MZ

Leaching with Subsurface Drip
IrrigaƟon under Saline, Shallow
Groundwater CondiƟons

S

S

:V

Blaine Hanson,* Jan W. Hopmans, and Jirka Šimůnek
One op on for coping with the high soil salinity levels caused by saline, shallow groundwater condi ons along the westside of the San Joaquin Valley is to convert from sprinkler or surface irriga on methods to drip irriga on. Experiments
in commercial fields revealed that subsurface drip irriga on of processing tomato (Lycopersicon esculentum Mill. var.
esculentum) is highly profitable under these condi ons compared with other irriga on methods. The experiments also
showed that li le or no field-wide leaching occurred, based on the conven onal or water balance approach to es ma ng the leaching frac on (LF), yet soil salinity measurements showed considerable leaching around the drip lines.
Actual LFs could not be calculated because LF, soil salinity, soil water content, and root density all varied with distanceand depth around the drip lines. Therefore, we conducted a numerical modeling study using the HYDRUS-2D computer
simula on model to evaluate leaching with drip irriga on under saline, shallow groundwater condi ons for diﬀerent
amounts of applied water, water table depths, and irriga on water salinity, described by the electrical conduc vity of
the irriga on water (ECiw). Results showed that LF values ranged from 7.7 to 30.9% as applied water amounts increased
from 60 to 115% of the poten al evapotranspira on (ETpot) for the ECiw = 0.3 dS m −1 irriga on water, even though thewater balance method showed no leaching for applied water amounts equal to or smaller than ETpot. The spa ally varying soil we ng pa erns that occur under drip irriga on caused the localized leaching, which was concentrated near
the drip line.
A: AW, applied irrigation water as a percentage of potential daily evapotranspiration; EC, electrical conductivity or salinity; ET,
evapotranspiration; LF, leaching fraction.

A

400,000 of irrigated land along the west side of
the San Joaquin Valley of California continues to be plaguedby saline soil conditions caused by saline, shallow groundwater(California Department of Water Resources, 2005). Artiﬁcial
subsurface drainage is not an option for controlling the salinity
and waterlogging problems caused by the shallow groundwater
because after more than 30 yr of research to date (2007), no economically, technically, and environmentally feasible drain waterdisposal methods exist. Thus, the drainage and salinity problems
must be addressed by better management of irrigation water
to control groundwater levels and reduce subsurface drainage,
increased crop water use of shallow groundwater without yield
reductions, and drainage water reuse for irrigation (Hanson andAyars, 2002). Schoups et al. (2005) concluded that continued
irrigation is not sustainable in these salt-aﬀected areas withoutchanging management practices.
B. Hanson and J.W. Hopmans, Dep. of Land, Air and Water Resources,
Univ. of California, Davis, CA 95616; J. Šimůnek, Dep. of Environmental
Sciences, Univ. of California, Riverside, CA 92521. Received 22 Mar. 2007.
*Corresponding author (brhanson@ucdavis.edu).
Vadose Zone J. 7:810–818
doi:10.2136/vzj2007.0053© Soil Science Society of America
677 S. Segoe Rd. Madison, WI 53711 USA.
All rights reserved. No part of this periodical may be reproduced ortransmi ed in any form or by any means, electronic or mechanical,
including photocopying, recording, or any informa on storage andretrieval system, without permission in wri ng from the publisher.
www.vadosezonejournal.org · Vol. 7, No. 2, May 2008810

One option is to convert from furrow or sprinkle irrigation
to drip irrigation. Drip irrigation can apply water both precisely
and uniformly at a high irrigation frequency compared withfurrow and sprinkler irrigation, thus potentially reducing subsurface drainage, providing better soil salinity control, and increasing
yield. The potential is not only governed by the technology, butalso by the design, installation, operation, and maintenance ofdrip systems. The main disadvantage of drip irrigation is its installation cost, which based on experience in California ranges from
US$1500 ha−1 to US$2500 ha−1. For drip irrigation to be at least
as proﬁtable as the other irrigation methods, more revenue fromhigher yields and reduced irrigation and cultural costs must occur.
Between 1998 and 2003, experiments in commercial ﬁelds
on the west side of the San Joaquin Valley evaluated the eﬀect ofsubsurface drip irrigation on processing tomato under saline, shallow groundwater conditions (Hanson and May, 2004; Hanson etal., 2006a). Subsurface drip irrigation of processing tomato underthese conditions was highly proﬁtable compared with sprinkle
irrigation. The average proﬁt of the three drip-irrigated ﬁelds
was $1195 ha−1 higher than for sprinkle irrigation. As a result,subsurface drip irrigation of processing tomato has increased considerably in the salt-aﬀected areas.
A key to the proﬁtable subsurface drip irrigation of tomato
under saline, shallow groundwater conditions is salinity control
in the root zone, which involves leaching salts from the root zone
by applying irrigation water in excess of the soil moisture depletion. The leaching fraction, used to quantify leaching adequacy,
is deﬁned as the amount of water that drains below the root zone
divided by the amount applied.

T
1. Seasonal applied water and evapotranspira on, and fieldwide leaching frac ons calculated from a water balance for thefour commercial sites, iden fied by the designa ons of BR, DI, DE,
and BR2.

Leaching fractions are determined in several ways for commercial ﬁelds. One approach is to measure the average root zone
soil salinity and the salinity of the irrigation water and then use
appropriate charts or equations to determine the leaching fraction(Ayers and Westcott, 1985). Under drip irrigation, however, soil
salinity, soil water content, and root density all vary around thedrip line, and as a result, uncertainty exists in the average root zone
salinity and thus, the leaching fraction under drip irrigation.
A second approach commonly used is the water balancemethod, which calculates the ﬁeld-wide amount of leaching as thediﬀerence between seasonal amounts of applied water and evapotranspiration. The crop evapotranspiration frequently is assumed
to equal the potential evapotranspiration (ETpot), calculatedusing crop coeﬃcients and a reference crop evapotranspiration(Allen et al., 1998).
Field-wide leaching fractions were calculated for the commercial ﬁelds of the previously mentioned experiments using
the water balance approach. The actual evapotranspiration wasdetermined using canopy growth rates and a calibrated computer evapotranspiration model. These calculations showed little
or no ﬁeld-wide leaching at most of the sites (Table 1), whichsuggests inadequate salinity control and raises questions about
the sustainability of drip irrigation under these saline, shallow
groundwater conditions.
Soil salinity measurements in the commercial ﬁelds (Fig. 1
and 2), however, clearly showed that substantial leaching occurred
near the drip lines because of the wetting patterns under drip
irrigation, and that the leaching was highly concentrated near the
drip line. It was found that soil salinity near drip lines depended
on the depth to the saline groundwater, the electrical conductivity(EC) or salinity ofthe shallow groundwater (ECgw), theirrigation watersalinity (EC iw ),and the amount of

F . 1. Measured distribu ons of soil salinity around drip lines for (A) average watertable depth = 2 m, irriga on water electrical conduc vity (ECiw) = 0.3 dS m−1, andgroundwater electrical conduc vity (ECgw)= 8 to 11 dS m−1; (B) water table depth
between 0.61 and 1 m, ECiw = 0.3 dS m−1,
and ECgw = 5 to 7 dS m−1; and (C) watertable depth between 0.61 and 1 m, ECiw
= 1.1 dS m−1, and ECgw = 9 to 16 dS m−1.
The dots are the drip line loca ons. Values
are EC of saturated extracts (dS m−1).

Year

1999
2000
2001
1999
2000
2001
2000
2001
2002

Seasonal applied
Seasonal
Leaching frac on
water
evapotranspira on———––––––––—— cm ———––––––––——
%
BR
41
52
0
43
54
0
52
58
0
DI
56
64
0
74
64
13.1
58
68
0
DE
73
61
13.6
56
59
0
BR2
59
62
0

applied irrigation water (AW). For a water table depth of about 2
m, soil salinity was low and relatively uniform throughout the soil
proﬁle even though ECgw ranged from 7.9 to 11.1 dS m−1 (Fig.
1A). Irrigation water salinity was 0.3 dS m−1, which reﬂected
the salinity of the surface irrigation water commonly used in
the salt-aﬀected area. For water table depths of 0.6 to 1 m, soil
salinity was the smallest near the drip line, but increased considerably with distance and depth from the drip lines for a condition
where ECiw = 0.3 dS m−1 and ECgw = 5 to 7 dS m−1 (Fig. 1B).
The observed high salinity levels in the soil proﬁles were due toupward ﬂow of the shallow saline groundwater. An ECiw = 1.1 dS
m−1 increased soil salinity near the drip line (Fig. 1C), and thevolume of leached soil near the drip line increased as the amountof applied irrigation water increased for a water table depth ranging from 0.46 to 0.6 m (ECiw = 0.52 dS m−1, ECgw = 8 to
11 dS m−1; Fig. 2). Because of the shallow groundwater depth,
the irrigation water displaced the shallow groundwater directly
beneath the drip line. The experiments inthe commercial ﬁelds also showed that aseasonal water application about equal
to the seasonal crop evapotranspiration
provided suﬃcient leaching near the drip
lines, yet minimized water table ﬂuctuations due to irrigation.
The soil salinity data from the commercial fields indicated that the waterbalance approach is not appropriate fordrip irrigation and that estimating actual
leaching fractions with drip irrigation
under saline shallow groundwater conditions using the traditional approaches
may
be diﬃcult if not impossible because
F . 2. Eﬀect of amount of applied irriga onofthespatial variability of soil salinity
water on the measured distribu ons of soil
under these conditions. Therefore, wesalinity around the drip line for (A) 589 mm
of applied water (about equal to the seasonal
conducted a numerical modeling study
evapotranspira on of processing tomato), andusing the HYDRUS-2D computer sim(B) 397 mm of applied water. Irriga on waterulation model to evaluate leaching withelectrical conduc vity (EC) = 0.52 dS m−1 and−1subsurface drip irrigation under saline,
groundwater EC = 8 to 11 dS m . The dots areshallow groundwater conditions for difthe drip line loca ons. Values are EC of saturated extracts (dS m−1).
ferent amounts of applied water, water

www.vadosezonejournal.org · Vol. 7, No. 2, May 2008

811

table depths, and irrigation water salinity and to estimate leaching
fractions with subsurface drip irrigation under these conditions.
The HYDRUS-2D model has been previously used in studies ofwater and chemical movement under drip irrigation using theidentical model domains and boundary conditions that were usedin this study (Gärdenäs et al., 2005; Hanson et al., 2006b).
This research speciﬁcally addresses the threat of soil salinization in the western San Joaquin Valley, where traditional
approaches to salinity and water table control through installation of subsurface drainage systems and drainage water disposal
facilities are not feasible. We believe that our study provides new
insights on the behavior of drip irrigation under saline, shallow
groundwater conditions.

of the dripper was assumed to be 1.1 cm and the irrigation discharge 11.975 L d−1. The drip irrigation was simulated assuming
an inﬁnite line source, which was shown previously by Skaggs
et al. (2004) to be a good representation of this drip irrigation
system. The line-source approach also was used in the previously
mentioned studies on water and chemical movement.
The zero pressure water table resulted in a constant pressure
boundary at the bottom of the domain. This boundary condition allowed drainage water to ﬂow out of the domain without
raising the water table, assuming that suﬃcient natural groundwater drainage was occurring. This approach provided an estimateof the potential leaching fraction below the drip line. A similar
response of little or no change in the water table depth due toirrigation was found in the previously mentioned drip irrigation
experiments in the commercial ﬁelds, except when overirrigation
occurred, suggesting adequate natural drainage at these locations.
Simulations were conducted during a 42-d period for watertable depths of 50 and100 cm, ECiw of 0.3, 1.0, and 2.0 dS m−1,
and AW amounts equal to 80, 100, and 115% of the potential
daily evapotranspiration rate (ETpot = 7.5 mm d−1). For theECiw = 0.3 dS m−1 scenario, we included a water applicationof 60% of ETpot. These ECiw values reﬂect the range found in
the commercial ﬁelds. Surface water with ECiw values generally
between 0.3 and 0.5 dS m−1 is the most common source of
irrigation water in the salt-aﬀected areas of the valley, hence our
focus was on the smaller irrigation water salinity. In some cases,
groundwater may be used with ECiw values generally between
1 and 2 dS m−1. The ECgw was assumed to be 8 dS m−1 forthe 100-cm water table depth and 10 dS m−1 for the 50-cm
depth, based on EC measurements of the shallow groundwater inthe commercial ﬁelds. Selected irrigation frequencies were twice
per week and daily for the 100- and 50-cm water table depth
scenarios, respectively. These parameters of water table depth,
groundwater and irrigation water salinity, and applied water wereselected because they are representative of the commercial ﬁelds.
Spatial distribution of salinity in the transport domain was simulated using the convection–dispersion equation for a nonreactive
tracer. Such simulations cannot account for complex processes
such as precipitation or dissolution of solid phases (e.g., gypsum
or calcite) or cation exchange.
Hydrostatic pressure head conditions were assumed for the
initial conditions. Initial soil water salinities were based on ﬁeld
measurements made in the spring, before drip irrigation, andMaterials and Methods
Soil water and soil water salinity distributions around thedrip line were modeled with the computer simulation model
HYDRUS-2D (Šimůnek et al., 1999). This software package
can simulate the transient two-dimensional movement of waterand nutrients in soils. In addition, the model allows speciﬁcation of root water uptake, which aﬀects the spatial distributionof water and soil water salinity between irrigation cycles. Fora detailed description of the application of HYDRUS-2D, werefer to the studies by Gärdenäs et al. (2005) and Hanson et al.
(2006b). The database of HYDRUS includes parameter values
that specify soil hydraulic properties of the loam soil type of thissimulation study, which was typical of the soils in the previously
discussed commercial ﬁelds. The van Genuchten–Mualem model(van Genuchten, 1980) described the soil water retention andunsaturated hydraulic conductivity relationships with the following hydraulic parameters: saturated hydraulic conductivity (Ks) =24.96 cm d−1, residual soil water content (θr) = 0.078, saturationsoil water content (θs) = 0.43, n = 1.56, α = 0.036 cm, and l =0.5. Longitudinal dispersivity was considered to be 5 cm andmolecular diﬀusion was neglected. Simulation output included
the spatial and temporal variations of soil water content, soil
water salinity, soil water pressure head, and the total water andsalt mass in the simulated soil proﬁle.
The simulated subsurface drip irrigation system design characteristics were typical of the drip systems used for processing
tomato, with a drip line depth of 20 cm and an emitter spacingof 30 cm. The simulated model domain was 100 cm deep and
75 cm wide (one half of the bed spacing used for tomato production; Fig. 3), representing the dominant presence of roots
based on ﬁeld observations. The transport domain was discretized
into 5000 ﬁnite elements with very ﬁne grid around the dripper(0.2 cm) and gradually increasing elements farther from the drip
(up to 4 cm). The simulations assumed zero water ﬂux boundary conditions along the vertical sides of the soil domain, a zeropressure groundwater table at either the 50- or 100-cm depthwith a hydrostatic pressure head distribution below and above
the groundwater table, and an atmospheric boundary conditionat the top of the domain, where ET = ETpot for unstressed root
water uptake conditions. It was assumed that the crop was fully
mature with a canopy coverage (percentage of soil shaded by the
canopy at midday) of 90% and that ET represents transpiration
only (i.e., evaporation was assumed to be zero). Drip irrigation
was represented using the system-independent time-variable ﬂux
boundary condition along the dripper circumference. The radius
www.vadosezonejournal.org · Vol. 7, No. 2, May 2008

F . 3. Model domain for the subsurface drip irriga on system.
812

were determined from EC measurements of the saturated soil
extract (ECe), the volumetric soil water content (θ), and the saturation percentage (SP) of collected soil samples. The SP was 53
and 62% for the 100- and 50-cm water table depths, respectively.
These values were measured in the commercial ﬁelds for similarwater table depths. The initial soil water salinity (ECsw) was calculated from:EC sw = (EC e )( SP ) θrow, Fig. 5). Water content also increased down to about 50 cm
deep directly below the drip line. Just before the next irrigation (t= 3.5 d), substantial drying occurred around the drip line becauseof root water uptake. Little root water uptake occurred beyond
about 20 cm from the drip line because of the low root density.
Similar wetting and drying cycles occurred throughout the total
simulation period of 42 d.
For the 50-cm-deep water table scenario (bottom row, Fig.
5), water content was at saturation below the 50-cm depth. Just
after irrigation (t = 0.28 d), soil water content was maximum near
the drip line. Before the next irrigation (t = 1 d), water content
decreased near the drip line, with the lowest water content near
the surface. Soil water content patterns throughout the simulation period (t = 40.28 d) were similar.

[1]

These initial conditions reﬂect the eﬀects of rainfall, sprinkle
irrigation (preplant and stand establishment), evaporation, andupward ﬂow from the shallow groundwater on soil salinity before
the start of drip irrigation.
The root distribution in Fig. 2 was described using the modelof Vrugt et al. (2001), with most roots extending 35 cm laterally
and 30 or 40 cm vertically for scenarios with the groundwatertable 50 and 100 cm deep, respectively (Fig. 4). The following
parameters of the Vrugt et al. (2001) model were used: Zm = 40
cm, Xm = 35 cm, x* = 5 cm, px = 1, z* = 25 cm, pz = 2. The reducing eﬀects of both soil water pressure head (Feddes et al., 1978)and osmotic head on root water uptake were included, assumingthat their eﬀects were multiplicative. The following parameters of
the Feddes et al. (1978) model were used: h0 = −1, hopt =−2, h2,high = −800, h2,low = −1500, h3 = −8000 cm; r2,high =0.5 cm d−1, and r2,low = 0.1 cm d−1. The threshold model(Maas, 1990) was used to described the osmotic eﬀects
using a threshold ECe = 2.5 dS m−1 and a slope of 9.9%.
These root distributions were used for all scenarios
because little information is available on the eﬀect of agiven scenario on root distribution, although in reality
the distributions may diﬀer with the amount of AW, theECiw, and the water table depth. Even though the root
distributions were constant, the spatial distribution of theroot water uptake varied between scenarios because of theeﬀects of both pressure head and osmotic
potential on root water uptake.
Observation nodes were selected at
speciﬁc locations in the domain to monitor
levels of soil water salinity and content withtime, as simulated by the HYDRUS model.
These nodes were located at distances of 0,
10, 20, and 30 cm from the center of thetomato bed and at depths of 0, 10, 20, 35,
and 60 cm, for a total of 20 nodes.

Soil Water Salinity

The initial ECe values varied between 5.5 dS m−1 (top 0.15
m of soil) and 8.7 dS m−1 (water table) for the 100-cm watertable scenario (data not shown). Converting the ECe data to soil
water EC (ECsw) resulted in corresponding initial ECsw values
varying from 7.1 dS m−1 near the soil surface to 8.0 dS m−1 (ECof the shallow groundwater) at the water table. Thus, the initial
ECsw values were nearly constant with depth (top row, Fig. 6).
F . 4. Root distribu on

used for the HYDRUS2D simula ons for the
100-cm water table depth
scenarios. Units on thescale are the percentagesof the total roots.

Results
Soil Water Content Pa erns

Because of the hydrostatic initial condition, the initial soil water content (t = 0)
increased with increasing depth for boththe 100- and 50-cm water table depth scenarios (Fig. 5). At the end of the ﬁrst drip
irrigation (t = 1 d) of the 100-cm watertable scenario, soil water content increased
substantially near the drip line, extending
to near the soil surface and about 30 cm
laterally at the depth of the drip line (topwww.vadosezonejournal.org · Vol. 7, No. 2, May 2008

F . 5. Simulated distribu ons of the volumetric soil water content in the domain for both the

100-cm water table depth scenario (upper contour plots) and the 50-cm water table depth
scenario (lower contour plots). Values in the contour plots are the mes (days) during thesimula on period. The black dots are the drip line loca ons. Water applica on = 100% andirriga on water electrical conduc vity = 0.3 dS m−1.
813

After the ﬁrst irrigation of the 100-cm
water table depth scenario (t = 1, top row,
Fig. 6), considerable localized leachingoccurred near the drip line, with ECsw
values smaller than 1 dS m−1 immediately adjacent to the drip line. Soil salinity
increased with distance from the emitter
and with soil depth. Salt leaching occurred
to about the 55-cm soil depth and to about
25 cm laterally away from the drip line.
Before the next irrigation, ECsw increasedslightly near the drip line because of root
water uptake. In addition to a downward
displacement of a low salt zone, a zone ofhigher soil salinity formed near the soil surface, extending to about 25 cm from thecenter of the soil bed. As time progressed(t = 39.4 d), leaching around the drip line
increased, with most of the leaching below
the drip line (Fig. 6). Simultaneously, F . 6. Simulated distribu ons of soil water salinity for the 100-cm water table depth scenario
the higher salt zone near the soil surface (upper contour plots) and the 50-cm water table depth scenario (lower contour plots). Values
increased in size, with salt concentrations in the contour plots are the mes (days) during the simula on period. The black dots are the drip
−1higher than those of the initial soil condi- line loca ons. Applied water = 100% and irriga on water electrical conduc vity = 0.3 dS m .
tions. No leaching occurred beyond about
50 cm from the drip line.
For the 50-cm water table depth scenario (bottom row, Fig.
100%), as would be expected (bottom row, Fig. 7). Values of6), the initial ECsw values increased from about 5.5 dS m−1 near
ECsw near the drip line were <1 dS m−1 for the low-salt irriga−1
the soil surface to about 10 dS m (EC of the groundwater)tion water (ECiw = 0.3 dS m−1), but increased to values >3 dS
at the water table depth. At the end of the ﬁrst irrigation (t =m−1 for ECiw = 2 dS m−1. The soil volume of high salinity near
0.28, bottom row, Fig. 6), localized leaching occurred aroundthe soil surface also increased as the ECiw increased, as would be
the drip line to about 10 cm horizontally from the drip line. The
expected. Similar behavior occurred for the 50-cm water table
volume of leached soil was smaller than that for the deeper water
depth scenarios (data not shown).
table scenario at the end of the ﬁrst irrigation due to the smaller
A cyclic behavior in ECsw with time was found at mostamounts of water applied during the daily irrigations. Just before
observation nodes for the 100-cm water table depth scenario,
the next irrigation (t = 1, bottom row, Fig. 6) aslight increase in ECsw occurred near the drip
line. As time progressed, the volume of leachedsoil increased, with applied water displacing
the shallow groundwater at depths below 50
cm. Much of the leaching occurred at depths
below the drip line (t = 40.28, bottom row, Fig.
6). Some leaching occurred above the drip line,
although salt accumulated near the soil surface.
The zone of accumulated salt extended nearly
30 cm from the center of the bed.
Increasing the amount of AW increased
the leached zone for the 100-cm water table
depth scenarios (top row, Fig. 7). Even for water
applications of 60% of ETpot (severe deﬁcit irrigation), however, some leaching occurred near
the drip line. Increasing the amount of AW
slightly aﬀected the salinity pattern above thedrip line, but greatly increased the leached zone
below the drip line. Similar behavior occurred
for the 50-cm water table depth scenarios (dataF . 7. Simulated distribu ons of soil water salinity (ECsw) at the end of the simula on
not shown). This behavior of both scenarios was period for (top row) amounts of applied water equal to60, 80, 100, and 115% of thesimilar to that of the ﬁeld data (Fig. 2).
poten al evapotranspira on (irriga on water electrical conduc vity [ECiw] = 0.3 dS m−
Increasing the ECiw increased ECsw near 1); and (bo om row) distribu ons of ECsw for ECiw = 0.3 dS m−1, 1.0 dS m−1, and 2.0 dS−1
the drip line for the 100-cm scenario (AW = m (applied water = 100%). The black dots are the drip line loca ons.
www.vadosezonejournal.org · Vol. 7, No. 2, May 2008814

reﬂecting the irrigation and subsequent root water uptake (Fig.
8, A1–D1). Values of ECsw near the surface increased with additional irrigations to values larger than the initial condition. At
the 10-cm depth, large ﬂuctuations occurred at all distances, butthey decreased in magnitude as distance increased. At 20 cm
deep (depth of the drip line), ECsw quickly decreased at the bed
center, reﬂecting the eﬀect of ECiw. The decrease became smaller
as distance from the bed center increased. Below the drip line,
ECsw decreased with time for all distances to values considerably
smaller than the initial condition because of leaching. The rateof decrease was smaller as the distance and depth below the drip
line increased.
The cyclic behavior was less pronounced for the 50-cm watertable depth scenarios because of the daily irrigations and smaller
water applications per irrigation (Fig. 8, A2–D2). Near the soil
surface, ECsw increased to levels greater than the initial condition.
The rate of soil salinity increase was largest at the bed center, andwas more gradual with time as lateral distance increased. At 10
cm deep, ECsw decreased with time except at the 30-cm distance,
while at 20 cm deep, ECsw decreased with time at distances near
the bed center but increased at distances ≥20 cm. Below the
drip line depth, ECsw decreased with time regardless of distancefrom the bed center. These trends were similar for the other water
applications and irrigation water salinities. At depths below the

drip line, however, ﬁnal values of ECsw depended on the amountof applied water and ECiw.
Salt Mass

The numerical solution provided the ﬁnal (t = 42 d) salt massin the soil domain from mass balance computations computedfrom salt input by the incoming irrigation water, salt leachingat the bottom of the simulated soil domain, and the initial saltmass. The total salt mass in the proﬁle decreased with time (Table
2 for the 100-cm water table depth) for all scenarios, with thesalt decrease proportional to applied irrigation water; however,diﬀerences were only minor for the ECiw = 2.0 dS m−1 scenario.
The mass balance results were similar for the 50-cm water table
depth (data not shown).
Above the drip line, the salt mass increased with time (Fig.
9A). For a given ECiw, the increase was the highest for the smallest water application. As expected, the salt mass above the drip
line also increased as ECiw increased, although only slight diﬀerences in the ﬁnal total salt mass occurred for the 80, 100, and110% ET scenarios at ECiw = 2 dS m−1. Similar results wereobtained for the 50-cm water table depth scenarios (data notshown). Our simulation results indicated that for deﬁcit water
applications, water ﬂow and salt transport into the zone above thedrip line was partially driven by capillary forces, causing upwardF . 8. Soil water salinity (ECsw) data with me at the observa on nodes for distances of (A1) 0, (B1) 10, (C1) 20, and (D1) 30 cm from the cen-ter of the bed at depths of 0, 10, 20, 35, and 60 cm for the 100-cm water table depth scenario (applied water [AW] = 100%, irriga on waterelectrical conduc vity [ECiw] = 0.3 dS m−1); and for distances of (A2) 0, (B2) 10, (C2) 20, and (D2) 30 cm from the center of the bed at depthsof 0, 10, 20, 35, and 60 cm for the 50-cm water table depth scenario (AW = 100%, ECiw = 0.3 dS m−1).

www.vadosezonejournal.org · Vol. 7, No. 2, May 2008815

2. Rela ve salt mass balance for the 100-cm water table
depth scenario. Values are expressed as a frac on of the ini almass, which was the same for all scenarios. The added salt is thesalt in the irriga on water. The amount of leached salt is the saltthat moved across the bo om of the domain.

Tsalt mass as AW increased. This salt decrease, however, was lesspronounced as the salinity of the applied water increased. Similar
results were obtained for the 50-cm water table depth, although
diﬀerences between ECiw values for a given amount of applied
water were slightly larger than those diﬀerences for the 100-cm
depth scenario (data not shown).

AppliedIni alAdded
Leached
Final
water
%
Irriga on water electrical conduc vity (ECiw) = 0.3 dS m −1
60
1
0.02
0.05
0.97
80
1
0.03
0.15
0.88
100
1
0.04
0.26
0.77
115
1
0.05
0.34
0.70
ECiw = 1.0 dS m −1
80
1
0.11
0.19
0.91
100
1
0.13
0.30
0.82
115
1
0.15
0.38
0.76
ECiw = 2.0 dS m −1
80
1
0.21
0.25
0.95
100
1
0.26
0.37
0.89
115
1
0.30
0.45
0.85

Water Balance and Leaching

Water uptake by roots showed a strong cyclic eﬀect between
irrigations for the 100-cm water table depth scenario (Fig. 10A).
Water uptake increased and time ﬂuctuations decreased as AW
increased except for AW = 60%. The increase in the root uptake
with AW reﬂects the larger values of soil water content near the
drip lines. Root water uptake decreased as the salinity of theirrigation water increased, because of increasing salt stress eﬀects.
We note that for no-stress soil conditions, root water uptake must
be equal to 7.5 mm d−1 (ETpot). The results of the 50-cm watertable depth scenarios (Fig. 10B) were similar; however, ﬂuctuations between the daily irrigations were much smaller due to theirrigation frequency. Maximum root water uptake values between
irrigations for a given amount of applied water and ECiw were
smaller than those of the deeper water table scenarios.
Water uptake increased slightly with time for the ﬁrst 10 dfor AW = 100 and 115%, suggesting that leaching of the rootzone increased during that time period. The higher values of AW

and lateral salt transport, which resulted in higher salt masses
about the drip line for the deﬁcit irrigation conditions than for
the well-watered conditions.
As expected, the salt mass decreased with time below the
drip line (Fig. 9B) because of leaching, thereby decreasing theF . 9. The ini al and final rela ve salt masses for water applica ons
F . 10. Water uptake fluxes by roots for (A) 100-cm and (B) 50-cm
water table depth scenarios (applied water = 60, 100, and 115%;
irriga on water electrical conduc vity = 0.3 dS m−1). Posi ve values indicate water flow out of the domain; nega ve values indicate
water flow into the domain.

of 60, 80, 100, and 115% and irriga on water electrical conduc vity (ECiw) = 0.3, 1.0, and 2.0 dS m−1 for (A) above and (B) below
the drip line depth. Values are expressed as a frac on of the ini almass. Data for the 60% water applica on was determined for ECiw
= 0.3 dS m−1 only.
www.vadosezonejournal.org · Vol. 7, No. 2, May 2008816

= 115% may reﬂect more leaching for that scenario, althoughhigher soil water contents probably were also a factor. For AW= 60%, water uptake decreased with time during the ﬁrst 15 d,which may reﬂected limited leaching in the root zone along withsmaller soil water contents.
The water ﬂux at the domain bottom (i.e., leaching) showeda cyclic behavior caused by the periodic irrigations (Fig. 11A forthe 100-cm water table depth scenario), with ﬂux values increasing as the AW increased. Occasionally, water ﬂux was negative for
AW = 60%, indicating upward water ﬂow from the ﬁxed groundwater table. As the salinity of the irrigation water increased, theleaching amount increased because of reduced water uptake (datanot shown). A trend of decreasing water ﬂux occurred for the ﬁrst
10 to 15 d, but thereafter, little trend was found.
A cyclic water ﬂux behavior was also found for the 50-cmdepth scenarios (data not shown) although water ﬂux values wereconsiderably higher than for the 100-cm water table depth scenarios. Moreover, upward water and salt ﬂuxes were signiﬁcantly
higher than for the 100-cm water table depth scenarios.
As expected, salt transport across the bottom of the soil
domain was cyclic as well (Fig. 11B), with larger salt ﬂuxes as
AW increased. Solute ﬂuxes decreased with time, with the largest
decrease for AW = 115%. This behavior reﬂects a decrease in theamount of salt transported from the domain with time as the
localized reclamation of the soil proﬁle progressed.
Cumulative amounts of AW, root water uptake, and leaching at the bottom of the soil domain are presented in Table 3.
Seasonal root water uptake decreased as AW decreased, suggesting that shallow groundwater contribution to the crop’s water
demand is limited by both the soil’s hydraulic conductivity andassumed root zone distribution. As a result, we expect crop yield
to decrease as well. Experimental data obtained in the commercial
ﬁelds (Hanson and May, 2004; Hanson et al., 2006a) conﬁrmed alinear relationship between tomato yield and applied water, withyield decreasing as AW decreased.
We deﬁne the localized leaching fraction (LLF) as the actual
leaching fraction representative of the local irrigated root domain
near the drip line. The LLF values ranged from 7.7 to 30.9% as
AW increased from 60 to 115% for the 100-cm water table depth
scenario (ECiw = 0.3 dS m−1) and from 11.3 to 36.1% for the
50-cm water table depth scenario (ECiw = 0.3 dS m−1) (Table
3). As the salinity of the irrigation water increased, LLF values
increased due to reduced root water uptake for the higher salinity
levels (Table 3). The LLF values were generally higher for the 50cm water table depth scenarios due to reduced root water uptake
compared with the deeper water table scenarios (Table 3). Similar
LLF values were obtained using a water balance approach withthe drainage amount equal to the diﬀerence between cumulative
applied water and root uptake, as calculated by HYDRUS-2D.

3. Amount of applied water, root uptake, drainage, andlocalized leaching frac on (expressed as a percentage) for diﬀerent amounts of applied water (expressed as a percentage of thepoten al crop evapotranspira on), diﬀerent water table depths,
and diﬀerent irriga on water electrical conduc vi es (ECiw).
T

Applied
water
%
60
80
100
115
80
100
115
80
100
115
60
80
100
115
80
100
115
F . 11. Fluxes at the bo om of the domain of (A) water and (B)

80
100
115

solute for diﬀerent water applica ons (irriga on water electricalconduc vity = 0.3 dS m−1). Posi ve values indicate water flow out ofthe domain; nega ve values indicate water flow into the domain.
www.vadosezonejournal.org · Vol. 7, No. 2, May 2008

817

AppliedRoot
Drainage
Leaching
water
uptakefrac on———————— cm2 ————————%
Water table depth = 100 cm, ECiw = 0.3 dS m −1
1410
1350
109
7.7
1890
1580
328
17.3
2360
1760
600
24.5
2710
1850
837
30.9
Water table depth = 100 cm, ECiw = 1.0 dS m −1
1890
1440
431
22.8
2360
1610
719
30.4
2710
1700
962
35.4
Water table depth = 100 cm, ECiw = 2.0 dS m −1
1890
1260
585
30.9
2360
1390
899
38.1
2710
1480
1150
42.4
Water table depth = 50 cm, ECiw = 0.3 dS m −1
1414
1263
161
11.3
1886
1474
418
22.2
2357
1642
724
30.7
2711
1738
979
36.1
Water table depth = 50 cm, ECiw = 1.0 dS m −1
1886
1360
531
28.1
2357
1508
853
36.2
2711
1597
1116
41.2
Water table depth = 50 cm, ECiw = 2.0 dS m −1
1886
1197
690
36.6
2357
1326
1029
43.7
2711
1407
1301
48.0

Discussion

occurs. These results show that assumption is not true. Becauseof the spatially varying soil water wetting around drip lines, theirrigation eﬃciency, deﬁned as the ratio of the cumulative root
water uptake to the applied water, was 74.6 and 69.7% for the
100- and 50-cm water table scenarios, respectively. Very highirrigation eﬃciencies occurred only under severe deﬁcit irrigation conditions. Because of high frequency irrigation, however,the volume of drainage per irrigation is small and the drainage
is distributed evenly across the irrigation season. As a result of
this behavior, the natural subsurface drainage in the previously
discussed commercial ﬁelds appeared to be suﬃcient to prevent
groundwater intrusion into the root zone.

Our results showed that the water balance approach to determining leaching fractions is inappropriate for drip irrigation.
Both the simulation and experimental data by Hanson and May(2004) and Hanson et al. (2006a) demonstrated that considerable localized leaching occurs near the drip lines, including underdeﬁcit irrigation conditions (AW < ETpot). We showed that forwater applications of 100% or smaller (ECiw = 0.3 dS m−1), the
LLFs ranged from 7.7 to 11.3% for AW = 60%, 17.3 to 22.2%
for AW = 80%, and 24.5 to 30.7% for a 100% water application.
Localized leaching fractions increased as the salinity of the irrigation water increased because of the corresponding reduction of
the root water uptake. When using the conventional deﬁnition of
LF, as generally applicable to low-frequency sprinkle and surface
irrigation methods, computations of LF for water applications
equal to 100% of ETpot or smaller will result in zero leachingvalues using the water balance approach.
The spatially variable soil wetting patterns that occur underdrip irrigation lead to localized leaching below the drip line, inspite of applying irrigation water at rates <ETpot. The localizedleaching is concentrated near the drip line, as indicated by the
ECsw patterns (Fig. 6), thus resulting in a potential for a greater
reduction of salts in the root zone than would occur for sprinkle
or surface irrigation methods for a given AW amount.
Our simulation results also showed salt accumulation above
the drip line, near the soil surface, potentially negatively aﬀecting crop establishment. To prevent this, periodic leaching isneeded by either seasonal rainfall or a preplant sprinkle irrigation, moving the accumulated salt below the drip line and awayfrom the rooting zone.
The simulation results also showed that larger water applications per irrigation event at a relatively low irrigation frequency
reclaim the soil faster than smaller applications at a higher frequency (Fig. 6). Thus, several large applications of about 24 heach should be applied for new drip systems installed in a saline
soil to quickly reclaim the soil near the drip line.

References
Allen, R.G., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop evapotranspiration.
FAO Irrig. Drain. Pap. 56. FAO, Rome.
Ayers, R.S., and D.W. Westcott. 1985. Water quality for agriculture. FAO Irrig.
Drain. Pap. 29. Rev. 1. FAO, Rome.
California Department of Water Resources. 2005. Agricultural drainage reduction and reuse program. Available at www.owue.water.ca.gov/agdrain/index.cfm (veriﬁed 18 Feb. 2008). California Dep. of Water Resources,
Sacramento.
Feddes, R.A., P.J. Kowalik, and H. Zaradny. 1978. Simulation of ﬁeld water use
and crop yield. John Wiley & Sons, New York.
Gärdenäs, A., J.W. Hopmans, B.R. Hanson, and J. Šimůnek. 2005. Two-dimensional modeling of nitrate leaching for diﬀerent fertigation scenarios under
micro-irrigation. Agric. Water Manage. 74:219–242.
Hanson, B.R., and J.E. Ayars. 2002. Strategies for reducing subsurface drainage in irrigated agriculture through improved irrigation. Irrig. Drain. Syst.
16:261–277.
Hanson, B.R., R.B. Hutmacher, and D.M. May. 2006a. Drip irrigation of tomato and cotton under shallow saline ground water conditions. Irrig. Drain.
Syst. 20:155–175.
Hanson, B.R., and D.M. May. 2004. Eﬀect of subsurface drip irrigation on
processing tomato yield, water table depth, soil salinity, and proﬁtability.
Agric. Water Manage. 68:1–17.
Hanson, B.R., J. Šimůnek, and J.W. Hopmans. 2006b. Numerical modeling of
urea–ammonium-nitrate fertigation under microirrigation. Agric. Water
Manage. 86:102–113.
Maas, E.V. 1990. Crop salt tolerance. p. 262–304. In K.K. Tanji (ed.) Agricultural salinity assessment and management. ASCE Manuals and Rep. on
Eng. Practice 71. Am. Soc. Civil Eng., New York.
Schoups, G., J.W. Hopmans, C.A. Young, J.A. Vrugt, W.W. Wallender, K.K.
Tanji, and S. Panday. 2005. Sustainability of irrigated agriculture in the
San Joaquin Valley, California. Proc. Natl. Acad. Sci. 102:15352–15356.
Šimůnek, J., M. Šejna, and M.Th. van Genuchten. 1999. The HYDRUS-2D
software package for simulating two-dimensional movement of water, heat,and multiple solutes in variable saturated media. Version 2.0. IGWMCTPS-53. Int. Ground Water Modeling Center, Colorado School of Mines,
Golden.
Skaggs, T.H., T.J. Trout, J. Šimůnek, and P.J. Shouse. 2004. Comparison of HYDRUS-2D simulations of drip irrigation with experimental observations.
J. Irrig. Drain. Eng. 130:304–310.
van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898.
Vrugt, J.A., M.T. van Wijk, J.W. Hopmans, and J. Šimůnek. 2001. One-, two-,and three-dimensional root water uptake functions for transient modeling.
Water Resour. Res. 37:2457–2470.

Conclusions
This analysis using numerical simulations of various subsurface drip irrigation scenarios included two shallow groundwatertable depths, a range of irrigation water and groundwater salinities, and various water application values. The studies provided
valuable insights into salt leaching processes with subsurface
drip irrigation under saline, shallow groundwater conditions,
supported by experimental data for similar soil and irrigationwater conditions. The main conclusions derived were that (i) salt
reclamation near the drip lines occurs quite rapidly after the onsetof irrigation; (ii) large applications per irrigation event reclaim
the soil more quickly than do smaller applications at a higher frequency; (iii) the size of the leached soil region near the drip line iscontrolled by the amount of applied water; (iv) soil salinity of theleached soil zone increases as the salinity of the irrigation water
increases; (v) localized salt leaching around the drip line occurs
even for deﬁcit irrigation conditions; and (vi) the conventional
method for calculating leaching fractions, used for sprinkle andsurface irrigation methods, is inappropriate for drip irrigation.
A common assumption is that an amount of applied water
equal to 100% ETpot results in an irrigation eﬃciency of 100%for drip irrigation, and that little drainage below the root zone
www.vadosezonejournal.org · Vol. 7, No. 2, May 2008818