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Hydroclimatic Index
for drought monitoring in
the Colorado River Basin
USA
Decision Center
for a Desert City
Arizona State
University
Tempe, Arizona
Institute for the Study
of Planet Earth
University of Arizona
Tucson, Arizona
National Oceanic and
Atmospheric Administration
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Instituting
Multi-Scale Hydroclimatic Indices in Drought Monitoring and Mitigation in Arizona (NOAA Project nos. NA07OAR4310455
& NA07OAR4310460)
Principal Investigators
Funding Program/Agency
Funding Partner
Project Objective
The goal of the project is to enhance the drought
monitoring tools available to decision-makers by transitioning a new,
effective, and easily understood drought index to stakeholders. The
operational tool will be handed off to the Arizona Drought Monitoring
Committee (ADMC), which
advises the Governor of Arizona on State drought status, and it will be made
available for import into the drought portal of the United States National
Integrated Drought Information System (NIDIS). The new drought
index addresses specific concerns of stakeholders by (1) contrasting
precipitation with climatic demand for water, (2) specifying the time periods
most appropriate for monitoring different forms of drought, and (3) providing
data with fine-scale spatial resolution.
Hydroclimatic Index
Soil moisture is the focus of many drought indices, but the
Hydroclimatic Index (HI) stops short of representing soil moisture in its
characterization of the hydroclimatic condition.
Only a few operational soil moisture sensors exist in the southwestern United States,
and the alternative of simulating soil moisture through time is difficult, as
climate conditions, land surface characteristics (soil type, vegetation type,
topography) and the amount of moisture relative to
the soil’s capacity combine to control soil moisture. The HI simply
represents the difference between precipitation (P) and potential evapotranspiration (PE) (P-PE) through time at a given
location. PE is the climatic demand for water, or that amount of evapotranspiration that would occur from a grass-covered
soil for which soil moisture is maintained at capacity. Negative P-PE values
indicate the amount by which the climatic demand for water can not be met by
precipitation and actual soil moisture would decline if not irrigated.
Positive values represent the amount of excess water from precipitation that
would recharge soil moisture, percolate to ground water, or run to streams
and reservoirs through overland flow or interflow.
Aggregates of P-PE are constructed for periods that
represent short-term conditions (1-, 3-month), intermediate conditions (6-,
12-month), and long-term conditions (24-, 36-, 48-month). Aggregate values
are then converted to percentiles to form the HI. The HI can then be
stratified into drought categories that represent levels of drought
intensity.
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Proposal &
Reports
Presentations
Ellis AW (2007) Instituting the Hydroclimatic Index in Monitoring
Drought across the Colorado
River Basin. United States Geological Survey 9th Biennial
Conference of Research on the Colorado Plateau, October, Flagstaff, Arizona
(View)
Ellis AW (2008) Defining Drought Occurrence within the Colorado River Basin. Annual Meeting of the
Association of American Geographers, April, Boston, Massachusetts
(View)
Publications
Ellis AW, Goodrich GB, Garfin GM (2008) Temporal Patterns in the
Spatial Extent of Drought across the Colorado River Basin
during the Past Century. Submitted to the International
Journal of Climatology, July 2008. (View)
Research Products
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