WSP07896

• File Number: 8449.600
• Description: Chatfield Reservoir
• State: CO
• Basin: South Platte
• Date: 2/1/1976
• Author: M W Bittinger & Asso
• Title: Preliminary Feasiblity Estimate of the Potential Use of the Conservation Pool of Chatfield Reservoir
• Water Supply Pro - Doc Type: Publication

.

The interpretation of the preceding tables is as follows: Column 1

of the tables shows the probability of experiencing the net annual lake

evaporation shown in column 3 of the same table. Column 2 shows the expected

return period of the same events in years. The limits of the actual annual

evaporation in acre-feet units shown in column 4 were obtained from the

minimum and maximum surface areas of the conservation pool. Column 5 shows

the expected annual evaporation volume in acre-feet that is most likely

to occur computed with the surface area corresponding to the 22,000 acre-

feet content. Column 6 shows the incremental evaporation volume in acre-feet

between the successive probability levels, Column 7 displays the cumulative

incremental evaporation volume of column 6. Some examples may illustrate

the use of these tables and point out the significance of the displayed

quanti ties.

The third line of Table 3 shows that at a probability level of 0.1,

which has a 10-year return period or a frequency of once out of 10 years,

the net evaporation will be 31.3 inches. Expressed volumetrically it is an

annual water loss of 3180-3550 acre-feet depending upon the content of the

conservation pool. Since the reservoir storage has.an equal likelihood of

being above or below the 22,000 acre-feet content, the most likely annual

evaporation loss is 3340 acre-feet. Comparing this loss with that shovm in

the first line of the same table, a difference of 555 acre-feet is recognized

from the last column of the table. The operational significance of this

difference is that if the operation of the evaporation replacement program

is based upon the 0.5 probability level, then once in every 10 years a deficit

of 555 acre-feet can be expected. The same operation would result in a

deficit of 935 acre-feet once in every 100 years as shown in the last column

of the fifth line of the table.

Tables 4 and 5 are identical in their setup to Table 3--the only differ-

ence being that instead of considering one single year's evaporation volumes,

three and five consecutive years' average annual evaporation figures were

used as the basis of the analysis, respectively. These tables were prepared

to yield indications concerning dry year sequences as were experienced during

the 1930's and the mid-1950's. Table 4 shows that once every 100 years three

consecutive years may occur with an average annual evaporation of 3425 acre-

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M. W. BITTINGER AND ASSOCIATES. INC.