Water Year Update: March 2021

Hydrologist Bart Nijssen and climatologist John Abatzoglou report on the 2021 water year, and use the Climate Toolbox to tell the story

Bart Nijssen and John Abatzoglou

Daylight hours are increasing rapidly and the end of winter is in sight. So it is an ideal time to do a quick recap of the water year to date. And of course we’ll use the Climate Toolbox to tell that story.

As its name implies, the Climate Toolbox allows one to investigate the climate and hydrology of a given region, such as the Northwest. Many of the tools are applicable well beyond our region, providing information across the western United States or even all of the continental United States. The tools not only visualize outputs from weather and hydrological models, but also include and display observations collected by many agencies. We’ll showcase some of these tools as we summarize the water year thus far.

The 2021 water year started on October 1, 2020. Hydrologists use October 1 as the start of the water calendar because around that time, the northern hemisphere snowpack is at a minimum, soil moisture is pretty low, and reservoirs are drawn down. Many of the tools in the Climate Toolbox reflect the water year. For example, the Climate Mapper allows one to examine conditions since October 1. We used this capability to depict the mean daily temperature and total precipitation anomalies for the current water year (Figures 1 and 2). The anomaly represents the deviation of the current year from the 1981-2010 mean. 

Mean daily temperature anomaly since the start of the 2021 water year on the basis of gridMet data as displayed by the Climate Mapper tool
Figure 2. Total precipitation anomaly since the start of the 2021 water year on the basis of gridMet data as displayed by the Climate Mapper tool. The anomaly is the difference in the total precipitation relative to 1981-2010.

Figure 1. Mean daily temperature anomaly since the start of the 2021 water year on the basis of gridMet data as displayed by the Climate Mapper tool. The anomaly is the difference in the mean daily temperature relative to 1981-2010.

Figure 2. Total precipitation anomaly since the start of the 2021 water year on the basis of gridMet data as displayed by the Climate Mapper tool. The anomaly is the difference in the total precipitation relative to 1981-2010

Although February was relatively cold in the Northwest, the water year to date has been warmer than average. Since the start of the water year, precipitation has been slightly above normal in much of Washington and northern Idaho, and slightly below normal in Oregon and southern Idaho. In addition to being relatively cold, February was relatively wet, especially west of the Cascade Range, in eastern Oregon, and in parts of Idaho. 

Snowpack conditions reflect these precipitation and temperature patterns. We used the Historical Water Watcher tool to compare snow observations on the first day of every month from December 2020 through March 2021 (Figure 3). Snowpacks generally were below normal on December 1, recovered somewhat by January 1, and were lower than historic conditions during January, but then increased considerably during a relatively cold and wet February. As a result, snow water equivalent (the amount of water contained in the snowpack) at the beginning of March was above normal in much of the Northwest, especially in Washington, northeastern Oregon, and central Idaho. 

Snow conditions in the Northwest

Figure 2. Total precipitation anomaly since the start of the 2021 water year on the basis of gridMet data as displayed by the Climate Mapper tool. The anomaly is the difference in the total precipitation relative to 1981-2010

The same story can be told with the Historical Climate Scatter tool in the Climate Toolbox. This tool compares conditions during the current water year to those during all water years since 1979 for a specific point location, county, watershed, or any selected rectangular area. One can customize the tool outputs by selecting which conditions to display along the horizontal and vertical axes. We selected a rectangular box that roughly coincides with Oregon to compare the October-December precipitation total to the October-December mean temperature (Figure 4). 

Precipitation and temperature during the first three months of the water year (October-December) as displayed by the Historical Climate Scatter tool.

Figure 4. Precipitation and temperature during the first three months of the water year (October-December) as displayed by the Historical Climate Scatter tool. The value for 2020 overlaps that for 2002. Boundaries of Oregon represented by a rectangular bounding box (42N – 46N, 124.75W-117W).

This plot reveals that in Oregon, October-December of water year 2021 was warmer and drier than average, and considerably drier than the same period during the 2015 water year (Figure 4) – which brought widespread drought to the Northwest. However, January precipitation was near normal in 2021, and, as noted above, February 2021 was both colder and wetter than normal. As a result, snowpack in Oregon increased from below normal in early December to above normal by early March.

So where does that leave us? Drought in south-central Oregon and southern Idaho eased somewhat during February, and streamflow and reservoir storage may recover further as snow starts to melt. It currently is unclear whether that will be sufficient to erase the moisture deficits in these regions. 

Seasonal outlooks for March through May from NOAA’s Climate Prediction Center indicate a greater than normal likelihood of below-average temperature and above-average precipitation for Washington and northern Idaho, consistent with the moderate La Niña. Unfortunately, the hydrologic outlook remains somewhat murky for places currently in drought, including Oregon and southern Idaho.  

Clockwise from top left: water year accumulated precipitation (data from gridMET), observed snow water equivalent (data from Natural Resources Conservation Service), reservoir storage (data from Natural Resources Conservation Service), and streamflow (US Geological Survey WaterWatch) as displayed by the Historical Water Watcher tool.

Figure 5. Clockwise from top left: water year accumulated precipitation (data from gridMET), observed snow water equivalent (data from Natural Resources Conservation Service), reservoir storage (data from Natural Resources Conservation Service), and streamflow (US Geological Survey WaterWatch) as displayed by the Historical Water Watcher tool. Colors indicate current-year conditions relative to the historical record (Precipitation: 1979-2015, Natural Resource Conservation Service: 1991-Present).


Bart Nijssen is a CIRC team member, hydrologist, and leading member of the University Washington’s surface hydrology group.

John Abatzoglou is a CIRC team member, climatologist, and professor at the University of California, Merced.


Featured Image: Mean daily temperature anomaly since the start of the 2021 water year on the basis of gridMet data as displayed by the Climate Mapper tool. All rights reserved.


Acknowledgments: The Climate Toolbox is funded in part through the NOAA Regional Integrated Sciences and Assessments (RISA) program and National Integrated Drought Information System (NIDIS).

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