The science behind drought declarations

When the Washington Department of Ecology issued a statewide drought declaration on April 8, there were a lot of questions, and even some skepticism, about the analysis that informed the drought emergency.

Is it really an "emergency"? Did Ecology issue the declaration too early? Was a drought declaration even necessary, given that a wetter-than-normal winter left many of our state's reservoirs full?

Questions are perfectly normal. It obviously can feel a little weird to be talking about drought during spring when the rivers are high and everything is green and growing.

Drought declarations, however, are guided by Washington state laws, and informed by a lot of very detailed science, so it's important to understand the work that goes on behind the scenes before an emergency is declared.

In Washington, a drought is declared when there's an expectation we'll have less than 75% of our normal water supply and there is a potential for undue hardship to water users or the environment. That's the legal threshold set in state law.

What goes into determining our "normal water supply" and "undue hardship" is complex, however.

For today, let's focus on the first criteria: 75% of normal. That's what we call the "hydrologic threshold."

Sounds very sciencey right?

And it is in fact extremely sciencey. Determining whether that 75% threshold has been crossed requires a huge number of instruments, measurements, models and analysis.

You put that all together, and you begin to see that a drought declaration is really the end product of a year-long conversation between Ecology and a host of scientists from a variety of state and federal agencies.

Let's dig in to some of these data sources.

Snowpack

Anyone who reads our water supply blogs is familiar with how much we love to talk about snowpack. It's our state's frozen reservoir - a key driver of our water supply and, in a good year, what keeps streamflows cool enough for salmon to swim upstream.

We often use the term "snow water equivalent" or SWE. Sometimes we simplify that to "snowpack." When we use these terms, we're talking about how much usable water is stored as snow in various basins. We know how much there is because data gathered by the Natural Resources Conservation Service's snow telemetry, or SNOTEL, stations.

NRCS explains the station's functions this way: "A standard SNOTEL site measures and records hourly data on snow depth, snow water equivalent, precipitation, air temperature, soil moisture, and soil temperature. These data are free and available to the public."

NRCS Hydrologist and Water Supply Specialist Toby Rodgers is a regular presenter at Ecology's monthly Water Supply Availability Committee Meetings. He said SNOTEL sites are an important tool for water management and planning.

"SNOTEL sites provide essential data used routinely for a number of purposes and interests including water supply forecasts, resource management, research, and even recreation," Rodgers said. "The SNOTEL network is a robust system of stations that are routinely maintained and updated in order to provide the most reliable data to all of the interested stakeholders."

A look at a recent SNOTEL data report paints an interesting picture that shows while precipitation is at, near, or even above normal in some basins, that "snow water equivalent," or SWE, is far below normal.

Take the Upper Yakima Basin as an example: The basin is at 9% of normal SWE compared to the average of the last 30 years. Again, that doesn't mean it has 9% of its maximum snowpack. It means that compared to snowpack levels averaged over the past 30 years, this year's SWE is just 9% of where it usually is. That's bad.

Our state's drought threshold is 75% normal. Despite low snowpack, the basin actually had 109% of normal precipitation during the winter months. So, where did all that extra water go?

Streamflows and groundwater

Statewide, this past winter was the fourth-warmest on record, going back to 1895. In a warm winter, precipitation in snow-dependent basins typically comes down as rain instead of snow and then runs off into streams, creating higher than normal winter streamflows. This was seen in our historic flooding in December.

Unfortunately, winter is not when we need the water the most. When it runs off in the winter, it's not around when we need it in spring and summer. This results in streamflows getting abnormally low. Streams with less water heat up faster, making conditions too warm for salmon and other fish.

"I often use a tea kettle analogy to describe this," said Caroline Mellor, Ecology's statewide drought lead. "When there's less water in the tea kettle, it takes less time to boil. The same applies to lower streamflows - that water gets hotter faster. This impacts fish passage and survival."

If the extra rain doesn't run off into streams, it can get sucked up by soil thirsty from four years of soil moisture deficits.

How do scientists keep track of streamflow and soil moisture?

The U.S. Geological Survey's Washington Water Science Center collects streamflow data from more than 380 streamflow gauges, drone footage, and on-site observations to measure and report streamflow. They also use their understanding of the connection between surface water and groundwater to create models of groundwater availability and how much water is needed to recharge depleted aquifers.

According to the USGS Washington Water Science Center, "In most areas of Washington, groundwater is hydraulically connected to nearby rivers, lakes, and wetlands. We study the groundwater-surface water interface to better understand how much water is moving from above and below ground. Knowledge of ground and surface water exchange helps managers protect these interconnected resources."

Seasonal projections

Seasonal forecasts are another factor to consider when determining whether the technical definition of drought will be met. We get our historical climate and seasonal climate information from the Washington State Climate Office at the University of Washington, or WASCO.

For the seasonal forecasts, WASCO scientists compile temperature and precipitation forecasts from the National Oceanic and Atmospheric Administration Climate Prediction Center and from national and international seasonal forecast models.

WASCO also monitors the status and forecasts for El Niño/La Niña in the tropical Pacific which can impact Washington weather. The weather and seasonal climate patterns that occur each year are put into historical context using long records of observations primarily from NOAA, but other federal agencies and universities as well. WASCO has developed multiple tools to analyze recent climate data.

"While not always relevant to monitoring for drought each individual season, longer-term climate projections are also provided by the Washington State Climate Office," said Deputy State Climatologist Karin Bumbaco. "This year's drought, for example, with a warmer than normal winter, near-normal precipitation, and much less snowpack than usual, is the mechanism for drought that climate models project will occur much more frequently by the 2050s."

Data transparency matters

We keep all our data and sources prominently displayed on our statewide conditions page. We have links to the Washington State Climate Office, NRCS, NOAA, the U.S. Drought Monitor and more.

If you want to hear directly from the scientists and water managers who gather and present water supply data, we have a monthly meeting where representatives from multiple state and federal agencies discuss current conditions, present detailed analysis of their data and offer forecasts.

These meetings are recorded and available for public viewing on our Water Supply Availability Committee page.