BOARD01425

• Board Meeting Date: 3/29/1999
• Description: ISF Section - Methodology Report
• Board Meetings - Doc Type: Memo

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Discussion of Existing Methodologies and their use in Colorado

R2CROSS
Nehring (1976) concluded that the R2CROSS methodology is appropriate as "a first level
of evaluation". He also recommends that it be used on "headwater streams at high elevations" and
on streams "where encroachment by diversion, pollution, arid development is not anticipated as a
serious problem." The CWCB staff agrees that the single-transect R2CROSS methodology is the
best available methodology for developing instream flow recommendations on the majority of
Colorado's coldwater streams. It provides a site-specific, cost-effective, quantification of the
streamflow required to "preserve the natural environment to a reasonable degree."

R2CROSS-based instream flow recommendations on coldwater streams should be verified
against recommendations resulting from other methodologies whenever possible. For example, on
gaged stream reaches, R2CROSS-based instream flow recommendations can easily be compared
with recommendations that result from the Tennant Method using stream gage records from the
USGS, the Colorado State Engineers Office, or other accepted sources. In addition, output from the
R2CROSS staging table can be used to generate a plot of wetted perimeter versus discharge. This
plot can then be used to compare an R2CROSS instream flow recommendation against one derived
from the Wetted Perimeter method. The verification of R2CROSS recommendations against the
Tennant and Wetted Perimetermethods could be accomplished with very little additional effort or
expense. When the resulting recommendations are similar, this verification process would add
additional confidence to the R2CROSS-based instream flow recommendation.

The CWCB and CDOW staff should also work to improve the hydraulic component of the
R2CROSS model. Nehring (1979) suggested a "tendency of the R2CROSS model to overestimate
the average velocity by an average of 45% from measured field values" and that "some action should
be taken to balance measured water stage levels and field velocity measurements." The tendency
for R2CROSS to overestimate instream velocities at modeled flows less than the field-measured
discharge is most likely due to the assumption of constant roughness in the current version of
R2CROSS. Relative roughness, or resistance to flow, within a stream tends to increase as flow
decreases. Therefore, it is expected that the assumption of constant roughness in the R2CROSS
model would result in inflated estimates of predicted velocities as modeled flows decrease. The
opposite would be anticipated at modeled flows greater than the field-measured discharge as the
assumption of constant does not allow flow resistance to decrease at higher flows, as it would in the
stream.

The staffs have generally addressed this modeling constraint by collecting field data during
times of low flow. Collecting data at flows near the resulting recommendation minimizes the need
for excessive extrapolation of modeled results. Staff is also in the process of investigating various
techniques for enhancing the R2CROSS model to include a variable roughness factor.

The CWCB and CDOW staffs should also cooperate to verify and document the biologic
links to the R2CROSS hydraulic model. As stated previously, CDOW began developing "seasonal

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