Agricultural Water Management 115 (2012) 242–251

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Agricultural Water Management
journal homepage: www.elsevier.com/locate/agwat

Minimizing nitrogen leaching from furrow irrigation through novel fertilizer
placement and soil surface management strategies
Altaf A. Siyal a,b,∗ , Keith L. Bristow c , Jirka Šimůnek daDepartment of Land and Water Management, Faculty of Agricultural Engineering, Sindh Agriculture University, Tandojam, Sindh, Pakistan
Endeavour Foundation Fellow with CSIRO Land and Water, PMB Aitkenvale, Townsville, QLD 4814, Australiac
CSIRO Sustainable Agriculture National Research Flagship and CSIRO Land and Water, PMB Aitkenvale, Townsville, QLD 4814, AustraliadDepartment of Environmental Sciences, University of California Riverside, Riverside, CA 92521, USAba r t i c l e

i n f oArticle history:
Received 8 April 2012
Accepted 12 September 2012
Keywords:
Irrigation efﬁciency
Water and nutrient use efﬁciency
Water savings
Solute transport
Nitrate leaching
HYDRUSa b s t r a c t
Inappropriate soil, water and fertilizer management in irrigated agriculture can result in environmental
problems, including groundwater pollution with nitrates. Furrow irrigation is widely used around theworld and is considered as a major source of nitrate leaching. Improved soil, water and fertilizer management practices are needed to improve the production and environmental performance of furrow irrigatedagriculture. This paper describes results of a simulation study using HYDRUS-2D to assess opportunities
to improve irrigation efﬁciency and reduce the risk of nitrate leaching from furrow irrigated systems. It
focuses on the commonly used practice in Pakistan where irrigation water supply is turned off once thewater level in the furrow has reached a pre-determined depth. The study involved analysing the impact
of fertilizer placement on nitrate leaching from a loamy soil subjected to three different soil surface treatments. Fertilizer placements included placing the fertilizer on the bottom of the furrow (P1 ), sides of the
furrow (P2 ), bottom and sides of the furrow (P3 ), on the sides of the furrow near to the ridge top (P4 ), andon the surface in the middle of the ridge top (P5 ). The soil surface management treatments included theoriginal soil (So ), compacting the bottom of the furrow (Sc ) and placing a plastic sheet on the bottom of
the furrow (Sp ). Results showed water savings varied with application rate and soil surface management,with soil surface management strategies Sc and Sp yielding water savings of 17% and 28% relative to So
for a water application rate of 1800 L h−1 for a 100 m long furrow. Leaching of nitrogen for this case was
reduced from 33% for So with fertilizer placement P1 to 1% by compacting the bottom of the furrow (Sc )and to zero loss by placing a plastic sheet on the bottom of the furrow (Sp ). By changing the fertilizer
placement for So from P1 to P2 , P3 , P4 , and P5 , nitrogen leaching was reduced from 33% to 2%, 15%, 0%,
and 0%, respectively. Results of this study demonstrate that placing nitrogen fertilizer on the sides of the
furrow near the ridge top (P4 ) or on top of the furrow at the centre of the ridge (P5 ) maximize the retention of nitrogen fertilizer within the root zone. Results of this study also demonstrate that enhancements
in irrigation efﬁciency, particularly in coarser soils with high inﬁltration rates can be achieved throughcompacting the bottom of the furrow or by placing a plastic sheet on the bottom of the furrow before
applying irrigation.
© 2012 Elsevier B.V. All rights reserved.

1. Introduction
The wide spread occurrence of high levels of nitrate in surface and groundwater is of growing concern (Angle et al., 1993;
Mailhol et al., 2001; Gärdenäs et al., 2005; Liu et al., 2005; Waddell
and Weil, 2006). This has resulted in increasing scrutiny of andpressure to improve agricultural practices, especially in irrigated∗ Corresponding author at: Department of Land and Water Management, Faculty of Agricultural Engineering, Sindh Agriculture University, Tandojam, Sindh,Pakistan. Tel.: +92 343 835 8031.
E-mail address: siyal@yahoo.com (A.A. Siyal).
0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.agwat.2012.09.008

systems to reduce the amount of nitrogen entering our water systems (Bar-Yosef, 1999; Gärdenäs et al., 2005; Jones and Olson-Rutz,
2011). The quality of soils, ground and surface water resources
is always at risk in areas where agricultural production is dominated by irrigation such as in Pakistan and many other semiarid
and arid regions of the world. Excessive and improper applicationof nitrogenous fertilizers with irrigation water leads to an increasein nitrate concentration in the ground and surface water in theseareas (Hayens, 1985; Waskom, 1994; Mitchell et al., 1994). One of
the most common contaminants found in groundwater worldwide
is nitrate (NO3 − ), an oxidized form of dissolved nitrogen.
Traditional cropping practices in Pakistan, as in many otherregions of the world, involve application of signiﬁcant quantities

A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

of nitrogenous fertilizers and large quantities of irrigation water,which can result in signiﬁcant leaching of nitrate due to excess
deep drainage. This not only decreases the availability of nitrogento plants but also creates environmental problems by degrading
the quality of the groundwater. The problem of nitrate leachingis aggravated in agricultural areas when soils are coarse textured(Waddell and Weil, 2006).
In about 90% of the irrigated area of the world, crops are irrigatedwith surface irrigation methods, most often using furrow irrigation(Tiercelin and Vidal, 2006), which is considered as a more water use
efﬁcient method than basin and border irrigation methods. Furrow
irrigation is commonly used in arid, semiarid and subhumid regionsof the world to irrigate vegetables and row crops. Wylie et al. (1994)
reported furrow irrigated corn as a major source of nitrate leaching
and groundwater pollution. As much as 40% of the available nitrate
can leach out of the root zone from a clay loam soil (Artiola, 1991).
Hence, there is a need to improve water, fertilizer and soil surface
management strategies for furrow irrigation to increase irrigation
efﬁciency and reduce nitrogen losses by leaching to groundwater(Bar-Yosef, 1999).
Leaching of nitrate below the root zone can be affected by a range
of factors, including application rates and timing of applications(Robbins and Carter, 1980). A ﬁeld study conducted by Benjaminet al. (1998) showed that placing fertilizer in the non-irrigatedfurrow in alternate furrow irrigated systems increased water use
efﬁciency and reduced fertilizer leaching. However, Musick and
Dusek (1974) and Crabtree et al. (1985) found a decrease in yield ofdifferent crops when alternate furrow irrigation systems were compared with conventional furrow irrigation systems. Mailhol et al.
(2001) reported that fertilizer application near the top of the ridge
followed by heavy irrigation has a beneﬁcial impact on both yieldand nitrogen leaching. Kemper et al. (1975), Hamlett et al. (1990),
Clay et al. (1992) and Waddell and Weil (2006) also found that byplacing fertilizer near the top of the ridge, corn crop yields increased
and the risk of nitrogen leaching decreased. Most of the research
to date on reducing leaching of nitrogen from furrow irrigation
has focused on fertilizer placement (Benjamin et al., 1998; Lehrsch
et al., 2000; Mailhol et al., 2001; Waddell and Weil, 2006) andmanaging water application and water depth (Abbasi et al., 2003,
2004; Mailhol et al., 2007). Little work has been done on reducing
deep drainage and solute leaching from furrow irrigation using a
combination of soil surface management and fertilizer placement
strategies.
When the water inﬁltration rate is very high and advancement of
the furrow wetting front is slow (often a problem in coarse textured
soils), the bottom of the furrows (but not the sides) is sometimes
compacted before irrigation to reduce the inﬁltration rate (Yonts,
2007). Compaction of the bottom of the furrow can be achievedin a number of ways, including driving a tractor along the furrows
to decrease porosity (increase soil density). Another way to minimize (and mostly eliminate) inﬁltration and deep drainage directly
below the furrow is to place impermeable plastic sheets on thebottom of the furrow. This will increase irrigation efﬁciency bydiverting the direction of water ﬂow directly into the side wallsof the furrow, thereby eliminating direct downward ﬂow below
the plastic cover and providing maximum opportunity for water to
inﬁltrate into the side of the furrow and be drawn up into the ridgeby capillarity.
Placement of fertilizer plays an important role in nutrient uptake
by plants and leaching of nutrients below the root zone. In Pakistan,nitrogen fertilizers, in granular form, are usually broadcast manually onto the bottom of the furrow and sides, which can result inleaching of nitrate below the root zone before it can be extracted
by plants. Normal furrow irrigation practice in Pakistan involves
diverting water from a watercourse or ﬁeld canal to the furrow
until the water level reaches a particular depth in the furrow at

243

which point the water supply is turned off. In this paper we referto this depth as the ‘switch-off depth’.
The main objective of precision fertilizer placement is to makenutrients easily accessible to roots but without causing damage
to the young seedlings, especially during the early stages of plant
growth (Jones and Jacobsen, 2009). Achieving this requires careful placement of fertilizer within furrow irrigated systems to retain
as much nitrogen as possible within the root zone for as long as
possible to maximize plant uptake. The objective of this study
therefore was to evaluate different strategies to improve irrigation
efﬁciency and minimize deep drainage and nitrogen leaching fromfurrow-irrigated systems. To do this we used the HYDRUS-2D
model (Šimůnek et al., 2008) to analyze different nitrogen fertilizer
placements in combination with modiﬁcations of the soil surface
at the bottom of the furrow. Computer modelling is an efﬁcientmethodology for analysing alternative water and fertilizer management options for different irrigation systems (Abbasi et al., 2003),
and HYDRUS-2D has been widely used as an effective tool for modelling water and solute transport in a variety of soil geometries andirrigation systems (Mailhol et al., 2001; Cote et al., 2003; Gärdenäs
et al., 2005; Crevoisier et al., 2008; Siyal et al., 2009; Siyal andSkaggs, 2009). We did not include plants and root water uptake
in this study, which allowed us to assess the worst case scenario interms of fertilizer placement and soil surface modiﬁcation on deep
drainage and nitrate leaching.

2. Materials and methods
The HYDRUS-2D software package, which includes a user
friendly interactive graphics-based interface (Šejna et al., 2011),
uses numerical methods to solve the Richards’ equation forsaturated–unsaturated water ﬂow and the convection–dispersion
equation for solute transport. We used HYDRUS-2D in the present
study to analyze water ﬂow and nitrogen transport through a crosssectional furrow subjected to different fertilizer placements andsoil surface management strategies.
2.1. Governing equations for water and solute transport
For Darcian two-dimensional water ﬂow in a variably saturated
medium, the Richards’ equation can be expressed as:
∂
∂
=
∂t
∂xi

 
K

Kij

∂h
+ Kiz
∂xj


(i, j = 1, 2)(1)where xi (i = 1, 2) are the spatial coordinates (L), t is time (T),  iswater content (L3 L−3 ), h is the pressure head (L), K is the hydraulicconductivity (L T−1 ), and Kij are components of the hydraulic conductivity anisotropy tensor. Since we assume isotropic porous
media with x1 = x and x2 = z being the horizontal and vertical coordinates, respectively, the conductivity tensor is diagonal with theentries Kxx and Kzz equal to one.
Solute transport was described using the advection–dispersion
equation:
∂c
∂
=
∂t
∂xi



∂c
Dij
∂xj


−

∂qi c
∂xi

(i, j = 1, 2)(2)

where c is the solution concentration (M L−3 ), qi represents waterﬂux density (L T−1 ), and Dij is the dispersion tensor (L2 T−1 ), withstandard expression for the longitudinal (εL ) and transverse (εT )dispersivities (L) (e.g. Bear, 1972). Eq. (2) is valid for non-reactive
transport, so adsorption and precipitation/dissolution of the solutes
are ignored.

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A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

2.4. Initial conditions
The initial conditions were speciﬁed in terms of the soil waterpressure head h(x, z) and were set to decrease linearly with depth,
from −300 cm at the top of the ﬂow domain (z = 0) to −200 cm at
the bottom of the ﬂow domain (z = −100) (Fig. 1).
2.5. Boundary conditions

Fig. 1. Initial soil water pressure head (cm) in the ﬂow domain and boundary conditions imposed on the ﬂow domain during simulations.

2.2. Soil hydraulic properties
The soil hydraulic properties (water retention (h) and hydraulicconductivity K(h)) needed in Eq. (1) are described using the van
Genuchten–Mualem model (van Genuchten, 1980; Mualem, 1976):
(h) =⎧⎨ r +⎩

s − r[1 + |˛h|n ]

m

h<0

(3)

h≥0

s
1/m m

K(h) = Ks Se [1 − (1 − Se

) ]

2

(4)

where  r and  s denotes the residual and saturated water contents(L3 L−3 ), respectively; Ks (L T−1 ) is the saturated hydraulic conductivity, ˛ (L−1 ) and n are empirical coefﬁcients affecting the shapeof the hydraulic functions,  is a pore connectivity parameter,m = 1 − 1/n, and Se is effective saturation:Se =(h) − rs − r(5)

The HYDRUS-2D soils catalogue contains hydraulic property
parameters for a wide range of soil types and we used a medium
textured loam for this study. The soil hydraulic parameters for theloam soil were  r = 0.078,  s = 0.43, Ks = 1.04 cm h−1 , ˛ = 0.036 cm−1 ,n = 1.56 and  = 0.5 (Carsel and Parish, 1988).
2.3. Flow domain
We simulated water ﬂow and solute transport in the root zoneof a furrow irrigation system represented by a ﬂow domain 100 cm
wide (representing two halves of a 60 cm furrow and a complete
40 cm ridge at the upper boundary) and 100 cm deep (Fig. 1). A
ﬁnite element mesh was generated using the MESHGEN module
within HYDRUS-2D. Total number of nodes and triangular elementsin the ﬂow domain were 4437 and 8651, respectively. The ﬁnite
element grid was adjusted such that the size of the elements were
smaller near the soil surface and the furrows where water ﬂuxes(inﬁltration and surface evaporation) were most rapid, and larger
near the no-ﬂow and free drainage boundaries along the sides andthe bottom of the ﬂow domain.

The boundary conditions used to simulate water and solutetransport in the furrow irrigated system are given in Fig. 1. We did
not consider the effect of a water table so nodes at the bottom of theﬂow domain (z = −100 cm) were assigned a free drainage boundary
condition (unit hydraulic gradient in the z direction). Due to symmetry, the nodes representing the left and right boundary of theﬂow domain were set as ‘no ﬂux boundaries’ as no ﬂow or solutetransport occurred across these boundaries. The ridge surface andthe unsubmerged sides of the ridge were speciﬁed as ‘atmosphericboundaries’ (Šejna et al., 2011). Here water evaporates from thesoil surface at a constant potential rate (= 0.016 cm h−1 ) as long as
the surface soil water pressure head is above a certain thresholdvalue (= −15,000 cm in our study), and then switches to a constantpressure head (−15,000 cm) when the soil surface dries out to thethreshold value.
The bottom and both sides of the furrow were assigned a special boundary condition, newly implemented into HYDRUS-2D forthis study. The actual conditions along this boundary were adjusted
dynamically depending upon the water level in the furrow and conditions in the ﬂow domain (root zone). The part of the furrow thatwas below the water level was assigned a pressure head boundary
condition and the part that was above the water level was assigned
either a seepage face or atmospheric boundary condition, depending upon the saturation conditions in the ridge (see Appendix for a
detailed description of this boundary condition).
The initial water level in the furrow was set to zero, and it
increased once water ﬂowed into the furrow. A water application
rate of 1800 L h−1 furrow−1 for a 100 m long furrow was used. This
corresponds with a Qs ﬂux of 90 cm2 h−1 in Fig. A1 and an area
based application rate of 18 mm h−1 . Once water entered the furrow it started inﬁltrating into the soil from the bottom and sidesof the furrow. The criterion for controlling irrigation was based on
the commonly used practice in Pakistan, where once the furrowﬁlls to a particular depth, 10 cm in this study, water ﬂow into thefurrow is switched off. Inﬁltration then continued until all of thewater in the furrow had entered the soil. When water inﬁltrationceased, the boundary condition switched from the pressure headboundary condition to the atmospheric condition.
2.6. Initial and boundary conditions for nitrogen transport
The ﬂow domain was considered nitrogen free at the beginning of the simulations, i.e. c(x, z) = 0, except for those areas where
fertilizer was placed to simulate the effects of different fertilizer
placements (see next section for detail about actual placementof the fertilizer). Nitrogen was applied at 150 kg N ha−1 , which istypical of what is commonly practiced in furrow irrigated systems in many parts of the world, including Pakistan (FAO, 2004).
The irrigation water was nitrogen free. Solute ﬂux out of the ﬂow
domain could occur only from the bottom free drainage boundary. A third-type (Cauchy) boundary condition was speciﬁed at the
seepage, atmospheric and free drainage boundary conditions.
The diffusion coefﬁcient Do of nitrate in solution at 25 ◦ C wastaken as 0.068 cm2 h−1 as reported by Beven et al. (1993) and usedby Mailhol et al. (2001, 2007) in their simulations with HYDRUS2D. The longitudinal dispersivity (εL ) is usually considered equal to
one-tenth of the proﬁle depth and transverse dispersivity (εT ) equalA.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

245

Fig. 3. Water pressure head at the bottom of the furrow following initiation ofirrigation for the different soil surface management strategies (So , original soil; Sc ,compacted soil; Sp , plastic covered soil).

hydraulic conductivity of the top 2 cm of soil was decreased to 1/5thof the original uncompacted value (i.e. 0.208 cm h−1 ) (Douglas andCrawford, 1993; Schwen et al., 2011). Soil compaction in the ﬁeld
can be achieved in a number of ways, including by tractor wheels orwith the Eversman v-shaped wheel as suggested by Yonts (2007).
Placement of plastic on the bottom of the furrow (Sp ) was simulated by setting a no ﬂux boundary condition at the bottom of the
furrow, so that inﬁltration occurred only through the sides of the
furrow. The simulations were run for 120 h (5 days) for all treatments.
3. Results and discussion
3.1. Water application
Fig. 2. Schematic showing (a) the different fertilizer placements (Pn ) and (b) soil
surface management treatments (Si ) analysed in this study (P1 , fertilizer at the bottom of furrow; P2 , fertilizer on the sides of furrow; P3 , fertilizer on the bottom andsides of the furrow; P4 , fertilizer on the furrow sides near the ridge top; P5 , fertilizer
on the top at the centre of the ridge and So , original soil; Sc , compacted soil; Sp ,plastic covered soil).

to one-tenth of εL (Beven et al., 1993; Cote et al., 2003; Phogat et al.,2011). For the present study longitudinal and transverse dispersivities were therefore set at 10 cm and 1 cm, respectively. Nitrogenvolatilisation and adsorption by the soil matrix was not consideredin this study (Huygens et al., 2007).
2.7. Soil surface management and fertilizer placement strategies
Opportunities to maximize nitrogen retention in the ﬂow
domain (root zone) and hence minimize leaching were analysed
for ﬁve different fertilizer placements. These involved placing thefertilizer at the bottom of furrow (P1 ), on the sides of the furrow(P2 ), on the bottom and sides of the furrow (P3 ), on the furrowsides near the ridge top (P4 ) as suggested by Kemper et al. (1975),
Mailhol et al. (2001) and Waddell and Weil (2006), and on the top
at the centre of the ridge (P5 ) (Fig. 2(a)). The nitrogen fertilizer was
mixed into the top 1–3 cm of soil to ensure that the same concentration and total amount of nitrogen (150 kg N ha−1 ) was applied toeach of the P1 –P5 treatments.
We also analysed different soil surface management strategies in combination with the fertilizer placement strategies. These
included original soil (So ) as the control, compaction of the bottomof the furrow (Sc ) and placement of a plastic sheet on the bottomof the furrow (Sp ) as shown in Fig. 2(b).
The saturated soil hydraulic conductivity for the original soil
(So ) was 1.04 cm h−1 . For the compacted soil (Sc ) the saturated

The soil water pressure head at the bottom of the furrow for thedifferent soil surface management strategies with an application
rate of 1800 L h−1 (100 m furrow)−1 is given in Fig. 3. The plots showthat the water level in the furrow starts rising following initiation ofirrigation. When the water level reaches the target depth of 10 cm(after 6.7 h for So ) the irrigation is switched off. Water remaining
in the furrow continued to inﬁltrate into the soil, with the furrow
emptying after 16 h. Similarly, for Sc and Sp , water application continued until the water level in the furrow reached 10 cm after 5.6
and 4.8 h respectively, at which time the irrigation is switched off.
Water remaining in the furrow continued to inﬁltrate, with the
furrow emptying after 18.7 and 23.8 h for Sc and Sp , respectively.
As expected, the plastic covered furrow (Sp ) took the least time
to reach the 10 cm switch-off depth (4.8 h) while the Sc treatment
took 5.6 h and the So treatment took 6.7 h. These relatively short differences in application times resulted in signiﬁcant savings in termsof water applied, with the depth equivalents of water inﬁltrationfor So , Sc and Sp equalling 120, 100 and 86 mm, respectively. This
demonstrates water savings of 17% and 28% for the compacted andplastic covered soil scenarios, respectively, compared with the original soil (Table 1). These water savings mean less water was takenfrom the water supply, leaving more water to be used elsewhere,
either as environmental ﬂows or for other productive purposes.
Fig. 4 shows the time periods for irrigation, inﬁltration and redistribution for the different soil surface management strategies up to
120 h. It shows that the irrigation periods are 6.7, 5.6 and 4.8 h for
So , Sc and Sp , respectively. Inﬁltration starts at the beginning of theirrigation event and continues after irrigation has been turned off
until all the water residing in the furrow has inﬁltrated. The inﬁltration periods were 15.8, 18.7 and 23.8 h for So , Sc and Sp , respectively.
Even though the least amount of water is applied in the Sp treatment (Table 1), this treatment retains water within the furrow the246

A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

Table 1
The impact of application rate on millimeter equivalent of water applied, drained and retained in the ﬂow domain and the % savings in water applied under the compacted(Sc ) and plastic covered (Sp ) soil surface management strategies compared to the original (So ) practice. The highlighted row is the 1800 L h−1 (100 m furrow)−1 ﬂow rate usedas the base case in this study.
Application rate
(L h−1 (100 m
furrow)−1 )

1200
1400
1600
1800
2000

So

Sc

Sp

Applied(mm)

Drained(mm)

Retained(mm)

Applied(mm)

Drained(mm)

Retained(mm)% Savedrelative toSo

Applied(mm)

Drained(mm)

Retained(mm)% Savedrelative to
So

187
150
132
120
112

58.0
25.0
12.0
5.5
3.5

129.0
125.0
120.0
114.5
108.5

136
117
107
100
95

12.5
3.5
1.3
0.6
0.4

123.5
113.5
105.7
99.4
94.6

27.3
22.0
18.9
16.7
15.2

106
96
90
86
84

0.38
0.20
0.18
0.18
0.18

105.6
95.8
89.8
85.8
83.8

43.3
36.0
31.8
28.3
25.0

longest (Fig. 3) as there is no vertically downward inﬁltration fromthe bottom of the furrow. Redistribution continues to take place
after all water has inﬁltrated from the furrow in response to gravity(mostly vertically downwards) and capillarity (mostly horizontallyand vertically upwards into drier parts of the ﬂow domain, andespecially into the ridge).
Further investigation of the effect of evaporation losses fromthe water remaining in the furrow until it has all inﬁltrated into
the soil is needed in follow up studies. One way to minimize theseevaporative losses would be to irrigate in the evening or at night toavoid irrigating during daylight hours when evaporative demandis highest.
3.2. Water ﬂux and soil water content
Fig. 5 shows the water ﬂuxes at the bottom boundary (freedrainage boundary) of the ﬂow domain as a function of time frominitiation of irrigation under the different soil surface management strategies. It shows that for soil surface management strategy
So water started draining from the bottom of the ﬂow domain
35 h after initiating irrigation, with the drainage ﬂux increasing to
1.11 cm2 h−1 after 120 h. For Sc , water started draining from thebottom of the ﬂow domain 57 h after initiating irrigation with thedrainage ﬂux increasing to 0.21 cm2 h−1 after 120 h. For Sp , water
started draining from the ﬂow domain after 89 h with the drainage
ﬂux increasing to 0.017 cm2 h−1 after 120 h. The drainage ﬂuxes
after 120 h were 5 and 65 times smaller for Sc and Sp , respectively,
than for So . This, together with the reduced water input, resulted
in total water savings of 17% and 28% over the 120 h for the Sc andSp treatments, respectively, when compared with So (Table 1).

Fig. 4. The time duration of irrigation, inﬁltration and redistribution under different
soil surface management strategies (So , original soil; Sc , compacted soil; Sp , plastic
covered soil).

The movement of the wetting front horizontally into the ridgeand vertically downwards is shown in Fig. 6. It is clear from thisﬁgure that for the So scenario gravitational forces combine strongly
with the capillary forces to cause downward vertical ﬂow of water
to dominate compared with horizontal ﬂow. The wetting front inthis case reached a depth of 83 cm 30 h after initiation of irrigation.
For the Sc scenario, ﬂow in the vertical and horizontal directions
is similar early on (up to 5 h), but gravitational forces again start
dominating over capillary forces as irrigation progresses. For this
scenario, water reached a depth of 70 cm after 30 h, compared to
83 cm for the So scenario. There is thus a reduction of about 16% in
the vertical water movement in Sc compared to So .
Placing plastic on the bottom of the furrow (Sp ) prevents direct
vertical inﬁltration from the bottom of the furrow. This forces thewater to ﬁrst ﬂow horizontally into the ridge before it can ﬂow
downwards in response to gravitational forces. This exposes moreof the inﬁltrating water to capillary forces associated with driersoil in the ridge and can reduce the vertical downward movement of water. In this case the wetting front reached a depth of
55 cm after 30 h, which is a reduction of about 21% in the vertical water movement compared to Sc . It is worth highlightingthat the different soil surface treatments had a bigger effect on
the downward vertical ﬂow than the horizontal ﬂow (Fig. 6), withsome of this effect reﬂecting the different total amounts of water
applied. These results are consistent with those of Cook et al. (2009)

Fig. 5. Water ﬂux (cm2 h−1 ) from the bottom free drainage boundary as a functionof time from initiation of irrigation under the different soil surface management
strategies (So , original soil; Sc , compacted soil; Sp , plastic covered soil).

A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

who analysed water ﬂow in permanent raised beds and showedthat the horizontal wetting front extended further into the bed
when the base of the furrow was compacted compared with nocompaction.
We calculated the effect of the 17% and 28% reductions in water
applied for treatments Sc and Sp compared with So by assessing
the total drainage from the ﬂow domain and total water remaining
within the ﬂow domain 120 h after initiation of irrigation (Table 1).
The initial water held in the ﬂow domain was 180 mm for all treatments. Fig. 7 shows the soil water content distribution within theﬂow domain at 0 h (initial water content) and 120 h after initiation

247

of irrigation for each of the soil surface treatments. The So treatment
has the highest level of non-uniformity with wetter zones below the
furrows and drier zones below the ridge. The Sc and Sp treatments
show much more uniform distributions of water with depth acrossthe ﬂow domain. These data together with that in Table 1 conﬁrm
that compacting the base of the furrow or placing a plastic sheeton the bottom of the furrow can deliver signiﬁcant water savings
without major reductions in total soil water held within the ﬂow
domain.
Furrow irrigation efﬁciencies, which can vary from 30% (Fahong
et al., 2004) to 65% (Arabiyat et al., 1999) are very low compared
to well managed hi-tech subsurface drip irrigation systems (Ayars
et al., 1999). Furrow irrigation is however still considerably cheaper
than subsurface drip systems and is therefore likely to remaina major form of irrigation for many years to come. Given thisreality it is essential that every effort is made to keep improving
the efﬁciency of furrow irrigation and as we have shown there
are real and practical opportunities to do this. An advantage ofthe strategies presented here is that they could be easily implemented by farmers around the world, including those with limited
resources.

3.3. Effect of rate of water application
As the criterion for applying water to the furrow was set by ﬁlling the furrow to a preset depth, 10 cm in this study, the rate ofwater application (and amount of inﬁltration) will be affected by
the time needed to reach this switch-off depth. The lower the rateof water application, the more time it will take to reach the switchoff point and hence the more time there will be for inﬁltration. The
effect of application rates of 1200, 1400, 1600, 1800 and 2000 L h−1(100 m furrow)−1 on the total amount of water applied, drained andretained within the ﬂow domain was also analyzed, with results
given in Table 1. It shows that with increasing rates of water application the percent water savings of Sc and Sp compared with So
decreases.

Fig. 6. Horizontal and vertical movement of the wetting front as a function of time
following initiation of irrigation for the different soil surface management strategies(So , original soil; Sc , compacted soil; Sp , plastic covered soil).

Fig. 7. Soil water content (cm3 cm−3 ) distributions within the ﬂow domain (rootzone) initially and 120 h after initiation of irrigation for soil surface treatments So ,Sc and Sp (see text for further explanation).

248

A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

Table 2
Effect of fertilizer placement on the total amount of nitrogen leached from the bottom boundary (100 cm) of the ﬂow domain 120 h after the initiation of irrigation. Fertilizer
placements P1 –P5 and soil surface management strategies So , Sc and Sp are described in the text.
Fertilizer placement

Original soil (So ) (kg N ha−1 )Compacted soil (Sc ) (kg N ha−1 )Plastic covered soil (Sp ) (kg N ha−1 )P1 (furrow bottom)
P2 (furrow sides)
P3 (furrow bottom & sides)
P4 (furrow sides near the ridge top)P5 (centre of ridge)

50.00
3.00
23.00
0.00
0.00

1.40
0.05
0.62
0.00
0.00

0.00
0.00
0.00
0.00
0.00

Fig. 8. Nitrogen concentration (mg cm−3 ) distributions within the ﬂow domain for different soil surface management strategies and fertilizer placements.

A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

249

3.4. Nitrogen leaching

4. Conclusions

3.4.1. Effect of nitrogen fertilizer placement and soil surface
management strategies on nitrogen leaching
The effect of nitrogen fertilizer placement and soil surface management strategies on nitrogen leaching is summarized in Table 2.
Results show that for So the maximum amount of nitrogen leached(50 kg ha−1 ) occurred from fertilizer placement P1 (nitrogen at
furrow bottom) followed by fertilizer placement P3 (nitrogen on
furrow bottom and side) (23 kg ha−1 ). The least amount of nitrogen loss occurred from fertilizer placements P4 and P5 (nitrogenon side or top of the ridge). Similar trends were observed for soil
surface management strategy Sc . This demonstrates that direct contact of the fertilizer with inﬁltrating water such as in P1 , P2 and P3
will lead to more nitrogen leaching. Minimising direct contact will
therefore reduce the risk of nitrogen leaching as has been previously reported (Lehrsch et al., 2000, 2008). In terms of soil surface
management strategy Sp , there is little impact of fertilizer placement on nitrogen leaching. This is because most of the water thatinﬁltrated stayed in the ﬂow domain (root zone), with very little of
the water that came in contact with the nitrogen leaving the ﬂow
domain.

Results of this study demonstrate the effect of soil surface management and fertilizer placement strategies on nitrogen leachingfrom furrow irrigated systems and highlight practical opportunities
for improvement in water and fertilizer use efﬁciency. They showthat a 30–35% loss of nitrogen from the root zone for the original
soil (So ) with fertilizer placement P1 (fertilizer placed just beneaththe surface of the soil on the bottom of the furrow) can be reduced
to 1–2% by compacting the bottom of the furrow and to zero loss
by placing a plastic sheet on the bottom of the furrow. Similarly, a
30–35% loss of nitrogen from the ﬂow domain for the original soil
(So ) with fertilizer placement P1 can be reduced to 2%, 15%, 0% and
0% by changing fertilizer placement to P2 , P3 , P4 and P5 respectively.
Placing fertilizer on the furrow sides near the ridge top (P4 ) and on
the top at the centre of the ridge (P5 ) have the lowest risk of nitrogen leaching. This demonstrates the importance of minimising theexposure of nitrogen fertilizer to downward ﬂowing water.
Irrigation efﬁciency of furrow irrigated systems, especially oncoarse textured soils, can be improved by compacting or placing aplastic sheet on the bottom of the furrow. For the soil and irrigation
application rates used in this study compacting the bottom of the
furrow can result in water savings as high as 25%, while placing aplastic sheet on the bottom of the furrow can lead to water savings as high as 40%. Implementing combined fertilizer placement
and soil surface management strategies as outlined in this study
will improve the production and environmental performance offurrow irrigated systems and thereby contribute to meeting both
environmental and food security objectives.
We purposely focussed on one irrigation cycle and omitted
plants and root water uptake in this study to explore the interactions and differences between the soil surface conditions andfertilizer placements and the role of gravity versus capillarity withas few ‘complicating’ factors as possible. This allowed us to focus
on soil surface modiﬁcations that are likely to provide the mostopportunity for improving water and nutrient use efﬁciency. The
impact of soil type, furrow design, different switch-off depths andother strategies for turning off irrigation water, multiple irrigation
cycles, plant water uptake and evaporation from water ponded in
the furrow along with other aspects will be addressed in follow up
studies.

3.4.2. Nitrogen concentration remaining within the ﬂow domain
The effects of soil surface management and fertilizer placement strategies on nitrogen leaching are demonstrated graphicallyby plotting the spatial distribution of nitrogen concentration(mg cm−3 ) in the ﬂow domain 120 h after initiation of irrigation(Fig. 8). This ﬁgure shows that nitrogen in the P1 /So treatment ismost vulnerable to leaching, with very little nitrogen migrating
towards the central plane of the ﬂow domain and up into the ridge.
This contrasts strongly with the P1 /Sp treatment where most of thenitrogen is retained directly below the plastic covered furrow andthe zero concentration at the bottom of the ﬂow domain highlightsthat no nitrogen has reached this depth and leached from the ﬂow
domain.
Fertilizer placement P2 highlights the beneﬁt of having slightly
drier conditions in the ridge prior to irrigation in all three soil surface management treatments, with capillary forces pulling waterand hence nitrogen up into the ridge. The spatial distribution of
nitrogen with higher concentrations in and immediately below theridge and very low concentrations directly below the furrow highlights the effect of the dominant downward movement of water
below the furrow in P2 /So . This contrasts with treatment P2 /Sp
where water has to ﬁrst ﬂow into the ridge then down and aroundthe plastic, resulting in high concentrations of nitrogen in the ridge
but a much more uniform distribution of nitrogen with depth acrossthe ﬂow domain.
Nitrogen concentration proﬁles show that all the nitrogen fertilizer is retained within and below the ridge to depths of 50–60 cm forall soil surface treatments with fertilizer placement P4 . Treatment
P4 /Sp shows the greatest lateral and vertical spread compared with
P4 /Sc and P4 /So , which again highlights the potential beneﬁt of using
plastic on the bottom of the furrow. The nitrogen concentration
proﬁles also show that all the nitrogen fertilizer is retained within
the ridge for all soil surface treatments with fertilizer placement
P5 .
The above results demonstrate that the alternative soil surface
management strategies involving compaction and use of plastic
on the bottom of the furrow not only save water but also playa key role in retaining greater amounts of nitrogen within theﬂow domain for longer periods of time. The beneﬁts of thesepractices are particularly noticeable when fertilizer is placed at
the bottom of the furrow (P1 ) or at the bottom and sides of the
furrow (P3 ).

Acknowledgments
This work was supported in part by the Sindh Agriculture
University, Tandojam, Pakistan, CSIRO Sustainable Agriculture
National Research Flagship and CSIRO Land and Water. We thankin particular the Australian Government Department of Education,
Employment and Workplace Relations (DEEWR) for providing an
Endeavour Research Fellowship to Professor Altaf Ali Siyal and the
Sindh Agriculture University, Tandojam, Pakistan, for approving
leave to allow Prof. Siyal to undertake his fellowship with CSIROLand and Water in Townsville, Australia.

Appendix A. Special boundary condition at the bottom andsides of the furrow, implemented for this study
Analyzing furrow irrigation with a special boundary condition
that accounts for changes in the water depth in the furrow required
adjustments to the HYDRUS code and an additional input ﬁle. Thisboundary condition is shown schematically in (Fig. A1).
HYDRUS calculates the volume of water in the half furrow (S)and its rate of change (dS/dt) depending on inﬂow and outﬂow to250

A.A. Siyal et al. / Agricultural Water Management 115 (2012) 242–251

Fig. A1. Schematic showing a half furrow and implementation of a special boundary
condition accounting for variable water depth in the furrow (a is the half-width of
the bottom of the furrow, hw is the water level in the furrow, Qin the water inﬁltrationfrom the furrow into the soil, Qs is the water supply rate, ˛ is the slope of the ridge
side and c is the half-width of the water surface).

determine the water level in the furrow (hw ) via the following massbalance equations:
S=

1
1
hw
hw (a + c) = hw a + a +
2
2
tan ˛

= hw a +h2w
2 tan ˛dS
= Qs (t) − Qin (t)
dt
a

(i)
(ii)

dhw
hw dhw+=tan ˛ dt
dt

a+

hw
tan ˛

dhw
= Qs (t) − Qin (t)
dt

(iii)

This can be expressed in its ﬁnite difference form as:
a+

hw
tan ˛

j+1

j

hw = hw +j+1

j

hw − hw= Qs (t) − Qin (t)tt tan ˛jhw + a tan ˛(Qs − Qin )

(iv)
(v)

where a is the half-width of the bottom of the furrow, Qin the waterinﬁltration from the furrow to the soil proﬁle across the furrowjwalls, Qs is the water supply rate, t is the time step, and hw andj+1
hw are water levels in the furrow at the previous and current
time steps. The relevant parts of the boundary below and above
the water level in the furrow were then assigned the time–variable
pressure head (Dirichlet) and seepage face boundary conditions,
respectively. HYDRUS then calculated which part of the seepage
face boundary was active (with prescribed zero pressure head)
and which was inactive (with prescribed zero ﬂux or atmosphericboundary conditions). The supply rate (Qs ) and the maximum waterlevel that can be reached before water supply is stopped are speciﬁed by the user.
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