California Agriculture
Volume 63, Number 3

2009

Page 131

Drip irrigation provides the salinity control
needed for profitable irrigation of tomatoes in
the San Joaquin Valley
Blaine R. Hanson
Jan W. Hopmans

Don E. May

Jirka Simunek

Robert B. Hutmacher

Copyright c 2009 by Regents of the University of California, unless otherwise noted. This
article is part of the collected publications of California Agriculture. California Agriculture isarchived by the eScholarship Repository of the California Digital Library.

Abstract
Despite nearly 30 years of research supporting the need for subsurface drainage-water disposal facilities, the lack of these facilities continues to plague agriculture on the San Joaquin
Valley’s west side. One option for coping with the resulting soil salinity and shallow water-tableproblems is to convert from furrow or sprinkle irrigation to drip irrigation. Commercial fieldstudies showed that subsurface drip systems can be highly profitable for growing processing
tomatoes in the San Joaquin Valley, provided that the leaching fraction can achieve adequate
salinity control in the root zone. Computer simulations of water and salt movement showed
localized leaching fractions of about 25% under subsurface drip irrigation, when water applications equaled the potential crop evapotranspiration. This research suggests that subsurface
drip irrigation can be successfully used in commercial fields without increasing root-zone soil
salinity, potentially eliminating the need for subsurface drainage-water disposal facilities.
Keywords: drip irrigation, salinity, processing tomatoes
Suggested Citation:
Blaine R. Hanson, Don E. May, Jirka Simunek, Jan W. Hopmans, and Robert B. Hutmacher(2009) “Drip irrigation provides the salinity control needed for profitable irrigation of tomatoes
in the San Joaquin Valley”, California Agriculture: Vol. 63: No. 3, Page 131.
http://repositories.cdlib.org/anrcs/californiaagriculture/v63/n3/p131

Review ArticletDrip irrigation provides the salinity control needed forprofitable irrigation of tomatoes in the San Joaquin Valley
by Blaine R. Hanson, Don E. May, Jirka
Šimů nek, Jan W. Hopmans and Robert B.
Hutmacher

Despite nearly 30 years of researchsupporting the need for subsurface
drainage-water disposal facilities, thelack of these facilities continues toplague agriculture on the San Joaquin
Valley’s west side. One option for

Subsurface drainage systems and drainage-waterdisposal methods are not needed for properly
designed and managed drip irrigation systems.

coping with the resulting soil salinity
and shallow water-table problems is
to convert from furrow or sprinkle
irrigation to drip irrigation. Commercial field studies showed that
subsurface drip systems can be highly
profitable for growing processing
tomatoes in the San Joaquin Valley,provided that the leaching fraction
can achieve adequate salinity control
in the root zone. Computer simulations of water and salt movement
showed localized leaching fractionsof about 25% under subsurface drip
irrigation, when water applications
equaled the potential crop evapotranspiration. This research suggeststhat subsurface drip irrigation can be
successfully used in commercial fields
without increasing root-zone soil
salinity, potentially eliminating theneed for subsurface drainage-waterdisposal facilities.

The lack of widespread subsurface
drainage-water disposal facilities
continues to plague agriculture along
the west side of the San Joaquin Valley. Despite more than 30 years of
research, drainage-water disposal
methods that are economically, technically, politically and environmentallyfeasible have not been implemented. In
some areas, land retirement has been
the result.

Subsurface drip irrigation is allowing San Joaquin Valley tomato growers to apply water precisely
and uniformly, increasing yields and reducing the runoff of saline drainage water.

A UC study (Schoups et al. 2005)
concluded that a salt balance must be
maintained in the root zone for productive cropping systems to continue, andirrigation without improved management practices cannot be sustainedin the San Joaquin Valley. The only
options available to address salinity and drainage problems without
retiring land are: (1) reducing drainage through the better managementof irrigation water; (2) increasing theuse of shallow groundwater for crop
irrigation without any yield reductions; and (3) reusing drainage water.
All three methods require adequate
salinity control in the root zone. This
study is an example of the first option;

as a result, subsurface drip irrigationis commonly used in salt-affected soils
for processing tomato production. The
second option has been proposed, butlittle information exists on its use by
growers. The California Department of
Water Resources is promoting the third
option, but its use is limited and still inan experimental stage.
One way to implement option one isto convert from furrow or sprinkle irrigation to drip irrigation. Drip irrigation
applies water precisely and uniformly athigh frequencies, potentially increasing
yield and reducing root-zone soil salinity and drainage. These advantages are
not only governed by the technology, butalso by the design, installation, opera-

http://californiaagriculture.ucanr.org • July–September 2009 131

Subsurface drainage systems and drainage-waterdisposal methods are not needed for properly
designed and managed drip irrigation systems.
rigation of cotton assume an additional
economic risk.
In 2008, the Westlands Water
District — which encompasses morethan 600,000 acres of farmland inwestern Fresno and Kings counties —reported 37,396 acres of cotton and
86,011 acres of processing tomatoes,
now the largest single crop acreage;
cotton production has decreased substantially in recent years (Westlands
Water District 2009). Because processing tomatoes are a higher value cropthan cotton, subsurface drip irrigation offers potentially higher profits.
However, unlike cotton, tomatoes aremoderately sensitive to soil salinity,
and reduced tomato yields can result.
The threshold electrical conductivity(EC), which represents the maximum
root-zone soil salinity at which yield
is not reduced, is 2.5 deciSiemens permeter (dS/m) for tomato compared to
7.7 dS/m for cotton (Mass and Grattan
1999).

Andros Engineeringtion and maintenance of drip systems.
The main disadvantage of drip irrigation
is its high installation cost, which ranges
from $600 to $1,000 per acre. Subsurface
drip irrigation, commonly used for processing tomatoes, involves placing drip
lines 8 to 12 inches below the soil surface
directly below the plant row; surface
drip irrigation involves placing the drip
lines on the soil surface.
In the late 1980s, two large-scalecomparisons of subsurface drip andfurrow irrigation were conducted incotton under saline, shallow groundwater conditions (Fulton et al. 1991; Styles
et al. 1997). Drip irrigation consistently
resulted in higher cotton yields withless water application than furrow irrigation. However, the profit with furrow irrigation was much higher at one
location, and drip irrigation was only
slightly more profitable at the other.
The cost of the drip systems played the
major role in their profitability. As aresult, growers who convert to drip ir-

Specialized equipment (shown here, by Andros Engineering) is used to install drip tape
8 to 12 inches below the soil surface, at a cost of about $600 to $1,000 per acre. Despite thisprice, studies show that improved irrigation efficiency and yield benefits increase profits forgrowers in the San Joaquin Valley, compared with sprinkle or furrow irrigation.

132

CALIFORNIA AGRICULTURE • VOLUME 63, NUMBER 3

Between 1998 and 2003, experiments in commercial fields in theWestlands Water District, on the San
Joaquin Valley’s west side, evaluatedsubsurface drip irrigation of processing tomatoes under saline, shallow
groundwater conditions. In addition,
starting in 2006, computer simulations using the HYDRUS-2D model(Šimůnek et al. 1999) evaluated leachingwith subsurface drip irrigation underthese conditions. This model has beenused previously in studies of waterand chemical movement under drip irrigation (Gärdenäs et al. 2005; Hanson,Šimůnek, et al. 2006). We present a review of this research.
Commercial field experiments

Experiments in three commercial
fields (sites BR, DI and DE) comparedsubsurface drip irrigation to sprinkle
irrigation (Hanson and May 2003, 2004).
Drip systems ranged from 40 to 80
acres each in area, and sprinkle irrigation was used for the rest of the fields.
Water table depths ranged from 2 to
6 feet. Electrical conductivity ranged
from 0.3 dS/m for irrigation water from
Westlands Water District to 1.1 dS/m forwell water, and from 4.0 to 16.4 dS/m inthe shallow groundwater. A small-scale,randomized, replicated experiment was
conducted in each drip-irrigated field
to investigate the relationship between
yield, soluble solids (a measure of yield
quality) and applied water. The soil
type was clay loam at the three experimental sites.
We found that subsurface drip irrigation was highly profitable for processing tomatoes under these shallow,
saline groundwater conditions compared to sprinkle irrigation. Average
yields were 40.5 tons per acre for subsurface drip irrigation versus 33.9 tons
per acre for sprinkle irrigation, with
$484 per acre more profit on averagefor drip than sprinkle irrigation. The
average difference in soluble solids
between the two irrigation methods
was not significant. The small-scale
experiments showed increased yieldand decreased soluble solids as applied
water increased.
Yields of the drip-irrigated fields
were monitored for 2 more years afterTABLE 1. Seasonal applied water,
evapotranspiration and leaching fractions
calculated from a water balance for fourcommercial sites

Year*

Seasonal
applied water

SeasonalET†. . . . . . . . inches . . . . . . . .
BR
1999
2000
2001
DI
1999
2000
2001
DE
2000
2001
BR2
2002

Leaching
fraction‡
%

16.0
16.8
20.5

20.3
21.4
22.9

0
0
0

22.2
29.0
22.9

25.1
25.2
26.6

0
13.1
0

28.8
22.1

24.2
23.1

13.6
0

23.2

24.3

0

* BR, DI, DE and BR2 are site designations for thecommercial fields.
† Evapotranspiration.
‡ Zero values indicate no leaching, which occurred becauseseasonal applied water values were smaller than seasonalevapotranspiration.

Determining leaching fractions

Salinity control is needed in theroot zone to maintain profitable subsurface drip irrigation of tomatoes insalt-affected soils. This can be achievedby leaching or flushing salts from theElectrical conductivity (dS/m)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Plant row
–5

A

–10

Drip line

–15
–20
–25

Depth (inches)water was 8 to 10 dS/m (the thresholdfor cotton is 7.7 dS/m).
At all commercial sites, tomato yields
increased as applied water increased.
Factors contributing to this finding
included higher soil-water content andreduced root-zone soil salinity due tolarger zones of low salt around the drip
lines as more water was applied. Cottonyields, however, were unresponsive tothe amount of applied water, reflecting cotton’s salt tolerance and ability
to utilize saline, shallow groundwater(Wallender et al. 1979). Consequently,
contributions by the saline, shallow
groundwater to crop evapotranspiration should be minimized for tomato
and maximized for cotton.
Soil salinity levels around the drip
lines depended on the depth to groundwater, salinity of shallow groundwater,
salinity of irrigation water and amountof applied water. For a water table
depth of about 6 feet, relatively uniform soil salinity occurred throughout
the profile, with values smaller than
the threshold electrical conductivity oftomato (fig. 1A). For water table depths
less than about 3 feet, relatively low
levels of soil salinity occurred near
the drip line, but values increased tohigh levels beyond the wetting pattern due to the upward flow of shallow
groundwater (figs. 1B and 1C). Higher
soil salinity occurred near the drip line
when the salinity of the irrigation water increased (fig. 1C). Larger amounts
of applied water increased the zone of
low-salt soil near the drip line, evenwhen shallow water tables had depthsof less than 2 feet (fig. 2).
At all sites, water table depth
showed little response to drip irrigation, except when overirrigation occurred during one year at site BR (datanot shown). A subsequent reduction
in applied water at that site caused thewater table to decline due to reduced
percolation and the natural drainage ofshallow groundwater.

–5

B

–10
–15
–20
–25

–5

C

–10
–15
–20
–25
–25 –20 –15 –10 –5

0

5

10

15 20

25

Distance from drip line (inches)

Fig. 1. Soil salinity/electrical conductivity (EC)around the drip line for water depth of about(A) 6 feet, EC irrigation water = 0.3 dS/m, EC
groundwater = 8 to 11 dS/m; (B) 2 to 3 feet, EC
irrigation water = 0.3 dS/m, EC groundwater = 5
to 7 dS/m; and (C) 2 to 3 feet, EC irrigation water= 1.1 dS/m, EC groundwater = 9 to 16 dS/m.

Electrical conductivity (dS/m)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Plant row

Drip line

A (23.2 inches)–10

Depth (inches)the first year. Yields remained highexcept for one site, which had 2 yearsof reduced yields due to late plantings.
We did not find any trends toward yield
reductions with increased soil salinity
near the drip lines, which ranged fromvalues less than, to higher than, thethreshold electrical conductivity of 2.5
dS/m for tomatoes.
At a fourth commercial field (siteBR2), a small-scale, randomized-block,
replicated experiment evaluated the
response of tomato and cotton yields
to different amounts of applied water under very shallow groundwaterconditions of 18 to 24 inches (Hanson,
Hutmacher, et al. 2006). The soil type
was clay loam. Tomato yields ranged
from 34.6 tons per acre for 15.6 inchesof applied water to 42.8 tons per acrefor 23.2 inches, even though nearsaturated, highly saline soil occurred
at only 18 inches deep. At 23.2 inches,water application is about equal to theseasonal evapotranspiration or cropwater use for tomatoes. However, cotton yields did not respond when water
was applied at amounts equal to or
greater than about 40% of the potential seasonal evapotranspiration. The
electrical conductivity of the irrigationwater was 0.5 dS/m and of the ground-–20

B (15.6 inches)
–10

–20

–30 –25 –20 –15 –10 –5

05

10 15 20 25 30

Distance from drip line (inches)

Fig. 2. Soil salinity/electrical conductivity (EC)around the drip line for water depth of about
18 to 24 inches, EC irrigation water = 0.5 dS/m,
EC groundwater = 8 to 10 dS/m, for water
applications of (A) 23.2 and (B) 15.6 inches.

http://californiaagriculture.ucanr.org • July–September 2009 133

Electrical conductivity (dS/m)a daily evapotranspiration rate of 0.29
inches per day, but the actual simula0
tions varied by applied water amounts–5and irrigation frequency. The applicaDrip–10 linetion rate was constant during the simu–15
lation period for a particular scenario–20
consisting of a water table depth, an–25
irrigation water salinity and an applied–30
water amount.
–35
We simulated two irrigations per
A
B
C
D
week
for a 40-inch water table depth,
0 5 10 15 20 25
0 5 10 15 20 25
0 5 10 15 20 25
0 5 10 15 20 25
and daily irrigations for the 20-inch
Distance from drip line (inches)
depth. These frequencies reflect thoseFig. 3. Soil-water salinity/electrical conductivity (EC) around the drip lines at (A) start ofused in the commercial field experisimulation period (T = 0 day), (B) just after first irrigation (T = 1 day), (C) just before second
ments (Hanson et al. 2003). The drip
irrigation (T = 3.5 days) and (D) just after last irrigation (T = 39.5 days). Applied water = 100%line was 8 inches deep, and electricalevapotranspiration; EC irrigation water = 0.3 dS/m.
conductivity of the shallow groundwater was 10.0 and 8.0 dS/m for theroot zone — applying irrigation waterlong drip irrigation can be sustained
20- and 40-inch water table depths,
in amounts exceeding the soil moisture under saline, shallow groundwater
respectively, based on measured levels
depletion. The leaching fraction is used conditions. The soil salinity data, howto quantify leaching adequacy, and isever, clearly showed that because of the in the commercial fields. The initialsoil-water salinity levels at the start
derived from the ratio of the amount of wetting pattern under drip irrigation,of the simulation period were based
water that drains below the root zone to leaching was highly concentrated nearon samples collected in spring, prior
the amount of water applied.
the drip line (referred to as “localized
Leaching fractions can be determined leaching”). The soil salinity data also in- to drip irrigation. The simulated root
several ways. One approach is to meadicated that the water balance approach distribution was based on field dataof rooting patterns for drip-irrigatedsure the average salinity of the root-zone is not appropriate for drip irrigation
tomatoes at the UC West Side Researchsoil and irrigation water, and then use
and that estimating actual or localized
appropriate charts or equations to deter- leaching fractions under drip irrigation and Extension Center (Hanson and
May 2007).
mine the leaching fraction. However, soil may be difficult and also inaccurate. It
Simulated reclamation (salt removal)salinity, soil-water content and root den- is reasonable to expect that the salinof soil near the drip line was rapid,
sity all vary around the drip line, result- ity patterns reflect long-term behavior,and the simulated salinity patterns
ing in uncertainty about the accuracy of as long as adequate salinity-control
were consistent with those found inroot-zone soil salinity.
measures (sufficient leaching and nothe commercial fields (fig. 3) (Hanson
A second approach commonly
groundwater intrusion into the root
et al. 2008). The simulations predicted
used is the water balance method, byzone) prevent salts from accumulatingthat the volume of reclaimed soil would
which a fieldwide amount of leachingin the root zone.
increase over time, with most reclamais calculated as the difference between
Computer simulations
tion occurring below the drip line, andthe seasonal amount of applied water(measured with a flow meter) and evapoBecause of the difficulties in estimat- that salts would accumulate near thesoil surface. Large seasonal applicationstranspiration. Because actual evapoing actual leaching fractions for theof water would increase the zone oftranspiration in a given field is usually
drip-irrigated commercial fields, we
unknown, it is frequently estimated usused the computer model HYDRUS-2D lower-salinity soil near the drip lines,
consistent with our field data. But theing crop coefficients and a reference crop (Šimůnek et al. 1999) to simulate thelarger amounts would have little effectevapotranspiration value obtained from movement of water and salt in soil unon the volume of reclaimed soil above
the California Irrigation Management
der drip irrigation for a 42-day period
the drip line. As expected, salinity near
Information System (CIMIS).
and quantify drainage below the root
the drip line would increase as irrigaWe calculated fieldwide leachingzone. Simulations were conducted fortion water salinity increased. The root
fractions for the commercial fields
water table depths of 20 and 40 inches;using the water balance method.
irrigation water salinities of 0.3, 1.0 and uptake of soil water would decrease as
applied water decreased, suggesting
Evapotranspiration was determined
2.0 dS/m; and applied water at 80%,
using canopy growth rates and a cali100% and 115% of potential evapotrans- the potential for decreased yields withdecreasing water applications, as wasbrated computer model. These calcupiration. For 0.3 dS/m irrigation water,
lations showed little or no fieldwide
we conducted an additional simulation found in our commercial field data forprocessing tomatoes.
leaching at most of the sites (table 1),of applied water at 60% of potential
The actual or localized leachingwhich suggests inadequate salinity
evapotranspiration. The depth of apfractions for the 40-inch water table
control and raises questions about how plication per irrigation was based on
1

2

3

4

5

6

7

8

Depth (inches)

0134

CALIFORNIA AGRICULTURE • VOLUME 63, NUMBER 3

9

10 11 12

scenarios were 7.7% for the 60% water
application treatment, 17.3% for the 80%
treatment, 24.5% for the 100% treatment
and 30.5% for the 115% treatment. As
irrigation water salinity increased, theactual leaching fraction increased asa result of reduced root-water uptake.
Even for water applications equal to or
smaller than 100% of potential evapotranspiration, drainage occurred below
the root zone due to the spatially variable wetting under drip irrigation.
A common assumption is that applying water at amounts equal to 100%of potential evapotranspiration resultsin irrigation efficiency of 100%, definedas the ratio of cumulative root-water
uptake to applied water. In cases ofdrip irrigation at 100% of potential
evapotranspiration, little drainage below the root zone is assumed to occur.
However, the computer simulations
showed that this assumption is nottrue. Because of spatially varying soilwater wetting around the drip lines, irrigation efficiency was 74.6% and 69.7%
for the 40- and 20-inch water tablescenarios, respectively, with the 100%water application. Very high irrigation
efficiencies occurred only under conditions of severe deficit irrigation.
Because of high-frequency irrigation, the volume of drainage per irrigation was small and drainage wasdistributed evenly over the irrigationseason. As a result, natural subsurface
drainage in the commercial fields was
sufficient to prevent groundwater intrusion into the root zone.

In both studies (field experiments
and computer simulations), considerable localized leaching occurred aroundthe drip lines, due to the wetting patterns of subsurface drip irrigation. The
localized or actual leaching fractions
determined from the computer simulations were about 25% to 30% for a water
application equal to 100% of potential
evapotranspiration.
Under subsurface drip irrigation ofprocessing tomatoes, localized leaching is highly concentrated near the
drip line, resulting in relatively low
soil-salinity levels in areas where root
density is highest. The water balance approach for estimating leaching amounts
is inappropriate for drip irrigation because of such localized leaching.
The computer simulations showedthat reclamation around drip lines insaline soil would be rapid. Predicted
reclamation was faster for relatively
infrequent large water applications perirrigation than for smaller applications.
The low-salt zone around the drip line

increased as the amount of applied water increased, and soil salinity aroundthe drip line increased as salinity of theirrigation water increased.
We found that very high irrigation
efficiencies under drip irrigation can
only be obtained by substantial deficit
irrigation, in contrast to the frequent assumption that drip irrigation is nearly
100% efficient for water applications
equal to about 100% of potential evapotranspiration.
Sustainable drip irrigation

The key to sustained subsurface
drip irrigation of processing tomatoes
in salt-affected soils is profitability,
which in turn depends on salinity control in the root zone. This requires irrigating with relatively low-salt water;
applying sufficient irrigation water foradequate localized leaching; leachingsalts that accumulate around the drip
line; and preventing saline, shallow
groundwater intrusion into the rootzone. The following are recommenda-

Leaching and efficient drip systems

The field research and computer
simulation modeling demonstrated
that subsurface drip irrigation of processing tomatoes is highly profitablecompared to sprinkle or furrow irrigation under saline, shallow groundwater conditions. Tomato yields increased
as applied water increased, and cottonyields were unaffected. These tomato
yield results suggest that root uptakeof saline, shallow groundwater should
be minimized to prevent yield reductions, while the cotton yield results
indicate that substantial root uptakeof the saline groundwater can occur
without yield reductions.

To minimize the uptake of shallow, saline groundwater — which can affect tomato yields —sufficient irrigation water must be applied in the root zone to ensure adequate leaching.
Above, filters, pumps and fertilizer tanks are part of drip irrigation systems.

http://californiaagriculture.ucanr.org • July–September 2009 135

tions for subsurface drip irrigation ofprocessing tomatoes under conditionsof the San Joaquin Valley’s west side:
Water applications. Seasonal water
applications should be about equal to
the seasonal evapotranspiration, which
is 25.5 inches in the San Joaquin Valley(Hanson and May 2006). This provides
sufficient localized leaching. Higher
applications could raise the water table,causing saline, shallow groundwater
intrusion into the root zone. Smaller applications reduce tomato yields.
Salinity of irrigation water. The electrical conductivity of irrigation water
should be about 1.0 dS/m or less; higher
levels may reduce yields.
Irrigation frequency. From daily
to two or three irrigations per weekshould occur after the start of drip irrigation (Hanson et al. 2003). Daily
irrigations are recommended for very
shallow, saline groundwater conditions.
The amount of water application perirrigation should be determined using
appropriate crop coefficients (Hanson
and May 2006) and the reference crop
evapotranspiration from CIMIS.
Salt leaching. Periodic leaching ofsalt accumulated above buried drip
lines will be necessary with sprinkle irrigation for stand establishment, if winter and spring rainfall is insufficient.
System maintenance. Drip irrigation
systems should be designed for a highuniformity of applied water, and should
be properly maintained to prevent emitter clogging.
Drainage-water disposal

Can drip irrigation eliminate theneed for expensive subsurface drainage
systems and drainage-water disposal
methods? We believe the answer is yes,since no subsurface drainage systems
were used at our sites. Subsurface drip
irrigation continues to be used at thesesites along with many other fields along
the San Joaquin Valley’s west side.
Drip irrigation resulted in littlechange to the water table at these sites(except at site BR, where overirrigation
occurred), and the computer simulations revealed that drainage or percolation below the root zone would occur.
The field data indicated that small ap136

plications of water per irrigation andrelatively uniform distribution of irrigations over time, coupled with natural
subsurface drainage, prevented groundwater intrusion into the root zone. Thisfinding suggests that, for the conditionsin these fields, subsurface drainage
systems and drainage-water disposal
methods are not needed for properly
designed and managed drip irrigation
systems.
These results indicate that subsurface drip irrigation of processing
tomatoes — a higher value, moderately
salt-sensitive crop compared to cotton — is sustainable in the salt-affectedsoils that we studied. Similar results
might be expected for crops of similarvalue that are moderately salt sensitiveand suitable for drip irrigation, such as
melon. Drip irrigation of salt-tolerant
crops such as cotton, sugar beets andgrain may not be profitable because of
their relatively low cash value. While

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little research has been conducted in the
San Joaquin Valley on drip irrigation ofsalt-sensitive crops under saline conditions, a literature review of numerous
studies on drip irrigation of vegetable
crops (Hanson et al. 2008) showed that
drip irrigation may be a sustainable
practice for salt-sensitive crops.

B.R. Hanson is Irrigation and Drainage Specialist,
Department of Land, Air and Water Resources,
UC Davis; D.E. May is Farm Advisor Emeritus, UC
Cooperative Extension; J. Šimů nek is Professor of
Soil Physics and Hydrologist, Department of Environmental Sciences, UC Riverside; J.W. Hopmans
is Professor of Water Management, Departmentof Land, Air and Water Resources, UC Davis; andR.B. Hutmacher is Extension Cotton Specialist, Department of Plant Sciences, UC Davis, and Director, UC West Side Research and Extension Center.
Support for this project was provided by the UC
Prosser Trust Fund; the Westlands Water District;and Farming D and Britz Farming Company, bothof Five Points, Calif.

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CALIFORNIA AGRICULTURE • VOLUME 63, NUMBER 3