Chris Lynch: Forecasting Future Water Supplies in the Yakima Basin

Chris Lynch: Forecasting Future Water Supplies in the Yakima Basin

Lake Cle Elum, one of the five reservoirs on the Yakima system.

As water becomes increasingly scarce across the West, the need to accurately forecast how much will be available is becoming ever more crucial to efficient water management. The Bureau of Reclamation’s Yakima Field Office has a distinctive method of forecasting water supply known as the Total Water Supply Available (TWSA) forecast. Born out of litigation and the Yakima basin’s unique characteristics, the system allows Reclamation to predict how much water will be available in total, how much will be available for each of the various water rights in the basin, and how much will be available for fish conservation and other uses. In this interview, Chris Lynch, the river operations engineer for the Yakima Field Office, tells Irrigation Leader how the forecast system came to be, how water supply levels are calculated, and how the system allows the field office to better serve all the users who depend on the Yakima for their water needs.

Irrigation Leader: Please tell us about your background and how you came to be in your current position.

Chris Lynch: I was born and raised for 10 years on a small farm in Pennsylvania before moving to Colorado. There, I came to understand the value of water because it was arid. I graduated from Colorado State University’s civil engineering master’s degree program with an emphasis on hydraulics and hydrology. I got my first job with the U.S. Army Corps of Engineers in Seattle, where I worked in hydrology and water management for 12 years. I then moved over to the drier side of the state of Washington, east of the Cascade Mountains in the Yakima basin. I’ve been here for about 24 years, managing the water in the Yakima basin and doing studies to improve operations, water supplies, and fish flows.

Irrigation Leader: Please tell us about the history of the TWSA forecast.

Chris Lynch: The concept arose in 1945 out of a lawsuit. The court decision is known as the 1945 consent decree. The basic issue was the priority of distribution of storage water. The basin followed prior appropriation doctrine, but a large number of the water rights, often referred to as junior rights, have a priority date of May 10, 1905, the date of the authorization of the Yakima Project. The other class of rights, often referred to as senior rights, have priority dates earlier than May 10, 1905. The 1945 consent decree states that the senior rightholders receive their water first from the total water supply, including storage water, and the junior rightholders receive whatever remains of the total water supply. The senior rights are nonproratable, while the junior rights are proratable. Junior rightholders share any shortage equally. To make these calculations, a method of determining the total water supply was necessary.

The water supply was generally healthy from 1945 to 1972. It wasn’t until 1973, when the Yakima basin was in a significant drought and was running out of water, that Reclamation had to dust off the TWSA method and start using it.

Irrigation Leader: How is the TWSA determined?

Chris Lynch: The TWSA has three components. The first is system storage: the storage level in the five reservoirs that make up the system on the first of the month for which we’re running the TWSA. Of the three components, only system storage is known instead of estimated.

The second component is an estimate of return flow from the first of the month through the end of September, based on the historical experience of the irrigation districts in certain water supply conditions. There is a bit of a feedback loop, because return flow relies on the water supply in general. We run the TWSA estimate, and if it’s a drought year, we may adjust the estimated return flow downward. If it’s a wet year, we may increase the estimated return flow a bit.

The third component, and the main forecasted part, is the runoff forecast above the Yakima River near Parker, located just below the city of Yakima. We estimate all the runoff from the mountains, through the reservoirs, and from the whole area above the Yakima River near Parker.

Irrigation Leader: Does the return flow come from irrigation use?

Chris Lynch: Exactly, and that’s why the return flow varies with the water supply and are lower in dry years. During a shortage, the proratable districts won’t get as much water, and consequently there won’t be as much going out on the fields or flowing through their canals, and in turn there will not be as much water returning to the river. In good water supply years, the irrigators will have more water, which can result in more return flow.

The Pigtail Peak snotel station.

Irrigation Leader: How do you measure things like snowpack and predict seasonal precipitation?

Chris Lynch: Snowpack is a really good metric to use for the runoff forecast because it is like money in the bank, saved up all winter and released in the spring and summer. Along with snow runoff forecasts, we also use precipitation and flow data. We have what we call snotel stations located in the mountains that record and feed us hourly precipitation and snow water data (that is, data on the amount of water in the snowpack). We work closely with the Natural Resources Conservation Service (NRCS), which runs most of the snotel sites. Folks at the Yakima Field Office also go out into the mountains monthly in winter and manually measure the snow water to verify that the automated numbers coming in are accurate. We also have snow courses that do not have automated recording devices, so we must go out and manually measure the amount of water in the snowpack.

Lake Kachess one of the five reservoirs in the Yakima system.

Lake Keechelus one of the five reservoirs in the Yakima system.

Most of the runoff forecasts use snow data, precipitation data, and antecedent flow data. Our colleagues in Reclamation’s Columbia–Pacific Northwest Regional Office in Boise produce a main numerical forecast using a multiple linear regression method. We also use runoff forecasts produced by the NRCS and the National Weather Service’s Northwest River Forecast Center. We use all these to develop the runoff forecast for the TWSA estimate. We make both low and high runoff forecasts to help account for uncertainties and estimate errors. The low-end forecast and the TWSA based on it are of particular interest to the irrigators for contingency planning in dry years.

It is difficult to forecast future precipitation beyond about 10 days, and forecasts of 10 days or more are only used in the Northwest River Forecast Center’s runoff forecast. We do look at seasonal precipitation and temperature forecasts made by the National Weather Service’s Climate Prediction Center. These are not used directly in formulating the TWSA forecast, but are primarily used in reservoir modeling and reservoir operations planning for minimum instream flow, outmigration flow, and flood control operations.

Irrigation Leader: What is the yearly cycle of the TWSA forecast?

Chris Lynch: We do a forecast at the beginning of each month from April to July. We will also add forecasts in August and September during water-short years when we need to refine the distribution of water. We try to get the forecast out to the public as early as possible each month, but we must wait until after the first of the month to get all the necessary data. In April and May, the TWSA is used to set the volume of water released from the reservoirs for spring outmigration flows for fish. We also use the TWSA forecast to set the summer minimum instream flow at the Yakima River near Parker gage.

Irrigation Leader: What operational decisions does Reclamation make on the basis of the forecast?

Chris Lynch: Of course, one of the major decisions is setting the proratable irrigation districts’ water supply. The irrigation water supply is equal to the TWSA minus two estimated factors: the water that will flow out of the basin unused by irrigation and the amount of water left in the reservoirs on September 30. We then subtract the senior rightholders’ water from the irrigation water supply and are left with the water available for the juniors. If it’s less than their entitlements—a shortage—we divide it by their entitlements to define the proration rate. For example, in 2005, it was 42 percent. This rate is then applied to each proratable irrigation district’s entitlement to determine its individual water supply for the season. The irrigation districts then use that in their own decisions and planning. We also determine the minimum instream target flow for the Yakima River near Parker, as defined in title 12 of the Yakima River Basin Water Enhancement Program legislation. The lower the water supply, the lower the target flow; the higher the water supply, the higher the target flow.

As I said earlier, we also use the TWSA to determine volumes of water we release in April and May for fish outmigration. The higher the TWSA, the more water we will release.

The other factor that comes into play in all this is the date when the allocation begins. In a water-short year, prorationing begins when the irrigation districts’ demand is greater than the amount of water that is incidentally available in the river. When the districts’ demand outstrips the amount of water available, we must release water from the reservoirs. When that date, known as the proration date, is set, we rerun the TWSA for the period from the proration date to September 30. We will actually do two each month thereafter. The main forecast of interest for setting the proration rate covers the period from the date prorationing begins to September 30. We also continue to do the first-of-the-month to September 30 forecasts for comparison to other years and to set target flows. For example, if prorationing begins on May 7, then each month after that, the main forecast will cover the period from May 7 to September 30. This will define the irrigation districts’ water budget for the season. We update it each month, even though the period remains the same, because our estimate of the runoff portion of the TWSA will improve as the season progresses and the supply becomes firmer. This also allows the districts to better manage their allocation. If we kept switching the start date, then the water they would have saved would be redistributed to everybody else and not necessarily benefit them.

Irrigation Leader: What decisions do farmers, irrigation districts, and other water users make on the basis of the forecast?

Chris Lynch: What I’ve heard for many years is that the districts that have a water shortage will use the TWSA forecast to lay out a plan to make it through the year with as little damage to their stakeholders as possible. They’ll have their board meetings and determine how they will budget their water—for instance, whether they will turn off their water for a period early in the season to save water or run steady at a certain rate and exhaust their supply before the end of the season. The farmers give their irrigation district managers feedback on how they prefer the water be managed based on when they really need the water and when they might be able to survive not having it. This is strongly influenced by the predominant crops that are grown in a particular district.

Irrigation Leader: Has the TWSA process needed to be updated or altered based on changing weather climatic conditions?

Chris Lynch: Not really. It’s just a way to identify how much water is in the basin. However, the runoff forecast part of the TWSA may need to be improved or adapted as we see different climatic cycles. I mentioned earlier that there were no significant droughts from 1945 to 1972, but several in the 1970s, some minor ones in the 1980s, and some severe ones in the 1990s and 2000s. We go through cycles, and of course, one of our concerns is climate change in the future. Climatic cycles and climate change need to be considered in the runoff forecast portion of the TWSA. If we have earlier snow melt, we may need better early forecasts. It may also require an earlier proration date, although that’s not something that would call for a change in the method.

Irrigation Leader: Have there been any new technologies that have affected how you develop or use the forecast?

Chris Lynch: We are working with our regional office to update our runoff forecasts; we’re always hoping to do a better job on that side of things. The National Oceanic and Atmospheric Administration is providing a new technology to estimate snow coverage in the basin using satellite imaging combined with snotel station data. There are a finite number of snotel stations providing point measurements of snow water spread throughout and near the basin, so they provide only a sample, a representation, of how much snow water is actually in the mountains. We’re developing forecasts using the new snow coverage maps, which in a sense provide more information on the actual water stored in the snow. In 2020, the satellite imaging technology allowed us to see that we didn’t have as much lowelevation snow as our snotel stations indicated but did have more high-elevation snow. That improved our forecasting.

In addition, our regional office has developed a new tool that puts all the basin data in one place, which will help us reanalyze our old forecasts and develop new methods that we hope will improve our runoff forecasting.

Irrigation Leader: What is your vision for the future of the TWSA forecast?

Chris Lynch: The TWSA forecast works well for the way the basin is set up right now. I would like to see continued improvements in the runoff forecasting methodology, including improvements based on the snow coverage maps and possibly a more empirical consideration of return flow. If new storage projects are built in the basin, we need to protect existing water rights and preserve the total water supply process to make sure existing rights are not negatively affected by the implementation of new projects.

Chris Lynch is the river operations engineer at the Bureau of Reclamation’s Yakima Field Office. He can be reached at clynch@usbr.gov.