Equivalent cropping area and whole farm water balance approaches to reduce net recharge to shallow...

ARTICLE

Equivalent cropping area and whole farm water balanceapproaches to reduce net recharge to shallow saline groundwaterfrom rice based cropping systems

Shahbaz Khan

Received: 11 December 2006 / Accepted: 23 April 2007 / Published online: 5 June 2007

� Springer-Verlag 2007

Abstract Sustainability of irrigated agriculture in the

semi-arid regions such as the rice growing regions of the

southern Australia could be under serious threat due to

recharge to saline groundwater leading to secondary sa-

linisation. This paper presents two approaches to reduce net

recharge to shallow saline groundwater from ponded rice

cropping systems using a soil, water and groundwater

management model–SWAGMAN model. First approach

applies the concept of growing deep-rooted crop (crop

consuming water from soil and shallow water table) over

an area equivalent to recharge from a unit area of rice crop

to maintain the total water balance. The second approach

involves achieving whole farm water balance for average

and wet climatic conditions to find out optimum cropping

pattern to minimise recharge from rice based system. Re-

sults of both approaches are sensitive to water table depth

and regional groundwater outflow rates. The first approach

was applied to determine an optimal mix of rice/lucerne

crop to keep water tables and average soil salinity changes

less than 0 m/year and 0.015 dS/m, respectively. The area

of lucerne required to match the net groundwater rise

caused by 1 ha of rice is 1.27, 2.45 and 3.55 ha for 1, 0.5

and 0.2 ML/ha regional groundwater outflow capacity.

Results from second approach revealed that it is possible to

control net recharge under above average rainfall using a

suitable whole farm cropping mix e.g. 50 ha rice–wheat

rotation, 19 ha rice, 25 ha of lucerne and 136 ha of fallow.

Keywords Groundwater recharge � Salinity � Shallow

water table � Rice based system � Optimum cropping mix �Equivalent cropping area � SWAGMAN

Introduction

Sustainability of irrigated agriculture in the rice growing

regions of the Murray Darling Basin (MDB) Australia fa-

ces secondary salinisation challenges due to high recharge

levels under current irrigation practices, water trading out

of the area and hydraulic loading not being maintained at

sufficient levels to keep root zone free of salts (Khan et al.

2006). For the rice growing regions in the MDB ponded

rice contributes 40–50% of the accessions to the ground-

water, and the other major sources are other irrigated crops

and pastures, channel leakage and rainfall (Leslie 1992).

Rice production in rotation with winter and summer

crops and annual subterranean clover pastures is the major

land use in the Coleamablly Irrigation Area (CIA) located

in the MDB (Fig. 1). Prior to irrigation, water table levels

under the CIA were 15–20 m deep. During 1960s 333

farms were allocated 79,000 ha of land and irrigation be-

gan in the CIA. In a good water allocation year the total

area under rice in the CIA can be over 20,000 ha using

between 12 and 16 ML/ha (1,200–1,600 mm) of irrigation

water.

In the CIA the rice fields are ponded with water between

120 and 180 days, therefore, the potential for excessive deep

drainage/groundwater recharge is higher than intermittently

irrigated crops or pastures. Rice growers have been under

considerable pressure to increase the water use productivity

(t/ML) and to reduce the water use of rice-based systems

to fulfill environmental and socio-economic require-

ments. One way to increase rice water use productivity is to

S. Khan (&)

CSIRO Land and Water and Charles Sturt University,

Locked Bag 588, Wagga Wagga, Australia

e-mail: [email protected]

123

Paddy Water Environ (2007) 5:143–152

DOI 10.1007/s10333-007-0078-6

reduce annual deep drainage or recharge below rice fields

(Humphreys et al. 2006). Leslie (1992) recommended

adoption of a rice industry target recharge of 1 ML/ha

(100 mm). The target for upper limit of deep drainage is

currently 200 mm however in many rice fields signifi-

cantly higher deep drainage than this target occurs

(Beecher 2005).

A number of options have been investigated by various

researchers of on-farm rice growing restrictions to mini-

mise groundwater recharge (Humphreys et al. 1994). These

restrictions include soil-based criteria, paddock rice water

use limits, exclusion of land from rice growing and limits

on the intensity of rice growing. Rice cultivation is con-

fined to soils with at least 2–3 m of continuous medium to

heavy clay (i.e. >45% clay) in the top 3.5 m based on one

soil profile analysis per 4 ha (i.e. a 200 m grid). Irrigation

water use must not exceed a target set at the end of each

rice season based on actual net evaporative demand. (Rice

water use target = ETo – rainfall + 400 mm) (ETo refer-

ence crop evapo-transpiration). This criteria is usually as-

sessed at the end of the rice season. In addition, the total

area of rice that can be grown each year is restricted, and

rice area and water use are closely monitored by the water

delivery authorities. The ‘‘rice environmental policy’’ is

defined and implemented by the water delivery authorities

as part of their overall community led Land and Water

Management Plans and is reported on annual basis through

the mandatory environmental report of the irrigation area.

There is considerable variation in percolation rates across

soil types and within fields (van der Lelij and Talsma 1978;

Beecher et al. 2002) and small highly permeable areas can

make a large impact on total percolation losses also re-

ferred to as shallow groundwater recharge.

As an alternative to current rice water use and soil

restriction policy in the CIA this paper aims to achieve

whole farm water balance through an appropriate mix of

recharging and discharging land uses which can limit re-

charge to the groundwater outflow capacity of the under-

lying aquifer systems. Such an approach can provide

cropping flexibility to rice growers as well as manage the

water table and salinity problems of the irrigation area by

restricting net recharge to zero.

Case study area

This paper deals with the issue of rising levels of water

table and salinity as a result of rice growing with particular

reference to Coleambally Irrigation Area (CIA). The CIA is

located to the south of the Murrumbidgee River. The irri-

gation area was developed during the 1960s to make use of

water diverted westward as a result of the Snowy Moun-

tains Hydro-Electric Scheme. Irrigation water is used for

crops such as rice, wheat, barley, oats, canola, soybeans,

maize, sunflowers, lucernes, grapes, prunes and pastures

for sheep and cattle.

The historic groundwater rise trends in CIA are shown

in Fig. 2. The trends show rapid rises in the 1970s and the

early 1980s after the introduction of irrigation in the early

1960s.The groundwater levels and salinity for September

2002 in the CIA is given in Figs. 3 and 4, respectively.

There has been an overall water table decline (1.0–3.0 m)

since 2002–2003 season due to low water allocations and a

relatively dry climate. The spatial distribution of total

groundwater outflow in the CIA is given in Fig. 5 based on

groundwater flow analysis by Khan (2005) which proposed

that CIA can be divided into five sub-regions for better

recharge management. These groundwater outflow rates

were computed using aquifer lithology and piezometric

levels. Around 15GL (1 GL = 1 Million Cubic Meter) is

discharged through capillary upflow in shallow water table

areas e.g. in the southern Coleambally Irrigation Area. The

low groundwater outflow rates, shallow water tables and

high groundwater salinity especially in the Southern CIA

may result in salinisation of land and pose a risk to the

Fig. 2 History of groundwater changes in CIA

Aust r aliaMurray Darling Ba s i nPrincipal rive r s

N

EW

S

Brisbane

S y dne yC I A

Me l b o urn e

Ad e laide

P e r t h

Darwin

H o bar t

Mur r a y R

Darling

R

Fig. 1 Location of the Coleambally irrigation area, Australia

144 Paddy Water Environ (2007) 5:143–152

123

agricultural productivity and environmental sustainability.

There is a need for the implementation of on-farm and/or

regional groundwater recharge management in this area.

Taking into account farmer preferences; modelling and

practical approaches are needed to identify combinations of

on-farm and regional options to achieve desired environ-

mental objectives, and to evaluate the tradeoffs between

environmental and economic objectives. As rice is only one

of many irrigation activities with potential to influence

water tables at the farm and regional scale, solutions for

controlling water tables therefore need to be derived from

the consideration of a complex mix of biophysical and

socioeconomic factors, over shorter and longer time

frames.

This paper aims at finding out appropriate mix of

recharging and discharging land uses which can limit re-

charge to the regional groundwater outflow capacity of the

underlying aquifer systems.

Data analysis

Decile method groups monthly precipitation occurrences

into deciles and provides an accurate statistical measure-

ment of precipitation and drought. Decile analysis of long-

term monthly rainfall data for CIA is shown in Table 1 and

its qualitative interpretation (Gibbs and Maher 1967) is

given in Table 2.

The total rainfall for the seventh decile is 462 mm

whereas the median (fifth decile) rainfall is 368 mm which

means the above average rainfall is 94 mm greater than the

median.

The long-term average reference evapo-transpiration is

around 1,160 mm over the rice-growing season (October–

February). The rainfall over the same period varies from

less than 100 mm to over 300 mm. The reported rice water

use ranges between less than 1,000 mm and over

1,600 mm indicating significant recharge for high water

use farms.

Methodology

A range of approaches ranging from the identification of

leaky paddocks or sites with excessive recharge to

achieving a whole farm water balance through a better mix

Fig. 3 Depth to piezometric level for CIA, September 2002

Fig. 4 Groundwater salinity for the CIA (all piezometers), Septem-

ber 2002

370000 380000 390000 400000 410000 420000 430000

6120000

6130000

6140000

6150000

6160000

6170000

0

0.25

0.5

0.75

1

ML/haML/ha

Fig. 5 Groundwater outflow rates in the Coleambally irrigation area

(yellow spots are areas with limited data or points closest to a

pumping bore)

Paddy Water Environ (2007) 5:143–152 145

123

of recharging and discharging crop mixes, can be used to

reduce accessions from rice based systems.

Crop/rice ratios to maintain total water balance

In this approach economic viability of the farming system

is maintained by allowing same area of rice to be grown if

the farmer is willing to put the discharging crops to miti-

gate net recharge.

Under most hydro-geological conditions the ponded rice

fields cause saturation of underlying soils and recharge to

the water table whereas crops such as lucerne can tap

shallow groundwater to meet major proportion of their crop

water requirements (causing net groundwater discharge

from the water table). If crops grown across the entire farm

or area are considered on the basis of their recharge to and

discharge from the shallow water tables, an optimal mix of

recharging and equivalent discharging crops can be deter-

mined to keep water tables and soil salinity at desired

levels while maintaining economic viability. The dis-

charging crops such as lucerne which is a deep rooted

perennial crop can also be used to provide a dewatering

buffer around the rice area as well as balance total recharge

through discharge (Fig. 6). In this approach the water table

rise is dissipated through lateral flow from under the rice

crop to the adjoining land.

Figure 7 shows a water balance framework of approach

for determining optimum mix of crops utilising ground-

water from water table and the rice. In this framework

SWAGMAN Farm model (Khan et al. 2003) is used to

determine the equivalent discharging crop areas to balance

recharge from a unit area of rice.

Whole farm water balance approach

In the second approach, ten cropping pattern scenarios

were undertaken for a single farm with an area of 230 ha in

order to analyse water table and salinity impacts of dif-

ferent rice based irrigation systems. These scenarios in-

cluded 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. For these scenarios fifth and

seventh deciles of rainfall were considered (Tables 5 and

6). These scenarios included crop mixes given in Table 3.

Table 1 Rainfall analysis using decile method on the Coleambally rainfall data (1920–2003)

Decile January February March April May June July August September October November December Total

First 22.86 22.35 20.32 33.78 10.92 4.06 11.43 101.09 0 0 14.48 2.54 243.83

Second 0 5.84 53.59 10.16 29.97 10.92 20.07 5.59 31.5 72.64 34.04 3.05 277.37

Third 17 12 25.4 58.9 14.4 23.5 36.9 44.5 12.6 26.6 5.3 46.3 323.4

Fourth 0.1 45.2 0 14.1 17.9 20.9 63.4 53.3 11.6 58.9 30.4 34.5 350.3

Fifth 0 0.76 9.65 27.94 26.42 22.61 44.7 33.02 8.64 68.58 10.92 114.3 367.54

Sixth 13.72 0 1.27 1.52 31.5 148.59 88.65 19.05 9.91 20.07 7.37 76.96 418.61

Seventh 54.3 58.6 5.8 75.4 66 21.4 61.3 44.9 38.4 25.3 3.7 6.8 461.9

Eighth 152.65 14.22 20.07 12.45 84.84 3.3 37.85 27.43 80.77 5.33 31.75 26.92 497.58

Ninth 4.4 5.1 32.7 44.3 33.6 19.2 12.8 60.5 42.7 121.2 65.5 123.9 565.9

Tenth 74.68 0.76 123.7 46.23 115.32 64.01 81.03 19.56 47.5 154.69 14.99 9.4 751.87

Table 2 Definition of decile rainfall bands

Decile range Interpretation

1 Very much below ‘‘average’’

2 Much below ‘‘average’’

3 Below ‘‘average’’

4 Slightly below ‘‘average’’

5 and 6 ‘‘Average’’

7 Slightly above ‘‘average’’

8 Above ‘‘average’’

9 Much above ‘‘average’’

10 Very much above ‘‘average’’

Gibbs and Maher (1967) Fig. 6 Rice–lucerne crop mix to control water table


123

The irrigation levels for average (fifth decile) and wet

(seventh decile) years used for these crops are given in

Table 4.

The effect of initial water table depth on net recharge

was explored for each of the scenarios, with starting ground

water levels of 1 and 3 m employed. These levels were

assumed to be consistent over the whole areas of the farm

with no local perturbations anticipated for this application.

Shallow groundwater pumping was not included and the

runoff from the farm/paddocks was considered to be min-

imal due to the drainage water being recycled back onto the

farm. Three regional groundwater outflow rates (0.2, 0.5

and 1 ML/ha) were used to explore cropping options for

the southern, central and northern parts of the study area.

The initial volumetric soil water content for the farm was

considered to be 0.3.

SWAGMAN Farm model in conjunction with ground-

water outflow rates from a three dimensional MODFLOW

model was applied to simulate above cropping options.

SWAGMAN (Salt WAter and Groundwater MANagement)

Farm is a farm-scale hydrologic economic model that

integrates agronomic, climatic, irrigation, hydrogeological

and economic aspects of irrigated agriculture. The model is

capable of determining optimum mix of land use to keep

water table and soil salinity within acceptable limits while

maximising the economic returns. Alternatively the model

can simulate water and salt balance and economics of a

given cropping preference. MODFLOW is a 3D finite

difference based numerical model which simulates

groundwater flow dynamics. MT3D model works in con-

junction with MODFLOW and simulates solute (salts in

this case) concentration in groundwater. Details of asso-

ciated modelling methodologies can be found in Khan et al

(2003, 2007).

No

Yes

Rooting depth & ETc Cropping season

SWAGMAN Farm

Salts & discharge/recharge to WT

Soil, Crop & WT data Irrigation & climate data

MODFLOW + MT3D GW quality data Aquifer properties & GW flow boundary conditions

Water balance at different scales

YesNo

Optimum crop mix & layout

Identify crops discharging from shallow WT against rice

WT & Salinity Improved?

Economically viable?

Fig. 7 Framework for optimum

mix of crops that utilise

groundwater from shallow

water table and rice

Table 3 Land-use areas (ha) and cropping patterns for selected

scenarios

Scenario Land-use areas (ha)

1 Rice 69, fallow 161

2 Rice 69, wheat 75, winter pasture 25, fallow 61

3 Rice 69, wheat 25, lucerne 25, fallow 111

4 Rice 69, wheat 50, lucerne 25, winter pasture 25,

fallow 61

5 Rice 69, soybeans 25, corn 25, fallow 111

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

7 Rice 19, rice/wheat 50, lucerne 25, fallow 136

8 Corn 200, fallow 30

9 Corn 100, wheat 100, fallow 30

10 Corn/wheat 100, fallow 130

Table 4 Irrigation levels (ML/ha) for selected scenarios

Crop Irrigation requirement (ML/ha)a

Seventh deciles

of rainfall

Fifth deciles

of rainfall

Rice 13 14

Rice/Wheat (R–W) 14.5 15.5

Wheat 4 5

Winter Pasture 4 5

Lucerne 7 8

Soybeans 7 8

Corn 9 10

Corn/Wheat (C–W) 10.5 11.5

a 1 ML/ha is equivalent to 100 mm of water


123

Results and discussion

Crop/rice ratios to maintain total water balance

Rice with lucerne

Using median rainfall and evapo-transpiration values in the

CIA, average starting volumetric soil water content of 0.3

for the self mulching clays and three groundwater outflow

rates of the underlying aquifers (0.2, 0.5 and 1.0 ML/ha)

ratios of an optimal mix of rice and lucerne crops were

determined to keep water tables and average soil salinity

changes less than 0 m/year and 0.015 dS/m, respectively.

The groundwater salinity was assumed as 4 dS/m. For

these scenarios irrigation levels of 14 ML/ha for rice and

8 ML/ha for lucerne was assumed.

Figure 8 shows that the optimum cropping ratio is

affected by the depth to water table and groundwater out-

flow rates. For a given groundwater outflow rate the lu-

cerne/rice ratio increases from 2 to 3 m depth and then it

decreases from 3 to 5 m. This is due to higher capillary

upflow by lucerne for depths less than 3 m and greater soil

storage capacity below 3 m depth. For lower regional

groundwater outflow rates higher lucerne/rice area ratios

are required e.g. for a 3 m depth to the water table, area of

lucerne required to match the net groundwater rise caused

by 1 ha of rice is 1.27, 2.45 and 3.55 ha for 1, 0.5 and

0.2 ML/ha groundwater outflow, respectively.

Rice with wheat

An optimal mix of rice and wheat crops were also deter-

mined to keep water tables and average soil salinity changes

less than 0 m/year and 0.015 dS/m, respectively using same

conditions as that of rice with lucerne. The irrigation levels

of 14 and 3 ML/ha were, respectively assumed for rice and

wheat. Figure 9 shows that the optimum cropping ratio is

affected by the depth to water table and regional ground-

water outflow rates in a similar fashion as for rice with

lucerne. For example, for a 2 m depth to the water table,

area of wheat required to match the net groundwater change

caused by 1 ha of rice is 2.6 ha, and 5.2 ha for 0.5 and

0.2 ML/ha groundwater outflow, respectively.

Whole farm water balance approach

The proposed cropping scenarios were modelled for two

soil types (self mulching clays and transitional red brown

earths), two rainfall patterns (fifth and seventh decile

bands), two groundwater depths (1 and 3 m) and three

groundwater outflow rates (0.2, 0.5 and 1 ML/ha)

(10 · 2 · 2 · 2 · 3 = 240 model runs) to explore effect of

different factors on net recharge and salinity changes. This

analysis is based on irrigation water use provided in Ta-

ble 4.

Overall similar net recharge and salinity trends were

observed for both soil types with higher net recharge values

for the transitional red brown earths. The results for these

scenarios for fifth decile are given in Tables 5 and 6 and

summarized below for three groundwater outflow based

zones of CIA.

Implications for Southern CIA

For 0.2 ML/ha groundwater outflow (typical for the

Southern CIA) all scenarios result in a net positive recharge

except Scenario 7 with 50 ha rice–wheat rotation, 19 ha

rice, 25 ha of lucerne and 136 ha of fallow land. The first

five scenarios with 69 ha of rice crops without a crop after

rice result in the higher rates of groundwater recharge. For

1 m depth to water table there is a net accumulation of salt

resulting in 0.4–0.7 dS/m increase in groundwater salinity.

For the deeper groundwater depths the net recharge is able

to keep the root zone free of salts. Scenarios 8, 9 and 10

show Corn–Wheat rotation is a better option in terms of

relatively less recharge (still the recharge is around 1 ML/

ha) however, there is a greater risk of salinisation if the

water table is at 1 m depth from the ground surface.

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3 3.5 4

Lucerne/Rice Ratio D

epth

to

Wat

erta

ble

(m)

1 ML/ha0.5 ML/ha0.2 ML/ha

Fig. 8 Lucerne/rice ratio at different depths to water table

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 2 4 6 8 10

Wheat/Rice Ratio

Dep

th t

o W

ater

tab

le (

m)

0.5 ML/ha0.2 ML/ha

Fig. 9 Wheat/rice ratio at different depths to water table


123

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123

Implications for central and western CIA

The 0.5 ML/ha regional groundwater outflow rate is quite

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

best cropping option to control groundwater recharge ap-

pears to be Scenario 7 with 3 m depth to water table with

the lowest net recharge and net increase in salinity. Other

options such as scenario 10 with a corn–wheat rotation

again gives lower net recharge (0.4–0.6 ML/ha). All other

options result in net recharge greater than 0.7 ML/ha.

Implications for Northern CIA

The regional groundwater outflow rates of 1 ML/ha and

water table depth of 3 m roughly represent 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). In this region if rice–wheat rotation

with some area under lucerne or a corn–wheat rotation is

practiced the net recharge can be reduced to below or very

close to the target levels.

The above scenarios were repeated using the same

parameters for crop mix, irrigation requirements and soil

properties for above average rainfall scenario (seventh

decile) and average irrigation levels provided in Table 4.

The results (not shown here) reveal that if the actual irri-

gation levels can be decreased (e.g. rice water use to

around 12 ML/ha or corn to 8 ML/ha) many of the studied

scenarios will achieve a net recharge balance under above

average rainfall years.

Conclusions

The ponded rice fields cause saturation of underlying soils

and recharge to the water table. The equivalent cropping

area concept involves recharge mitigation through the use

of discharging crops such as lucerne, which is a deep-

rooted perennial crop, to provide a dewatering buffer

around the rice growing areas. The modelling results show

that it is possible to determine optimal mix of rice/lucerne

and rice/wheat crops to keep water tables and average soil

salinity changes less than 0 m/year and 0.015 dS/m,

respectively using median rainfall and evapo-transpiration

values. Initial water table depth and the regional ground-

water outflow capacity are the key factors determining area

of deep rooted vegetation required to mitigate groundwater

recharge from 1 ha of rice. For lower regional groundwater

outflow rates higher lucerne/rice area ratios are required

e.g. for a 3 m depth to the water table, area of lucerne

required to match the net groundwater change caused by

1 ha of rice is 1.27, 2.45 and 3.55 ha for 1, 0.5 and 0.2 ML/

ha groundwater outflow, respectively. Similarly for lower

regional groundwater outflow rates higher wheat/rice area

ratios are required e.g. for a 2 m depth to the water table,

area of wheat required to match the net groundwater

change caused by 1 ha of rice is 2.6 ha, and 5.2 ha for 0.5

and 0.2 ML/ha groundwater outflow, respectively.

An alternative to the equivalent crop area approach is to

achieve whole farm water balance. In this paper ten crop-

ping scenarios were modelled for two soil types (self

mulching clays and transitional red brown earths), two

rainfall patterns (fifth and seventh decile bands), two

groundwater depths and three groundwater flow rates to

explore effect of different factors on net recharge and

salinity changes. Comparative analysis of these scenarios

suggests it is possible to control net recharge under average

rainfall and water allocation years using a suitable cropping

mix e.g. 50 ha of rice–wheat rotation, 19 ha rice, 25 ha of

lucerne.

Acknowledgments Project funding from the Coleambally Irrigation

Cooperative Limited is acknowledged. Inputs from the Coleambally

Irrigation Cooperative Limited Environmental Committee provided

useful sensibility checks for this work.

References

Beecher HG (2005) Better prediction of groundwater recharge from

rice growing, Research Report No. P1102FR09-05 Cooperative

Research Centre for Sustainable Rice Production. Yanco Agri-

cultural Institute, Yanco, ISBN 1876903440

Beecher HG, Hume IH, Dunn BW (2002) Improved method for

assessing rice soil suitability to restrict recharge. Aust J Exp

Agric 42:297–307

Gibbs WJ, Maher JV (1967), Rainfall deciles as drought indicators.

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doi: 10.1007/s10666-006-9081-3. http://www.dx.doi.org/

10.1007/s10666-006-9081-3

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123

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