CHEMICAL AND ISOTOPIC TRACER EVALUATION OF WATER MIXING AND EVAPORATION IN A DAMMED TEXAS RIVER DURING DROUGHT
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Copyright 2016 John Wiley & Sons, Ltd. The interaction between drought and river regulation is monitored to better understand river flow mixing, evaporation and surface-groundwater exchange in changing regional climates and in increasingly regulated waterways. This study compared Brazos River stable isotope (18O and D) and electrical conductivity values with reservoir, creek and aquifer samples in the Brazos watershed, the largest watershed in Texas. The combination of tributaries, rainfall and the Brazos River Alluvium Aquifer, on the one hand, and the Lake Whitney reservoir, on the other hand, represent endmembers of dilute run-off water and evaporated saline water, respectively. A simple isotope mixing model that uses monthly river discharge, Lake Whitney discharge, historical monthly precipitation 18O and pan evaporation accurately reconstructs river 18O (0.5 on average). Data and isotope balance modelling support continued evaporation of 18O-enriched Lake Whitney water as it flows downstream, although the most evaporation took place in Lake Whitney. The difference between river and precipitation 18O, or 18ORIV-PPT, here a measurement of degree of evaporation, ranged from 0.1 for a small creek, to 1.7 for the Brazos River, to at least 2.7 in Lake Whitney. This study indicates that drought in regulated rivers may enhance reservoir discharge dominance in river flows during peak drought conditions when combined run-off and baseflow dominance would be expected in a similar undammed river. Copyright 2016 John Wiley & Sons, Ltd.
