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The following message is forwarded to you by srevett@ag.gov (list as the
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>From: srevett@ag.gov
>Date: Wed, 20 Nov 96 14:29:12 -0400
>To: dreynold@cd-eso.water.ca.gov
>Subject: Re: Measuring Evapotranspiration
>In-Reply-To: <9611201922.2F3154@cdeso134.water.ca.gov>
In <9611201922.2F3154@cdeso134.water.ca.gov>, on 11/20/96
at 11:22 AM, dreynold@cd-eso.water.ca.gov said:
>I recently attended a class where the statement was made that neutron
>probes can not be used to measure ET. If this is true I would assume the
>person would say the same about TDR.
>I have become certified to use a neutron probe for two reasons, 1) We
>have one and 2) I thought I might be able to get some studies going to
>update crop ET numbers.
>I realize that it would be better to use lysimeters, but.. they are
>expensive and not readily available and I would guess expensive. It
>would be great if I could talk DWR into installing its own but not in my
>lifetime.
>Someone must have done comparison studies with probes of various types
>and lysimeters.
>I ask this group to give your opinions on this subject. I am asking the
>question regarding measurement of real-time ET not as an irrigation
>scheduling tool.
>Thanks in advance,
>dean
>********************************************************
>Dean Reynolds
>Associate Land and Water Use Analyst
>California Department of Water Resources
>3251 S Street
>Sacramento, Ca. 95816
>916-227-7602
>916-227-7600 Fax
>dreynold@water.ca.gov
Dean,
Starting with the proposition that all evapotranspiration (ET)
measurements are the result of some kind of calculation of the water
balance in the field, we can make some statements about ET measurement
methods.
The water balance is usually represented by an equation of the sort:
ET = deltaS + I + P - R - D
where deltaS is the change in storage of water in some chosen soil profile
depth, I is irrigation as a depth, P is precipitation as a depth, and R
and D are runoff and deep drainage as depths. It gives a more general
flavor to the equation if we replace D with F for flux across the bottom
of the profile - this recognizes that flux across the profile bottom may
be in either direction.
Weighing lysimeters control D by not allowing it or by measuring it
directly. Modern weighing lysimeters usually incorporate a vacuum
drainage system or a system to provide measurement water inflow and
outflow if high water tables must be replicated in the lysimeter. Also,
most weighing lysimeters have some sort of lip that rises above the soil
surface and that eliminates small runoff events including runoff from
irrigation.
The change in storage is measured by direct weighing.
Measuring the change in storage by measuring soil water content can easily
be as precise as measuring it by direct weighing (though not with neutron
scattering, see below). That isn't the problem. The problem is with
uncontrolled and unmeasured flux across the bottom of the profile.
You can use neutron scattering (NS) to measure ET if:
1) You measure deeply enough that you can close the water balance at the
bottom of the profile. That is, you want to measure a good ways below the
bottom of the part of the profile that is wetted by irrigation. Also, the
water contents in the lower part of the profile should be low enough that
hydraulic conductivity is almost nil. Some researchers have escaped this
limitation (at least in theory) by
a) measuring the hydraulic conductivity as a function of soil water
potential,
b) measuring the soil water potential as a function of water content
(ignoring hysteresis),
c) measuring the water content at at least two places deep in the
profile, and d) calculating the flux at the bottom of the profile from
these data and incorporating the flux into the water balance.
Jim Wright (1982) stated that neutron scattering measurements had to be
take to below 2 m in his southern Idaho location to obtain good results.
2) The interval over which you want to measure is long enough to make the
error in water content in near surface readings a negligible part of the
overall water balance.
3) You carefully calibrate the NS gauge and take readings at small depth
increments (I use 20 cm increments but 15 cm or even 10 cm increments can
add some precision to the value of profile water content).
Assuming negligible or measurable (or at least estimable) deep drainage
losses you can obtain quite good seasonal, monthly and even weekly ET data
using NS alone.
If you have a high water table you will have to calculate fluxes across
the bottom of the profile - this can be difficult and is prone to many
errors - the fluxes can be quite high.
If by "real-time" you mean hourly or half-hourly ET data then NS alone is
not going to do the trick. But then, you can't use NS in unattended mode
anyway. An operator must be present under current and foreseeable
regulations.
But!
I showed (Evett, 1993) that quite accurate ET data (good enough for daily
ET) could be obtained by combining TDR measurements of soil water content
in the top 30 cm (with probes placed horizontally in the soil at depths of
2, 4, 6, 10, 15, 20 and 30 cm) with NS measurements at deeper depths. The
ET data were from precision weighing lysimeters (0.05 mm resolution). For
diurnal fluxes the TDR data give good correspondence to lysimeter data
(half-hourly intervals). My TDR probes only went to 30 cm depth. Deeper
probes would cover more of the root zone (I covered about 90% of the root
zone of winter wheat) and give accurate data even without NS measurements.
White and Zegelin (1995) showed good correspondance between weighing
lysimeter measured soil water balance and that measured by TDR.
The big problem with all methods that don't restrict deep drainage is the
estimation of that deep drainage. Weighing lysimeters, by their nature,
eliminate any flux across the lower part of the profile. They have other
problems but not the deep drainage one.
REFERENCES:
Evett, S.R. 1993. Evapotranspiration by soil water balance using TDR and
neutron scattering. In Management of Irrigation and Drainage Systems,
Irrigation and Drainage Div./ASCE, July 21-23, 1993, Park City, Utah. pp.
914-921.
White, I., and S.J. Zegelin. 1995. Electric and dielectric methods for
monitoring soil-water content. Chapter 22 in L.G. Wilson, L.G. Everett,
and S.J. Cullen (eds) Handbook of Vadose Zone Characterization &
Monitoring. Lewis Publishers, Boca Raton. pp. 343-385.
Wright, J.L. 1990. Comparison of ET measured with neutron moisture meters
and weighing lysimeters. Pp. 202-209 in Irrigation and Drainage:
Proceedings of the National Conference. Durango, Colorado, July 11-13,
1990. ASCE, New York.
Steve Evett srevett@ag.gov
USDA-ARS, P.O. Drawer 10, Bushland, Texas 79012 U.S.A.
(1/2 mile W., Interstate-40 S. access road)
Tel. 806-356-5775, FAX: 806-356-5750
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-- End of forwarded message
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Steve Evett srevett@ag.gov
USDA-ARS, P.O. Drawer 10, Bushland, Texas 79012 U.S.A.
(1/2 mile W., Interstate-40 S. access road)
Tel. 806-356-5775, FAX: 806-356-5750