An appropriate definition of the winter withdrawal requires consideration of
regional conditions and characteristics. For example, in an Illinois study by LaTour
[35], winter withdrawals were computed as an average of November through April
withdrawals and assumed outdoor use was 80 %. For Ohio, Indiana, and Wisconsin,
Shaffer [22] defined the winter withdrawals as the average of December through
February withdrawals and assumed that outdoor use is 100 % consumptive. In this
study, because water main breaks are common in the winter months due to freezing
conditions, a system’s winter withdrawal in a given water year is computed as the
minimum of the December, January, and February total withdrawal, with outdoor
use assumed to be 100 % consumptive.
The winter base rate method has some obvious limitations. It ignores consumptive use that does not vary seasonally, as might occur for a public supply system that
supplies a large industrial facility whose consumptive use is constant throughout
the year. It would not give accurate results for a system with significant seasonal
variations in user population, for example, a community that experiences a large
influx of tourists during the summer months. In addition, no systematic evaluations
of the method have been conducted, partly because an alternative methodology is
not available for the PWS use type. However, Shaffer [22] did apply the method to
commercial as well as public supply users, and for commercial facilities, she
compared results from the winter base rate method with those obtained from the
simple water balance approach, Eqs. (1a) and (1b). Shaffer found that the annual
consumptive use coefficient for both Ohio and Indiana from the winter base rate
method was 30 %, which fell between the median and average of the annual
consumptive use coefficients for facilities in Ohio computed using Eq. (1b),
which were 17 and 42 %.
3.6 Configuration of the Water Balance Conveyance Model
As described above, this study estimates consumptive use primarily by application
of non-site-specific consumptive use coefficients to water withdrawal quantities
rather than by using return flow data. The coefficients are obtained from
literature values or, in the case of PWS, computed using the winter base rate
method. In a few instances when sufficient data were available, the water balance
equation, Eq. (1b), was used. However, return flows are often discharged at
locations some distance away from original withdrawal points. Therefore, in
order to estimate consumptive use by this study’s spatial unit of analysis, the
river segment, a means of representing the routing of water from withdrawal points
to discharge points was needed. An available water routing model and database
structure, the New England Water-Use Data System [39] was considered, but it was
determined that its data needs were too intensive for a region the size of the upper
Potomac Basin.
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J. Ducnuigeen et al.
regional conditions and characteristics. For example, in an Illinois study by LaTour
[35], winter withdrawals were computed as an average of November through April
withdrawals and assumed outdoor use was 80 %. For Ohio, Indiana, and Wisconsin,
Shaffer [22] defined the winter withdrawals as the average of December through
February withdrawals and assumed that outdoor use is 100 % consumptive. In this
study, because water main breaks are common in the winter months due to freezing
conditions, a system’s winter withdrawal in a given water year is computed as the
minimum of the December, January, and February total withdrawal, with outdoor
use assumed to be 100 % consumptive.
The winter base rate method has some obvious limitations. It ignores consumptive use that does not vary seasonally, as might occur for a public supply system that
supplies a large industrial facility whose consumptive use is constant throughout
the year. It would not give accurate results for a system with significant seasonal
variations in user population, for example, a community that experiences a large
influx of tourists during the summer months. In addition, no systematic evaluations
of the method have been conducted, partly because an alternative methodology is
not available for the PWS use type. However, Shaffer [22] did apply the method to
commercial as well as public supply users, and for commercial facilities, she
compared results from the winter base rate method with those obtained from the
simple water balance approach, Eqs. (1a) and (1b). Shaffer found that the annual
consumptive use coefficient for both Ohio and Indiana from the winter base rate
method was 30 %, which fell between the median and average of the annual
consumptive use coefficients for facilities in Ohio computed using Eq. (1b),
which were 17 and 42 %.
3.6 Configuration of the Water Balance Conveyance Model
As described above, this study estimates consumptive use primarily by application
of non-site-specific consumptive use coefficients to water withdrawal quantities
rather than by using return flow data. The coefficients are obtained from
literature values or, in the case of PWS, computed using the winter base rate
method. In a few instances when sufficient data were available, the water balance
equation, Eq. (1b), was used. However, return flows are often discharged at
locations some distance away from original withdrawal points. Therefore, in
order to estimate consumptive use by this study’s spatial unit of analysis, the
river segment, a means of representing the routing of water from withdrawal points
to discharge points was needed. An available water routing model and database
structure, the New England Water-Use Data System [39] was considered, but it was
determined that its data needs were too intensive for a region the size of the upper
Potomac Basin.
154
J. Ducnuigeen et al.