, inflow on the Yuba River, and inflow into Folsom Lake on the American River, so it’s a combination of those four major rivers in the Sacramento basin.  Not included in this index is the runoff that results from precipitation on the valley floor itself, so that would be in addition to these numbers.  It also does not include the flows we would expect on the San Joaquin system, so overall, the picture is a lot bigger than even these numbers indicate.

The significance of the Sacramento four river index is those inflow forecasts are the basis for the year type on which most of the Bay Delta standards are based, such as wet, above normal, below normal, dry, or critically dry,” he said.  “Those are all based on those inflows into those reservoirs and the Sacramento Valley.

To date, we’re running 282% of average in terms of runoff,” he said.  “That also tells you that the storms have been warmer than average because we’re 200% of average on precipitation but we’re 282% in terms of runoff, so typically more of the precipitation would have fallen as snow than it did this year and would runoff later in the year.”

With respect to where we were last year, last year there was about average precipitation but runoff was only 6.7 MAF versus the 22.1 MAF, so over three times more runoff so far this year than last year, Mr. Leahigh said.  Last year, the total water volume was 17.4 for the entire year, so we’re well in advance of that.  In 1977, for the entire year, there was only 5.1 MAF.   The largest was in 1983, which was 37.7 MAF.  “There are about even chances that we would surpass that 1983 total at this point,” he said.

In terms of water storage, the state has been able to recover any deficits in terms of surface storage; however, groundwater storage is another story.  “Up and down the state, most every reservoir has been in flood control operations, so we have essentially been releasing many times over what’s been coming in,” Mr. Leahigh said.  “I noted that for Oroville, the entire lake’s volume we’ve passed through in just six weeks.  For Delta outflows just since January 1st , 26 MAF of outflow went out the Golden Gate, so 26 Folsom Reservoirs just since January 1st of water that had to be passed through for flood control purposes in order to continue to hold that vacant storage for snowmelt and for any subsequent precipitation events that could occur from this point forward.”

He said that reservoir storage on the San Joaquin system is very high right now, so they are working to create as much of a hole in the tributary reservoirs in order to absorb that snowpack when it does start to melt off a little bit later this spring. So San Joaquin flows as measured at Vernalis will stay very high for some time.

Mr. Leahigh noted that San Luis Reservoir is full this year.  It is off-stream storage, which is filled by pumping from the Delta from both the State Water Project and the Central Valley Project export facilities.  He noted that filling San Luis Reservoir has been a much rarer event since the 2008-09 Delta smelt and the NMFS biological opinions.

Prior to that point, we probably filled San Luis about 80% of the time; that’s flipped and it’s closer to 20% of the time that we’re actually able to capture enough excess flows in the Delta in order to fill San Luis Reservoir,” he said.  “We were successful in doing that this year because of the very wet conditions and very favorable in terms of flows in the Delta, so there was of course that large amount of excess flow to capture, and that’s going a long ways towards supporting the State Water Project allocation this year.”

The State Water Project allocation is currently at 60%, which was also the final allocation last year.  “We’re looking for opportunities to increase that,” Mr. Leahigh said.  “We’re going to be reevaluating that in terms of these very high March 1st snow survey and runoff results.  We have some uncertainties with Lake Oroville, but in years like this, there are a lot of other flows that are coming into the Delta so that we’re much less reliant on storage from Lake Oroville in the very wet years, so we’re looking for opportunities to increase that allocation as we move forward in the spring.”

To conclude, Mr. Leahigh showed a comparison of the this year’s drought monitor compared to last year at about the same time.

Last year at about this time, virtually all of the state was in some type of drought or abnormally dry condition, and at peak, about 46% of the state was in the most extreme or exceptional drought condition; as of a week ago, that has completely flipped.  Now, none of the state is in any of the conditions that would be classified as extreme or exceptional in terms of drought.  About 7% of the state is classified as being in some type of drought and about another 15% in abnormally dry conditions.

If you want to look at a positive side to all of this rain, that’s been the positive there,” Mr. Leahigh said.  “Of course, this is also the year where we’ve had some opportunities to start recharging some of those depleted groundwater storages, but that of course takes years, just because of the capacity and the infiltration rates to get those groundwater levels back up, so that’s years in the process.  But surface storage has been recovered.”

For everyone who works in water in the state, we really need to think about the language that we use and the deions that we use around the water issues that we’re facing,” said Chair Armando Quintero.  “When I see these drought maps, I think that for the public it would be great to have the visual that shows the groundwater conditions at the same time, because there continues to be the confusion that the drought’s been addressed in terms of surface water, but in groundwater, we still have a severe problem.  I think that it could be of value of all of us and for the citizenry of the state to really understand that and understand where we have to put our efforts as a state, because as these GSAs are forming of course, we hope to see much improved intentional recharging of the groundwater basins.

The other thing is do you know how this year might be characterized in terms of a 100 year storm or a 100 year winter or a 200 year winter … do you have a number like that?,” asked Mr. Quintero.

That’s a little difficult one to get at,” said Mr. Leahigh.  “Those return frequencies are typically associated with shorter duration events.  You can look at it at pretty much any duration, for the historical record that we have goes back for the precipitation a little over 100 years, for the runoff, it’s back to 1924, but in those terms, forecasts are that we’ll come very close to being the wettest for any of those, at least within the last 100 years.”

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