WSP12086

• File Number: 8449.800.A
• Description: Metro Study SB 96-74
• State: CO
• Basin: South Platte
• Date: 4/1/1998
• Author: CWCB
• Title: Denver Basin and South Platte River Basin Technical Study, Technical Addendum 1 and 2, Senate Bill 96-074
• Water Supply Pro - Doc Type: Publication

Chapter 1: South Platte Basin Inventory

March 21,1998
Page 1-12

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Chanoes in St. Vrain. Bio Thompson and Cache La Poudre Flows,

Growth in municipal and industrial water use in these South Platte
tributary basins has resulted in increased return flow volumes and a change in
return flow patterns due to the fact that municipal water use is generally less
consumptive than irrigation use and because there are proportionately more
winter season return flows due to wastewater discharges.

Future M&I water use in these basins will also utilize 48,000 acre-feet per
year of Windy Gap project water, which will add return flows to the South Platte
basin, While return flows from Windy Gap water are fully reusable, these return
flows were included in our estimate of future Kersey flows and accounted for
separately as part of the reusable portion of future flows.

Future urban growth in this region will also result in increases runoff from
impervious surfaces (RIS).

These changes in flows were estimated by quantifying the total change in
municipal water use within the St. Vrain, Big Thompson and Cache La Poudre
Basins between 1950 and 1980 and between 1980 and 2025 based on best
available information and projections. Municipal and domestic water use within
this northern region has increased from approximately 48,000 acre-feet per year
in 1950 to approximately 105,000 acre-feet per year in 1980. By the year 2020,
municipal use is project to reach approximately 260,000 acre-feet per year
(NCWCD, 1991). The net increase in return flows due to: 1) changes of irrigation
water rights and CBT units from irrigation uses to municipal uses, 2) use of
additional Windy Gap supplies, and 3) increased RIS is estimated to be
approximately 76,000 acre-feet per year between 1950 and 2025 and 60,000
acre-feet per year between 1980 and 2025. These increases were added to the
Kersey gage flows incrementally between 1950 and 1980 using monthly
coefficients representative of RIS (discussed in Chapter 4) and of the difference
between typical municipal and irrigation return flow patterns.

Also, since 1953 the CBT project has delivered an annual average of
approximately 230,000 acre-feet of supplemental water supply to agricultural,
municipal and rural domestic water users within the Northern Colorado Water
Conservancy District. Approximately one half of this supplemental supply has
returned to the South Platte above the Kersey gage. This can be seen in the
historical records for the stream gages at the mouths of the Cache La Poudre,
Big Thompson and St. Vrain rivers. While the monthly pattern of CBT return
flows varies from year to year, stream flows out of these South Platte tributaries
have generally increased in all months of the year. A monthly CBT return flow
pattern was estimated by calculating the return flow resulting from 190,000 acre-
feet per year of CBT deliveries using a simplified irrigation depletion model and

Hydrosphere Resource Consuttan!s, 1002 Walnu! Sune 200, Boulder, CO 80302