WSP01533

• File Number: 8282.600.20
• Description: Colorado River Interim Surplus
• Basin: Colorado Mainstem
• Date: 12/23/1993
• Author: USDOI/BOR
• Title: Draft Reclamation Proposed Surplus Guidelines
• Water Supply Pro - Doc Type: Publication

after year 2006 as the critical period extends out to 20 years and the average use over the
20 year critical period increases.

Once the surplus threshold storage requirement increased up to the Flood Control Surplus
capacity of Lake Mead, it was held constant from then on. Also the shortage threshold was
then assumed to be set at elevation 1120 feet to protect elevation 1050 feet.

Testing this strategy the model runs generally found that with the current level of depletions,
and storage over the near term, the Colorado River system is in a fill and spill mode. With
the current level of depletions and the critical period of record extending for 12 years, the
system is capable of allowing surplus while providing the capability of maintaining scheduled
deliveries of 7.5 maf to the lower basin users, through the most 12-year critical period of
record. However, as the basin developS the critical period of record extends out and the
storage required increases significantly.

Using the Critical Period Strategy, the change in percent of Lake Mead surplus capacity,
over time, is shown on in Figure 10. This can be compared with the 70 Percent to
90 Percent Capacity Trended By Use Strategy displayed on Figure 11, or compared with the
70 Percent Assurance of Avoidance Strategy displayed on Figure 12.

Several runs were made to compare this methodology with the more familiar percent capacity
threshold strategies, and the percent assurance of avoidance strategies. The Critical Period
Method is compared with the Flood Control Surplus Strategy on Figure 13, with the
70 Percent Assurance of Avoidance Strategy on Figure 14, with the 80 Percent Capacity
Surplus Threshold Strategy on Figure 15, and with the 70 Percent Capacity to 90 Percent
Capacity Trended By Use on Figure 16.

The Critical Period Strategy provides more surplus opportunities during the early years of
lower development than do the 70 Percent Assurance of Avoidance, 80 Percent Capacity, and
the 70 Percent to 90 Percent Capacity Trended By Use strategies.

Once the average states use exceeds 11.8 maf the critical storage requirement increases
sharply, as the critical period of record extends out to 20 years, and the surplus threshold
capacity rises quickly to the Flood Control Surplus capacity level. The likelihood of
surpluses will falloff rapidly, as the critical period extends out, and as the strategy protects
against the potential for impacting the target protection level.

The potential for shortage under the Critical Period Strategy is the same as a Flood Control
Strategy. While the strategy protects a target elevation of 1050 feet it also provides
additional protection against a shortage determination. The strategy uses the existing
reservoir conditions when determining the storage required for the critical period. For
example: Had the system just gone through the six worst years on record the strategy would
still require that a surplus could not be determined unless existing storage was adequate to
provide for a critical period drawdown. This is consistent with 602(a).

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