The US Geological Survey (USGS) Texas Cooperative Fish and Wildlife Research Unit, in collaboration with Texas Tech University and the USGS Texas Water Science Center, recently completed a study of trends in water quality and quantity in major reservoirs of the Brazos and Colorado River basins located at the southern edge of the Great Plains. The study examined water quality, major contributing-stream inflow, storage, local precipitation (rainfall), and basin-wide total water withdrawals between 1965-2010 (before the recent drought and flood events). The objective of the study was to determine historical patterns in reservoir water attributes for application in management and planning for future needs. The study revealed declining trends in stream inflow and storage in most of the reservoirs examined.

The summer of 2011 was exceptionally hot and dry across much of the southern Great Plains. Extreme heat and drought conditions were commonplace in the region and for Texas, 2011 came to represent the worst single-year drought on record. The Texas Comptroller of Public Accounts described the 2011 drought as possibly "? one of the most devastating economic events in [the state's] history." Estimates of economic losses in agriculture and associated activities for the year ran as high as $8.7 billion. By the fall of 2011 water storage in Texas reservoirs had collectively declined to less than 60 percent of full capacity, and conditions were far worse in arid and semiarid west Texas.

Conditions improved somewhat in 2014, but in 2015 rain fell hard. In the spring and early summer of 2015, torrential downpours were routine. Extreme precipitation and flooding were common in the region and for Texas, May 2015 represented the wettest May since recordkeeping began. This flooding resulted in human fatalities and economic losses estimated at $3 billion in Texas and Oklahoma.

What should people worry about ? too little water? Too much? Both? While answers to these questions are important to mitigate and adapt to the possibility of changing conditions, they cannot be found by looking only at what has happened in the last few years. One exceptionally dry year followed shortly by an exceptionally wet year does not say much about the future. Certain surface water attributes, such as reservoir storage (water volume), can also fluctuate within a single year due to seasonal changes in water availability and management operations. Reliable projections of future water conditions require an understanding of past trends over the long term.

Constructed reservoirs provide a surface water supply for human consumption, irrigation, hydroelectric power, flood control, and recreational activities, including sport fisheries. They can also provide an important buffer to mitigate the effects of drought.

The largest annual decreases in storage (adjusted for median values) were observed in the upper reach of the Colorado River in west Texas, the driest portion of the study area. Median and extreme levels of local precipitation, however, did not change appreciably during the study period suggesting that changes in precipitation were not responsible for the declining inflow and storage. The study did not look at all possible reasons but two variables analyzed emerged as potential contributors to the declining surface water levels: temperature and water withdrawals.

The study recorded an increase in at least one of three water temperature indicators (overall temperature, hottest month temperature, or coldest month temperature) in the majority of reservoirs examined. Increasing temperature could partly explain the decreasing inflow and storage because as water temperature increases, so does the rate of evaporation and water loss. A study conducted by the Texas State Climatologist reported that air temperatures over Texas increased during the 20^th century and especially since the 1960s. A study led by Texas Tech University scientists in collaboration with the Texas Cooperative Fish and Wildlife Research Unit found that water temperature in Texas reservoirs is strongly and positively associated with air temperature, even when the influence of season is removed.

Increased water withdrawals could also partly explain declining levels of reservoir inflow and storage, as total water withdrawals (ground and surface combined) generally increased in both basins during the study period, and especially in the Colorado River basin. Water chemistry (quality) remained relatively constant over the long term except for phosphorus levels (eutrophication), which generally increased during the study period.

The Texas Water Development Board estimates that the human population and corresponding water demand in Texas may increase by 82 percent and 21 percent, respectively, between 2010 and 2060. This scenario would likely result in increased competition for water resources among different users, terrestrial and aquatic wildlife included. This study's finding that declining inflow and storage in some of the reservoirs occurred in the absence of detectable changes in precipitation creates an additional concern. If current climate-change projections of future decreases in precipitation over the southern Great Plains are correct, this would represent a new stressor to the hydrology of the region that could potentially increase the geographic scope or magnitude of negative trends in inflow and storage. On the other hand, even if future rainfall patterns remain steady in the region, as they did during the study period, a continuing positive trend in air/water temperatures would continue to pose a risk to water supplies.

Information derived from studies conducted by Cooperative Research Unit scientists and their cooperators may help reservoir and natural resource managers identify research needed to better understand past trends in surface water attributes and to assist in projections of future conditions and in mitigation efforts.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Reynaldo Patino, reynaldo.patino@ttu.edu, Unit Leader, USGS Texas Cooperative Fish and Wildlife Research Unit at Texas Tech University.