A cross disciplinary framework for linking farms with regional groundwater and salinity management...

A cross disciplinary framework for linking farms withregional groundwater and salinity management targets

Shahbaz Khan *, Tariq Rana, Munir A. Hanjra

International Centre of Water for Food Security, Charles Sturt University, Cooperative Research Centre for Irrigation Futures,

CSIRO Land and Water, Locked Bag 588, Wagga Wagga Campus, NSW 2678, Australia

a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 5 ( 2 0 0 8 ) 3 5 – 4 7

a r t i c l e i n f o

Article history:

Received 21 April 2007

Accepted 4 September 2007

Published on line 29 October 2007

Keywords:

Groundwater

Irrigation

Net recharge

Salinity

SWAGMAN model

Waterlogging

a b s t r a c t

Irrigation delivers major benefits in food security and human development. Irrigation also

leads to waterlogging and salinity which threaten the sustainability of irrigated agriculture

and pose major socioeconomic and environmental risks. The issue can be addressed by

limiting net recharge to groundwater such that the water and salt keep natural equilibria.

Often the information on net recharge within catchments is unavailable, particularly at

lower spatial scales such as the farm or paddock; this offers little guidance for on-farm land

and water management decisions—basic decisions that ultimately impact regional net

recharge and waterlogging and salinity dynamics. This paper develops a cross disciplinary

framework based on the concept of net recharge for setting paddock scale targets and to link

these to the regional targets and community’s goals for sustainable irrigation management.

A management model, cast in a dynamic programming format to integrate a detailed

hydrological model with an economic model was applied to estimate the productivity,

profitability and sustainability of irrigated agriculture in a region of the Murray Darling Basin

in Australia. SWAGMAN1 Farm model was used to determine paddock scale net recharge.

This interactive model enables an individual farmer to choose a profit optimizing crop mix

while lowering net recharge; this in turn leads to a win-win outcome for all farmers. The net

recharge metric can be used for the conversion of diffuse source groundwater recharge to a

point source recharge at paddock scale, enabling the definition of private property rights to a

common pool problem and assigning individual responsibilities for its management—a

vexing issue and a new concept for the commons literature. Net recharge shows significant

spatial and temporal variation which warrants a targeted/zonal approach to address the

issue. Regional and targeted strategies and actions to address the issue are identified. Apart

from its applied and action research orientation, the development of paddock scale net

recharge metric is perhaps the most significant conceptual contribution of this research

which can lead to shared management of groundwater aquifers.

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1. Backdrop

During the second half of the 20th century the global food

system has responded to the doubling of world population by

more than doubling food production. Crop yield growth and

* Corresponding author. Tel.: +61 2 69332927; fax: +61 2 69332647.E-mail address: [email protected] (S. Khan).

0378-3774/$ – see front matter # 2007 Elsevier B.V. All rights reservedoi:10.1016/j.agwat.2007.09.005

intensification have been the dominant factors, and irrigated

agriculture has played a major role. About 40% of the global

harvest came from just 20% of croplands that are irrigated

(FAO, 2003; Khan et al., 2006). Irrigation has delivered multi-

faceted social benefits, virtually shielding irrigated commu-

d.

mailto:[email protected]

http://dx.doi.org/10.1016/j.agwat.2007.09.005

Table 1 – Irrigated lands damaged by salinity in the top four irrigators and the world

Country Postel (1991) Ghassemi et al. (1995)

Irrigated areadamaged (Mha)

Share of irrigatedland damaged (%)

Irrigated areadamaged (Mha)

Share of irrigatedland damaged (%)

India 20.0 36.0 7.0 16.6

China 7.0 15.0 6.7 14.0

US 5.2 27.0 5.6 28.6

Pakistan 3.2 20.0 4.2 24.0

World 60.2 24.0 45.4 20.0

1 A recent review can be found in Croke et al. (2006) and Khannaand Malano (2006).


nities against the episodes of hunger and famine. The benefits

of irrigation are many: irrigation boots productivity and

improves food security, generates employment and income,

improves nutrition, health and education and fosters human

development and enhances equity in favor of the poor

households particularly where they have good access to land

and water resources and related rural infrastructure and

services (Hanjra et al., in press).

Irrigation delivers major benefits in food securityand human

development. Poorly managed irrigation can also have unin-

tended environmental consequences and social disbenefits

(Hussain and Hanjra, 2003, 2004). About 1/3rd of global irrigated

land have lower productivity due to poorly managed irrigation

causingwaterlogging and salinity.Annually about 10 million ha

(Mha) are lost to salinisation of which about 1.5 Mha are

irrigated lands. Estimates differ widely for various irrigation

systems (Table 1). Cumulative global productivity loss due to

land degradation over three decades has been estimated at 12%

of total production from irrigated, rainfed and rangeland or

about 0.4% per annum (World Bank, 2003, p. 85). National costs

of dryland salinity in Australia are estimated at AUD$

130 million per annum in lost agricultural production, AUD$

100 million per annum in infrastructure damage and AUD$ 40

per annum in the loss of environmental assets (Hajkowicz and

Young, 2002). Data spanning 1971–93 from India and Pakistan

Punjabs show that intensification of land and water resources

caused resource degradation, slowing overall productivity

growth (Murgai et al., 2001). For Pakistani Punjab these data

show that resource degradation has reduced over all produc-

tivity growth from technical change, education and infra-

structure investments by 1/3rd. Estimates for the left bank main

canal of Tungabhadra project in south west India show that

land degradation alone accounted for about 15% of the system’s

productive potential (Janmaat, 2004). Many of the high-

potential irrigated areas such as Punjabs in India and Pakistan

and parts of the Yellow River basin in China are now

experiencing signs of stagnation in crop productivity growth,

over-use of water resources, pest infestations, and buildup of

toxic salts—the menace that had undermined the stability of

several ancient irrigation societies and that places ours in

jeopardy as well (Khan et al., 2006; Postel, 1999). China, India,

Iran, and Pakistan are among the countries where a significant

share of the irrigated land is now jeopardized by scarce river

water, groundwater depletion, a fertility-sapping buildup of

salts in the soil, or some combination of these factors,

threatening rural industries and livelihoods of millions.

Irrigation induced waterlogging and salinity reduces agri-

cultural production and imposes other economic, social, and

the environmental costs (Houk et al., 2005; Ragan et al., 2000;

Wichelns, 1999, 2002). The costs are often not confined to a

paddockor private property but spread over a wider scale. There

can be a range of off-farm impacts including damages to roads

and infrastructure, loss of aesthetic values and reduced

biodiversity, etc. (Characklis et al., 2005). Other impacts include

displacement of labor, out migration, income disparity, and

overall reduction in food output. These off-site costs or

externalities have important policy implications regarding land

and water resources management and how tax dollars are

deployed to mitigate salinity and create social benefits for all.

Sustainable irrigation management remains a daunting task

(Khan et al., 2006). A range of complex natural resource

management issues are challenging traditional irrigation

management approaches across irrigated systems in Australia

(Clarke et al., 2002; Proust, 2003). In many cases the loss of

agricultural productivity impacts resource using industries

while the costs of changing management practices and

ensuring the future sustainability of land and water resources

as well as ecosystem health and biodiversity conservation

appear substantial. Smedema and Shiati (2002) provide a review

of the salinity hazards of irrigation development in arid areas.

Waterlogging and salinity are twin evils of irrigation that

can be managed, not controlled. Watertable elevations and

their adverse impacts on crop productivity are sensitive to the

farmer’s crop mix and irrigation practices (Ashraf and Saeed,

2006; Tarboton et al., 2004). Improved land and water

management practices thus hold the greatest promise to

salinity management (Kahlown and Azam, 2002).

Worsening salinity issues across old established irrigation

systems has prompted the Australian governments, indus-

tries and communities to investigate alternative to promote

sustainable water use. Develop of effective salinity manage-

ment strategies require a through knowledge of the technical

and biophysical derivers of the issue as well as underpinnings

of embedded social and economic structures and environ-

mental factors that can be leveraged to address the problem.

Unfortunately such knowledge is incomplete1 or unavailable

at lowest spatial scales, say a paddock or farm—the basic

decision making unit whose actions impact land and water

management and salinity dynamics.

This study develops a linked hydro-economic and biophy-

sical framework that can be used by the irrigators to select a

crop mix to optimize economic returns while meeting paddock

scale net recharge targets thus maintaining groundwater tables

and salinity within desirable limits. The geospatial modelling

along with economic optimization enables farmers to limit

Map 1 – The location of study settings.

a g r i c u l t u r a l w a t e r m a n a g e m e n t 9 5 ( 2 0 0 8 ) 3 5 – 4 7 37

irrigation to levels that best suit the soil, climate and ground-

water dynamics at the paddock scale and then links these

individual decisions to wider zonal groundwater recharge

targets—a zero rise in watertable. This work can serve as a

basis for designing tradeable recharge credits at paddock scale;

to link these targets/credits to overall groundwater and salinity

management goals; to assign private property rights to a shared

common pool issue; to craft institutions for effective recharge

management; and to promote a rational dialogue between the

farmers and other stakeholders to develop a shared-vision for

sustainable irrigation management.

2. The setting and data

The Coleambally Irrigated Area (CIA) located in the southern

Murray Darling Basin in Australia served as settings for this

study (Map 1). The CIA covers about 80,000 ha and is managed

by a cooperative—Coleambally Irrigation Cooperative Ltd.

Fig. 1 – The net recharge concept. Note: the upward pointing ar

from deep groundwater aquifer to shallow aquifer, leading to a

is the situation in some zones in the Murray Darling Basin, Au

Water supplies are regulated, provided either by irrigation

companies through a channel network or can be pumped by the

farmers directly from the rivers or creeks. Water rights are

defined by an existing water supply contract between the

cooperative and State Water and between individual irrigators

and the cooperative. The CIA is unique in the southern Murray

Darling Basin in that it nearly uses all its water on annual crops.

Furthermore, the CIA is relatively distant from rivers with

relatively little salt export downstream through either surface

or sub-surface flows compared to most irrigated areas.

September to February are the irrigation months and March

to August are the non-irrigation months. Rice remains the

major crop, normally grown in ponded water from sowing or

from thethree-leaf stage. Often leguminous pastures or dryland

crops are grown in rotation with rice to help improve soil

fertility and limit pesticide use. The drainage water from rice

fields is recycled to maximize the utility of irrigation water and

minimize off-farm impacts. Due to limited water available for

irrigation, area caps for rice on each farm are applied—the

ability of the soil to pond water without excessive recharge to

groundwater or environmental effects to other lands are the

factors considered for rice caps, but there have been limited

scientificknowledge toguide this vision. Hence, soil salinisation

duetorisingwatertablesremainsamajor concern inthesetting.

This paper builds on previous hydrogeology studies in the

CIA and develops the concept of net recharge for setting

paddock scale targets to avert watertable rise and guide

community vision for sustainable irrigation management in

the CIA. Net recharge occurs when the quantity of irrigation

water applied exceeds the evaportanspiration (ET) needs of

crops, leaching requirements of soil, and water movements

within underlying groundwater systems (Fig. 1). Net ground-

water recharges/discharges are the principal driver of rising/

falling watertables which in turn has implications for salinity

and drainage.

This study developed a simple spatial model to estimate

net recharge at two time points, namely March and September

row for the leakage refers to flow due to artesian pressure

rise in shallow ground watertable in the rootzone (and this

stralia).


every year. A groundwater surface for the entire CIA was

developed using the contouring software ‘SURFER’. The

groundwater table data from 800 piezometers located in

settings monitored between 1994 and 2004 was used. Net

recharge was estimated by computing the volume change

between the two surfaces. The resulting soil volume was then

multiplied by the effective porosity (5%) to calculate the

change in the volume of groundwater.

The interpolation method called Krigging available within

the SURFER software was used with a linear variogram model

to derive shallow groundwater surfaces. The data was gridded

to easting from 378,590 to 420,875 m, and northing from

6,120,000 to 6,162,175 m. The resultant mesh was then blanked

out using a digitised boundary map of the CIA. The blanking

operation meant that data outside the boundary of the CIA

was ignored in any calculations. Piezometric data gridded and

blanked thus formed each piezometric surface and these

surfaces were determined for March and September of each

year. The groundwater surface of succeeding period was

deducted from the surface of the previous period to compute

the net volume change between the two periods. If the volume

change was positive then net recharge had occurred; and if the

volume change was negative then net discharge had occurred.

The analysis was based in the following assumptions:

� T
he piezometers less than 15 m deep accurately reflect
watertable level. This holds mostly but some areas where

aquifer confinement occurs to a greater or lesser degree may

impact the validity of this assumption.

� T
he effective porosity value used (5%) will affect the
magnitude of the estimate but the estimate will not distort

the result between net recharge or discharge; and it will not

affect the trend over time as this methodology was based on

seasonal differences. Since this analysis uses a uniform

effective porosity for the whole area, errors will be created as

the piezometric rise/fall in a clayey soil and sandy soil but

will be assessed to represent the same volume change, when

in reality may not the case.

� T
he results obtained from the area wide analysis are
representative averages of the irrigation area, and can

Fig. 2 – Seasonal variations in net

therefore be strongly influenced by distinct fluctuations in

particular subregions of the CIA.

Net recharge was based on the following four estimates:

1. N

rec

et annual recharge (September): the volumetric watertable

difference between September of a given year and

September of previous year.

2. N
et annual recharge (March): same method as above but
March values substituted for September values to calculate

volumetric watertable difference.

3. N
et seasonal recharge (summer/irrigation): the volumetric
addition during irrigation season, calculated by the volu-

metric watertable difference between March of a given year

and September of the preceding year.

4. N
et seasonal discharge (winter/non-irrigation): the dissipated
volume of water during the winter months (non-irrigation

period) determined by the volumetric watertable difference

between March and September of any given year.

Paddock scale targets for various crop mixes were derived

using SWAGMAN1 Farm model (Khan et al., 2003), as

discussed latter. The modelling results support the conception

that paddock scale net recharge targets can serve as mile-

stones to address the twin issues of waterlogging and salinity

and to develop a shared-vision among stakeholders that could

serve as a road map for sustainable irrigation management.

3. Results

Net recharge analysis was carried out at various spatial scales

specifically at the irrigation system level; for various zones/

subregions; and at the farm level. The analysis spans a time

series of 10 years (1994–2004). The results are given below.

3.1. Net recharge analysis at the irrigation system level

As noted earlier, a positive value shows net recharge and a

negative value denotes net discharge. Fig. 2 shows seasonal

harge in the study settings.

Fig. 3 – Seasonal variations in net recharge, 1994–2004.


net recharge, winter rainfall and irrigation allocation (%).

Clearly net recharge occurs mainly in summer season and

discharge in the following winter season.

Net recharge and discharge years are given in Fig. 3. The

values below the 458 line show an overall net discharge; the

values above this line show net recharge to the groundwater

aquifer. The net recharge periods thus are 1996, 1997 and 1999;

the net discharge periods are 1995, 1998, 2001, 2002 and 2003.

Seasonal recharge and discharge for winter and summer are in

balance for the year 1994 and 2000. The large winter discharge

Fig. 4 – Different zones for water b

in 2002 was due to dry conditions in that year with a record

deficit of evapotranspiration minus rainfall. This relationship

could be useful in determining what levels are not to be

exceeded for net recharge management and what summer

recharge values can be absorbed by the groundwater aquifer

system during subsequent winter months or dry spell.

The annual net recharge estimated over a longer time

frame of 30 years (including summer and winter seasons)

provides an alternative analysis to recharge trends which

exhibit same overall trend, omitted here for brevity. The

March to March period analysis shows lesser fluctuation than

the September to September period. This seems plausible

because March to March trends are dominated by the recent

irrigation activities and not complicated by the following

winter conditions. Overall this analysis suggests that intra-

period net recharges are significant; irrigation periods are

associated with recharge and winter periods with discharge.

Net recharge patterns for various zones may however vary

significantly, as shown below.

3.2. Net recharge targets for different zones

In contrast to earlier assumption that area wide analyses are

representative averages of the CIA, net recharge in particular

subregions/zones may vary due to distinct geophysical

conditions and fluctuations. The spatial patterns in net

alance in the study settings.


recharge were analysed to assess the departures from this

assumption. The analysis depicted five distinct zones within

the CIA. The net recharge targets for these five zones in CIA

were derived for the period 1999–2000 using MODFLOW and

APSIM models calibrated by Khan et al. (2004) (Fig. 4). For each

of the five zones, the water balance was computed for the

upper Shepparton formation using the water budget analysis.

The water budget thus provides estimates of net inflow, net

outflow, total recharge, and vertical leakage from the upper

Shepparton to lower Shepparton formations.

On the basis of these calculations the total vertical leakage

from shallow to deeper aquifers was estimated at 31.0 GL. This

estimate was twice the previous estimate on vertical outflow

capacity of the aquifers in the CIA (Khan et al., 2004). This

denotes the decline in deeper aquifer pressures due to

continued groundwater pumping. The net lateral flow from

the upper Shepparton formation was small due to the smaller

transmissivity of this aquifer. If the total transmissivity of the

Shepparton formation was considered the total lateral outflow

would be around 15 GL, an estimate in accord with the

previous studies (Khan et al., 2004). The total estimated

recharge for 1999/2000 was 55.70 GL. Considering the lateral

and the vertical outflow capacities (15 + 31 GL), the total

recharge reduction requirement was 9.7 GL or 0.12 ML/ha on

average (note: 1 ML/ha = 10 cm/ha). If the total recharge to the

Shepparton formation could be managed according to the

vertical leakage between aquifers, the watertable would

remain static or decline over time.

The upper limits of net recharge for various zones were

computed by adding the vertical leakage to the net local

outflow and are given in the last row of Table 2.

Water balance results for these five zones show a

consistent pattern of groundwater recharge during the

irrigation period and groundwater discharge during the

non-irrigation period. The highest vertical groundwater

Table 2 – Net recharge targets for five zones in the study setti

Period Water balance component

March 99–August 99

(non-irrigation period)

Horizontal inflow (ML)

Horizontal outflow (ML)

Net recharge (+)/discharge (�) (ML/ha)

Leakage (ML/ha)

Total recharge (ML)

Leakage (ML)

Net recharge (ML)

September 99–February 2000

(irrigation period)

Horizontal inflow (ML)


Net recharge (+)/discharge (�) (ML/ha)

Leakage (ML/ha)

Total recharge (ML)

Leakage (ML)

Net recharge (ML)

Yearly total Horizontal inflow (ML)


Total recharge (ML)

Leakage (ML)

Net recharge (ML)

Upper limit of total recharge (ML)

Note: 1 ML/ha = 10 cm/ha.

leakage was around 0.2 ML/ha/6 months for Zone 1, which

also had the lowest net recharge due to relatively more

groundwater depth and groundwater pumping in and around

the zone. On the other hand, Zones 3 and 4 had the lowest

vertical groundwater leakage associated with high recharge

rates. Zones 2–4 had the highest risk of soil salinisation due to

net vertical capillary upflows. These estimates show that total

CIA shallow to deep groundwater outflow capacity was around

15,000 ML/season or 30,000 ML/year. This implies that there

was a need to reduce total recharge by around 20,000–

25,000 ML/year for the CIA as a whole—through a combination

of on-farm and zonal management options discussed latter.

Based on these zonal data a cap could be worked out. A cap

may be effective to limit the net recharge to sustainable levels

but the cap must reflect the extent and impact of individual

contributions to the aquifer. The cap would be ineffective if

potential contributors are excluded, covers several unlinked or

partially linked commons, or fails to reflect the spatial

impacts. In the case of CIA, the scientific information shows

that there are several commons, some of which are partially

linked (Fig. 4). The CIA wide cap may reduce the total volume

of net recharge but fail to reduce the impacts in discharge

zones (Fig. 5). A system wide cap may reduce some better

quality net recharge that would be beneficial to deeper

aquifers and hence reduce potential future groundwater

reserves. This suggests that sub-regional caps/strategies are

required for effective recharge management.

3.3. Impacts of extreme events on the net recharge targets

Fig. 5 depicts the impact of dry conditions on groundwater

balance. It shows that recent dry conditions combined with

higher groundwater pumping and low water allocations

caused a net decline of 0.5–2 m in shallow groundwater

aquifers in the setting over 1 year period. Fig. 4 shows relative

ngs

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5

133 39 155 46 72

296 147 202 151 99

0.11 �0.16 �0.1 �0.02 0.12

0.19 0.17 0.15 0.12 0.13

2613 �2075 �2656 �375 1556

4513 2205 3984 2250 1686

�1900 �4280 �6640 �2625 �130

147 27 160 49 81

304 179 246 167 119

0.27 0.69 0.85 0.47 0.69

0.17 0.27 0.16 0.14 0.14

6413 8948 22578 8813 8948

4038 3502 4250 2625 1816

2375 5446 18328 6188 7132

280 66 315 95 153

600 326 448 318 218

9026 6873 19922 8438 10504

8551 5707 8234 4875 3502

475 1166 11688 3563 7002

8871 5967 8367 5098 3567

Fig. 5 – Total groundwater decline (m) from March 2002 to March 2003 in the study settings.


locations of pumping wells and groundwater outflow rates in

shallow aquifers. It should be mentioned that areas of greatest

groundwater decline are located in close proximity to the deep

groundwater pumping wells. This could also be explained by

relatively higher vertical leakage between the Shepparton and

Renmark formations, as shown by the recent analysis of

Coleambally pumping data by Khan et al. (2004).

The results of the net recharge analysis spanning the past

10 years for non-irrigation and irrigation periods show that the

1997/98 season had the highest net recharge. This could

therefore be considered as the extreme event for the study

area. By using this net recharge estimate in the calibrated

MODFLOW model of the CIA (Khan et al., 2004), the simulation

was run for the next 30 years to analyse the impact of such

extreme events on the depth of watertable in the western

Coleambally irrigation area. Fig. 6 shows the time dependent

increase in the western CIA under 0–0.5 m depth to watertable

Fig. 6 – Simulated watertable depth rise in

for the 30-year scenario stochastically generated into the

future. Inclusion of stochastic events in net recharge analysis

remains critical, in view of the current prolonged drought in

the setting and predicted climate change. Such analysis can

help to understand system’s resilience to natural shocks and

to estimate the cost of human adaptations required to mitigate

the potential adverse impacts (John et al., 2005).

3.4. Sustainable irrigation management options fordifferent zones

The above analysis shows significant spatial patterns in net

recharge and discharge to warrant demarcation of the entire

CIA into five distinct zones. At present the net recharge areas

are around 30% of the CIA where a focus on recharge reduction

alone can help to address the issue; for others zones a more

diversified approach would be required. This would involve (i)

the western region of study settings.


implementing both on-farm and zonal management options

for the groundwater discharge zones and (ii) the need for

rational sharing of costs between the net groundwater

recharging farms, discharging farms and the wider commu-

nity.

Modelling results show that the overall vertical leakage

capacity between the shallow and deeper aquifers was around

30,000 ML/year. To maintain or lower current groundwater

levels, the total recharge needs to be less than the overall

vertical leakage capacity of aquifers. Since groundwater levels

in some regions, e.g., central, southern and western parts are

already within the root zone, it requires initially a reduction in

groundwater recharge to less than the outflow capacity. On

the sub-regional/zonal basis, the water balance indicated

wider environmental benefits from sustainable resource

management such as reduced saline flows to natural streams,

and protection of ecosystem services within and beyond

the CIA.

Net groundwater discharge zones located usually in low

lying areas, such as around Bull Road, are impacted by both

regional and local groundwater inflows. An immediate

remedial action would be to avoid growing rice within

100 m of these depressions because groundwater levels are

already too close to the surface and any hydraulic loading in

the vicinity could direct lateral flows to these depressions. In

addition to this local action, the regional saline groundwater

flows towards these depressions must be intercepted.

The lower panel in Fig. 7 presents the location map of the

selected piezometers to help visualize the trends in ground-

water dynamics in five different zones within the CIA. The

land and water management options for sustainable irrigation

management differ widely among the zones, as discussed

below.

3.4.1. The options for Zone 1The estimates show that the total groundwater outflow

capacity of this zone was around 7000 ML/year. The piezo-

metric levels near the eastern edge of this zone are very close

to the groundsurface whereas those near the western edge

have groundwater levels deeper than 5 m. The groundwater

levels in this zone show an overall declining trend (Fig. 7) and

the downward leakage rates in this region are expected to

increase further in future. One option would therefore be to

convert rice growing areas with high water use to non-ponded

crops such as maize to help improve the overall water use

efficiency.

3.4.2. The options for Zone 2The total groundwater outflow capacity in this zone was

around 4000 ML/year. Some of the groundwater levels away

from the Coleambally deep groundwater bore are very close to

the ground surface (Fig. 7). There exists a need to implement

net recharge management on a priority basis to all farms:

reduce total on farm recharge to less than 0.5 ML/ha using

winter cropping options; and limit rice water use to levels

below the rice ET + 1.0 ML/ha. For the eastern edges of this

zone where shallow groundwater salinity values are below

2500 dS/m, spear point pumping and conjunctive use of

surface and groundwater sound promising to address the

issue.

3.4.3. The options for Zone 3The total groundwater outflow capacity of this zone stands at

about 7000 ML/year. The groundwater levels close to the

intersections between Zones 2 and 3 are near the ground

surface and the groundwater levels at the western edge are

well below the root zone. Although groundwater levels have

shown a declining trend recently (Fig. 7), this remains a net

groundwater importing/discharge zone and thus the highest

salinity risk area due to higher capillary outflows from the

saline watertable surface. This denotes the need to implement

net recharge management strategies on a priority basis with

an objective to reduce total recharge by 0.6 ML/ha/year.

Options include winter cropping, on farm water use efficiency

improvements and shallow groundwater pumping. However,

due to higher salinity of shallow groundwater the drainage

water reuse options may be limited. Drainage options

combined with the serial biological concentration of salts

(Khan et al., 2007a) may be considered in the eastern and

middle parts of this zone.

3.4.4. The options for Zone 4The total groundwater outflow capacity for this zone was

around 5000 ML/year. Despite dry climate and low water

allocations over the past few years this area has not shown

any dramatic decline in the piezometric levels (Fig. 7)

therefore confirming a very low rate of vertical leakage

between the shallow and deeper aquifer. Like Zone 3 this

zone faces the highest salinity risk due to shallow water-

tables and higher capillary outflows through the soil. This

points to the need to implement net recharge management

strategies on a priority basis with an objective to reduce total

recharge by 0.5 ML/ha/year. The strategies could include

winter cropping, conversion from annual pastures to

lucerne, integrating agroforestry and perennial pastures

(Dunin, 2002; Turner and Ward, 2002) and on-farm water use

efficiency improvement and shallow groundwater pumping.

Rotation of rice paddies to leach out salts from the root zone

should also be considered as an option (Bouman and Tuong,

2001). This zone has some potential for good quality shallow

groundwater pumping at the eastern edges. However, due to

higher salinity of shallow groundwater drainage from most

of this zone the reuse options may be limited. Drainage

options combined with the evaporation basins and serial

biological concentration of salts can be considered in the

middle parts of this zone.

3.4.5. The options for Zone 5The total groundwater outflow capacity in this zone was

around 3000 ML/year. The piezometric levels at the extreme

western edges of this zone have shown a declining trend over

the last few years indicating a good impact of deep ground-

water pumping and reduced lateral inflows (Fig. 7). Hence, net

recharge management should be implemented with a target

to reduce overall recharge by 0.4 ML/ha and to enhance

vertical leakage by encouraging deeper groundwater pump-

ing. Local improvements in this zone combined with

improvements in Zone 4 would help achieve the long-term

sustainability. For instance trees can provide localized

discharge from deeper groundwater systems (Hatton et al.,

2002)

Fig. 7 – Location map of the selected piezometers and historic groundwater trends for various zones in the study settings.

Note: the piezometers locations are marked on the x-axis and time points on the y-axis.


3.5. Net recharge targets at the farm level

The previous analysis shows significant spatial patterns in

groundwater recharge and has articulated some spatial

strategies for sustainable recharge management. In any

irrigated system, a paddock or farm remains the basic unit

to make land and water management decisions—the deci-

sions that determine net recharge at farm level and add up to

determine the net recharge at the zonal vis-a-vis the system

level. However, the biophysical and hydrological and eco-

nomic information to support management decisions at the

paddock scale is generally not available. Furthermore, the

factors that underpin land and water management decisions

at the farm level may not be uniform across farms, due to

heterogeneity in resource endowments and personal prefer-

ences for certain farming types that may in turn impact net

recharge on individual farm. This calls for farm specific

information to address the net recharge issue head on. A

unified approach across farm, guided by a common vision for

sustainable land and water management, would ensure a win-

win outcome for all. This nevertheless requires farm scale net

recharge analysis.

Salt Water And Groundwater MANagement (SWAGMAN1

Farm) model was used to compute paddock scale net recharge

Table 3 – Land use areas (ha) and cropping patterns forthe modelled scenarios

Scenario Land use areas (ha)

1 69 rice, 161 fallow

2 69 rice, 75 wheat, 25 winter pas-

ture, 61 fallow

3 69 rice, 25 wheat, 25 lucerne, 111

fallow

4 69 rice, 50 wheat, 25 lucerne, 25

winter pasture, 61 fallow

5 69 rice, 25 soybeans, 25 corn, 111

fallow

6 19 rice, 50 rice/wheat, 50 corn, 111

fallow

7 19 rice, 50 rice/wheat, 25 lucerne,

136 fallow

8 200 corn, 30 fallow

9 100 corn, 100 wheat, 30 fallow

10 100 corn/wheat, 130 fallow


targets. The SWAGMAN Farm model computes the lumped

estimates of water and salt balance component for the

cropping and fallow periods for a range of broad acre crops

such as rice, soybean, maize, sunflower, fababean, canola,

wheat, barley, hay and grazed lucerne, annual pastures,

perennial pastures as well as dry land wheat and uncropped

areas, for different irrigation, soil and climatic and hydro-

geological conditions. The model then computes the gross

margin/profits from alternative cropping combinations, sub-

ject to net recharge constraints. This state of the art hydro-

economic model thus clearly captures economic and environ-

mental tradeoffs in adopting different land and water

management options at farm scale, to help farmers decide

sustainable irrigation intensities while optimising profits.

SWAGMAN1 Farm is a lumped water and salt balance

model which integrates climatic, agronomic, hydrogeological

and irrigation, and economic aspects of irrigated agriculture

under shallow watertable conditions at a farm scale (Khan

et al., 2007b). The model has a dedicated standalone and web

based user-interface to help input data and seamlessly

visualise results. This model has been used to evaluate

management options such as net recharge management for

the control of shallow watertables, focusing specifically on

managing the net recharge beneath the root zone in relation to

the vertical and lateral regional groundwater flows. The water

and salt balance computations for each crop were validated

after detailed monitoring (Edraki et al., 2003). The farmer’s

decision set shows crop mix, net recharge to groundwater and

expected profits.

Regional groundwater investigations, surface–ground-

water interaction models of the irrigation regions and the

SWAGMAN1 Farm model are strategic developments in

natural resource management, with emphasis on providing

real solutions for real people in real catchments. For instance,

Coleambally Irrigation has structured its environmental

management plan for on-farm net recharge management

using SWAGMAN1 Farm model. This innovative decision tool

can help promote sustainable land and water management

on-farm; it could also serve as a milestone for defining net

recharge targets/credits to individual farmers that could

contribute to achieving the overall groundwater and salinity

management targets at catchment scale and promote a

rational environmental management dialogue between the

farmers and other stakeholders.

3.5.1. Application of SWAGMAN1 Farm model to themanagement of net recharge targetsThe SWAGMAN1 Farm model was used to analyse watertable

and salinity impacts of different rice based irrigation systems.

Ten scenarios were generated for a single model farm of

230 ha. For these scenarios 5th deciles of rainfall were

considered (1920–2003). These scenarios included single crops

grown in individual paddocks as well as rotations such as rice/

wheat and corn/wheat in the same paddock over a given year

shown in Table 3; and information on irrigation levels for

these crops.

The effect of initial watertable depth on net recharge was

explored for each of the scenarios, using a starting water level

of 1.0 and 3.0 m. These levels were assumed to be consistent

over the whole area of the farm with no local fluctuations. No

groundwater pumping was included and the runoff from the

farm/paddocks was considered minimal due to the water

being recycled back onto the farm. Three regional ground-

water outflow rates of 0.2, 0.5 and 1.0 ML/ha were used to

explore options for the southern, central and northern parts of

the study area. The initial water content for both soils was

considered to be 3.0%.

3.5.2. Implications for on-farm land and water managementdecisionsTo explore the effect of different factors on net recharge and

salinity, the proposed cropping scenarios were modelled for

transitional red brown soil type, 5th decile band of rainfall, two

groundwater depths (1.0 and 3.0 m), and three groundwater

flow rates (0.2, 0.5, and 1.0 ML/ha) thus generating 60 model

runs (10 � 1 � 1 � 2 � 3 = 60). The model results are given in

Table 4 and summarized below.

3.5.3. Implications for southern region

For 0.2 ML/ha groundwater outflow (typical of the Southern

CIA) all scenarios result in a net positive recharge except

scenario 7 with 50 ha rice–wheat rotation, 15 ha rice, 25 ha of

lucerne and 136 ha of fallow. The first five scenarios with 69 ha

of rice without a crop after result in the higher rates of

groundwater recharge. For 1.0 m depth to watertable there

was a net accumulation of salt resulting in 0.4 to 0.7 dS/m

increase in groundwater salinity. For the deeper groundwater

depths the net recharge kept the root zone free of additional

salts. Scenarios 8–10 show that corn–wheat rotation was a

better option due to a lesser recharge (still the recharge was

around 1.0 Ml/ha) however there was greater risk of salinisa-

tion from shallow watertable, at 1.0 m depth from the ground

surface.

3.5.4. Implications for central and western regionThe 0.5 ML/ha regional groundwater outflow rate remains

quite typical for the central and western parts of the CIA. The

best cropping option to control groundwater recharge appears

to be scenario 7 with 3.0 m depth to watertable with the lowest

net recharge but net increase in salinity. Other options such as

scenario 10 with a corn–wheat rotation again give lower net

Table 4 – Watertable and salinity impacts of different rice growing scenarios for transitional red brown earth soils using 5th decile band for rainfall data

Scenario Depth ofwater

table (m)

Net recharge/discharge underland use (ML)

Net recharge (ML/ha) Salt fromirrigation

(tons)

Net salt intoroot zone(tonnes)

Ave watertable

change (m)

Ave saltconcentrationchange (dS/m)

Leakageat 0.2

(ML/ha)

Leakageat 0.5

(ML/ha)

Leakageat 1.0

(ML/ha)

Leakageat 0.2

(ML/ha)

Leakageat 0.5

(ML/ha)

Leakageat 1.0

(ML/ha)

1 1 177 108 �7 0.8 0.5 0.0 106 486.2 �1.0 0.8

3 260 191 76 1.1 0.8 0.3 106 4.8 0.6 0.0

2 1 355 286 171 1.5 1.2 0.7 164 184.2 0.2 0.3

3 262 193 78 1.1 0.8 0.3 164 44.8 0.9 0.0

3 1 121 52 �63 0.5 0.2 �0.3 132 661.9 �1.2 1.1

3 270 201 86 1.2 0.9 0.4 132 30.4 0.6 0.0

4 1 224 155 40 1 0.7 0.2 161 510.4 �0.6 0.8

3 262 193 78 1.1 0.8 0.3 161 43.2 0.9 0.0

5 1 322 253 138 1.4 1.1 0.6 149 335.2 �0.4 0.5

3 324 255 140 1.4 1.1 0.6 149 8.5 1.1 0.0

6 1 94 25 �90 0.4 0.1 �0.4 168 335.2 �0.4 0.5

3 170 101 �14 0.7 0.4 �0.1 168 3.3 1.4 0.0

7 1 �225 �294 �409 �1.0 �1.3 �1.8 132 737.4 �1.5 1.2

3 42 �27 �142 0.2 �0.1 �0.6 132 16.8 0.6 0.0

8 1 587 518 403 2.6 2.3 1.8 192 90.6 0.6 0.1

3 466 397 282 2.0 1.7 1.2 192 0.9 2.6 0.0

9 1 288 219 104 1.3 1.0 0.5 154 90.6 0.6 0.1

3 210 141 26 0.9 0.6 0.1 154 58.2 1.1 0.0

10 1 �14 �83 �198 �0.1 �0.4 �0.9 125 391.7 �0.6 0.6

3 �14 �83 �198 �0.1 �0.4 �0.9 125 3.84 0.7 0.0

Note: bold-faced number show the upper and lower values for respective column; tonnes refers to Mg = 1000 kg; and 1 ML/ha = 10 cm/ha.

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recharge (0.4–0.6 ML/ha). All other options result in net

recharge greater than 0.7 ML/ha.

3.5.5. Implications for northern region

The regional groundwater outflow rates of 1.0 ML/ha and

watertable depth of 3.0 m roughly represents the groundwater

conditions in the Northern CIA. For these conditions cropping

scenarios 7 and 10 result in lowest net recharge (�0.3 and

0.1 ML/ha). If rice–wheat rotation with some area under

lucerne or a corn–wheat rotation were practiced the net

recharge can be reduced very close or below the target levels.

3.5.6. On-farm actions matterThe model results in Table 4 suggest that on-farm land and

water management decisions have a strong bearing on net

recharge and salinity dynamics. For instance, net recharge/

discharge could vary from �1.8 to 2.6 ML/ha; salts from

irrigation from 106.0 to 192.0 tonnes (tonnes refers to

Mg = 1000 kg); net salts into the root zone from 0.9 to

661.9 tonnes; salt concentration change under land use from

0.0 to 1.2 dS/m; and average water table change from �1.5 to

2.6 m—depending on farm scale land and water management

actions. Coordinating farmer’s production decisions based on

sound scientific frameworks such as SWAGMAN Farm model

offers the best pathway to achieve sustainable irrigation

management.

4. Summary and conclusions

Shallow groundwater salinity induced by irrigation can be

managed by keeping irrigation applications below the net

recharge—a concept that links the evapotranspiration needs

of crops, leaching requirements of soil, and water move-

ment within underlying groundwater systems. Often net

recharge within a catchment/basin is unknown or existing

knowledge is incomplete, particularly at lower spatial scales

such as the farm or paddock; this offers little guidance for

on-farm land and water management decisions that

together impact basin wide net recharge and waterlogging

and salinity dynamics.

This paper developed the concept of net recharge for

setting paddock scale recharge targets and to link these to the

regional targets and community’s goals for sustainable

irrigation management. A management model, cast in a

dynamic programming format to integrate a detailed hydro-

logical model with an economic model was applied to estimate

the productivity, profitability and sustainability of irrigated

agriculture in a region of the southern Murray Darling Basin in

Australia. SWAGMAN1 Farm model was used to determine

paddock scale net recharge. This interactive model enables an

individual farmer to choose a profit optimizing crop mix while

lowering net recharge through better irrigation management.

A unified action by all irrigators across farms within the

catchment can lead to a win-win outcome for all farmers.

The results of biophysical and hudrogeological modelling

reported in this study provide evidence of considerable spatial

variations in net recharge across different zones within the

study area, requiring a spatially targeted approach to address

the recharge issue. This implies that achieving overall

recharge targets for the irrigation area as a whole alone

would be ineffective—one size fits all approaches would be a

blunt policy tool that would do more harm than the good, say

be eliminating some beneficial recharges/return flows. It also

implies that sustainable irrigation management requires a

more informed and science-based policy response for effec-

tively targeting the issue. Thus, recharge reduction strategies

offer promise in those zones where net recharge occurs;

limiting discharge would be desirable for the net discharge

zones; and different salinity mitigation strategies would be

called for in zones of shallow groundwater and high salt

concentration. The modelling results show that on-farm land

and water management actions offer the greatest promise to

mitigate salinity and waterlogging.

To guide on-farm land and water management decisions,

an explicit attempt was made to determine paddock scale net

recharge targets. SWAGMAN1 Farm model was used for this

purpose. This model enables farmers to determine paddock

scale net recharge targets while optimizing profits. But

alternatively irrigators optimize productivity and profitability

subject to a zero net recharge constraint, promoting sustain-

able irrigation management. The paddock scale targets can

then be linked to overall recharge targets at catchment scale to

meet community’s vision for sustainable irrigation manage-

ment.

The development of paddock scale net recharge metric is

perhaps the most significant contribution of this study. The

management of paddock scale net recharge target across

farms and optimization of profits achieves a win-win outcome

for all farmers. The recharge management metric enables the

conversion of diffuse source recharge to point source recharge

at paddock scale, enabling the conversion of private property

rights to a common pool issue and assigning individual

responsibilities for its management. To help refine this work,

future research should calibrate the model in other irrigation

systems facing waterlogging and salinity issues as well as

cover additional dimensions such as:

� H
ydro-economic ranking of on-farm and regional options
for regional groundwater management.

� H
ydrological assessment of downstream environmental
benefits of net recharge management emanating from

promising on-farm and regional groundwater management

options.

� D
esign and testing of market based instruments for sharing
environmental costs and benefits among the stakeholders.

This way forward would entail building enduring partner-

ships between biophysical scientists, economists and other

professional as well governments, irrigators and wider

community to help promote the development of sustainable

irrigation management tools and practices.

Acknowledgements

The authors wish to acknowledge funding support by the

Coleambally Irrigation Cooperative Limited, the Cooperative

Research Centre for Irrigation Futures and the Cooperative

Research Centre for Sustainable Rice Production, Australia.


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