2024-06-18_HYDROLOGY - M1977436

• Permit No: M1977436
• IBM Index Class Name: Hydrology
• Doc Date: 6/18/2024
• Doc Name: Substitute Water Supply Plan
• From: Division of Water Resources
• To: DRMS
• Email Name: NCG
• Email Name: JLE
• Media Type: D
• Archive: No

Holcim South Platte Combined SWSP June 18, 2024
Plan IDs 3614, 4773, 3624, 4772, 4616, 3437, 3376, 3650, 3668, 5475, 5829, &t 6243 Page 10 of 26
St. Vrain River
Replacement Source Projected 2024 Yield Comments
acre-feet
City of Longmont Lease 34.50 Annual lease
-Lighthouse Cove Pond Recharge 13.41 1.5 shares Smith Et Emmons Ditch and free river
City of Longmont Lease
Holcim has entered into a water lease agreement with the City of Longmont under which
Longmont will provide up to 375 acre-feet annually of fully consumable water to offset depletions
which may regularly occur during the mining of the Irwin-Thomas Pit. A copy of the lease agreement,
dated October 1, 2021, is attached. Holcim has separately requested that the City of Longmont
provide 10 acre-feet to offset depletions at Holcim's other sites. Deliveries under this lease are
available from either the outfall of Longmont's municipal wastewater treatment plant or the
confluence of Spring Gulch and Saint Wain Creek. Deliveries will be made to St. Vrain Creek during
the period of April through October 2024 pursuant to the attached delivery schedule.
Smith Et Emmons Ditch/Lighthouse Cove Pond Recharge
Holcim owns 1.5 of 8 total outstanding shares in the Smith &t Emmons Ditch Company. The 1.5
Smith &t Emmons Ditch Company shares were historically used to irrigate 68 acres of pasture grass.
The pro-rata diversions available at the farm headgate for the 1.5 shares used on the property were
estimated to total 224.21 acre-feet per year, based on the average headgate diversions for the Smith
&t Emmons Ditch (WDID 0600553) for the years 1950 to 1998, assuming a 10% ditch loss. Using a
Modified Blaney-Criddle analysis the potential crop consumptive use for the 68 acres was determined
to be 108.7 acre-feet. Assuming a farm efficiency of 55%, the average consumptive use was
determined to be 91.65 acre-feet for the 1.5 shares. Total return flow obligations from the use of the
1.5 shares were calculated as 132.56 acre-feet per year by subtracting the historical consumptive use
from the pro-rata amount of diversions available at the farm headgate. One-half of the return flows
(66.28 acre-feet) were assumed to occur as surface return flows and one-half of the return flows
(66.28 acre-feet) were assumed to occur as subsurface return flows. The monthly and annual
volumetric limits for the farm headgate deliveries are shown in the table below.
Table E- Monthly and Annual Farm Headgate Delivery Limits (acre-feet)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
0 0 0.22 3.40 35.24 53.76 79.64 38.93 11.86 1.15 0 0 224.21
Subsurface return flows were lagged to the river using the AWAS stream depletion model with
the following parameters:
• Distance from the centroid of the irrigated property to the river (X) = 800 ft
• Alluvial aquifer width (W) = 9,300 ft
• Specific yield (S) = 0.2
• Transmissivity (T) = 50,000 (gpd/ft)
The resulting return flow factors are shown in Table F below.
Table F- Monthly Surface Water (SW) and Groundwater (GW) Return Flow Factors