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Maximizing the Utility of Available Root Zone Soil Moisture Data for Drought Monitoring Purposes in the Upper Colorado River Basin and Western High Plains, and Assessing the Interregional Importance of Root Zone Soil Moisture on Warm Season Water Balance

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February 8, 2016
Peter Goble
Hosted by Russ Schumacher (advisor), Scott Denning, Jose Chavez (Civil and Environmental Engineering)

Abstract

Root Zone Soil Moisture (RZSM) data has both drought monitoring and seasonal forecasting applications. It is the lifeblood of vegetation, a large component of the hydrologic system, a determining factor in irrigation scheduling, and works to govern the means by which energy imbalances are settled between land and atmosphere. The National Integrated Drought Information System (NIDIS) has worked in conjunction with the Colorado Climate Center to improve regional drought early warning through enhanced monitoring and understanding of RZSM. The chief goals of this research have been threefold: 1. To develop operational products that can be used to improve the weekly drought monitoring process in the Upper Colorado River Basin and eastern Colorado. 2. To engineer a soil moisture core measurement protocol that is reliable within ten percent of the true volumetric water content value. This protocol, if successful on a local plot, will be expanded to alpha testers around the United States and used by the USDA for drought monitoring as well as NASA for ground validation of the Soil Moisture Active Passive (SMAP) Satellite. 3. To expose the seasonality and spatial variability of positive feedbacks that occur between RZSM and the atmosphere across the Upper Colorado River Basin and western High Plains using reanalysis data from the North American Land Data Assimilation System Phase-2 (NLDAS).

Results from statistical modeling of post-processed NLDAS data from the last 30 years point primarily to a time frame between May and July in which soil moisture anomalies become significantly correlated with seasonal temperature and precipitation anomalies. This time of year is partially characterized by a climatologic maximization of downwelling solar radiation and a northward recession of the polar jet, but also precedes the anticipated arrival of the North American Monsoon. Correlations appear to be due to a climatologically-expected transition between energy-limited and moisture-limited evaporation regimes that propagates from south to north and from low elevations to high elevations over the course of the first half of the summer. Coupling between RZSM and precipitation exudes a substantial amount of interregional variation, but if issued at the optimum time of year, seasonal precipitation forecasts can be improved by 4-14%. Furthermore, careful monitoring of RZSM before and during this time of year can improve drought early warning capability by flagging areas that are at risk of experiencing exacerbated drought through soil moisture feedbacks.