Significance of Groundwater Divide in Water Accounting Return Flow Calculation for Lower

A formal rule is being developed that requires Reclamation to identify the location and quantify the amount of water pumped from Colorado River aquifers. Implementing the rule will support the Contract administration and Water Accounting requirements for use of Colorado River water. To more accurately account for this water, the location of possible groundwater divides must be ascertained when a portion of the lands are drained by drainage ditches. The goal of this effort is to develop a framework to help Reclamation implement and support water account requirements. The main objectives are to (1) delineate the location of possible groundwater divides, and determine how the location of the divide might evolve over time ;(2) estimate the amount of groundwater return flow to the river; and , (3) conduct uncertainty and sensitivity analyses of groundwater divide locations and how they could influence the overall water budget in general. The results from Objective (3) will help determine which variables and parameters are the most influential to the uncertainties of water budget estimates, so that future field characterizations are better targeted, reducing uncertainties in water accounting. DRI will estimate how groundwater divides evolve in response to variations in river stage, water diversions from the river, and crop rotation. The potential modeling tools can range from simple analytical solutions (e.g., the Dupuit solutions) to generic numerical groundwater flow models. The modeling approach will be selected based on available data and on the developed conceptual models and governing equations. As long as information about river stage and application of irrigation water is available, the model will forecast alterations in the groundwater divide location, and hence the amount of return flows.