Agricultural Water Management 74 (2005) 219–242
www.elsevier.com/locate/agwat

Two-dimensional modeling of nitrate leaching forvarious fertigation scenarios under micro-irrigation
A.I. Gärdenäs a, J.W. Hopmans b,*, B.R. Hanson b, J. Šimůnek caSwedish University of Agricultural Sciences, Department of Soil Sciences, 75007 Uppsala, Swedenb
Department of Land, Air and Water Resources, University of California,
123 Veihmeyer Hall, Davis, CA 95616, USAc
University of California, Department of Environmental Sciences, Riverside, CA 92521, USA
Accepted 25 November 2004

Abstract
The regular application of nitrogen fertilizers by irrigation is likely responsible for the increase innitrate concentrations of groundwater in areas dominated by irrigated agriculture. Consequently,
sustainable agricultural systems must include environmentally sound irrigation practices. To reduce
the harmful effects of irrigated agriculture on the environment, the evaluation of alternative irrigationwater management practices is essential. Micro-irrigation offers a large degree of control, enabling
accurate application according to crop water requirements, thereby minimize leaching. Furthermore,
fertigation allows the controlled placement of nutrients near the plant roots, reducing fertilizer losses
through leaching into the groundwater. The presented two-dimensional modeling approach provides
information to improve fertigation practices. The specific objective of this project was to assess the effectof fertigation strategy and soil type on nitrate leaching potential for four different micro-irrigation
systems. We found that seasonal leaching was the highest for coarse-textured soils, and conclude that
fertigation at the beginning of the irrigation cycle tends to increase seasonal nitrate leaching. In contrast,
fertigation events at the end of the irrigation cycle reduced the potential for nitrate leaching. For all
surface-applied irrigation systems on finer-textured soils, lateral spreading of water and nitrates wasenhanced by surface water ponding, causing the water to spread across the surface with subsequent
infiltration downwards and horizontal spreading of soil nitrate near the soil surface. Leaching potential
increased as the difference between the extent of the wetted soil volume and rooting zone increased.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Nitrate leaching; Fertigation scenarios; Micro-irrigation; Numerical modeling* Corresponding author. Tel.: +1 530 752 3060; fax: +1 530 752 5262.
E-mail address: jwhopmans@ucdavis.edu (J.W. Hopmans).
0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2004.11.011

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A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

1. Introduction
The quality of soils, ground and surface waters is specifically vulnerable in climatic
regions where agricultural production is possible only by irrigation such as in California(USA) and in many other (semi-) arid regions of the world. The regular excessive
application of nitrogen fertilizers with irrigation water is likely responsible for the increasein nitrate concentrations of groundwater resources in these areas. Specifically, non-point
source pollution of nitrate in groundwater is a major problem in many areas of California
such as the Salinas Valley, Santa Maria Valley, and along the eastside of the San Joaquin
Valley. As a result, nitrate concentrations in groundwater exceeds the drinking water
standard in these areas. Therefore, alternative irrigation water and soil management
practices are needed that tactically allocate water and fertilizers to maximize theirapplication efficiency, by minimizing fertilizer losses through leaching towards thegroundwater (Bar-Yosef, 1999).
The shape of the wetted soil volume under micro-irrigation and the spatial
distribution of soil water, matric potentials, and nitrate concentrations are dependent on
many factors, including soil hydraulic properties, emitter discharge rates, spacing, andtheir placement (above or below the soil surface), irrigation quantity and frequency, cropwater uptake rates and root distribution patterns. Water and nutrients should not beapplied in areas where roots are absent, or at a rate higher than the roots can possibly
take up. In general, root development under drip irrigation is constrained to the soilvolume wetted by the emitters, with root length density decreasing with depth (Goldberg
et al., 1971; Stevens and Douglas, 1994; Michelakis et al., 1993). A better understandingof the interactions of irrigation method, soil type, crop root distribution, and uptake
patterns and rates of water and nutrients will provide improved means for properand efficient micro-irrigation water management practices (Hopmans and Bristow,
2002).
Micro-irrigation (drip emitters, drip tape, and micro-sprinklers) has the potential ofprecisely applying water and chemicals both in amount and in location throughout a field.
Studies have shown the potential of high field-wide uniformity of applied water andchemicals to be higher when compared with other irrigation methods, provided that the
micro-irrigation systems are properly designed, managed, and maintained. For example,
the studies by Alsinan (1998) and Schwankl and Prichard (2001) demonstrated that fieldwide uniformity of applied chemicals is partly controlled by the on and off times of
injection, considering the travel time of applied water to move through the end-of-the-line.
Micro-irrigation systems can be designed and operated so that water and nutrients areapplied at a rate, duration and frequency, so as to maximize crop water and nutrient uptake,while minimizing leaching of nutrients and chemicals from the root zone of agricultural
fields. While high field-wide uniformities are possible under micro-irrigation, thedistribution of both water and nitrate about the drip line is very non-uniform. Both soil
moisture content and chemical concentration will be the highest near the drip line after
application, but will redistribute thereafter as controlled by soil physical properties.
Because of these types of non-uniform wetting patterns, it is possible that percolation
below the root zone and nitrate leaching occurs, despite that applied irrigation water isequal or less than crop ET. It is therefore essential to use a two-dimensional modelingA.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

221

approach to develop optimal fertigation practices. The leaching potential can bechanged by the timing of the fertigation, relative to the irrigation water application. A
specific example of this was presented by Cote et al. (2003), demonstrating that fertigation
at the beginning of the irrigation cycle might reduce nitrate leaching under specificconditions.
There are few soil and crop specific guidelines for designing and managing irrigation/
fertigation systems that minimize nitrate leaching, considering typical non-uniform
distributions of soil solution nitrate and crop uptake. Some studies have investigated the
distribution of fertilizer about the dripline (e.g. Clothier and Sauer, 1988; Mmoloawa
and Or, 2000), but few studies have investigated the effect of fertigation management/
irrigation management on the spatial distribution and crop availability of supplied
nitrogen (e.g. Somma et al., 1998; Mailhol et al., 2001; Li et al., 2004). Currently, thegrower’s incentive for adopting improved fertigation practices may be limited, sincefertilizer costs are only a small fraction of the total production costs and changes inproposed fertigation practices may not affect crop yield. However, when energy costs
and groundwater contamination regulations are incorporated, improved fertigation
practices may be essential.
The presented study will assist operators of micro-irrigation systems to better manage
fertigation, as well as their design, especially, to minimize nitrate leaching. The specificobjective of this study was to evaluate the controls of fertigation strategy and soil type on
nitrate leaching potential for four different micro-irrigation systems, each associated with atypical crop. The selected combinations of micro-irrigation system and crop are
representative for California conditions.

2. Materials and methods
The modeling of water flow and fertigation scenarios was conducted using an adapted
version of the computer simulation model, Hydrus-2D (Šimůnek et al., 1999). This
software package can simulate the transient two-dimensional or axi-symmetrical threedimensional movement of water and nutrients in soils. In presented applications, we
solely considered nitrate as applied by the micro-irrigation systems through fertigation.
In addition, the model allowed for specification of root water and nitrate uptake,affecting the spatial distribution of water and nitrate availability between irrigation
cycles. The soil hydraulic properties parameters for the various soil types that are
required for the simulation model were available in the model’s database. For each soil
type and emitter type, the spatial patterns of water content and nitrate concentration
were determined for various fertigation strategies. These strategies included different
injection durations, different injection times relative to the irrigation set time, anddifferent concentrations. Model simulations will be presented for four differentpressurized irrigation systems; each associated with a typical crop. These are surface
drip tape (SURTAPE; strawberry), subsurface drip tape (SUBTAPE; processing
tomatoes), surface drip emitter (DRIP; grape), and micro-sprinkler (SPR; citrus). Thesetypes were chosen as they represent a typical variety of commonly used micro-irrigation
systems in California.

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2.1. Micro-irrigation modelingConsidering two-dimensional soil water flow, the water flow equation is written as:




@u
@
@h
@
@h
@K
¼
Kr
Kz
WUðh; r; zÞ
þ

@t @r
dr
@z
@z
@z

(1)

where u is the volumetric soil water content (L3L3), K defines the unsaturated hydraulicconductivity function (LT1), h is the soil water pressure head (L), r is the lateralcoordinate, z is the vertical coordinate (positive downwards), t is time (T) and WU(h,r, z) denotes root water uptake (T1). Both K and WU are functions of u and/or h. The
subscripts r and z allows for the possibility to include soil anisotropy, i.e. to simulate waterflow with the unsaturated hydraulic conductivity function being different for the r- and zdirection. Two-dimensional flow was used for both the surface and subsurface drip tape
(SURTAPE and SUBTAPE) systems. The axi-symmetrical form of Eq. (1) was used tosolve for the radial flow equation to simulate water and nutrient distribution for both thesurface drip emitter (DRIP) and micro-sprinkler (SPR) system. For simplicity of notation,
we use the same spatial coordinate notation, r, for both radial and horizontal directions. In
both cases, Eq. (1) was solved with the Hydrus-2D model (Šimůnek et al., 1999) using theGalerkin finite element method based on the mass conservative iterative scheme proposed
by Celia et al. (1990). We used the hydraulic relationships as defined by van Genuchten(1980).
For all simulated scenarios, it was assumed that lateral water flow along the boundaries
was zero (zero flux boundary condition), and that the bottom boundary is defined by a unit
vertical hydraulic gradient, simulating free drainage from a relatively deep soil profile. The
top boundary condition representing irrigation events vary according to the simulated
irrigation system. However, for all scenarios, the crop-specific potential evapotranspirativeflux (ETc) was computed from the product of potential evapotranspiration (ET0) and thecrop coefficient (Kc). In all scenarios, except for DRIP, it was assumed that the potential
evapotranspiration was equal to the potential crop transpiration, Tpot, thus neglecting
evaporation. For the DRIP scenario, the potential ET was partitioned into potential
evaporation (Ep = 0.05ETc) and potential transpiration (Tp = 0.95ETc).
During irrigation, the particular boundary conditions representing various irrigation
systems were of the flux type, controlled by the constant volumetric application rate (Q0,
L3T1) of the emitter of each micro-irrigation system. The Hydrus-2D code was adapted toallow for a time-variable ponded boundary condition by computing wetted surface area.
This was done by switching from a Neumann (flux) to a Dirichlet (head) boundary
condition if the surface pressure head required to accommodate the specified emitter flux
for a surface node, is larger than 0. A sufficient number of surface nodes are switched in an
iterative way until the entire irrigation flux Q0 is accounted for, and the radius of the wettedarea is obtained. Since the infiltration flux into the dry soil is larger for early times, thewetted area continuously increases as irrigation proceeds.
Numerical solutions were confirmed through comparison of the wetted radius with thesteady state analytical solution of Wooding (1968). In addition, the Hydrus-2D code wasadapted to compute positive soil water backpressure values for the SUBTAPE system

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

223(Lazarovitch et al., 2005) that develop if the flow rate out of the drip tape is larger than therate at which the discharged water can move into the wetted soil surrounding the drip tape.
The local root water uptake term in Eq. (1) was computed from:
WUðh; r; zÞ ¼ gðhÞRDFðr; zÞWT pot(2)where g(h) is the so-called soil water stress response function (dimensionless) of Feddes
et al. (1978) that reduces root water uptake from its maximum possible value because ofsoil water stress. RDF denotes the normalized root water uptake distribution, Tpot is thepotential transpiration rate, and W is the width or radius of the soil surface, associated withthe transpiration process (as controlled by the lateral root distribution). Both g(h) and RDF
are a function of the spatial position within the multi-dimensional root zone domain. The
RDF function was normalized, so that when integrated over the root zone (V) domain, itsvalue is equal to 1, i.e.
RDFðr; zÞ ¼ R

bðr; zÞ
bðr;
zÞdV
V

(3)

where b characterizes the dimensionless spatial distribution of unstressed root water
uptake. For the non-uniform cases, RDF was defined by (Vrugt et al., 2001):





z
r
bðr; zÞ ¼1
(4)1
eððpz =zm Þjz zjþðpr =rm Þjr rjÞzm
rm
where rm and zm define the maximum rooting length in the radial/horizontal direction anddepth direction (L), and pz, z*, pr, and r* are empirical parameters that can describe nonsymmetrical root geometries. The physical interpretation of these parameters is presented
in Vrugt et al. (2001). For the uniform root distribution cases, RDF = 1/Wzm. In thepresented simulations, we assumed that the root distribution is constant in time.
In Hydrus-2D (Šimůnek et al., 1999), solute transport is described by


@uc
@
@c
@q c
¼
uDij
(5)
 i NUðc; r; z; tÞ
@t
@xi
@xi
@xi
where the subscripts i and j denote either r or z, and c denotes the nitrate concentration insoil solution (ML3). The first term on the right side represents the solute flux due todispersion, the second term the solute flux due to convection with flowing water, and thethird term represents root nutrient uptake. The water flux q is computed from Eq. (1), Dij isthe dispersion coefficient (L2T1), and the NU term defines the local passive nitrate uptake(ML3T1) by plant roots, which is a function of time and the spatial coordinates. The
presented application assumes a single value for the longitudinal dispersivity of 5 cm (withthe transverse dispersivity being one-tenth of the longitudinal dispersivity), and neglects
molecular diffusion. For all applications, the initial relative nitrate concentration is 0. The
boundary condition representing fertigation is a third-type Cauchy boundary condition thatprescribes the fertigation rate during defined irrigation intervals. The solute flux is definedby the product of the spatially variable water infiltration rate, q0 (LT1), and the dissolved
nitrate concentration, c0 (ML3). We assume no lateral transport of nitrate outside thedomain boundaries. The cumulative leaching of nitrate mass out of the lower boundary iscontrolled by the nitrate concentration at that depth and the corresponding water flux

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density as computed from Eq. (1). At all times, the spatially distributed nutrient-uptake
term, NU, is computed from
NUðr; z; tÞ ¼ cðr; z; tÞWUðr; z; tÞ(6)using the local WU value as computed from solution of Eq. (2). In our simulations, we limit
ourselves to nitrate-N only, and neglect mineralization gains and denitrification losses.
2.2. Irrigation scenarios
A total of 80 fertigation scenarios for four different micro-irrigation systems, four
different soil types, and five fertigation strategies were examined to evaluate water andnutrient use efficiency and nitrate leaching. Each of the fertigation scenarios was precededby 56 days of flow-only simulations, to approach a ‘pseudo-equilibrium’ condition before
irrigation with fertigation started. This was done to ensure that the initial soil water regime
was not a factor in the transport and leaching predictions. The initial condition for thisinitialization period was set to a uniform soil water pressure head of 400 cm. The
subsequent simulation period with fertigation was 28 days. The four micro-irrigation/
cropping systems were (SPR) micro-sprinkler for citrus; (DRIP) drip emitter on the soil
surface for grape; (SURTAPE) surface drip tape for strawberry; and (SUBTAPE)
subsurface drip tape for processing tomatoes. All micro-irrigation systems were applied tofour different soil types: sandy loam (SL), loam (L), silty clay (C) and anisotropic silty clay
(AC). The soil hydraulic parameters for the loam and sandy loam were taken from Carsel
and Parish (1988) and for the silty clay from the Rosetta database (Schaap and Leij, 1998).
The difference between the C and AC soil is that the lateral hydraulic conductivity of the
AC soil was five times higher than for the C soil. This was accomplished by increasing thehorizontal Ks of the AC soil with a factor of 5. The l-value of the VGM expression was set
equal to 0.5 for all soils. Whereas the hydraulic parameter values are listed in Table 1, thecorresponding functions are presented in Fig. 1.
The five simulated fertigation strategies were based on recommendations of the microirrigation industry and grower’s practices. Schematically, the five different fertigation
strategies are presented in Fig. 2. These are (B) fertigation for a total duration of 2 h,starting 1 h after the beginning of the irrigation cycle; (M) fertigation for a total duration of
2 h, in the middle of an irrigation cycle; (E) fertigation for a duration of 2 h, starting 3 h
before irrigation cutoff; (M50) alternating irrigation, starting the first and last 25% of eachirrigation with fresh water, and fertigation during the remaining 50% in the middle of theirrigation cycle; and (C) continuous fertigation, with 1-h fresh water irrigation before andTable 1
Soil hydraulic properties for the four selected soil types
Textural classur (cm3 cm3)

us (cm3 cm3)a (cm1)n

Ks (cm/day)

Sandy loam, SL
Loam, L
Silty clay, C
Anisotropic clay, AC

0.065
0.078
0.111
0.111

0.41
0.43
0.481
0.481

0.075
0.036
0.01620.0162

1.89
1.56
1.32
1.32

106.1
24.96
9.61
Ks,z = 9.61, Ks,r = 48.1

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

225

Fig. 1. Soil hydraulic relationships between (A) log hydraulic conductivity and log soil water pressure head and(B) water content and log soil water pressure head.

after the fertigation period. The fresh water irrigations prior and after each fertigation arecommon practices that ensure uniformity of fertilizer application and flushing of the drip
lines. Fresh water time intervals were reduced to 25% for the SURTAPE irrigation becauseof its small irrigation interval (see Table 2). Each irrigation cycle was simulated for 28
days, including either four (SPR) or eight (DRIP, SUBTAPE and SURTAPE) irrigation
cycles with fertigation. The irrigation cycle length (P) and other crop-specific irrigation
characteristics are presented in Table 2. To illustrate long-term implications, the fertigation
strategies were simulated for an additional 28 days for the SUBTAPE irrigation system.

Fig. 2. A general schematic, illustrating the five fertigation strategies, where P denotes the irrigation cycle period(h). Applied concentration levels were selected such that the total applied mass was identical between scenarios.
The duration of strategies B, M and E is 2 h.

226

Table 2Irrigation system parameters
DRIP (grape)

SURTAPE (strawberry)

SUBTAPE (tomatoes)

907.2
2.03
1.8
7
669  669
0

90.72
1.63
1.5
3.5
183  304
0

15.967
10.64
0.13
3.5
20  76.2
0

11.975
2.66
1.15
3.5
30  150
20

Water demand
ET0 (cm/day)
Crop coefficient, Kc

0.7
0.65

0.7
0.85

0.5
0.7

0.7
1.06

Simulated domain
Width, W (cm)Radius, R (cm)Depth, Z (cm)

n.a.
334.5
200

n.a.
90
200

75
n.a.
61

75
n.a.
100

Root water uptake
The critical water pressure heads in Feddes et al.
(1978) model h1, h2, h3max, h3min, h4 (cm)10, 25, 400,
400, 8000

1, 2, 1000,
1000, 8000

10, 25, 200,
300, 8000

1, 2, 800,
1500, 8000

Root zone
Root distribution model
Maximum rooting depth, zm (cm)Depth with max root density, z* (cm)
Maximum lateral root extension, rm (cm)Distance r with max root density, r* (cm)Non-symmetry coefficients, pz and pr

Uniform
100
n.a.
334.0
n.a.
n.a.

Vrugt model
90
0
25
0
1.0, 1.0

Uniform
30.5
n.a
56
n.a
n.a

Vrugt model
100
25
75
0
1.0, 1.0

n.a. = not applicable.

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

SPR (citrus)
Irrigation
Discharge rate, Q0 (L/day)
Irrigation intensity, q0 (cm/day)
Irrigation time, P (day)Irrigation interval, DP (day)Emitter (d) and line (w) spacing (cm)Depth of emitter (cm)

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

227

2.3. Irrigation system parameters
The irrigation requirement, Qreq (L/day), was computed from the crop-specific potential
ETc (cm/day) and the irrigated soil area, A = dw, where d (cm) and w (cm) represent emitter
distance and irrigation line distance, respectively. Selected ET0 values were typical values
for CA irrigated systems in the regions. The applied irrigation volume per irrigation cycle, I[cm3], was estimated for each crop from Qreq, the irrigation interval, DP [day], and theirrigation efficiency, f i. For all irrigation systems, we assumed an irrigation efficiency of
85%. Finally, the irrigation cycle duration, P (per day), was determined from the total
irrigation volume, I, and the emitter discharge rate, Q0. Specific values for each of the four
micro-irrigation systems are presented in Table 2.
The irrigation layouts for each of the four micro-irrigation systems with characteristic
dimensions, including emitter and irrigation line spacing, are presented in Fig. 3, whereas
relevant irrigation application and model parameters are presented in Table 2. Both DRIP
(Fig. 3A) and SPR (Fig. 3D) are considered point sources so that radial geometry is
assumed. Because of the multiple outlets along the tape, both SUBTAPE (Fig. 3B) and
SURTAPE (Fig. 3C) were simulated using the line-source model with a rectangular
geometry. For SPR, we assumed a measured non-uniform water application with most of
the applied water occurring within a 1 m radius from the sprinkler head. More detailed
information about the simulation model and model parameters can be found in Hanson
et al. (2004), which is available upon request.
The selected parameters for Feddes et al. (1978) water stress response function weretaken from Van Dam et al. (1997), and are presented in Table 2. The values of the root
distribution parameters in Table 2 signify typical root systems for the four irrigatedcropping systems.

3. Results and discussion
The relative root distribution and temporal dynamics of water content and soluteconcentration for the four micro-irrigation systems are shown in Fig. 4A (DRIP and
SUBTAPE) and b (SURTAPE and SPR), for a variable number of observation points during
the 28-day irrigation period. In the interest of saving space, we only show these simulation
results for the loamy (L) soil and the beginning (B) fertigation scenario. The dynamics ofirrigation and fertigation is clearly demonstrated by the number of peaks. It appears thatwater content values have approached a quasi steady state for almost all observation points,
with the exception of the observation nodes at the bottom of 2-m deep profiles (DRIP and
SPR). Concentration values show that most dynamics occurs near the emitter or at the soil
surface, with concentrations increasing during the 28-day period for most other regions of
the soil zone.
The water balance results for all four micro-irrigation systems and soil types issummarized in Fig. 5, expressed in percent of total applied water for the 28-day period after
2 months initialization. The root zone storage was computed from the simulated root zone,
whereas drainage values include water storage values below root zone in addition to thesimulated water flux values at the bottom of the simulated soil domain. We conclude that

228
A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

Fig. 3. Irrigation layout of the four simulated micro-irrigation systems; (A) DRIP, (B) SUBTAPE, (C) SURTAPE, and (D) SPR.

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242
229

Fig. 4. Relative root distribution with location of observation nodes (left) and temporal variations in water content (middle) and nitrate concentration (right) for (A) DRIP
(four observation points) and SUBTAPE (four observation points) and (B) SURTAPE (three observation points) and SPR (six observation points).

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

Fig. 4. (Continued ).

230

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

231

Fig. 5. Water balance for all four irrigation systems.

most of the applied irrigation water is indeed beneficially used through root water uptake,with a value of about 80%. As expected, most drainage occurs in general for the coarsertextured soils. The slightly larger drainage values for the DRIP system are likely caused by
the high root density close to the soil surface and emitter, with relative low root density
below the 40 cm depth. In contrast the lowest drainage values were determined for the
SUBTAPE system. Over the 28-day period, soil water storage increased slightly for the
SPR and SUBTAPE systems, most likely because these two systems had not reachedpseudo equilibrium after the 2-month initialization period. The flow simulations assumed awell-maintained irrigation system, so that the simulated leaching data approximately agreewith the irrigation requirements that were based on an irrigation efficiency, f, of 85%.
The overall N-leaching results are presented in table format in Table 3. The effective
N-leaching was computed from the mass leaving the bottom of the simulated soil domain
during the 1-month simulation period, augmented with the change in N-storage in the soil
domain below the root zone, effectively quantifying the mass of nitrate moving out of theroot zone. Thus, it was assumed that the soil N below the root zone would not be available
for root uptake after the simulation period. We note that the total N-added is the samefor each fertigation strategy, but varies between micro-irrigation systems. However,irrespective of irrigation system, we assume a relative nitrate concentration of 1.0 for the
2-h fertigation scenarios. Average values of percentage N leached are system-specific foreach soil type, whereas the standard deviation values (S.D.) are an indication of the232

A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242

Table 3
Percentage of N leached as a fraction of the total N added
Soil typeFertigation strategy% N leached of total N added
SPR

DRIP

SURTAPESUBTAPE

SL

B
M
E
M50
C
Average
S.D. (CV, %)12.97610.2957.601
10.33610.31910.30
1.90 (18)22.57133.1525
13.4192
28.517832.0471
25.94
8.13 (31)23.11523.12022.68022.68523.32222.98
0.29 (1.3)

6.647985
8.650231
6.589420
7.898884
8.432919
7.64
0.98 (13)

L

B
M
E
M50
C
Average
S.D. (CV, %)

7.642
5.867
4.221
5.893
5.884
5.901
1.21 (20)

9.19587.20112.78206.68796.99896.573
2.33 (35)13.02012.27511.36812.13112.37912.2350.59 (5)0.191043
0.194873
0.138668
0.184335
0.192725
0.180
0.024 (13)

C

B
M
E
M50
C
Average
S.D. (CV, %)

1.641
0.896
0.431
0.951
0.912
0.966
0.43 (43)1.96370.45420.78130.91070.82150.986
0.57 (58)

3.596
8.140
13.84910.4496.582
8.523
3.88 (46)0.015284
0.015739
0.007894
0.014177
0.015451
0.014
0.003 (21)

AC

B
M
E
M50
C
Average
S.D. (CV, %)

0.204
0.100
0.042
0.109
0.103
0.112
0.059 (55)0.03660.01530.00720.01940.01590.019
0.011 (57)

4.302
8.649
14.232
10.6267.615
9.085
3.677 (40)0.000013
0.000008
0.000003
0.000008
0.000008
0.000
0.00 (0)variation between fertigation strategies for each system. The smaller the CV (%) value, theless impact fertigation strategy will have on controlling leaching potential. We note thoughthat the CV values have not much meaning for average leaching fraction values lower
than 1%.
3.1. Soil type effects
The results in Table 3 clearly show that soil type effects are much more important thanfertigation strategy or micro-irrigation system type. Across the board, the leachingpotential is much larger for the coarser-textured soils (types SL and L). Except for the
SURTAPE, leaching losses are small for the C and AC soil types, independent of fertigation
strategy. Also, we found that the SURTAPE irrigation scenario has the largest leachingpotential, independent of soil type or fertigation strategy. First, the root zone is shallow,
confined to the 30 cm depth. Hence, applied irrigation water will be lost if redistributed

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water moves below this depth. This is likely the main cause for the coarse-textured soils.
Secondly, for the finer-textured soil types, the infiltration capacity is limited, causing asignificant fraction of the applied water to move laterally across the soil surface into thefurrows. This water moves subsequently back into the soil; however, it is essentially lost
because the furrow depth is below the assumed maximum rooting depth (see also Fig. 9).
The lowest overall leaching losses for the AC soil are the result of lateral movement ofwater and dissolved fertilizer, thereby making more effective use of the whole rooting
system.
3.2. Irrigation system effects
In general, the leaching potential increases from SUBTAPE, SPR, DRIP to SURTAPE.
An exception to this general result is large leaching value for the DRIP system for the SL
soil, caused by the high hydraulic conductivity and low soil water storage capacity of thecoarse-textured soil in combination with limited lateral spreading of roots.
3.3. Fertigation strategy effects
In general, we found that the largest leaching losses occur most often for the B
fertigation strategy (in 9 out of 16 cases). This is so because its fertigation period is thesmallest (as are M and E) and nitrate is potentially available for leaching during thesubsequent irrigation period. A potential exception applies for the SUBTAPE irrigation
system, for which the leaching potential was reduced for the B-fertigation strategy, once
the fertigation period was extended from 28 to 56 days, especially for the coarse-textured
soils (results not shown). This result is in agreement with sandy soil simulations of Coteet al. (2003), who concluded that the higher retention of nutrients for the B strategy iscaused by the upward transport of nutrients during and immediately after the fertigation bycapillary movement. However, their general conclusion was based on irrigation in
extremely dry soils, neglecting root water uptake. The E scenarios are most efficient andgenerate the least leaching. However, we note that of the total of 16 soil type/irrigation
system combinations in Table 3, only six of those have an average leaching above 1%. Yet,fertigation strategy E generated the least leaching among the five strategies for these six
worst-case scenarios.
An exception to this general rule applies to the SURTAPE irrigation system where theleaching of the E strategy is relatively high for all soil types, in particular for the finertextured soils (C and AC). For these systems, the likelihood of surface losses by lateral flow
are the highest, thereby increasing the percent N losses. In addition, because of the short
irrigation times for the SURTAPE system (0.13 days in Table 2), differences between
fertigation strategies, as expressed by CV in Table 3, are the lowest for SURTAPE. Except
for the SURTAPE, leaching losses are small for the C and AC soil types, independent of
fertigation strategy. The type of fertigation has the largest effect for the DRIP system, most
likely because of its relatively small rooting zone size.
Additional variations in leaching between fertigation scenarios are likely caused bydifferences in spatial extent between the wetting soil volume and root domain. Leachinglosses are most likely to occur for irrigation/fertigation scenarios where the differences in234

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wetting and root volumes are large. It is therefore that some DRIP systems are mostvulnerable to leaching, and that leaching occurs for non-uniform water applications for the
SPR system.
3.4. Nutrient uptake
Although not shown here, root nutrient uptake accounted for about 40–60% of the total
N added, independent of fertigation scenario. Root nitrate uptake was the least for the SPR
irrigation system (20–30%), mostly caused by the highly non-uniform water application,
whereas roots extended laterally through the simulated soil domain. Consequently, bothwater and nutrients were less accessible for the roots further away from the sprinkler
system. Most of the applied nitrate (50–80%) was stored in the root zone, potentiallyvulnerable for leaching after 28 days, later in the growing season.
3.5. Specific results
The effects of soil type and fertigation strategy using DRIP on soil nitrate distribution ispresented in Fig. 6, using contour plots of relative nitrate concentration. These plots allow
for a qualitative interpretation of the simulation results, regarding leaching potential. All
results are representative for the end of the first irrigation cycle of duration P (Table 2).
Therefore, subsequent water and nitrate distribution will be controlled by additional
irrigation cycles, followed by soil water redistribution and root water and nitrate uptake.
Most results are intuitively clear, with nitrate occurring at larger depths for the coarsertextured soils. Nitrate concentrations are increasingly diluted for the fertigation strategies
that have longer nitrate application times (M50 and C) and longer periods of freshwater
application after fertigation (B and M). Hence, typically the highest nitrate concentrations
occur for the E fertigation strategy that applies the nitrate in a 2-h period at the end of theirrigation cycle. We note that the rootzone is concentrated in a 30 cm circle around the drip
line (Fig. 4A). Therefore, the E-strategy is most efficient, as it applies the nitrate to the roots
and minimizes leaching. Finally, the increased lateral spreading of the nitrate in the silty
clay soils is mostly caused by the increased ponded soil surface, as controlled by the
infiltration rate. Also, anisotropy clearly leads to enhanced lateral transport of water anddissolved nitrate, reducing nitrate leaching potential and making nutrients available near
the soil surface where roots are generally most abundant.
Similar contour plots for the SUBTAPE irrigation system are presented in Fig. 7. As for
DRIP, the highest nitrate concentrations occur near the emitter. Since the drip tape isburied, there is no enhanced lateral spreading of the nitrate across the wetted soil surface orfor the anisotropic clay soils, as was the case for DRIP. The relative root distribution for thissystem has most roots available for nutrient uptake in a circle of about 40 cm around thedrip tape, with a maximum root density at the 25 cm depth (Table 2).
Fig. 8 shows the concentration contour plots for the SURTAPE micro-irrigation system.
Most striking about these results is that the occasional ponding of the soil surface extends
beyond the bed surface for the finer-textured soils (C and AC), causing water accumulationin the furrow and allowing for infiltration of water with dissolved nitrate into the bed at the
40 cm depth. Since the rooting depth was limited to the 30 cm depth, this adds to increased

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Fig. 6. Contour plots of relative nitrate concentration for DRIP, four soil types and five irrigation strategies.

water and nutrient losses. In all other aspects, the SURTAPE system performs as the othersystems with the lowest leaching potential for the E fertigation strategy.
Many micro-sprinkler systems apply water non-uniformly, as the one that is used in our
simulations for which the concentration contour plots are presented in Fig. 9. Because we
assumed that roots were uniformly distributed in the lateral direction to a depth of 100 cm,
the non-uniform application leads potentially to large leaching losses in the soil domain
near the sprinkler head. However, as before, the E-fertigation strategy is likely the mostefficient as it eliminates leaching by fresh water after fertigation. If roots can take up thenutrients before the front reaches the 100 cm rooting depth, leaching losses will be small.
Both the C and AC (anisotropic clay) allow for increases lateral movement of the water and236
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Fig. 7. Contour plots of relative nitrate concentration for SUBTAPE, four soil types and five irrigation strategies.

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237

Fig. 8. Contour plots of relative nitrate concentration for SURTAPE, four soil types and five irrigation strategies.

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Fig. 9. Contour plots of relative nitrate concentration for SPR, four soil types and five irrigation strategies.

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dissolved nitrate, thereby making nitrate more available to the roots and reducing leachingpotential.
There is little information on soil nitrate distributions, both experimental and throughnumerical model simulations. Specifically, no field experimental data for various
fertigation strategies are available. The modeling study by Cote et al. (2003) demonstrated
that fertigation strategy can affect nitrate leaching. Modeling studies by Abbasi et al.

Fig. 10. Comparison of measured and simulated spatial water content distributions of SUBTAPE.

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A.I. Gärdenäs et al. / Agricultural Water Management 74 (2005) 219–242(2003a, 2003b) and Assouline (2002) showed that Hydrus-2D simulations of soil watercontent and solute distributions are reasonably close to measured values. As also
demonstrated by the laboratory study of Li et al. (2004), field nitrate distributions arehighly affected by the wetting patterns of the micro-irrigation system. A comparison ofwetting patterns between field measurements (Hanson, unpublished data) and Hydrus-2D
simulations for the subsurface drip irrigation of tomatoes is presented in Fig. 10. For both,soil water content values were maximal near the drip line and decreased with depth below
the drip line and with horizontal distance. Soil water content increased with depth above
the drip line. The experimental data showed some elongation in the vertical direction,
probably reflecting preferential flow channels in the soil, which were not simulated byHydrus-2D. Also Skaggs et al. (2004) concluded that Hydrus-2D predictions of watercontent distribution for drip tape irrigation agreed well with experimental observations. In
addition to not including soil structural effects, we realize that the presented model results
may be different if soil nitrogen chemistry is an important aspect of transport.

4. Conclusions
In summary, we report on the results of using the current state-of-the-art modeling of
micro-irrigation systems, with a specific emphasis on fertigation strategies that affectnitrate leaching. When comparing micro-irrigation systems and fertigation strategies, thesoil type must be considered. Seasonal leaching was the highest for the sandy loam soil andthe lowest for the silty clay soil.
Total seasonal leaching was the lowest for subsurface drip tape (SUBTAPE), forwhich water and fertilizers are effectively supplied to the rooting system of theprocessing tomatoes. Total seasonal leaching was the highest for the surface tape system(SURTAPE), when compared to the other micro-irrigation methods, mostly because of
the relatively shallow root depth of strawberries in comparison with furrow depth. We
conclude that leaching potential increases as the differences between the extent of thewetted soil volume and rooting zone increases. As expected nitrate distributions for the
micro-sprinkler (SPR) system were highly dependent on the non-uniform water
application pattern, as most of the irrigation occurred in a small area within 1 m distancefrom the micro-sprinkler.
We conclude that fertigation for a short time at the end of the irrigation cycle (strategyE) generally reduces the potential of nitrate leaching in micro-irrigation systems, with the
exception of surface drip and tape systems in clayey soils. Moreover, we also found that
fertigation strategy is not so important for SUBTAPE systems, except that the B strategy
may be favorable as it tends to retain nitrate above the drip tape by capillary action. The
leached zone for the surface tape (SURTAPE) was very small compared to the otherscenarios mainly because of the short irrigation time after fertigation. Long fertigation
times resulted in relatively uniform nitrate distributions in the wetted regions for all but one
irrigation system. For all surface-applied micro-irrigation systems on finer-textured soils,
lateral spreading of water and nitrates is enhanced by surface water ponding, causing thewater to spread across the surface with subsequent infiltration downwards and horizontal
spreading of soil nitrate near the soil surface.

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Acknowledgements
We acknowledge funding by the California FREP, Fertilizer Research and Education
Program, and stimulating discussions with Dr. Keith Bristow of CSIRO Land and Water in
Townsville, Australia, that initiated this research.

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