Over the headwaters of the Colorado River ΔTWS is typically positive during the winter and spring months (coinciding with maximum snowpack storage and minimal runoff) and negative during the summer and fall months (during times of high water demand and high evapotranspiration). Since 2002, the time period for water gains has shortened, and the total annual amount of water gained has decreased. This is inconsistent with observed data in the region2002 was the peak time of one of the worst droughts in recorded history in the area. Observations show increasing trends in accumulated precipitation, reservoir storage, and snowpack since 2002. This would suggest that trends seen in ΔTWS are the result of changes in deep groundwater storage or plant-water storage.
Pine beetles have killed much of the vegetation in this region since the beginning of the 21st century, so this study hypothesizes that the decreasing trend in ΔTWS is the result of the increasing fraction of vegetation unable to hold water. To test this theory, other regions around the Colorado River Basin are analyzed using GRACE estimates. Regions tested include areas that 1) are mainly affected by large reservoirs, 2) are mainly affected by croplands utilizing deep groundwater, 3) contain natural undisturbed land, unaffected by pine beetles or manmade changes, and 4) an entirely separate area in the western United States also affected by pine beetle kill. Results could show that a decreasing trend in ΔTWS is also evident in other pine beetle affected areas but not observable in regions dominated by other natural or manmade phenomena.
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