Actual evapotranspiration, flash droughts, water deficits, reduced vegetative growth, and wildfires: the effects of seasonally water-limited conditions in a changing climate

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The Southeastern U.S. experiences recurring hydrologic droughts, which can reduce water availability for human consumption and ecosystem services, leading to plant stress and reduced plant growth. This project examines relationships between drought and the water cycle in the Southeast with data from the Panola Mountain Research Watershed (PMRW) near Atlanta, Georgia and other Southeastern sites with varying land-use, vegetation types, catchment water storage, and climate. Results leveraged from these data are used to better understand the important factors affecting the water cycle, the occurrence of water deficits, and the onset of droughts.

Statement of Problem:

The interaction of actual evapotranspiration (AET) with soil moisture and groundwater storage during times of precipitation-deficits, where precipitation is less than potential evapotranspiration (PET), is important in understanding and quantifying water deficits (PET – AET). Based on water storage availability, these deficits can affect tree rooting zones to avoid stress and mortality. However, if water deficit frequency increases rapidly, trees may be unprepared and have insufficient time to adapt to the new climate conditions. Soil moisture status during water deficit conditions preclude deeper recharge, reducing stream baseflow that supports water availability for human consumption and ecosystem services. Rapid-onset, flash droughts are associated with severe depletion of soil moisture and water deficits and can increase the risk of wildfires. In November 2016, these precipitation deficit type flash droughts, common in the Southeastern U.S., caused wildfires in the Great Smoky Mountains and adjacent areas of Georgia, Tennessee, and North Carolina. The result of these rare but extensive fires was 14 deaths, 10,000 acres of burned land and towns, and substantial agricultural losses. Given the frequency of flash droughts in recent years and their impacts on agriculture and wildfire risk, we need to better predict the effects of future climatic change on water deficits and drought occurrence.

 

Why this Research is Important: 

Long-term research on the water cycle at PMRW, a small, seasonally water-limited, forested watershed, has provided insight in AET, water storage, water deficit, and flash drought interactions. Part of the USGS Water Energy and Biogeochemical Budgets (WEBB), PMRW serves as an ideal site for further research on these interactions as hydrologic and biogeochemical properties and processes have been extensively studied over its 33-year history. The storage component of the water budget is well constrained as a stream baseflow versus watershed storage relation has been developed and there are five continuous soil moisture profiles to assess shallow storage in various landscape positions. Although an eddy covariance system has been installed to directly measure AET and several years of data have been collected, these have not yet been fully analyzed. Our study helps improve understanding of how land use, vegetation type, watershed storage characteristics, and climate affect the severity of water deficits and the occurrence of droughts.

 

Average monthly water budgets for water years 1986 – 2015, Panola Mountain Research Watershed

Average monthly water budgets for water years 1986 – 2015, Panola Mountain Research Watershed, Stockbridge, Georgia

(Credit: Aulenbach. Public domain.)

Objective(s):

1. Assess the impacts of differences on the water budget, including whether estimates of watershed storage are accurate.

2. Determine how future climatic scenarios can affect the severity of water limiting conditions and the frequency of flash droughts.

3. Extend the knowledge gained at PMRW to other areas that experience water-limiting conditions and to contrasting land uses.

4. Combine the land characteristics versus climate relationships with land use and climate coverages to make spatial assessments of the potential for water deficits and droughts.

 

Methods:

Objective 1: Compare estimates of AET from published estimates from 30 years of monthly water budgets at PMRW with estimates using the eddy covariance system.

Objective 2: Using eddy covariance AET estimates combined with data from five soil moisture profiles at PMRW, quantify the watershed relationships between soil moisture and water deficits and the specific dynamics of flash drought development in the fall of 2016. Determine minimum time-frames during different times of the growing season (that have different PET) to develop water limiting and flash drought conditions from various soil moisture status starting points.

Objective 3: Using sites that have eddy covariance systems, quantify differences in water deficits depending on vegetation type, rooting zone depth, and soil characteristics while considering differences in climate. Use these results to infer the resilience of various vegetation types to climatic change scenarios by interchanging spatial differences in climate for changes in climate over time.

Objective 4: Use eddy covariance data to adjust spatial PET products to more accurately estimate AET. Use climatic change scenarios to predict where there are likely to be increases in severity in water deficits, droughts, and wildfires