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Update of the Accounting Surface along the Lower Colorado River

Project Number: 9671D3B
Project Chief: Stephen Wiele & Stan Leake
Cooperating Agencies: Bureau of Reclamation
Period of Project: July 2007 – July 2008

PROBLEM

Wilson and Owen-Joyce (1994) and Owen Joyce and others (2000) documented a method to identify wells in which pumping would constitute a diversion of Colorado River water under the decree, Arizona v. California, 1964. Their method defined the extent of the “river aquifer” adjacent to the river and floodplain, and an “accounting surface” in the river aquifer for comparison of ground-water levels to determine whether water pumped by a well has a source of tributary water or other inflow, or does not have a source of inflow and would be replaced by water from the river. The accounting surface in the river aquifer adjacent to the river or the floodplain is defined as the ground-water level that would exist if the river was the only source (and sink) for water in the aquifer. For river reaches that are not within reservoirs, the accounting surface slopes down valley in approximately the same way that the surface of the river slopes under conditions of a “standard” release of water into the lower Colorado River. For reaches adjacent to reservoirs, the accounting surface is a flat surface set to a standard pool elevation.

Prior to issuing a proposed rule to use the accounting surface, an update of the surface is proposed for the following reasons:

1. The river profile on which the accounting surface was based used a surface-water model from the early 1990s with some cross-sectional data from the early 1980s. Changes in channel geometry (scour and fill) and changes in diversion patterns since then could have changed river-surface profiles.
2. The accounting surface in parts of the Parker and Palo Verde areas was based on elevations of drains along the edge of the floodplain, and not the elevation of the river. Furthermore, some of the elevations used in the Palo Verde Valley appear to be based on a non-standard vertical datum, adding an error to the elevation of the accounting surface in that area.
3. Improved ground-water modeling methodology is now available that will allow efficient construction of accounting surfaces tied to the river in reaches not adjacent to reservoirs. The model-computed accounting surfaces would be an improvement in the older, hand-drawn surfaces.

OBJECTIVES

The objective of this project is to update the accounting surfaces that were originally published in Wilson and Owen-Joyce (1994) and Owen Joyce and others (2000). Adjacent to the non-reservoir reaches including Mohave Valley, Parker-Palo Verde-Cibola Valleys, the Laguna Dam area, and the Yuma area, accounting surfaces will have a general down-valley slope, as determined by ground-water models. Adjacent to reservoirs, accounting surfaces will have a uniform (flat) surface that corresponds to a particular reservoir pool elevation.

APPROACH

Ground-water models will be constructed for the river aquifer and flood plain in the following reaches: (a) Mohave Valley, (b) Parker-Palo Verde-Cibola Valleys, (c) the Laguna Dam area, and (d) the Yuma area. Lateral boundaries will be defined by the outer edges of the river aquifer as defined by Wilson and Owen-Joyce (1994) and Owen Joyce and others (2000). The models will be steady-state, and will consist of a single model layer to represent a slab of aquifer material with constant transmissivity. A version of the USGS ground-water model program MODFLOW (Harbaugh and others, 2000) will be used. Horizontal grid spacing will be no larger than 0.5 miles, resulting in calculations of the accounting surface elevation on that or a smaller interval, throughout the river aquifer and floodplain. The river will be the only represented boundary condition for flow into or out of the ground-water system. This boundary will use the River Package of MODFLOW with a riverbed conductance that allows the river to be well-connected with the underlying aquifer.

Initial river surface-elevation data for the models upstream from Laguna Dam will be based on profiles used to construct the original accounting surfaces. The need for an updated reference discharge and water surface elevations in the Colorado River will be examined in cooperation with Reclamation.

Example of initial heads predicted by the groundwater model undergoing development.

References

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: U.S. Geological Survey Open-File Report 00-92, 121 p.

Owen-Joyce, S.J., Wilson, R.P., Carpenter, M.C., and Fink, J.B., 2000, Method to identify wells that yield water that will be replaced by water from the Colorado River downstream from Laguna Dam in Arizona and California: U.S. Geological Survey Water-Resources Investigations Report 00-4085, 31 p.

Wilson, R.P., and Owen-Joyce, S.J., 1994, Method to identify wells that yield water that will be replaced by Colorado River water in Arizona, California, Nevada, and Utah: U.S. Geological Survey Water-Resources Investigations Report 94-4005, 36 p. and 19 map sheets.