SPDSS_Task58_Review Previous Estimates of Potential CU

• Title: SPDSS Task 58 - Review Previous Estimates of Potential CU
• Description: This memo reviews the methods used in the past to determine potential consumptive use.
• Decision Support - Doc Type: Task Memorandum
• Date: 8/5/2004
• DSS Category: Consumptive Use
• DSS: South Platte
• Basin: South Platte
• Contract/PO #: C153954
• Grant Type: Non-Reimbursable
• Bill Number: SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
• Prepared By: Leonard Rice Engineering

SPDSS Task 58 Memo
August 5, 2004
Page 19 of 22
recommended that the following methodology be used in deriving estimates of PCU in the
SPDSS study area where sufficient climate data exists.
1. Identify climate stations representative of regions that have data adequate to perform
a PCU calculation with the Standardized Penman-Monteith equation. Climate
stations recommended for use in the Standardized Penman-Montieth equations are
those in the COAGMET or NCWCD climate station networks.
2. Perform the PCU calculations at the regional climate stations using the Standardized
Penman-Monteith equation.
3. Perform similar PCU calculations at the same climate stations, for the same time
period, with the modified Blaney-Criddle method.
4. Derive local calibration coefficients (PCUstanaaraizea~CUmoaifiea s-c) to apply with the
modified Blaney-Criddle method for the historic period where climate data did not
exist to allow a Standarized Penman-Monteith calculation of PCU.
5. Operate the modified Blaney-Criddle method throughout the desired SPDSS study
period with the local calibration coefficients applied to estimate regional consumptive
use for the various crops.
Calibration b~ysimeter Results
Climate data needed to estimate consumptive use with the more detailed Penman based
consumptive use methods does not typically exist except at selected sites at lower elevations in
the SPDSS study area. Therefore, without detailed climate data being available at the higher
elevations in the SPDSS study area, it is proposed that local calibration of irrigated grasses at
high altitude be based on lysimeter data identified in this report. Consistent with the base used in
most of the identified lysimeter studies, coefficients would be suggested for a monthly form of
the original Blaney-Criddle method (Kt=1), already in the StateCU program.
T,awn ~TraSS
The predominant method in use in the State (by Division of Water Resource representatives and
consultants) for estimating consumptive use of lawn grass is the modified Blaney-Criddle with
modifications (revised Kt equation, elevation adjustments, revised crop coefficients) suggested
by Pochop (1984). Based on the investigations for this task, it is recommended that the
modifications as suggested by Pochop be implemented in the StateCU program for statewide use
in estimating lawn grass consumptive use.
Modified Blaney-Criddle with Adjustment for Elevation
As discussed earlier, much of the staff of the Division of Water Resources and several of the
consultants apply the modified Blaney-Criddle method with MRCS suggested adjustments for
elevation. It is recommended that the elevation adjustment be incorporated into StateCU and
used when the previous described three strategies (local calibration with more detailed method,
calibration with lysimeter data, or use of Pochop method for lawn grass) are not appropriate. To
provide information on the validity of using modified Blaney-Criddle with elevation adjustment,
several comparisons will be made between using this approach and PCU calculated using the
locally calibrated coefficients (derived from ASCE Standardized P-M or Lysimeter data).