Moisture Balance Drought Index (MBDI) Scenario Building for the Colorado River Basin (CRB)

Project ID: 5546
Principal Investigator: Mitchell Haws
Research Topic: Water Supply Forecasting
Funded Fiscal Years: 2010
Keywords: None

Research Question

This is a continuation of an existing Science and Technology (S&T) project (Project #8836, Hydroclimatic Index in Drought Forecasting and Climate Scenario Building for the Colorado River Basin, 2008). This will be the final year of activities under this research. The original research is as follows:

Drought causes an annual estimated impact of six to eight billion dollars, including rapid depletion of surface water supplies, record fire activity, massive drought-related forest mortality, and drastic reduction of cattle stock. Others include water use restrictions, need for a drought plan, and shortage sharing for the Colorado River basin (CRB) states. The Department of the Interior (DOI) identified all of the hot spots in the Water 2025 study. Drought is a key factor between water sufficiency and crisis. The primary goal of the proposed project is to produce a method for short-term (1-48 month) forecasts of drought across the CRB for operational use, employing a measure of drought that represents a significant improvement over previously used tools. The secondary goal is to employ Intergovernmental Panel on Climate Change (IPCC) expectations of temperature and precipitation changes in the CRB to produce long-term (50+years) assessments of the likelihood of CRB droughts that are deeper, and/or more pervasive, and/or longer.

The original research is nearly complete, but there are some activities that will make this more accessible to the general public:

* Stakeholder feedback to introduce product to the users

* Develop and enhanced interactive Web site for users. This will allow users to examine their specific service area and determine drought potential based on their input.

* Launch the Web site and trouble shoot any problems.

* Final Enhance Report which will include all additional data garnered in the 3 activities above.

Need and Benefit

While some recent studies examined hydroclimatic variability of the entire country or just the Western United States, a number of scientists focus on climate variability at regional and sub-regional scales. Climate controls over regional areas such as individual river basins in the Western United States are complex and often have mixed relationships to atmospheric and oceanic teleconnections. Defining drought by crude sub-regions introduces the potential for problems, but greater difficulty stems from the fact that efforts to portray drought are hampered by reliance on indices (e.g., Palmer Drought Severity Index [PDSI], Standardized Precipitation Index [SPI]) that are confusing and contain regional biases and limited relationship to the multiple dimensions of drought. SPI, a solution preferred by most climatologists due to a long list of criticisms of the PDSI, ignores half of the hydrologic equation the temperature-driven climatic demand for water. This is a critical problem in the Southwestern United States, where evaporative loss dominates the hydrologic budget in summer; thus, despite comparable seasonal totals, summer precipitation is rendered much less effective than winter precipitation.

The most recent scientific assessment of the IPCC reports that temperatures in the CRB, particularly the winter season values, are expected to rise more quickly than the globally averaged temperatures due to elevated greenhouse gas concentrations. Furthermore, IPCC suggests that winter season precipitation will experience a decline over the next 50 years of 5 to 20 percent. Detailed regional numerical simulations predict that by 2050, temperatures in the CRB will increase by 1.7 degrees Celsuis relative to 1950-1999 and precipitation will decline by 6 percent.

For years, scientists linked moisture conditions in the CRB with El NiƱo, Southern Oscillation, but a significant literature is growing linking moisture variations in the Southwestern United States to a variety of teleconnections, such as the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. Recent research used annual drought frequency to establish that vast areas of the Western United States are associated with multidecadal oscillations in the Pacific and Atlantic Oceans.

Hydroclimatic Index (HI): Developed by the principal investigator in Fall 2005. HI is based on the difference between hydroclimatic input (precipitation) and loss (evapo-transporation [ET]) on a monthly resolution using a widely accepted method of calculation. The HI is straightforward and easy to interpret so that it is transparent to nonexperts. Like the SPI, the HI can be calculated for an array of time scales to portray different forms of drought.

The HI is based on the premise that the relative hydroclimatic characteristics of a location are best expressed by the amount of moisture in the soil (usually simulated due to the poor distribution of soil moisture sensors and the limited record of existing sensors). Simulating soil moisture is fraught with problems, as the choice of functions to describe the relationship between calculated actual evapo-transpiration, the potential for evapo-transpiration, and soil field capacity is subjective; moreover, relationships that may verify well in one region of specific climate, soil, and vegetation type may not be appropriate for use in other regions. This critique has been applied to the PDSI, which simulates soil moisture.

The HI stops short of representing soil moisture in its characterization of the hydroclimatic condition; rather, it 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 ET that would occur if the soil were saturated. Negative values indicate the amount by which the climatic demand for water can't be met by precipitation.

Contributing Partners

Contact the Principal Investigator for information about partners.

Research Products

Not Reviewed

The following documents were not reviewed. Statements made in these documents are those of the authors. The findings have not been verified.

Hydroclimatic Index-MBDI Final Report Haws-Ellis.pdf (final, PDF, 818KB)
By Andrew Ellis and Mitch Haws
Publication completed on January 20, 2011

Drought monitoring/forecasting tools available in Arizona and the Southwest with a new, easily understood drought index now called "Moisture Balance Drought Index"(MBDI). The index contrasts precipitation with climatic demand for water across any geographic domain and to be used to specify the time periods most appropriate for monitoring different forms of drought to fill a knowledge gap identified by stakeholders. The index generates fine-scale resolution for the Colorado River basin.

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Last Updated: 6/22/20