Original Research

A.A. Siyal*
T.H. Skaggs
M.Th. van Genuchten

Reclama on of Saline Soils
by Par al Ponding: Simula ons
for Diﬀerent Soils
A tradi onal method of reclaiming salt-aﬀected soils involves ponding water on a field andleaching salts from the soil through a subsurface le drainage system. Because water andsalts move more slowly in areas midway between drain lines than in areas near the drains,achieving a desired level of desaliniza on across the en re field requires that ponding connue long a er areas close to the drains are already free of salts, thus causing an ineﬃcientleaching process that wastes water. A par al ponding method of leaching was recently
suggested to improve the leaching eﬃciency by up to 85%. In this study, we tested thepar al ponding method for its poten al to save water and me by simula ng the leaching of salts from salt-aﬀected profiles with various soil textures, le-drain depths, and soil
depths. Simula ons for laboratory sand tanks and field condi ons both showed that transport veloci es midway between drains are greater under par al ponding than under total
ponding because the local hydraulic head gradient is larger under par al ponding condions. As the ponded area increases toward the drain, water origina ng from areas near thedrain moves faster than water from midway between the drains. By adop ng par al ponding, water and me savings of 95 and 91%, respec vely, were found possible for a sandy
soil. The method also showed water savings of 84% when applied to a loam soil and 99% fora layered sand over loam soil but only 13% when applied to a layered loam over sand soil.

Problems of soil saliniza on and sodifica on are increasing in many irrigatedComplete surface ponding of tiledrained fields is an ineﬃcient methodof leaching salts because of large
diﬀerences in infiltra on that exist
across the field. Simulations with
HYDRUS-2D/3D were used to inves gate an alterna ve, water-conserving,
par al ponding, leaching strategy forvarious soil textures and profiles.

A.A. Siyal, Dep. of Land and Water Management, Sindh Agriculture Univ., Tandojam,
Pakistan; T.H. Skaggs, U.S Salinity Lab., 450
W. Big Springs Rd., Riverside, CA 92507; andM.Th. van Genuchten, Dep. of MechanicalEngineering, COPPE/LTTC, Federal Univ. of Riode Janeiro, Rio de Janeiro, RJ 21945-970, Brazil.
*Corresponding author (siyal@yahoo.com).

arid and semiarid regions where rainfall is insufficient to leach out salts from the vadosezone. An estimated 45 million ha out of a total 230 million ha (19.5%) of irrigated land
worldwide, and 32 million out of 1500 million ha (2.1%) of dry land, are salt affected tovarying degrees (Ghassemi et al., 1995). Some 0.25 to 0.5 million ha of land around theworld are lost from production every year as a result of soil salinization (FAO, 2002).
Salinity problems in Pakistan mirror these global trends. Irrigated agriculture in Pakistan
is mainly confined to the Indus plains. About 33 million Mg of salt per year is moved into
these plains by the Indus River and its tributaries. Of this total, about 24 million Mg isretained in the Indus basin each year, with 13 million Mg staying in the Punjab provinceand 10 million Mg in the Sindh province (Mughal, 2002). These provinces are experiencing severe irrigation-induced salinity problems. Approximately 6.3 million ha of land are
affected by salinity (Alam et al., 2000), 3.45 million ha due to irrigation (Pakistan Ministryof Food, Agriculture and Livestock, 2005). Productive land in Pakistan is being damagedby salinity at a rate of about 40,000 ha yr−1 (Alam et al., 2000). This suggests that fertile
land is being converted into unproductive, salt-affected soils at a rate of about 110 ha d−1.
Soil salinity may be depriving Pakistan of about 25% of its potential production of major
crops (World Bank, 1992)

All rights reserved. No part of this periodical may
be reproduced or transmi ed in any form or by anymeans, electronic or mechanical, including photocopying, recording, or any informa on storage andretrieval system, without permission in wri ng fromthe publisher.

A common practice in Pakistan and elsewhere is to leach salts from affected soils. Smallridges (called bunds) are constructed around an entire field, good-quality water is ponded
on the soil surface, and the salts are leached from the root zone to deep groundwater or to
nearby surface waters, either directly or via tile drains. Because of more rapid infi ltrationand shorter travel distances, areas above the drains in tile-drained systems are leachedfar more quickly using this method than areas midway between the drains (Youngs andLeeds-Harrison, 2000). Thus, to achieve a desired level of desalinization across an entire
field, ponding needs to continue long after areas close to the drains are already free of salts.
This process wastes large amounts of good-quality water that could be used for irrigationor other purposes.

2010, Vol. 9

www.VadoseZoneJournal.org | 486

Vadose Zone J. 9:486–495
doi:10.2136/vzj2009.0129
Received 19 Sept. 2009.
Published online 3 May 2010.
© Soil Science Society of America
5585 Guilford Rd. Madison, WI 53711 USA.

Calculations by Youngs and Leeds-Harrison (2000), using ananalytical solution for seepage to a tile drain, revealed that more
uniform and efficient leaching can be achieved by dividing thesalt-affected field into strips separated by bunds, and then flooding the field incrementally, starting from strips located midway
between the drains and progressing toward strips located overthe drains. Termed partial ponding, this method progressively
increases the area of the field under ponding. For the conditions
studied, Youngs and Leeds-Harrison (2000) found that the partial
ponding method required as much as 84% less water to leach asoil than uniform ponding and 76% less time. While their theoretical analysis was for coarse-textured soils, they suggested thatthe methodology should also be suitable for fi ne-textured soils.
The partial ponding concept was recently tested by Mirjat et al.
(2008) and Mirjat and Rose (2009) using laboratory sand tanks.
Their results were relatively close to the theoretical predictions by
Youngs and Leeds-Harrison (2000). The experiments by Mirjat
and Rose (2009) and Mirjat et al. (2008), however, were limited
to uniform sand. A need exists to test the partial ponding concept
for fine-textured and layered soil profi les. Computer models suchas HYDRUS-2D/3D (Šimůnek et al., 2006) that simulate waterand solute transport in two-dimensional, variably saturated media
would be appropriate tools for such an analysis.
The objective of this study was to use variably saturated flow modeling to evaluate the partial ponding leaching technique (Youngs
and Leeds-Harrison, 2000; Mirjat and Rose, 2009). We used
HYDRUS-2D/3D to evaluate the leaching of salts from sandtanks as studied by Mirjat et al. (2008), as well as to performseveral leaching simulations for field conditions. The simulations
considered full and partial ponding, initially saturated and unsaturated conditions, and various drain and soil depths, soil textures,and soil layering.

6 Numerical Simulaons

Governing Flow and Transport Equa ons

Simulations of the partial leaching problem using HYDRUS2D/3D were based on the standard Richards equation for flow
and the equilibrium advection–dispersion equation for solutetransport in a two-dimensional, variably saturated medium. Fora two-dimensional isotropic medium, the Richards equation is
given by⎤∂θ ( h )∂ ⎡⎢∂h=K ij ( h )− K i 2 ( h )⎥⎥⎢∂t∂ xi ⎢
∂x j
⎥
⎣

[1]

⎦

where θ is the volumetric water content [L3 L−3], h is the soil waterpressure head [L], t is time [T], xi (i = 1,2) are spatial coordinates[L] with x = x1 and z = x 2 in this study representing the horizontal and vertical (positive downward) coordinates, respectively,
and K ij(h) is the soil hydraulic conductivity tensor [L T−1]. For an
isotropic medium, the off-diagonal entries of K ij are zero, while themain diagonal entries (K11 and K 22) are equal to the unsaturated
hydraulic conductivity, K(h). For the soil hydraulic properties θ(h)and K(h), we used the equations of van Genuchten (1980):
θ(h ) = θr +θs − θr⎡1+ |α h|nvg ⎤ m
vg
⎣⎢
⎦⎥

(m = 1−1 nvg )m⎤2⎡
K ( h ) = K s Se ⎢1 − (1 − Se1/m ) ⎥
⎢⎣
⎥⎦

[2]

[3]

where θ r and θ s are the residual and saturated water contents,
respectively [L3 L−3], α vg [L−1] and nvg (dimensionless) are empirical shape parameters, Ks is the saturated hydraulic conductivity [LT−1], and Se is the effective saturation:
Se ( h ) =θ (h )− θr
θs − θr[4]

Assuming no sorption or decay reactions, the standard advection–
dispersion equation for solute transport is given by⎞⎟∂ (θC )∂ ⎛⎜∂C
⎜ θ Dij=− q i C ⎟⎟⎟⎜∂t∂ xi ⎜⎝
∂x j
⎠⎟

[5]

where C is the solution concentration [M L−3]; Dij is the dispersion coefficient tensor [L 2 T−1], described here using standard
expressions (e.g., Bear, 1972) involving the longitudinal (εL) andtransverse (εT) dispersivities [L] but without the contribution ofdiff usion in the liquid phase; and qi is the volumetric fluid flux
density vector [L T−1] given by the Darcy–Buckingham law.

Flow Domain
To enable comparisons with the partial ponding experiments of
Mirjat and Rose (2009), the first set of simulations used the flowdomain geometry used in their laboratory sand tank studies, i.e.,a rectangular cross-sectional (x,z) domain 100 cm wide and 15 cm
high. This geometry is 1/40th of the size of the tile drainage system
considered by Youngs and Leeds-Harrison (2000) in their theoretical analysis. Because of symmetry, only one side of the drain
was simulated. Figure 1 shows example flow domains and finiteelement meshes with 0.5-cm-diameter drains located at depths of
5 and 10 cm, thus mimicking the experimental setups of Mirjat
and Rose (2009). Similar flow domains and discretizations wereused for the different leaching scenarios considered in this study.
Additional simulations were done using field-scale dimensions: adrain spacing of 40 m, drain depth at 2 m, and an impermeable
layer at 6 m. The various scenarios are summarized in Table 1. Mostof the cases considered involved uniform soil profiles. We also studied two layered profi les (sand over loam and loam over sand), withthe boundary between the layers located at the depth of the drain.
The flow domain for all simulations was initially (at t = 0) assumed
to be uniformly saturated (h = 0), with a relative salinity of 1.0.

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Table 1. Simulated salt leaching scenarios.
Case

Soil depth

Drain depth

Soil type

Ponding method

C0

15

5

sand

complete

P0
C1

15

5

sand

partial

15

10

sand

complete

P1

15

10

sand

partial

C2

30

5

sand

complete

P2

30

5

sand

partial

C3

15

5

loam

complete

P3

15

5

loam

partialC4

15

5

sand over loam

complete

P4

15

5

sand over loam

partialC5155loam over sandcomplete

P5

15

5

loam over sandpartial

C6

600

200

sand

complete

P6

600

200

sand

partial—————— cm ——————

Fig. 1. Typical geometries and finite element meshes for the flow
domains used in the HYDRUS-2D/3D salt leaching simulations.

For the sand tank simulations, a constant pressure head of 0.5 cm(i.e., 0.5 cm of water ponding) was imposed on all or part of thesoil surface boundary, depending on the invoked leaching scenario,
and zero flux on the remainder. For the field-scale simulations, theponding depth was 20 cm. A third type (Cauchy type) boundary
condition was used to prescribe the concentration flux along theponded surface, with the incoming water being salt free (C = 0).
The bottom, right, and left boundaries were assumed to be zeroflux boundaries for both water and salt. The drain boundary was
specified as a seepage face for water flow and as a zero concentration gradient for solute transport.

Soil Hydraulic and Solute Transport Parameters
For the sand tank simulations, we assumed hydraulic properties similar to those of the Mirjat et al. (2008) experiments. The
hydraulic parameters in Eq. [2] and [3] were estimated with the
Rosetta pedotransfer functions (Schaap et al., 2001) using as input
soil texture (sand) and bulk density (1.63 g cm−3) as reported by
Mirjat et al. (2008) to obtain the following estimates: θ r = 0.05,
θ s = 0.34, Ks = 0.89 cm min−1, α vg = 0.031 cm−1, and nvg = 4.43.
The values for θ s and Ks estimated in this way compared favorably with the porosity (0.37) and saturated conductivity (0.84 cm
min−1) values estimated by Mirjat et al. (2008) (they did not estimate unsaturated hydraulic parameter values). For simulations on
the loam soil, we used soil textural class averages as estimated by
Carsel and Parrish (1988): θ r = 0.078, θ s = 0.43, Ks = 0.017 cm
min−1, α vg = 0.036 cm−1, and nvg = 1.56.
For the solute transport simulations, we assumed that for the simulated flow velocities and transport times, molecular diff usion was
negligible relative to hydrodynamic dispersion (Skaggs and Leij,
2002). The longitudinal dispersivity was set equal to one-tenthof the depth of the flow domain (i.e., εL = 1.5 or 3.0 cm), whichis consistent with various studies indicating that εL is about onetenth the scale of a transport experiment (Beven et al., 1993; Coteet al., 2003). The transversal dispersivity (εT) was assumed to be
one-tenth of the longitudinal dispersivity (e.g., Hanson et al.,2006). Solute sorption or decay were not considered.

Leaching Time Required with Complete Ponding
With complete ponding, hydraulic head gradients along the surface
are much greater in the vicinity of the drain than elsewhere, suchthat most of the drain flow originates from the area above the drain(Kirkham, 1949). Consequently, the area above the drain leaches
much more quickly than the area midway between drains. Let n be
the fraction of the total drain flow that originates from a narrowsection of the field that is midway between drain lines, has a widthof Δs, and is parallel to the drain line. Then, assuming piston flow,
the depth of leaching below the section during time t will be z =nQt/fΔs, where f is the saturated water content of the soil and Q
is the drainage flux [L2 T−1]. If Z is the targeted depth of leaching,then the time T required for desalinization isT=

f Δ sZnQ[6]The parameters n and Q can be computed with HYDRUS for agiven Δs and ponding depth. Note that in the case of complete
ponding, the target depth of leaching refers to the area midway
between drains.

Leaching Times Required with Par al Ponding
The partial leaching approach by Youngs and Leeds-Harrison(2000) is based on the premise that more efficient and uniform
leaching can be achieved by dividing the soil surface into strips

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separated by bunds, and progressively increasing the flooded areafrom strips midway between the drains toward the drains until
the whole area is flooded. Implementing this approach requires
estimates of the leaching times for each stage such that at the endof the final stage the cumulative leaching in each strip will equal
the desired target depth, Z. Youngs and Leeds-Harrison (2000)
and Mirjat and Rose (2009) provided a procedure for calculating the required flooding times, which we summarize here. For
more detail, see Youngs and Leeds-Harrison (2000) and Mirjat
and Rose (2009).
Assume that the drains are separated by a distance 2D and thatthe area between the drains is divided into 2N strips, such that the
strips have a width D/N. According to the strategy of Youngs andLeeds-Harrison (2000), the first strip midway between the drains
is flooded for a time period t1, after which the bund between thefirst and second strip is broken to allow flooding of the second strip.
Strips 1 and 2 are kept ponded for a time period t 2 , after which thethird strip is included and the three strips are flooded together fora time period t 3. The fourth strip is incorporated next, with theprocess continuing until all N strips are included and the wholearea is flooded.
The times ti needed to obtain uniform removal of salt to a target
depth Z across the whole area can be estimated from the systemof equations (Youngs and Leeds-Harrison, 2000)

Fig. 2. Illustration of the partitioning of the total drain flow underpartial ponding with four strips. Strip 1 is midway between the drains(x = 50), while Strip 4 starts at the drain (x = 0); Qi is the drain flow
when Strips 1 through i are ponded and ni,j is the fraction of Qi originating from the jth strip.

t3 =

t2 =

TQ 4 n4,1 ( n4,4 − n4,3 )[9]

Q 3 n3,3 n4,4
TQ 4 n4,1 ⎡⎣⎢ n3,3 ( n4,4 − n4,2 ) + n3,2 ( n4,3 − n4,4 )⎤⎦⎥[10]

Q 2 n2,2 n3,3 n4,4

t1 =TQ 4 n4,1 ⎣⎢⎡n3,3 (n2,1n4,2 −n2,2 n4,1 + n2,2 n4,4 )−n4,4 (n2,2 n3,1 −n2,1n3,2 + n2,1n3,3)+ n4,3 (n2,2 n3,1 −n2,1n3,2 )⎦⎥⎤
Q1n1,1n2,2 n3,3 n4,4

[11]

where T ≡ DfZ/n4,1Q4.

N∑ i = j ni , jQi tiZ=( j = 1,..., N )(D N ) f[7]

Table 2. Drain flux densities for partial and complete ponding scenarios.
Flux density

where Qi is the drain flow when Strips 1 through i are
1/4 area
1/2 area
3/4 areaComplete Increase
Soil texture
Cases
pondedpondedpondedpondingin flux†
flooded, and ni,j is the fraction of Qi originating from
2 min−1 ——————————————————cm
Strip j. For N = 2 (two strips of width D/N), partial
C0 and P0 1.340
1.720
2.420
4.100
3.06
ponding involves two strips in which the center strip is Sandleached fi rst for time t1, with n1,1 = 1 at rate Q1. The Sand
C1 and P1 2.534
3.266
4.605
6.180
2.44
bund between Strips 1 and 2 is then broken and the two Sand
C2 and P2 1.640
2.031
2.711
4.251
2.59
strips are leached together for time period t 2 , with the Loam
0.033
0.048
0.080
3.20
C3 and P3 0.025
flux in the center Strip 1 being n2,1Q2 and the flux in Sand over loam C and P 0.581
0.883
1.451
3.100
5.33
4
4
Strip 2 closer to the drain being n2,2Q 2 . Figure 2 shows Loam over sand C and P 0.165
0.245
0.295
0.331
2.00
5
5
schematically the partitioning of the flow for the case of† Comparison between 1/4 and complete ponding (times).
N = 4, which is the number of strips
used in our study. Using HYDRUS2D/3D, values of Q i and n i,j were Table 3. Fractions of the total drain flow originating from different strips under partial and complete
ponding, where n is the fraction of the drain flow when Strips 1 through i are ponded that originatedcalculated numerically. Results are from the jth strip.i,jshown in Tables 2 and 3, respectively.
The ponding times are found by solving Eq. [7] for the values of ti. For four
strips, these times are
t4 =T

n4,1
n4,4

[8]

1/2 area ponded 3/4 area ponded

Complete ponding

Soil texture

Case

1/4 areaponded, n1,1

n2,1

n2,2

n3,1

n3,2

n3,3

n4,1

n4,2

n4,3

n4,4

Sand

P0

1.0

0.19

0.81

0.06

0.12

0.84

0.01

0.03

0.14

0.82

Sand

P1

1.0

0.15

0.85

0.05

0.13

0.82

0.02

0.06

0.22

0.70

Sand

P2

1.0

0.24

0.76

0.11

0.18

0.71

0.04

0.05

0.13

0.78

Loam

P3

1.0

0.19

0.81

0.04

0.12

0.84

0.03

0.05

0.13

0.79

Sand over loam

P4

1.0

0.03

0.97

0.001

0.03

0.969

0.00

0.002 0.047 0.95

Loam over sand

P5

1.0

0.46

0.54

0.28

0.31

0.41

0.19

0.20

www.VadoseZoneJournal.org | 489

0.25

0.36

Leaching Simula ons
To simulate partial ponding in the sand tanks, the soil surface wasdivided into four equal strips of 12.5 cm each. At the beginning of
the simulation, the central strip midway between the drain lines(37.5–50 cm) was ponded for a period of time t1, after which theponded area was increased to 25 to 50 cm for a time period t 2 . The
flooded area was next increased to 12.5 to 50 cm and then to 0 to
50 cm, with leaching proceeding for times t 3 and t4, respectively.
The calculated pressure head and salinity distributions obtainedwith HYDRUS-2D/3D at t1 for the 1/4 area ponding scenario
were used as initial conditions for the 1/2 area ponded calculations. Similar distributions obtained at t 2 for the 1/2 area pondingcase were used as the initial conditions for the 3/4 area ponding,and so on. The procedure for the field-scale simulations was thesame except that the dimensions were 40 times larger, including
the ponding depth (20 cm).
For the complete ponding simulations, the entire surface of theflow domain was ponded at a constant pressure head (0.5 cm inthe sand tank simulations and 20 cm in the field-scale simulations).
The HYDRUS code was executed for the leaching times given by
Eq. [6] to reach the targeted leaching depth Z, with Z = 2 cm andΔs = 12.5 cm for the sand tank simulations and Z = 80 cm andΔs = 500 cm for the field-scale simulations.

6 Results and Discussion
Flow Paths

Figure 3 shows flow paths computed for partial and complete
ponding in sand tanks with drains installed at depths of 5 or 10 cm.
The flow paths in these figures were created with the HYDRUS
2D/3D particle tracking option. Shown are flow paths traced byindividual particles released simultaneously at equally spaced
points along the surface. The figures show the particle paths after
a fi xed time period (t = 400 min) such that particles along fasteror shorter paths had already reached the drain at that time, whileothers had not. The results indicate that particles originatingfrom the strip between 37.5 and 50 cm moved faster and covered
more distance with only one-fourth of the area ponded (Cases P0
and P1) compared with the other partial and complete ponding
scenarios. Similarly, flow along paths from the 1/2 area ponded
scenario moved faster than flow from the same area during 3/4 andcomplete ponding. This shows that increasing the ponded area substantially reduces flow velocities along streamlines from the strip
already ponded midway between drains. The reason is that expanding the ponded area toward the drains reduces the local hydraulichead gradient in the previously ponded areas. Thus as the area
increases, water originating from the strip near the drain moves
faster than water coming from midway between the drains. As
flooding progresses, the greatest leaching occurs under the newly
opened bunds, while leaching under the earlier bunds decreases
significantly. The results are similar for both drain depths considered and are consistent with the theoretical studies of Youngs andFig. 3. Flow paths for drains installed at 5- (Cases C0 and P0) and10-cm depths (Cases C1 and P1) in a sand soil subject to partial surface ponding scenarios.

Leeds-Harrison (2000) and the sand tank experiments by Mirjat
et al. (2008) and Mirjat and Rose (2009).
Figure 4 shows flow paths under complete ponding after the same
period of time (t = 2000 min) for a uniform sand, a uniform loam,
and the layered systems (sand over loam and loam over sand) withthe drain installed at the 5-cm depth (Cases C0, C3, C4, and C5,
respectively). The figure shows that in sand (Case C0), particles fromthe 0- to 37.5-cm area had already arrived at the drain, whereas thoseoriginating from 37.5 to 50 cm were still only halfway or less. For
the loam (C3) and the sand over loam (C4), flow paths from only
the 0- to 12.5-cm area had reached the drain. By contrast, particles
from the 12.5- to 37.5-cm area had traveled only very short distances,

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Fig. 5. Flow paths for tanks with an impermeable soil layer at 15- or
30-cm depth and a drain installed at the 5-cm depth (Cases C0 and C2).

Fig. 4. Flow paths for drain installed at the 5-cm depth for different soil textures (Cases C0, C3, C4, and C5) subject to complete
surface ponding.

whereas those from 37.5 to 50 cm had barely entered the soil. The
situation for the loam over sand (C5) layered soil is not that different from the sand in that flow paths from the 0- to 37.5-cm area
reached the drain while those from 37.5 to 50 cm were still only
halfway. Notice that the particles in the loam over sand system initially moved mostly vertically downward through the loam until
they reached the sandy soil, where they then started to move morehorizontally toward the drain. In comparison, for the sand over loam
profile (C4), flow paths for particles closest to the drain developed
a horizontal component more quickly and converged at the drain
without penetrating the underlying loam soil.

than 5 cm (Case P0). Table 2 shows that the drain discharge was
1.34 cm2 min−1 for P0, compared with 2.53 cm2 min−1 (1.89 times
larger) for P1. Moreover, most of the streamlines in P0 converged
from below the drain, whereas they reached the drain more fromthe top in P1. Similar trends were obtained for the 1/2 and 3/4
area ponding scenarios. The velocity vectors for P1 for complete
ponding were relatively large up to about 25 cm laterally from thedrain, but only up to about 12.5 cm for P0. The drain discharge
for P0 and P1 increased 3.1 and 2.4 times, respectively, when the
ponded area was increased from 1/4 to complete ponding. These
results are again consistent with the experimental results of Mirjat
et al. (2008).

Flow Velocity Vectors

For complete ponding with the drain at 5 cm, 82% of the water
entering the drain originated from the fi rst quarter of the area
next to the drain line (n4,4), 14% from the next quarter of the area(n4,3), 3% from the next area (n4,2), and only 1% from the area
midway between the drains (n4,1). These values, shown in Table 3
for P0, are essentially identical to those predicted by Youngs andLeeds-Harrison (2000) using their analytical solution (82, 13, 4,
and 1%, respectively). The HYDRUS-simulated values were also
close to the experimental results (75, 14, 7, and 5%, respectively)of Mirjat et al. (2008), except for the quarter midway between thedrains. With the drain at 10 cm, the fractions of flow from thefour strips were 70, 22, 6, and 2%, again very close to the experimental results of Mirjat et al. (2008). These numbers indicate thatsalts will be leached more uniformly from a coarse-textured profile
under complete ponding when the drain is positioned deeper, allother conditions being the same.

Figure 6 shows relative flow velocity vectors under partial andcomplete ponding with drains at 5 and 10 cm. The same velocity
scale factor was used for all plots, meaning that velocity vectorsof similar size in different plots represent the same flow velocity.
The plots indicate that under 1/4 area ponding, the flow velocities
under the outer strip were greater for the drain at 10 cm (Case P1)

As the depth below the drain increases, flow from the quarter
closest to the drain decreases, while it increases for the central
strip midway between the drains. The simulated results for theimpermeable layer at 30-cm depth (C2 in Table 3) show that
78% of the water reaching the drain originated from the fi rst

Figure 5 shows flow paths for drains installed at 5 cm in sand withimpermeable layers at 15 (Case C0) and 30 cm (Case C2). Having
a deeper soil profi le (C2) resulted in particles traveling faster andpenetrating deeper into the soil before converging at the drain.
Th is suggests that salts in tile-drained systems will leach quicker
in deeper soil profi les. These results again are consistent with theoretical calculations by Youngs and Leeds-Harrison (2000) for an
infinitely deep soil profi le (no impermeable layer).

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Fig. 7. Relative flow velocities in tanks having different soil textures,with drains at 5 cm and the soils subject to complete surface ponding.

Fig. 6. Relative flow velocity in a sand soil with drains installed at 5 or
10 cm under various partial and complete ponding scenarios.

quarter of the area next to the drain line, while 13, 5, and 4%
originated from the next three quarter sections. These results agreewith those obtained by Youngs and Leeds-Harrison (2000) for an
infinitely deep soil.
Figure 7 shows the effects of soil texture (sand vs. loam) and layering (sand over loam vs. loam over sand) on the flow velocity forcomplete ponding when the drain is again at 5 cm. For sand, thevelocity vectors from the 0- to 25-cm area were much larger thanthose coming from the 25- to 50-cm area. The drain flow rate fromthe sand (4.10 cm2 min−1) was much higher than that from theloam soil (0.080 cm 2 min−1). The loam had low flow velocities
throughout the profi le except very close to the drain. For complete
ponding of the loam soil, 79% of the drain discharge came fromthe first quarter section near the drain, followed by 13, 5, and only
3% from the next three quarters (Table 3, Case P3).

Velocity vectors in the sand-over-loam layered soil were large onlyin the fi rst quarter section (Fig. 7) and were very small in theremaining part of the soil profi le. The drain discharge in this casewas still relatively large (3.10 cm2 min−1), but with nearly all of
the water (95%) coming from the fi rst quarter section near thedrain (Table 3, Case P4) and very little (only 0.001%) from the area
midway between the drains. By comparison, the velocity vectors in
the loam-over-sand layered soil were much more uniform, except
in the sand just below the drain (Fig. 7). This case (P5 in Table 3)
exhibited the most uniform leaching pattern throughout the entire
cross-section, with 36, 25, 20, and 19% originating from the four
quarters, starting with the area near the drain.

Salt Leaching
Figures 8 and 9 show the relative salt concentrations remaining
in the profi le after leaching for the time period needed to leachsalt to a target depth of 2 cm (recall that the target depth applies
to the entire field with partial ponding and to the region midway
between the drains with compete ponding). To uniformly leach
the soil to 2 cm, 13.3% of the salt initially present needed to beleached from the sand and loam profi les (i.e., the salts initiallypresent in the top 2 cm of the 15-cm-deep profi le). The figure of
13.3% is a useful benchmark for assessing leaching for the different
scenarios. As expected, leaching was not uniform for the complete
ponding scenarios, with salts in areas next to the drains leached tomuch greater depths than those midway between the drains. At
the time the targeted depth midway between drains was reached,salts in areas next to the drain were leached to depths that werewww.VadoseZoneJournal.org | 492

Fig. 8. Relative salt concentration profiles in sand tanks after leachingfor pre-determined times for differing drain and soil profile depths.
White areas correspond to completely leached soil.

5.7, 6.8, 4.0, 8, 10, and 3.1 times deeper than the targeted depth
for cases C0, C1, C2 , C3, C4, and C5, respectively. As a result, 44
to 67% of the initial salts had leached out of the tank during the
optimum leaching time period. This extra leaching wasted muchgood-quality water. By comparison, far more uniform leaching wasachieved with the partial ponding method, where only 27 to 33%
of the initial salts had leached below the targeted depth of 2 cm.

Time and Water Savings with Par al Ponding
Figure 10 summarizes the potential for water and time savings withpartial ponding. Desalinization of a 15-cm-deep, uniform sand withthe drain at 5 cm using the partial ponding method required 95%
less water and 91% less time than the complete ponding method.
Desalinization of a 30-cm-deep sand resulted in water and timesavings of 87 and 78%, respectively. These results are very close to
those obtained by Youngs and Leeds-Harrison (2000) for their field

Fig. 9. Relative salt concentration profiles in tanks with different soil
textures after leaching for a predetermined time with drains installedat 5 cm. White areas correspond to completely leached soil.

scenario, which had a spatial scale about 40 times larger (drain spacing of 40 m and drain depth at 2 m). They calculated 98 and 92%
water and time savings, respectively, for uniform sand with an impermeable layer at 6 m, and 84 and 75% water and time savings for an
infinitely deep soil profile. Even more water and time savings (up to99%) were possible with partial ponding for the loam-over-sand case,but only 13% for the sand-over-loam layered profile.

Field and Laboratory Scales
Our calculations thus far used a scaled-down version of the fieldcase considered by Youngs and Leeds-Harrison (2000) so that we
could compare our results directly with the laboratory sand tankexperimental data of Mirjat and Rose (2009). We did additional
simulations for the larger scale, however, and found results that

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infi ltration rates were the same across the field. As the leachingprocess proceeded and drainage started, however, the partial ponding method again became more efficient, producing essentially thesame results in terms of salts being leached to the targeted depth ofleaching midway between the drains. When initially unsaturated,
67% of the initial salts were leached to the targeted depth with fullponding vs. 29% with partial ponding. This compares with valuesof 66 and 29% for full and partial ponding, respectively, when
the soil was initially saturated. Simulated salinity profi les for thisexample were approximately the same as those shown in Fig. 11.

Fig. 10. Predicted water and time savings with partial ponding fordifferent soil textures.

were essentially identical or very similar at the two scales. As an
example, Fig. 11 shows the results for complete and partial ponding(C6 and P6) for both the field scale (sand soil texture, drain spacingof 40 m, drain depth at 2 m, impermeable layer at 6 m) and thelaboratory scale (C0 and P0) of Mirjat and Rose (2009). The laboratory results are for 500 min of leaching and the field simulations for
20,000 min. The solute concentration profi les for the two scales in
the figure are basically scaled versions of one another.
Several other simulations using partial ponding produced very
similar results. These simulations included a case where, contrary
to the experiments by Mirjat and Rose (2009), the soil was initiallyunsaturated. For that example, we assumed a hydrostatic pressure
head profi le in equilibrium with a water table at the depth of thedrain (2 m). The initial water contents in the case of the sandranged from 0.34 at and below the water table to a value of 0.07
at the soil surface. The results of the simulation showed initially amuch more uniform leaching pattern, mostly because the initial

The work of Miller et al. (1965) suggested an interesting possibility
for further increasing the efficiency of partial ponding under field
conditions. In field experiments, Miller et al. (1965) found thatthe leaching efficiency was higher when intermittent ponding wasused instead of continuous ponding. A probable reason for thisfi nding is that with continuous ponding, part of the water willflow continuously through preferential flow paths, with limited
contribution to salt leaching. With intermittent ponding, however,more time would be available for solutes to diff use from relatively
immobile water to the preferential flow paths, thus increasing concentrations in the main flow paths when ponding would restart. A
leaching strategy that should be investigated in the future would
be to implement the partial ponding using intermittent ponding at
each stage of the leaching process rather than continuous ponding.

6 Conclusions

We used HYDRUS-2D/3D to analyze a partial ponding methodof leaching (Youngs and Leeds-Harrison, 2000) that saves considerable amounts of water and time to leach salts from the surface
horizons of tile-drained, salt-affected soils. The method was tested
for its potential to save water by simulating the leaching of salts
from a flow domain with various soil textures, tile-drain depths,
and soil depths. The simulation results
showed that streamlines originatingfrom midway between the drains traveled faster under 1/4-area ponding
than those from the same area under
1/2, 3/4, and complete ponding. The
reason is that the hydraulic head gradient midway between drains is larger
during partial ponding. When ponding was expanded to include the entire
field, water from areas near the drain
moved faster than water from midway
between the drains. Compared withcomplete ponding, partial ponding ofcoarse-textured soils resulted in water
Fig. 11. Comparison of salt concentration profiles for the full field case (drain spacing of 40 m, drain
and time savings of 95 and 91%, respecdepth at 2 m, impermeable layer at 6 m) and the scaled-down laboratory setup. Results are for comtively. The method also led to water
plete (Cases C0 and C6) and partial (Cases P0 and P6) ponding. White areas correspond to completelyleached soil.
savings of 84% for a loam soil, and 99%

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for a sand-over-loam layered soil but only 13% for a loam-over-sandsoil. Our numerical computations for flow in coarse-textured soils
under partial ponding were in very good agreement with previous analytical results given by Youngs and Leeds-Harrison (2000).
Our results indicate that partial ponding can be used to efficiently
leach salts not only from coarse-textured soils but also from finetextured and layered soils.

Acknowledgment

We acknowledge with gratitude Dr. Jirka Šimůnek, Univ. of California, Riverside,for valuable suggestions regarding the HYDRUS-2D/3D simulations.

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