WSP11637

• File Number: 8220.126.J
• Description: San Miguel Project
• State: CO
• Basin: San Juan/Dolores
• Water Division: 4
• Date: 9/1/1990
• Author: Boyle Engineering Co
• Title: San Miguel Project Water Supply Study Supplemental Report System Operations 1990 Irrigation Season
• Water Supply Pro - Doc Type: Report/Study

--:~

( ':
~

Subsequent to June 16, virtually all inftows have been stored and, as of mid-September,
reservoir storage stands at approximately 360 acre-feet. Inflows essentially stopped in early
August and reservoir evaporation and seepage is producing a net losing situation. At the end
of August storage stood at approximately 450 acre-feet, for a loss of about 90 acre-feet over a
two week period, The water in storage is being saved to make a fall stock pond run, As
explained for the Gurley system, the stock pond run is used to fill ponds which provide water to
livestock over the winter, Existing supplies are considered by ditch company personnel to
probably be adequate to make the pond run,

Examination of the inflow records show that approximately 1345 acre-feet of runoff was
captured over the April through July period, This represents approximately half of the 2600
acre-feet runoff volume of the inflow predicted for the Cone system by the March and April SCS
forecasts, The actual distribution over the season agreed very well with the predicted. As
predicted, approximately 50 percent of the season's total runoff had occurred by the end of
May,

Examination of the outflow records show that approximately 1430 acre-feet flowed out of the
reservoir, Of this amount, approximately 1250 acre-feet were for irrigation releases; the
remainder of the measured flows were from leakage lost through the outlet or seepage inflows
to the ditch, The increment of additional volume released above that captured over the April
through July period represents carry-over storage occurring from winter inflows,

System OperatIons Model Assessment

The development and underlying assumptions of the systems operation model were described
in the San Miguel Project Water Supply Study, Summary Report, Demand-Based System
Operations (June 1990). Key among the assumptions incorporated in the model to represent
the proposed operating plan were the following: operation of the reservoirs in response to crop
demands; headgate control on the Cone system; and implementation of an accounting system
which would track inflows. outflows, and storage on an individual account basis for both
systems to allow greater irrigator flexibility.

The system operations model, therefore, is a predictive tool representative of the proposed
operating plan. Because the operating plan was not implemented, an assessment of the model
in terms of a detailed comparison between actual and projected operations is of limited utility,
Never-the-Iess, a general comparison in broad terms is provided below to illustrate the utility of
the program in spite of the differences in detail between actual and proposed operations.

As presented in the San Miguel Project Water Supply Study, Summary Report. Demand-Based
System Operations (June 1990), on the basis of March forecasts, it was projected that if
attempts to irrigate all of the land were made, available supplies would likely be exhausted by
the end of June. If supplies were to be extended further into the season, only between 20 and
30 percent of the land should be irrigated. Even at the reduced level of Irrigation, available
supplies were projected to be exhausted by August.