7021

• UCREFRP Catalog Number: 7021
• Author: Oamek, G., et al.
• Title: Methodology For Analyzing Alternative Reservoir Shortage and Operating Criteria.
• USFW Year: n.d.
• USFW - Doc Type: \
• Copyright Material: YES

25,000 acre-feet in December, 17,000 acre-feet in January, and 5,500 acre-feet and 2,000
acre-feet in February and March, respectively.
The Tualatin Project has never experienced a significant water shortage since it was
developed. Intuitively, this suggests the marginal benefit of extra capacity in the
reservoir is near zero. As part of the ex post analysis, smaller reservoir capacities are
considered to observe how project benefits change with reservoir size. Capacities
considered range from 50 percent of the current size, about 29,500 acre-feet, and
incrementally increase up to its current capacity, 59,170 acre-feet. Operating criteria are
not defined for this case study; the shortage trigger and shortage criteria are set at zero
for all users.
Results of the model runs are summarized in Table 3. The first column details the
reservoir size considered and the last column shows cumulative changes in economic
benefits over the range. The middle columns summarize the marginal benefits of the
alternative reservoir sizes for the individual components.
Marginal benefits for irrigation go to zero at capacities greater than 40,680 acre-feet,
implying that capacity between this level and the current size has little economic value
to irrigators. Marginal benefits of additional capacity go to zero at sizes larger than
33,283 acre-feet for M&I uses. For water quality purposes, the point of zero marginal
benefits is reached at sizes larger than 48,076 acre-feet. It should be reiterated,
however, that despite carrying the name of the Tualatin Project, this case study is simply
for illustration and does not reflect actual conditions in the study area.
Figure 3 plots the marginal benefits of alternative reservoir sizes. Recall these are
marginal, or change in benefits, rather than total benefits. Marginal benefit curves are
typically downward sloping, implying diminishing marginal benefits as reservoir capacity
increases. Note that the curve meets the X-axis at a capacity slightly larger than 48,076
acre-feet. This indicates that regardless of reservoir construction and maintenance costs,
additional capacity above this level has little economic value. If marginal costs of
reservoir construction and maintenance were known, they could be plotted on in the
same graph as an upward sloping curve. The intersection of the marginal benefit curve
and the marginal cost curve would indicate the economically optimal reservoir capacity.
CONCLUDING REMARKS
The above case studies, and additional case studies involving the Dolores Project in
Colorado and the Cheney Project in Kansas, have demonstrated the adaptability of the
modeling system to a variety of hydrologic conditions and economic objectives. In
addition to examining alternative reservoir operating criteria and reservoir sizes, the
models can assess alternative hydrologic records, demand data, conveyance capacities,
water costs, and a host of other parameters affecting project management. Subsequent
case studies will be less hypothetical in nature, beginning with an operations study of
the Sevier River system. This case study will focus on reservoir operations, particularly
how alternative operating plans will affect marginal benefits of irrigated agriculture in
the Sevier basin.