C. L. Schultz, A. Seck, S. N. Ahmed;
Is Hot Drought a Risk in the US Mid-Atlantic? A Potomac Basin Case Study ; 18
June 2025 https://doi.org/10.1111/1752-1688.70031
The Potomac River is a major source of water for our region
and the only source of water for Washington, D.C. and Arlington, VA. Over 6
million people in Virginia, Maryland, Pennsylvania, West Virginia, and
Washington DC and the diverse ecosystems of the interstate area depend on the water
resources of the Potomac River basin. Responsible management of this resource is
necessary to ensure all our futures.
The Interstate Commission on the Potomac River Basin (ICPRB)
was created to protect and enhance the waters and related resources of the
Potomac River basin through science, regional cooperation, and education. The
ICPRB has no regulatory authority, but has teamed up with state, federal, and
local agencies, and private citizens to work towards a plan that will serve as
a roadmap for the sustainable use of this interstate resource now and into the
future.
A recently published article by Dr. Cherie Schultz and
colleagues in ICPRB’s Section for Cooperative Water Supply Operations on the
Potomac cited above, explores the future risks and impacts of hot drought
through the lens of the Potomac River basin. In the study which I have excerpted
from freely below, the researchers explain why estimating future water
requirements in the region is a challenge, but in the face of increased
variability in precipitation and temperatures it is essential for the
sustainability of our region.
As we prepare for the impacts of climate change (that we can
no longer stop), we must plan for adequate supplies of fresh water. Water is
essential for human life and well-being—necessary for households for drinking,
cooking, and sanitation, for industries, energy producers, and agricultural
producers—and for aquatic life that depends on freshwater ecosystems. Rivers
and streams are a primary source of water for human use especially in urban
areas like the DMV.
We size reservoirs and other infrastructure to meet the demands
of anticipated severe droughts and storms. Extreme events are key drivers in the
development of water management strategies. It can be expensive or catastrophic
to guess wrong. Thus, it is essential to
estimate changes and variability in stream and river flow to adequately assess future infrastructure needs.
Water supply planning, which relies on climate projections
at the regional scale, involves considerable challenges. First and foremost is
the lack of confidence in general climate model (GCM) projections of regional
precipitation, the primary driver of streamflow projections. But for a given
region, different GCMs can give very different precipitation projections,
resulting in wide ranges of estimates for future streamflow, not uncommonly
including disagreements on whether to expect an increase or decrease in stream
flow. (Green
Risks: Climate Models Biases and Limits). New infrastructure to mitigate
the impacts of climate change on water supply systems can be very costly and
hydrologic modeling studies based on a limited number of climate projections
may not be enough to compel decision makers to move forward and commit tax
dollars.
Other researchers (Zhang et al. (2023))
have argued that measured/observation-based statistically derived climate
response functions provides advantages over earth systems models in the
simulation of future stream flows. A major uncertainty in these simulations is
the response of streamflow to changes in temperature. Rising temperatures will
affect streamflow by increasing evaporative losses from soils and water
surfaces and by changing the timing and magnitude of snowmelt. Hydrological
droughts may become more severe due to increasing temperatures coupled with
natural variability in precipitation and result in extreme events which have
been characterized as “hot drought.” (Udall and Overpeck 2017;
Woodson et al. 2021).
In this study, the researchers at the ICPRB created an
approach for estimating future trends in annual streamflow for the Potomac
River. To project flows in extreme drought years, they projected changes in the
distribution function of annual streamflow under a future climate and using
machine learning to perform statical modeling to estimate the missing data to create a time series, of sufficient length
to compute extreme percentile values (100 year
droughts and 100 year floods), are created by pooling shorter time
series from multiple GCMs.
Flow in the freshwater portion of the Potomac River is
measured at the US Geological Survey (USGS) stream gage at Little Falls Dam
near Washington, DC located below the intakes of the metropolitan area
water suppliers and a above the head of tide in the Potomac estuary. With few
major impoundments in the 11,560 square mile drainage area above Little
Falls Dam, river flow is largely unregulated and highly variable. Some degree
of storage is provided by the underlying fractured bedrock groundwater aquifers, but
baseflow recession rates are typically on the order of months, so this storage
can be rapidly depleted during periods of low precipitation (Schultz
et al. 2014).
Precipitation above Little Falls averages 40.3 inches annually, with
evapotranspiration averaging 65%. Precipitation is fairly uniform throughout
the year, but river flow exhibits a pronounced seasonal variation due to higher
evapotranspiration rates during the March–September
growing season, which reduce both groundwater recharge and runoff (Trainer and
Watkins 1975).
Flow tends to be highest in the month of March, with a long-term mean of 1,421,768
cubic feet per minute, and lowest in September, with a long-term mean of 233,076
cubic feet per minute. The snowpack that may accumulate at higher elevations
during the winter months slightly increases median river flows in March and
April but does not persist long enough to have a significant impact on
summertime flows (Cummins et al. 2010).
Study results for the Potomac are consistent with past
findings that precipitation will increase in the Mid-Atlantic region as
temperatures rise (Shenk et al. 2021),
but also indicate that decreases in Potomac River flows and resulting
reductions in water supply availability may be experienced in some years.
Median temperature is projected to increase by 2.3° to 3.2°C, from the baseline
period, 1950–1979, to the planning period, 2040–2069, and by 2.0° to 4.8°C in
2080–2099, and median precipitation is projected to increase by 9%–12% and
11%–16% over the same time periods.
Applying statistical modeling, the data indicate that,
“future Potomac River flows will be impacted by ‘hot drought’, that is,
increasing drought severity caused by rising temperatures coupled with natural
variability in precipitation.” Even though precipitation amounts are expected
to increase by up to 16% by 2099, annual river flows may decrease by as much as
49% by the same year due to extreme heat. The increased rain does not mitigate
the conditions in the extremely dry and hot months. The scientists
modeling projected river flow changes of −3% to −26% by 2040–2069 and −2% to
−49% by 2070–2099 which is a wide range. These results can inform planning, but
need a higher degree of certainty before they can be depended on for investment decisions for the
Washington, DC, metropolitan area's cooperative regional water supply system.
In our region, assessments of water supply system reliability are largely
driven by the impacts of conditions comparable to a 100-year drought.
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