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San Diego State University (2016)

Hydrologic and geologic controls on groundwater recharge along the Mojave River floodplain aquifer

Whitney Anne Seymour

Titre : Hydrologic and geologic controls on groundwater recharge along the Mojave River floodplain aquifer

Auteur : Whitney Anne Seymour

Université de soutenance : San Diego State University

Grade : Master of Science (MS) 2016

Résumé
Groundwater is an extremely important water source for the state of California, particularly in arid regions such as the Mojave Desert. Over-pumping in the Mojave River Groundwater Basin has resulted in basin overdraft, and water is imported and artificially recharged to mitigate these effects. Though artificial recharge has helped maintain groundwaterlevels, stormflow runoff in the Mojave River is still important and is responsible for more than 80% of natural recharge to the Mojave River floodplain aquifer, the most productive aquifer in the basin. For this reason, it is important to understand processes governing recharge quantity and spatial distribution. The objectives of this study were to determine 1) how total stormflow volume, duration, and peak stormflow control the amount of groundwater recharge at a given distance downstream of where the Mojave River enters the floodplain aquifer, and 2) how geologically-mediated features, including depth to the water table and aquifer and channel morphology, control the amount and spatial distribution of recharge. Univariate and multivariate regressions were run using hydrologic and geologic variables to predict observed water-level rise for ten aquifer segments along the Mojave River, during 6 stormflow events. Results were compared to determine dominant controls on transmission loss and corresponding water-level rise for each segment. Of the two methods used to estimate water-level rise, inverse distance weighting (IDW) results were determined to give a more accurate representation of the system. IDW results indicate that magnitude is the dominant predictor of water-level rise for the entire floodplain aquifer. On a segment basis, recharge to the floodplain aquifer was controlled by different variables going downstream : pre-event water-level (segment 1) near the headwaters, duration (segment 4) and magnitude (segments 5-7) in the middle portion of the river, and peak flow (segments 8-10) towards the outflow of the river. Aquifer and channel variables were statistically significant for 5 out of 6 events. Additionally, a water balance was constructed for the three gaged reaches of the river. By combining reaches 1 & 2, the water budget was balanced with an error within 13% for all but one event. Transmission loss was the largest contributor to groundwater recharge along all three reaches, with artificial recharge and lateral inflow contributing only a very small portion to total observed recharge. Contrary to expectation, the largest water-level rise did not occur in the upstream reaches, where recharge was limited by a combination of channel width, aquifer width, and high pre-event water-levels.

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Page publiée le 4 octobre 2016, mise à jour le 7 août 2017