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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002098

Hydrosphere's Colorado River Model

December 29, 1993
Page 5

MODEL STRUCTURE

Simulation of the Colorado River system is achieved using a network flow programming
model. The model, like MODSIM (Shafer, 1979; Labadie et al., 1983) and its predecessor
SIMYLD (Texas Water Development Board, 1972), uses the out-of-kilter algorithm (Clasen,
1968; Barr et al., 1974) to perform, at each time-step, a static optimization that simulates water
allocation for a given system of priorities in a river basin network. The model is similar iri
structure to the model used by Brown et al (1988, 1990) in a study of the disposition of
streamflow increases from the Arapaho National Forest. The model performs three basic
tasks: (I) management of input and output data and run-control information, (2) conversion of
a problem stated in hydrological terms to one amenable to solution by the out-of-kilter
algorithm, and (3) solution of the network.

The network is a system of arcs and nodes. Each arc has an upper and lower bound
constraining the amount of water it can accommodate, and a "cost" associated with moving one
unit of flow along it. An arc may represent a river segment, reservoir storage, a hydropower
turbine, a consumptive use demand, or a system outflow (e.g" to the Gulf of California).
Costs are used to represent priorities, Mass balance is enforced by the algorithm by
constraining the sum of all inflows to each node to the sum of all outflows. There are no
feasible solutions of the network that violate mass balance,

At each time step, the algorithm finds the allocation of flows that minimizes the total
system cost or maximizes total system benefit, i. e. highest priority uses are served first. The
network diagram of Hydrosphere's Colorado River Model showing the construction of the arcs
and nodes is shown in Figure 2,

The optimization capability of Hydrosphere's Colorado River Model is used for.the
efficient simulation of the operation of the Colorado River system. Where system operating
policies can be represented by arc constraints and costs, they can be easily adjusted by
changing the parameters corresponding to them, hence the model is very flexible for examining
different institutional settings. In order to simulate the current institutional rules for water
allocation, the costs of the arcs are set to represent the relative priorities that currently exist
among diversions. Once a solution is found for a given time step, the reservoir contents
resulting from that solution are used as the beginning contents for the next month's simulation.

Many of the operations required by the Law of the River cannot be represented by arc
costs and constraints alone. For example, Lake Mead flood control operations, surplus
strategy operations, Powell-Mead equalization releases and other operations are in a class of
operations that are functions of system state variables, Thus a considerable amount of
computer code is devoted to implementing the Law of the River. This code is executed each
time step, adjusting arc constraints to reflect specific system operations.

Water movement and disposition in the Colorado River Basin is modeled by aggregating
inflows and outflows over time and space at the level of detail used in CRSM (Schuster, 1987).
Thus, a monthly time step was used; 14 basin reservoirs were included; inflows and flow
gains/losses were modeled at 29 points; 265 individual consumptive use points were
recognized; and more than 107 river reaches were modeled. This level of resolution gives the

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