WSP07957

• File Number: 8276.100
• Description: Grand Valley Unit-Colorado River Basin Salinity Control Project
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/21/1983
• Title: Grand Valley Salt Pickup Calculations - Draft Report
• Water Supply Pro - Doc Type: Report/Study

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CHAPTER III

RIVER BUDGET RESULTS

Physical changes in irrigation from year to year do not account for the

annual variations in the estimates of salt pickup from Grand Valley. The

variance from the mean is due to the inability to measure inflow and outflow

with aufficient accuracy. Even with the adjustment for unaccounted flow, the

variation about the mean salt load is significantly large.

Confidence in Results

The confidence limits on the recommended budget (Adjusted State Line),

are represented by the two outside sets of lines about the mean in Figure 5.

Both sets indicate the 95% confidence level in the mean. In other words, a

person can be 95% confident that the population mean annual salt pickup for

that budget lies between the confidence limits. The narrower set of limits

~ assumes that the 29 annual pickup values are independent samples. The widest

set accounts for the lag-l serial correlation in the annual salt pickup by

using the equivalent independent sample size instead of the actual sample

size.11 If the serial correlation is thought to be spurious, the narrow

limits can be used, but if it is genuine, the wide limits should be used.

The confidence limits using the equivalent independent sample size for the

four budgets presented in this report are displayed in Table 16. As can be

seen the confidence limits vary for each of the four budgets. The difference

in the confidence limits of the four budgets is due primarily to differences

in the lag-l serial correlation of their annual pickup values. It should be

11 For a discussion of confidence limits of the mean utilizing the
equivalent independent sample size, see Yevjevich, Vujica, Probability
and Statistics in Hydrology, Water Resources Publications, Fort Collins,
Colorado, 1972, pp. 186-190.

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