Regional Influences of Vegetation on Complex Droughts in North America

Problem Introduction — Extreme droughts diminish freshwater availability, with significant
consequences for the wellbeing of people and ecosystems. Surface vegetation is central to drought
because of its large influence on water and energy fluxes at the land-atmosphere boundary and
thus on soil moisture and streamflow. A key unknown is the extent to which vegetation and
droughts jointly shape each other: how vegetation specifically shapes regional drought
characteristics, and how vegetation-drought interactions evolve with increasing CO2 and climate
change. The mechanisms by which vegetation and drought are coupled and how that coupling
interacts with changes in climate and CO2 to alter future droughts must therefore be understood to
best inform risk management decisions in drought-prone regions.

Rationale for proposed work — Our proposed work is motivated by the need to advance
understanding of complex vegetation-drought coupling, which is critical for improving model
representations of processes affecting near- and long-term drought predictions.
Summary of Proposed Work — Our proposed research tests two hypotheses: (1) historical
terrestrial vegetation trends and variability have influenced the seasonal-scale characteristics of
historical extreme droughts in North America; and (2) Earth System Model (ESM) projections of
North American vegetation greening and land-surface drying are inconsistent with—and thus need
to be constrained by—the observed influence of vegetation on historical extreme droughts in North
America. The first hypothesis will be tested with observations and models for historical extreme
droughts, such as the recent California (2011-15), Texas (2010-11), and Great Plains-Midwest
(2011-2012) droughts. We will trace vegetation-drought feedbacks and how these complex
interactions shaped previous drought characteristics and influenced their predictability. We will
specifically quantify what role, if any, vegetation played in the onset, evolution, and termination
of historical severe seasonal-to-interannual droughts. The second hypothesis requires the use of
the latest generation of coupled ESMs from CMIP6, along with insights gleaned from our historical
analysis to characterize the influence of vegetation-drought interactions on future North American
drought risks.

Relevance to NOAA’s goal and to the competition — Our proposal targets “Competition 3,”
which seeks to improve our “understanding of how climate affects drought processes, […] the
relevant processes and feedbacks, and [link] this understanding [to] a more integrated
characterization of droughts and improved probabilistic predictions from seasons to decades.”
We will assess the role of vegetation in complex North American droughts using observations and
CMIP6 models. Our approach will identify the real-world role of vegetation in historical complex
droughts and the systematic model biases in droughts arising from vegetation to improve our
estimates of drought risks over the coming decades and enable drought stakeholders to mitigate
such risks. This goal aligns tightly with the efforts of the NOAA Drought Task Force (DTF), as
well as the NIDIS objectives of improving model predictions, and understanding drought
processes, causes, and associated complex interactions. We will also collaborate on this research
with Brad Udall of the Colorado Water Center and the USGS Southwest Climate Adaptation
Science Center, a key partner of NIDIS.

Award Announcement: https://news.dartmouth.edu/news/2020/08/justin-mankin-lead-national-drought-task-force

Climate Risk Area: Water Resources