WSPC04476

• File Number: 8283.200
• Description: Colorado River Basin-Colorado River Computer Models-Colorado River Decision Support System-RAY
• State: CO
• Water Division: 5
• Date: 12/29/1993
• Title: Hydrosphere-Colorado River Model-Technical Overview
• Water Supply Pro - Doc Type: Report/Study

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002103

Hydrosphere's Colorado River Model

December 29, 1993
Page 10

DEPLETIONS

Water use estimates used in Hydrosphere's Colorado River Model are stated in terms of
"depletion", the amount of water delivered for use less the amount of water that returns to the
river after use. Depletions at 265 points in the Colorado River basin are simulated in
Hydrosphere's Colorado River Model. Appendix A contains a listing of the depletion points.

Historic depletion estimates from 1982 to 1990, as well as future projected depletion
estimates are available for use in the model. These estimates have, for the most part, been
derived from data developed by the USBR for its 1991 Annual Operating Plan (AOP), dated
July 22, 1991 (USBR, 1991).

The model allows limited modification to the depletions via changes in the annual
amounts, return flow percentages, monthly distributions and relative priorities. The current
. version of the model does not allow the user to add or remove depletions from the network.
Future versions of the model will certainly have more demand resolution and may incorporate
more flexibility for network modification; these upgrades will be highly dependent on the types
of analyses sought by our customers.

The following subsections provide a general discussion of how the depletions are
handled in the model and more specific discussions of how CAP and MWD depletions are
represented.

Diversion. Depletion and Return Flow

The USBR's CRSM can simulate lagged return flows accruing to specified points below
the point of diversion. Return flow percentages, lag times (up to ten months from the' time of
diversion) and points of return are specified in the CRSM data files. In the 1991 AOP data set
all return flows are specified to return in the month of their diversion (Le. there are no lagged
returns). Return flows for about half of the approximately 120 diversions accrue above the
immediate downstream diversion point. Most of the rest of the return flows accrue before the
second downstream diversion point. In three cases return flows accrue to the next sequential
reach (as opposed to the next diversion point within the same reach) and there are intervening
inflows. In one case, an export from the Dolores River to the San Juan River, return flows
accrue to a completely different basin.

This representation of the system was simplified in developing Hydrosphere's Colorado
River Model. With one exception, the model simulates depletions at diversion points. The
exception is the Dolores/San Juan export that is simulated as a diversion and return flow.
Simulation of depletions represents an implicit assumption that return flows accrue above the
next diversion point with no lag. This assumption is correct for about half of the diversion
points in the CRSM data set. For the diversion points where this assumption is not correct the
impact will be to overstate water availability (understate environmental impacts) between the
diversion point and the point at which its return flows actually accrue. Because the overall
water budget is not affected, the principal result is a reduction in the precision with which the
spatial distribution of shortages are represented. Most of the diversions with remote return
flows are small, so that even this effect should be minimal.

Hydrosphere Resource COD.8ultants 1002 Walnut Suite 200 Boulder, Colorado 80302