2011-08-04_ENFORCEMENT - C1981008 ‎(4)‎

• Permit No: C1981008
• IBM Index Class Name: ENFORCEMENT
• Doc Date: 8/4/2011
• Doc Name: OSM Technical Review of PR6 Prime Farmland Yield - Morgan Property
• From: OSM
• To: DRMS
• Email Name: SB1
• Email Name: DAB
• Email Name: MLT
• Media Type: D
• Archive: No

•
average of 59% of fixed CO went toward shoot and leaf develop-
ment in the firsr year as compared to 84% during the second year.
It is assumed that much of the carbon that is fixed in photo-
synthesis in the early years is partitioned to root development
for the plant. Holt et al. (1975) found that there is a time in the
life of an alfalfa stand when an equilibrium is reached between
photosynthates used in producing new root tissue and the death
of old root tissue. Smeal et al. (1991) concluded that the amount
of water transpired to produce a given amount of total above and
below ground biomass remained constant over the study period,
but the amount of water transpired to produce a given above
ground biomass decreased until year 5 or 6 when the root system
was fully developed. However, consideration of the evaporative
portion of ET also affects the WUE and is subject to change as
the stand ages. A relatively low WUE occurs during the estab-
lishment year due to evaporation from low ground cover. After
the establishment year, full ground cover increases transpiration
but reduces evaporative loss. In older, thinning stands, ground
cover decreases and more water loss to evaporation can lead to
reduced WUE (Nelson and Smith, 1968).
When considering adopting deficit irrigation as a water con-
servation approach, the implications of stand age on alfalfa WUE
should be considered. Since WUE was not maximized until year
5 and remained constant afterward (Smeal et al., 1991), an alfalfa
producer may improve WUE by leaving a stand of alfalfa in
production for longer than the typical 4 to 5 yrproductionperiod.
Lengthening the life of the stand will reduce the frequency oflower
WUE found during establishment and the first few production
years. However, as stands age they decline in density and are at risk
for encroachment by weeds. Therefore, a balance between increas-
ing WUE and a thinning stand must be found.
Water Table Contribution
to Alfalfa Evapotranspiration
Alfalfa has an extensive root system with the ability to extract
water from deep within the soil profile. Alfalfa roots commonly
grow 2 to 4 m in length and under the right conditions, can grow
as deep as 9 m (Peterson, 1972). In the presence of a shallow water
table, the contribution to ET from the water table can be signifi-
cant. Documenting water uptake from shallow groundwater may
be important if controlled deficit or partial season irrigation of
alfalfa is promoted as a water savings approach (Putnam et al.,
2005). Additionally, water uptake from ground water may have an
effect on nearby streamflows which can impact other water users.
Research that quantifies the water table contribution to alfalfa
ET is very limited, especially under conditions of deficit irriga-
tion. A study in North Dakota performed over 2 yr by Benz et al.
(1984) used nonweighing lysimeters under three irrigation levels
(0.3, 0.8, and 1.3 times potential ET) and four water table depths
(46, 101, 155, and 210 cm) on sandy loam and loamy sand soils.
The ET was affected by both water table depth and irrigation
level. Contribution to ET from the water table ranged from 0%
for the 155 and 210 cm deep water table conditions to 57% for
the 46 cm water table depth at the 0.3 irrigation level.
Another study completed over 4 yr on the western slope of
Colorado by Kruse et al. (1993) used weighing lysimerers on a
fine sandy loam soil to evaluate the effects of three water table
depths (no water table, 60 cm, and 105 cm) and two levels of
water salinity (0.65 ds m -1 to simulate river water and 6 ds m
100
E, 90
80j
ti 70
. e 60
om so
.0 I— 40
30
I g o 20
03 10 •
t 0 1
2 0 100 200 300 400 500 500
120
100
80
60
40
r
0
on
0
0
8 s
8
A
Water Table Depth (cm)
.Benz et al le Kruse et al ♦Dardanetl &Collin
0
0
Precip. and Irrigation (cm)
y = 396.89x
r' = 0.60
•
•
y = - 38.361.n(x) + 184.39
r =0.63
0
oBenz et al •Kruse el al £Dardannelll & Collin
Fig. 4. Relationship between percent contribution to
evapotranspiration (ET) from the water table and water -table
depth (upper) and water from precipitation and irrigation for
alfalfa (lower).
to simulate saline groundwater). Over the course of the study at
the 60 cm water table depths, water from the water table made
up 62 and 76% of alfalfa seasonal ET for the 0.65 and 6 ds m )
water, respectively. At the 105 cm water table depths, the water
table supplied 27 and 28% of the alfalfa's seasonal ET for the
0.65 and 6 ds m water, respectively.
Dardanelli and Collin (2002) conducted a study in the
Argentine Pampas to determine the water table contribution. to
alfalfa water use. The study took place on four different sites, one
where alfalfa did not have access to a water table and three with
water tables varying from 2.0 to 4.0 m. The study used a water
balance approach from the site without a water table to estimate
ET and then used measured biomass yield to determine the local
WUE. At the sites with a water table, water table contributions to
ET were estimated by dividing the dry matter biomass yield by the
local WUE then subtracting the change in soil water for the top
1.2 m. The average contribution to ET from the water table was
21% at the lower depths. This study found that water table contri-
bution to ET increased as current seasonal rainfall decreased.
While additional research is needed to understand the
contribution to ET from shallow groundwater, a relationship
between ET contribution from the water table and depth to
the water table was estimated for alfalfa based on results of
published data (Fig. 4). The ET contribution declines rapidly
as water table depth increases to about 100 cm and then levels
off to about 20% for depths up to 600 cm. There is also a strong
negative relationship between contribution to ET from the
water table and the amount of irrigation and precipitation
(Fig. 4), suggesting that the potential of significant ET contri-
bution from a water table would increase under controlled defi-
cit or partial season irrigation. Evapotranspiration determined
on the basis of applied irrigation will underestimate actual crop
Agronomy Journal • Volume 103, Issue I • 201 1 49