DWR_2807910

• Title: STATE AND DIVISION ENGINEER'S FORUM ENGINEERING ASSUMPTIONS - ATTACHMENT: CROP COEFFICIENT MEMO
• Author/Source: DEAN SANTISTEVAN, P.E.
• Author/Source: DIVISION OF WATER RESOURCES
• Keywords: CROP COEFFICIENT, CROP GROWTH
• Document Type - Reference Library: Presentations
• Document Date: 4/15/2014
• Year: 2014
• Team/Office: Division 1 Office

MEMORANDUM
December 13, 2013
Issues with Selecting an Appropriate Crop Growth Stage Coefficient for the SCS Mod. Blaney
Page 22 of 34

note that by calibrating the Blaney
coefficients to signify ryegrass.

Results. The methodology outlined in
was achieved on the second iteration.
lawn type grass, ARS 1275 coefficients for pasture grasses were not considered.
resulting curve is shown below and also

Figure 20 – Comparison of crop curves

The shape of the DWR calibrated crop curve is similar to what Task Memo 59.1 reported, although lower
in magnitude. The resulting curves from the calibrated coefficients appear to follow the shape that
might expect to see for cool-season grasses, but
months (spring and fall). This phenomenon may be explained
response to high daytime temperatures and low night temperatures, similar to
high elevation arid sites. Pochop
underestimate consumptive use when there are high daytime temperatures and low night temperatures
due to the fact that the equation evenly weighs day and night temperatures. The Jensen Paper
that originally, “for long-time periods mean air temperature was considered to be a
radiation” and that “it is unfortunate that
as an index of solar energy instead percent of daytime hours [sic]
Criddle equation for monthly time periods, they introduced a climatic coefficient (
growth stage coefficient (kc) to separate out the climatic effects from the phonological. However, as
pointed out in Manual 70, the kc term may still contain a meteorological component and
an elevation adjustment is appropriate.

On the other hand, the ASCE Standardized equation uses solar radiation measurements and a
methodology for determining net solar radiation
surprising that if there is a significant climatic effect during the sho
using the SCS TR-21 equation that
during the spring and fall.

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Issues with Selecting an Appropriate Crop Growth Stage Coefficient for the SCS Mod. Blaney-Criddle Eqn.
note that by calibrating the Blaney-Criddle equation to The ASCE we are forcing the calibrated
The methodology outlined in Task Memo 59.1 was followed (TM59.1, p.12) and convergence
was achieved on the second iteration. Because pasture grasses in SCS TR-21 may actually represent a
1275 coefficients for pasture grasses were not considered. A comparison of the
also in Table 17 (Appendix A).

crop curves
crop curve is similar to what Task Memo 59.1 reported, although lower
The resulting curves from the calibrated coefficients appear to follow the shape that
season grasses, but the curve seems exaggerated du
This phenomenon may be explained, not by plant phenology, but
high daytime temperatures and low night temperatures, similar to what has been reported
noted that researchers have suggested the Blaney
when there are high daytime temperatures and low night temperatures
due to the fact that the equation evenly weighs day and night temperatures. The Jensen Paper
time periods mean air temperature was considered to be a good measure of solar
is unfortunate that Blaney and Criddle did not select extraterrestrial solar radiation
as an index of solar energy instead percent of daytime hours [sic].” When the SCS modified the Blaney
Criddle equation for monthly time periods, they introduced a climatic coefficient (kt) and independent c
) to separate out the climatic effects from the phonological. However, as
term may still contain a meteorological component and
appropriate.
the other hand, the ASCE Standardized equation uses solar radiation measurements and a
methodology for determining net solar radiation among many other climatic parameters
surprising that if there is a significant climatic effect during the shoulder months that are unaccounted for
equation that it will differ from the ASCE equation. This may explain
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Day of Year
Task Memo 59.1
DWR Calibrated Coefficients
ADJ. SCS TR-21 (from sea level)
SCS TR-21
Pochop (1984)
Criddle Eqn.
Criddle equation to The ASCE we are forcing the calibrated
.12) and convergence
21 may actually represent a
A comparison of the

crop curve is similar to what Task Memo 59.1 reported, although lower
The resulting curves from the calibrated coefficients appear to follow the shape that one
seems exaggerated during the shoulder
, not by plant phenology, but by the plant
what has been reported at
he Blaney-Criddle may
when there are high daytime temperatures and low night temperatures
due to the fact that the equation evenly weighs day and night temperatures. The Jensen Paper points out
good measure of solar
did not select extraterrestrial solar radiation
When the SCS modified the Blaney-
and independent crop
) to separate out the climatic effects from the phonological. However, as
term may still contain a meteorological component and may explain why
the other hand, the ASCE Standardized equation uses solar radiation measurements and a
among many other climatic parameters. It is not
that are unaccounted for
explain the spikes
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