QUANTIFYING WATER EXCHANGE BETWEEN THE BRAZOS RIVER AND THE BRAZOS RIVER ALLUVIAL AQUIFER USING HIGH TEMPORAL RESOLUTION MEASUREMENTS
To better understand the connection between these two water resources we measured groundwater discharge (Qgw) to a 24.3 km stretch of the Brazos River using high temporal frequency differential gaging (measurements taken every 20 minutes) from May 2015 - Feb. 2016. On 11 different occasions we also used two different mass balance methods to estimate Qgw based on specific conductance and endmember-mixing analyses (EMMA). Estimated Qgw obtained using the chemical methods consistently aligned with differential gaging estimates. We found that the Brazos River recharges the BRAA during rain events, and that Qgw is at its maximum (up to 50 m3/s) immediately following rain events. The chemical tracers identified this water as low in dissolved solutes and, therefore, short-term bank storage. Qgw decreased steadily with time since a rain event. The unconfined groundwater table remained high throughout both wet and dry conditions, whereas Qgw decreased to zero after 6-8 weeks of dry conditions. This implies that the river may be more connected to rechargeable bank storage reservoirs rather than the broader aquifer. Further work is needed to characterize the large scale connectivity between the Brazos River and the broader (10 km wide) BRAA.
This study is an important step in understanding temporal fluctuations in amount and direction of water exchange between a lowland river and its alluvial aquifer. Understanding such processes is invaluable in aiding managers of ground and surface water bodies to coordinate to efficiently appropriate limited water resources.