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As with Dean Reynolds, apologies to cross references with the IR L listings.
However I thought it appropriate to respond to both lists. Rick Allen
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I'd like to respond to Dean Reynold's questions concerning the application of
Kc procedures to a wetland. However, I am not familiar with the specific
study that Dean referred to.
First of all, I would suggest that evaporation from the open water areas of a
wetland can be
much different from that of the vegetated area and should be calculated
separately, as Dean suggested. This is especially true in a temperate
climate. In a temperature climate there may be portions of the winter where
the vegetation is dead and therefore becomes a "protective" mulch above the
wet soil or water and thereby reduces the Kc dramatically. I have measured a
Kc of less than 0.2 for dead cattail vegetations in northern Utah. During the
summer, the tall, lush cattail or bulrush vegetation can approach a Kc of even
1.4 in an arid climate due to the tall roughness and general "Oasis" affect of
transport of sensible heat and dry air from outside of the wetland.
Dean, you are also correct in presuming that the "Kc end" (end season) value
can be used until the "greenup" of the following year. However, in a freezing
climate, the Kc can go even lower than a typical Kcend (not a problem in
California). The value of the Kc during nongrowing season will of course
change with precipitation frequency and is best estimated using a daily
calculation time step and a Kc procedure that seperately includes the
evaporation of water from the soil and vegetation surface (interception).
Evaporation from water can be very different and is a strong function of the
water depth, the turbidity, and variation in temperature during the year.
Shallow (say less than 1 m) or turbid water will intercept solar radiation
near the water surface and therefore will facilitate the conversion of
radiation to evaporation at the surface in near real time (but will be
impacted by nighttime evaporation and carryover of heat from hot to cool
days). In deep water, however, the radiation from the sun is transmitted
deeply into the water and is converted directly into heat which can only be
transported to the water surface to supply evaporation by convection within
the water body (and a little conduction). This can be an extremely slow
process for deep, cold water bodies such as in the Rocky Mountain area of the
USA. A simple calculation using specific heat of water times the depth of
water will indicate the tremendous storage capacity for heat in deep lakes.
We have measured "Kc's" for evaporation from Bear Lake, Utah of less than 0.40
in the summer months due to the heat storage effect. You can contact
Professor Bob Hill at USU ("bobh@extsparc.agsci.usu.edu") for specific
information on the study. Bear Lake is quite deep, extremely clear, and has a
cold winter.
Much of the stored heat in Bear Lake returns to the surface in the fall months
as the lake cools. During this later period, the vapor pressure deficit of
the air is less, so that less of the energy is converted into evaporation as
it would be during summer, and more is used to warm the cooler air.
Therefore, over the course of a year, a smaller ratio of total solar radiation
is converted to evaporation for a clear, deep lake as opposed to a shallow or
turbid lake, and therefore the Kc is lower than for a shallow lake.
In the revision of the FAO 24 publication on Crop Evapotranspiration, we will
be suggesting two different Kc's. The first one is for shallow water bodies
(and "usually" water bodies near wetlands are shallow, fortunately, otherwise
there would be no emergent vegetation). The other set of Kc's is for deep
lakes. For open water less than 2m deep, or for all water bodies in tropical
climates having little change in water temperature, we suggest using a Kc of
1.05 for all months. This coefficient is based on the grass reference. For
deep water bodies (say greater than 2 m deep) in temperate climates with
winters, the FAO revision will suggest using a Kc = 0.65 for the spring and
summer and a Kc = 1.25 for the fall and winter. Of course, these average
values will vary, as discussed above, with actual depth, turbidity, and
variation in climate during the year. I welcome any comments on these
suggested values.
Now for the big question on the conversion between alfalfa and grass
reference. The FAO revision will suggest that the ratio of alfalfa reference
to grass reference (ETr/ETo) varies from about 1.05 for humid, calm conditions
to 1.2 for semi-arid, moderately windy conditions, and to 1.35 for arid, windy
conditions. The first condition is dominated by net radiation, which is
similar between the references. The latter two values are influenced more by
the differences in aerodynamic roughness and bulk surface resistance.
An equation for predicting the conversion between the two references that is
in the revision is the following:
ETr/ETo = 1.2 + [ 0.04 (u2 - 2) - 0.004 (RHmin - 45) ] (h/3)^0.3
where u2 is average wind speed at 2m in m/s, RHmin is daily minimum relative
humidity, %, and h is height of the alfalfa (generally 0.5 m is used for the
alfalfa reference). The "0.3" exponent is used on the h/3 term to indicate
the effect of roughness (height) on the impact of dryness and windiness on the
ratio. This same "adjustment" expression is used in the revision to adjust
Kc's for all crops for climate for use with the grass reference ETo.
The above equation happens to predict a ratio ETr/ETo = 1.24 at Kimberly,
Idaho, where u2=2.2 m/s during the summer period and RHmin averages 30
percent. This is similar to values of ETr/ETo that have been measured by Dr.
Jim Wright at Kimberly using weighing lysimeters.
RHmin can be predicted from daily or monthly dewpoint temperature as RHmin =
100 e°(Tdew)/e°(Tmax) where e°() is the saturation vapour function. If no
Tdew data are available, RHmin can be approximated closely enough as RHmin =
100 e°(Tmin)/e°(Tmax) where Tmin and Tmax are average daily minimum and
maximum air temperatures.
The revision of the FAO-24 (Doorenbos and Pruitt) publication should become
available in about November of this year (finally!!). It is in the final
draft at this time.
Rick Allen
Professor
Utah State University
(please note that I will be back in Logan, Utah in 5 days. Please use the
Email address ALLENRIC@cc.usu.edu to contact me. I am currently on leave to
the Katholic University, Leuven, Belgium.
p.s. The Kc for a wetland varies substantially with the "clothesline" effect
caused by occurrences of limited stands of wetland vegetation that are
surrounded by vegetation or other cover that is evaporating at a lower rate.
This is common for wetland vegetation that occurs along roadways or canals.
In this instance the Kc can go as high as 1.8.