Guest blogger Oregon State Climatologist Larry O’Neill describes Oregon’s drought.
Oregon, a state probably best known for its raging rivers and lush rain forests, is once again grappling with widespread drought. As of late July, Oregon Governor Kate Brown has approved drought declaration requests for fourteen Oregon counties (see Figure 1). Many of these Oregon counties have identified significant and severe drought impacts, especially to agriculture, livestock, and the availability of surface water, that is water found in streams, rivers, and reservoirs.
In this issues of Northwest Climate Currents, we breakdown how Oregon’s current drought developed and what we might expect in the weeks and months ahead. And because we think the best way to discover how drought is impacting our region is to let our region’s residents tell us what they’re experiencing, this post will illustrate how the Condition Observer Monitoring Reports (CMOR) and the Drought Impact Reporter (DIR), both freely available online tools for reporting drought impacts, have been used by agricultural and livestock producers to detail how this year’s drought has impacted them.
CMOR and DIR are both products of the National Oceanic and Atmospheric Administration‘s (NOAA) National Integrated Drought Information System (NIDIS) program, which provides funding for this blog series among other drought-awareness projects.
A Difficult Drought to Describe
In recent years, widespread drought has become something of a recurring theme for Oregon.
As droughts go, the story of the 2019/2020 drought is a doozy to describe.
The drought developed mainly during the fall of 2019 and persisted throughout the remainder of the water year. (A water year runs from October 1st of any given calendar year to September 30th of the following calendar year. ) Heavier rain events in January and later May led many observers to suspect drought was not going to manifest. However, in hindsight, we can now say there was a tendency to view those January and May rain events as more significant than they were in mitigating antecedent drought conditions. Then there was the issue of snowmelt and streamflow. In early spring, water was flowing well in Oregon’s rivers and streams. But as it turned out, this was due to the rapid melting of mountain snow. By late summer when it was needed most, available surface water was very low and unavailable to or rationed for many users, including farmers.
Let’s take this difficult story piece by piece starting with conditions in the fall of 2019 at the typical beginning of the Pacific Northwest’s wet season.
The Fall and the Water Year
Droughts are generally track per water year. This water year, beginning October 1st, 2019, started with Oregon in relatively good shape compared with previous years, according to the US Drought Monitor (Figure 2). In early October, about 10% of the state was listed as abnormally dry, the least severe form of drought listed by the Monitor.
This not-so-bad drought rating was followed that same month by a series of exceptionally moist frontal systems that doused Oregon with rainfall, clearing all abnormally dry conditions from the state like stagehands clearing a set. The downpour bred optimism for upcoming water supply conditions, and many thought a more typical rainy season was on the way, and with it, a welcome departure from the last decade or so of persistent drought. Unfortunately, that optimism was short-lived.
A Rain-Blocking Ridiculously Resilient Ridge
By mid-October, an extremely strong and persistent ridge of high pressure settled over the northwest Pacific Ocean. This ridge—something that is becoming more typical during fall and winter in recent years—obstructed and diverted the region’s typical seasonal storms farther north, sending our fall precipitation packing.
Due to their storm-blocking powers, persistent high-pressure ridges like this one have recently been named R3s, or Ridiculously Resilient Ridges, a term coined by climate researcher Daniel Swain in a 2015 paper on California’s 2012–2015 drought. Because they divert storms and moisture, R3 patterns often lead to dry conditions in the Pacific Northwest and California, and this is exactly what happened to Oregon in the fall of 2019.
During November and much of December—typically two of the wettest months in the Pacific Northwest—the presence of the R3 created very dry conditions throughout Oregon. If we tally up the numbers, we find that precipitation over most of the state for the 3-month period from October to December 2019 was less than 50% of normal (Figure 3), some of the driest conditions on record across much of the state for this 3-month period.
As a further consequence of the R3, our region’s typically reliable seasonal rain and mountain snow simply did not materialize until the end of December. By then, much of Oregon’s surface water supply was far below normal. Similar to what happened during the first half of the 2018/2019 water year, Oregonians were desperately hoping for a reprieve in the following months.
Drought Impacts Follow Holiday Hopefulness
Shortly after the holiday season, hopes for rain and snow rose anew as the anomalous ridge disappeared, allowing a series of cool, moist and potent Pacific weather systems to take aim at the Pacific Northwest. Washington state and northern Oregon benefitted most from the deluge. Most of the region had near-normal amounts of precipitation for the month of January.
By February 1st, 2020, parts of the Pacific Northwest were basking in near normal amounts of mountain snow, and other regions, such as the central and southern Oregon Cascades, had rebounded to over 80% of normal (Figure 4). The ample quantities of snow was one distinct feature of this wet season that differentiates this year’s drought from other recent droughts, most of which were characterized at least in part by a relative lack of snow, or what’s now frequently called a snow drought.
The 2014/2015 winter was just such a snow drought year. During the 2014/2015 winter snow drought, lack of mountain snow amplified water supply deficiencies into the following spring and summer. Hoping not to see yet another snow drought, Oregonians closely watching this water year’s climate and weather hoped the cool and wet conditions would continue into spring. This hope wasn’t far-fetched.
Abnormally wet spring seasons can substantially ease drought conditions in Oregon. For instance, just this last spring, a historic atmospheric river event in April 2019 added substantially to wet season precipitation and eased concerns about a potentially severe drought in the upcoming summer for much of Oregon. Would this year follow a similar pattern?
February 2020 brought another relatively prolonged dry period to Oregon, even as cooler-than-normal weather preserved our precious mountain snowpack. This meant that even with the wet month of January, Oregon’s surface water deficits caused by the historically dry fall were substantial and ultimately hard to overcome. As a consequence, reservoir levels in many parts of Oregon hit near record low levels, so much so that when irrigation allocations were announced near the end of February, many farmers ended up with well-below-normal irrigation allocations compared to previous years.
Mountain Snow and Spring Showers and Stream Flows…and Drought Impacts
The good news for March 2020 is that Oregon avoided any significant sustained high-elevation above-freezing temperatures or rain-on-snow events, which would have diminished mountain snowpack too early to capture and store it in area reservoirs. However, dry conditions continued across the state, leading to a flurry of impact reports in the CMOR and the DIR. Hopes for late season drought relief were beginning to fade. Reports from southwestern Oregon in particular pointed to a tough growing season ahead, with agricultural and livestock producers grappling with difficult planning decisions for the upcoming seasons. Additionally, a large-scale dry pattern returned to the Pacific Northwest. The question of whether Oregon’s drought would be a short-term phenomenon increasingly appeared settled: drought was here to stay. The new question became how severe it would be.
Because of the unique nature of how this drought evolved, it was difficult to assess past droughts for clues on how impacts from this drought would play out into the dry summer season. For that to make sense, let’s introduce some definitions.
A period of well-below-normal precipitation is referred to as a meteorological drought. At timescales of very roughly six months, the impacts of precipitation deficits on groundwater and streamflows induce what is known as a hydrological drought. Impacts from hydrological droughts are typically longer lasting and more acute, particularly on agriculture, livestock, and general surface water supply conditions.
By the end of March, it was abundantly clear that large parts of Oregon had entered a state of moderate to severe hydrological drought. If we look at streamflow percentiles in Oregon for March, we see just how far from the historical norm the state’s streams had gotten. By March 31st, 2020, much of western and central Oregon was experiencing extremely low streamflows, in many cases below the tenth percentile (Figure 5). Southwest Oregon was particularly hard hit as streamflows reached record lows for that date. This impacted agriculture acutely across the region’s Klamath Basin as water managers were forced to slash irrigation allocations to farmers due to low streamflows and low reservoir levels
Drought severity is often determined by assessing the percentiles of key drought indicators—such as low precipitation, low streamflow, and warm temperatures—over various timescales—such as months, seasons, and water years.
Tracking percentiles over a timescale helps you determine the relative frequency of occurrence of conditions relative to historical records. For instance, if you’re interested in understanding precipitation levels during the month of January 2020, it’s helpful to compare and rank those raw precipitation numbers to all the numbers recorded for January over a historical period, for instance 1981–2020. This is how and why we rank drought indicators using percentiles.
Starting at the end of March, streamflows throughout Oregon began to look better, but only superficially so. Abnormally warm conditions at the higher elevations led to snow melts that substantially boosted streamflows throughout much of the region. On April 1st , snow water equivalent (SWE) values across the Willamette and Klamath basins were near normal, according to measurements taken at mountain SNOTEL stations. (SWE measures the amount of liquid water in a given amount of snow.)
However, unseasonably warm temperatures that began in late March, led to a rapid and early melt out of mountain snow. In fact, snowpack at the Willamette and Klamath Basin SNOTEL stations melted out completely approximately three weeks earlier than normal. This early melt out combined with late spring precipitation events to produce above-average streamflows (see Figures 6and 7). Even with the rapid melt out, the snow conditions did temporarily moderate water supply conditions to some extent in those basins.
All of this extra streamflow, however, did not help out farmers. By the beginning of April, agricultural decisions about water allocation for the rest of the growing seasons were already being made based on the extraordinary dry conditions at that time. This meant that many agricultural impacts could not be easily fixed at a later date…when and if the precipitation arrived. As it turned out, some rain did come.
Spring did indeed bring May and June showers and thunderstorms to the Pacific Northwest. Much of Oregon received above-normal precipitation for the 30-day period from May 15th to June 15th.
While quite welcome, the rains did not even come close to rectifying the substantial water year precipitation deficits across much of the state, although water supply conditions in eastern Oregon—including Malheur county and the Wallowa mountains—did improve substantially.
For much of Oregon, however, the late spring rains were too little too late, merely providing little more than window dressing for the water year precipitation totals.
Current Drought Conditions
The most recent weekly depiction of Oregon in the US Drought Monitor (Figure 8) shows some regions in D3 drought, or extreme drought, and much of the state otherwise experiencing D1 or D2 drought conditions, or moderate and extreme drought respectively. Drought has increased in severity over the summer owing mainly to trends in streamflow and soil moisture conditions and in relatively above-average evaporative demand in southwest Oregon.
While summer is the dry season in Oregon and little rain falls anywhere, drought can also be driven by removal of surface water by evapotranspiration due to warm temperatures and depletion of groundwater due to pumping. This year’s drought is also evident in the water stored in Oregon’s reservoirs.
Figure 9 shows the percent capacity (blue) and percent average storage (yellow) for reservoirs in nine regions of the state on August 10, 2020 compared with historical conditions provided by US Bureau of Reclamation. Where drought is least severe, water storage is in relatively good shape, for instance, in eastern Oregon (Owyhee, Malheur, and Burnt), northeast Oregon (Umatilla), and northwest Oregon (Tualatin). Where drought is most severe, water storage is at low levels, for instance, in central and southwest Oregon.
Reservoirs on the Rogue project, which includes the Galesville and Applegate reservoirs in southwest Oregon, are only at 32% of average, levels which are at or near historic lows.
What Current Reservoirs Can Tell Us About Next Year
Monitoring drought severity throughout the summer is critical in assessing the state of surface water supply deficits as Oregon enters the next wet season. As things stand now, if Oregon experiences another subpar wet season after this one, we expect next summer’s drought will be more severe than this summer’s due to lack of recharge of groundwater and reservoir storage.
The state’s reservoirs are projected to have little or no carry-over of their current water into the next water year. That means if drought persists into 2021, the low carry-over will enhance drought impacts on agriculture and municipal water supplies next spring and summer.
What’s in Store for the Coming Water Year and Beyond
It is becoming increasingly apparent that the western United States is experiencing a prolonged period of aridification, or drying out, during which drought is becoming increasingly common. The last 20 years in Oregon is consistent with this trend.
While this blog tracked the complex details of this year’s drought, the story is really pretty simple: years of persistent drought are taking their toll on groundwater supplies and ecosystem health. This trend toward multi-year droughts is consistent with climate change projections in the coming decades.
This year’s drought was defined by a wet season that was very inconsistent with ups and downs in temperatures and precipitation, and just as importantly, the timing of that precipitation was inconstant from past years due the storm blocking of yet another R3 parking itself off our coast.
Larry O’Neill is the State Climatologist for Oregon. An associate professor for the College of Earth, Ocean, and Atmospheric Sciences (CEOAS) at Oregon State University, Larry serves on the state of Oregon Water Supply Availability Committee, Drought Readiness Council, and the Oregon Drought Monitor Advisory Committee. He is also the director of the Oregon Climate Service (OCS) and is part of the Oregon Climate Change Research Institute (OCCRI). His research involves investigating the impacts of air-sea interactions on Earth’s climate and variability of precipitation and evaporation.
About Northwest Climate Currents:
his post is part of an ongoing series we are calling Northwest Climate Currents. Northwest Climate Currents uses the Climate Toolbox (formerly the Northwest Climate Toolbox) and other online climate tools to help us understand and prepare for our region’s climate events.