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 10 of 22
A similar process was recently performed for determining crop consumptive use in the
Republican River Basin (within the South Platte Basin but outside of the SPDSS study area) by
Helton and Williamson consulting engineers for the State of Colorado. In this determination, 1)
PCU was calculated with the ASCE Standardized Penman-Monteith method for five recent years
in which adequate data was available, 2) PCU for the same recent years were also calculated by
the Hargreaves method and 3), adjustments (Penman-Monteith PCU/Hargreaves PCU) were
developed to apply to PCU estimates determined by the Hargreaves method for a study period of
1940-2002.
The process of calibrating a less precise and less data intensive method (i.e. modified Blaney-
Criddle or Hargreaves) with a more accurate but data intensive method (Penman-Montieth or
Modified Hargreaves) and application of the calibrated less precise method back in time appears
to be gaining consideration among consultants making applications in the South Platte Basin.
3. State Statutes Review
Colorado State Statutes were reviewed for possible requirements for use of a certain
consumptive use method. In the Colorado State Statutes, the only mentioned consumptive use
methodology is the modified Blaney-Criddle method. Provided below are the citations from the
Colorado Revised Statutes that direct a user to use the modified Blaney-Criddle method (i.e. SCS
TR21) in the calculations of depletions associated with wells. Underlined emphasis has been
added.
From Colorado Revised Statutes 37-92-308, (3)(b)(I)
"The request shall also identify for each well, including any augmentation wells: The permit
number and location; the prof ected use and volume of pumping; for all wells using the modified
Blaney-Criddle method to determine consumptive use, the projected number of acres and crops
to be irrigated; the anticipated stream depletions that affect the river after October 31, 2002, until
eighteen months after the date of the request in time, location, and amount, including a detailed
description of how such depletions were calculated, and shall list the identity, priority, location,
and amount of all replacement water sources to be used to replace stream depletions, including
both accretions and depletions attributable to any augmentation wells."
From Colorado Revised Statutes 37-92-308, (3)(c)(I)
"For those irrigation wells where diversions are not actually measured, the state engineer shall
determine the amount of stream depletions using actual data for the crops grown, acres irrigated,
surface water deliveries, and the modified Blaney-Criddle method."
These references to use of the modified Blaney-Criddle method are consistent with the vast
majority of determinations of PCU in the South Platte River Basin by the engineering
community as described in previous sections of this memo. It is interesting to note that the use
of the modified Blaney-Criddle method does not preclude the use of elevation adjustments or
locally calibrated crop coefficients.