increasing water productivity through deficit irrigation: evidence from the indus plains of pakistan

IRRIGATION AND DRAINAGE

Irrig. and Drain. 51: 87–92 (2002)

DOI: 10.1002/ird.39

INCREASING WATER PRODUCTIVITY THROUGH DEFICIT IRRIGATION:EVIDENCE FROM THE INDUS PLAINS OF PAKISTAN1

ASAD SARWAR1 AND CHRIS PERRY2*†

1 International Water Management Institute, Pakistan2 Wyclif, Croyde, Devon, UK

ABSTRACT

Water shortage shifts attention from the traditional concept of crop yield—production per unit of land—tothe productivity of water—production per unit of water. Whether this parameter is best increased throughresponsive, sophisticated irrigation scheduling or through predetermined schedules, providing limited water,has been a contentious issue for some years. This paper brings together modelling studies from Pakistan thataddress this issue, as well as raising the question of whether deficit irrigation threatens sustainability throughthe build-up of salts in the soil profile.

It is shown that under conditions of plentiful water, highest productivity is achieved when irrigation isprecisely scheduled to meet crop needs. However, when water is short its productivity is substantially increased(by almost 50%) by deficit irrigation. No salt problem occurs as long as amount of water applied throughirrigation is sufficient to meet 80% of the total crop evapotranspiration. Under limited water conditions, theproductivity achieved is little affected by setting predetermined irrigation schedules, allowing the operatingagency to concentrate on reliability of service. Finally, it is also shown that the salt build-up is independentof irrigation scheduling. Copyright 2002 John Wiley & Sons, Ltd.

KEY WORDS: deficit irrigation; productivity; irrigation scheduling; irrigation sustainability

RESUME

La rarefaction des ressources en eau explique pourquoi l’on ne porte plus l’attention sur le seul rendement(production par unite de terre cultivee) mais de plus en plus sur la productivite de l’eau. Savoir si l’ameliorationde ce parametre passe par un mode d’irrigation sophistique qui colle a la demande ou par des tours d’eau fixesune fois pour toutes delivrant des volumes d’eau limites est une question recurrente depuis de nombreusesannees. Cet article aborde cette question a travers la presentation de travaux de modelisation menes auPakistan. Il traite cette autre importante question: l’irrigation deficitaire ne menace-t-elle pas la durabilite dessystemes en favorisant l’accumulation saline dans le sol?

Il est prouve que, sans restriction sur l’eau disponible, le rendement maximal est atteint lorsque les arrosagessont calques precisement sur les besoins hydriques des cultures. Cependant, lorsque l’eau est appliquee enplus faible quantite, c’est-a-dire par une irrigation deficitaire, sa productivite est sensiblement augmentee (del’ordre de 50%). Dans de telles conditions, le rendement obtenu n’est que peu affecte par un tour d’eau fixe,ce qui permet a l’agence d’irrigation de se concentrer sur la fiabilite du service. Enfin, il est prouve que

* Correspondence to: C. Perry, Wyclif, Croyde, Devon EX33 INH. E-mail: chris [email protected]† Chris Perry is an IWMI Fellow.1 L’amelioration de la productivite de l’eau par irrigation deficitaire: preuve des plaines de l’Indus au Pakistan.

Copyright 2002 John Wiley & Sons, Ltd.Received 10 May 2001

Accepted 13 November 2001

88 A. SARWAR AND C. PERRY

l’accumulation de sels n’est pas un probleme pour les niveaux d’irrigation deficitaire qui maximisent laproductivite de l’eau. Copyright 2002 John Wiley & Sons, Ltd.

MOTS CLES: irrigation deficitaire; productivite; tour d’eau; durabilite de l’irrigation

INTRODUCTION

As the imbalance between supply and demand for water becomes more obvious (Seckler et al. 1998), irri-gation—the primary consumer of water in most water short countries—comes under simultaneous pressureto release water to other sectors, and to maintain or increase production to meet projected food and fibredemands. Irrigation is being asked to do more with less, increasing production per unit of water consumed.Historically, the focus has been on increasing the productivity of land—yield per hectare. Focusing on yieldper unit of water—kg m−3, or $ m−3 —is now a widespread challenge, heightened by the limited possibil-ities for extension of irrigation to other areas due to scarcity of land, scarcity of water, increasing costs ofdevelopment, and environmental concerns (Shanan, 1992).

Irrigation professionals fall into two broad schools of thought on how best productivity of water can beachieved: on the one hand, some argue that giving farmers the most responsive irrigation or access to wateras and when needed (Merriam, 1992; Plusquellec et al., 1994) will lead to rational and orderly behaviouramong farmers, and highly productive irrigation.

Others argue that it is unrealistic to promise a highly responsive, differentiated service to individual farmersin large surface systems with fragmented, smallholdings (Horst, 1998; Shanan, 1992; Albinson and Perry,forthcoming), and is in itself likely to lead to disorder if agreed schedules and quantities fail to materialise.This group also argues that deliberately providing farmers with less water than they might freely demand hasadditional benefits, encouraging selection of water-efficient crops, and careful attention to on-farm irrigationpractices.

Which of these two views is correct is an important question: while the majority of large-scale irriga-tion development is now complete, many existing systems are due for renovation and rehabilitation, and thequestion will always arise as to whether the “modernised” system should aim for responsiveness (which byimplication means greater expense, higher management input, and a complex specification of water entitle-ments) or for a simple system, with relatively little management intervention and a simple specification of“due share of available water” as the water right.

Where water is relatively abundant (for example, in Egypt) or where a project aims to grow very high valuecrops requiring extremes of reliability and accuracy in water delivery, the best choice will almost automaticallybe the more sophisticated option.1 Neither is the choice “black or white”—in a simple, proportional systemthe option to segregate special areas for separate treatment may be relevant—perhaps to meet specific soilconditions, or a traditional right to a particular service.

In many cases, however, the designer is faced with large, fairly homogeneous areas growing medium valuefield crops, with current or foreseeable shortage of irrigation supplies in relation to demand. This is the caseanalysed here, and the option we analyse is deficit irrigation—deliberate restriction of irrigation deliveriesbelow the potential full demand of the crop being grown.

MATERIALS AND METHOD

A large amount of research data on the effects of deficit irrigation on crop yields is available (Reta and Hanks,1980; Dierckx et al., 1988; English and Nakamura, 1989). However, there are hardly any data availableto quantify the effects of this water management practice on soil salinity and drainage needs. Similarly,the discussions on the water division are usually based on the comparison of crop yields, developmentcosts, management and infrastructural constraints and socio-political conditions (Merriam, 1992; Steiner andWalter, 1993; Wolters et al., 1997). However, very little attention is given to the long-term impacts ofthese proposed interventions on environmental parameters such as soil salinisation, water-table behaviour and

Copyright 2002 John Wiley & Sons, Ltd. Irrig. and Drain. 51: 87–92 (2002)

INCREASING WATER PRODUCTIVITY THROUGH DEFICIT IRRIGATION 89

drainage requirements. Therefore the debates on the advantages and disadvantages of this change remainambiguous due to the lack of necessary data to quantify this impact.

Three issues are addressed in this paper:

• Is deficit irrigation sustainable —does it have adverse effects on soil salinity and drainage needs, with aconsequent long-term decline in production?

• Is it productive —how much crop is produced per unit of irrigation water consumed?• How sophisticated must the service be to capture any benefits of deficit irrigation?

The results of two studies conducted in Punjab, Pakistan provide, the basis for the analysis. For both studies, aphysical-based transient soil water and solute transport model, Soil–Water–Atmosphere–Plant (SWAP) (vanDam and Feddes, 2000) was used. SWAP is a one-dimensional model to simulate water flow in a heterogeneoussoil–root system, which can be under the influence of groundwater. The model offers a wide range ofpossibilities to address practical questions in the field of agriculture, water management and environmentalprotection. The model has been successfully applied in many hydrological studies for a variety of climaticand agricultural conditions. Options exist for irrigation scheduling, prediction of depth to water-table, soilsalinity and leaching of nitrogen and pesticides. Before applying the SWAP model for these studies, it wascalibrated and validated for local conditions (Smets et al., 1997; Sarwar et al., 2000). For model calibration,potential evapotranspiration, irrigation and rainfall data were used to describe the upper boundary condition,whereas daily groundwater levels were used as the bottom boundary condition. Details of model calibrationand validation can be found in the original studies.

The first study demonstrated the effects of deficit irrigation on crop yields, soil salinity and water-tablebehaviour (Prathapar and Sarwar, 1999). In this study, the effects of three different irrigation regimes(supplying 100, 80 and 60% of potential evapotranspiration) on root zone salinity and water-table fluctu-ations were evaluated. Daily potential evapotranspiration values were calculated by the CROPWAT model(Smith, 1995), using actual climatic data collected from a field station located in south-east of the PunjabProvince. The irrigation requirement for each interval was calculated by subtracting cumulative rainfall fromcumulative evapotranspiration during that period. The first irrigation was applied to wheat, three weeks aftersowing, and to cotton six weeks after sowing. Subsequent irrigations were applied to wheat and cotton attwo-week intervals, in addition to the pre-sowing irrigations. Irrigations to wheat and cotton were stoppedtwo and six weeks before harvesting, respectively. This schedule is recommended by the Punjab AgriculturalDepartment (PAD) for the wheat–cotton agro-climatic zone and usually adopted by farmers. No extra waterwas applied to leach down salts.

Simulations were performed for shallow water-table conditions (i.e. 2.0 m depth) prevailing in the area.The water-table was initially considered at 2 m depth, and was allowed to fluctuate over the growing seasondepending upon the amount of irrigation and rainfall. The second study investigated the consequences ofdifferent water delivery schedules at the farm gate on crop production, soil salinity and water-table (Sarwaret al., 2001). In this study, a number of scenarios, representing the combinations of two water availabilitysituations—unrestricted and restricted—and three management approaches—fixed, flexible or on-demandschedules (as further defined below)—were evaluated.

Fixed schedule. These schedules are fixed at the beginning of the irrigation season. The quantity deliveredin each irrigation is constant and the timing of the delivery is predetermined. The approach is presently beingpractised in many irrigated areas of India and Pakistan. The behaviour of individual farmers in the fixedschedule is difficult to translate into an average condition. Therefore irrigation schedules recommended bythe PAD for wheat–cotton agro-climatic zone of the Punjab, Pakistan, were adopted for this study (Sarwarand Bastiaanssen, 2001). The depth of each irrigation was taken as 65 mm. The schedule was not adjustedannually to respond to rainfall or other climatic variations, and thus represents the most extreme scenario ofinflexibility.

Flexible schedule. The timing of irrigation deliveries is fixed at once every seven days, and the irrigationquantity for each interval is calculated by subtracting cumulative effective rainfall from the cumulative



potential evapotranspiration (ET pot) during that period. To obtain effective rainfall, a fixed factor of 0.85(Smith, 1995) was used.

On-demand schedule. This scenario allows the farmers to control the timing and quantity of irrigation.This situation was modelled by filling the root zone to field capacity whenever relative transpiration (Tact/Tpot)dropped below a value of 1.0. This criterion was used to optimise timing and amount of irrigation using theSWAP model. Maintaining Tact/Tpot at 1.0 ensures maximum crop yield (per unit of land).

For restricted irrigation supply scenario, the total irrigation depth was limited to 600 mm year−1. Thisis the amount of water available for crop growth the study area (WAPDA, 1989). Irrigations were adjustedto three water delivery schedules in such a way that the total amount of water applied in a year does notexceed 600 mm. For a fixed schedule, the application of this criterion resulted in three post-sowing irrigationsfor wheat and four to cotton with depth of each irrigation equivalent to 65 mm. For the flexible schedule,this amount of water could only meet 60% of total irrigation requirement. For the on-demand schedule, thisamount of water was only enough to accommodate Tact/Tpot up to 0.85. Simulations were performed for aperiod of 15 years considering actual rainfall and climatic data to derive meaningful average results. For thisstudy, zero flux at the bottom of the soil profile was used as the bottom boundary condition (Sarwar et al.,2000).

To evaluate the impact of different water delivery schedules on crop production and the environment, thefollowing performance indicators were used:

• Relative transpiration (T act/Tpot) is a proxy for crop yield per hectare.• Salt storage change (SSC = �C/Cinitial) is a measure of salt storage in the root zone. The salt storage

change over a certain period is �C, and Cinitial is the initial salt concentration at the onset of the timeframe considered.

• Deep percolation is an indicator of impact on the water-table.• Productivity of irrigation water supply (production per unit of irrigation supply Yact/Irr, kg m−3), where Yact

(kg ha−1) is the estimated actual crop production per unit area and Irr (m3 m−2) is the volume of irrigationwater applied per unit area.

This paper summarises and integrates the results of the two above-mentioned studies, and their implicationsfor system design and management.

RESULTS AND DISCUSSION

Effects of deficit irrigation on yield, productivity and sustainability

The results emanating from this study indicate that up to 15% yield reductions were observed whenirrigations were applied according to 60% of the total crop evapotranspiration. The salt build-up in the upper1 m of the soil profile for two irrigation regimes (supplying 100 and 80% of total crop evapotranspiration)was marginal. However, this is not the case when irrigations are applied according to 60% of total cropevapotranspiration. A saline layer develops between 50 and 100 cm depth of the soil profile, as the appliedwater is not enough to provide sufficient leaching. This shows that reducing irrigation supplies up to 60% oftotal crop evapotranspiration may give acceptable yields but will increase the changes of severe soil salinisationin the long run. Therefore, this deficit irrigation practice is not sustainable. Sarwar and Bastiaanssen (2001)have also shown that by adopting deficit irrigation practices, farmers can save up to 25% of water and increasethe productivity of water by 30% without any negative effects on crop yield and soil salinity.

Effects of irrigation scheduling on yield, productivity and sustainability

The response of three water delivery schedules, representing different levels of irrigation service flexibility,on crop yield per hectare and per unit of irrigation water were evaluated using the SWAP model.


INCREASING WATER PRODUCTIVITY THROUGH DEFICIT IRRIGATION 91

Unrestricted water supply. Table I summarises the results of the SWAP model output for the 15-yearperiod, where water supply is unrestricted. Table I shows that although for the on-demand schedule, relativecrop yields per hectare are 4–5% higher as compared to a fixed schedule, the increase in yield per unit ofwater is about 15% (0.74 kg m−3 compared to 0.61 kg m−3 for a fixed schedule). The results indicate thatan on-demand service requires least water (780 mm compared to 845 mm for a fixed schedule), minimisesdeep percolation (110 mm compared to 246 mm for fixed schedules). Soil desalinisation (SSC is substantiallynegative) took place in each case, with the extent of salt removal increasing with increasing deep percolation.

Restricted water supply. Table II summarises the results for a situation of restricted, fixed irrigationavailability (600 mm yr−1). The results show near uniformity in deep percolation (5–20 mm), yield perhectare (0.90–0.92 of potential) and salt build-up in the root zone, indicating that the performance of a fixedschedule, in conditions of water scarcity, is almost identical to the performance of an on-demand or flexibleschedule.

The yield per unit irrigation water ranges between 1.09 and 1.11 kg m−3 (compared to a maximum of0.74 kg m−3 under conditions of unrestricted water availability).

CONCLUSIONS

Due to population expansion, demand for food and fibre will continue to grow, while per capita wateravailability is diminishing.

In areas where water supplies for irrigation remain abundant and the objective is to maximise yield perhectare, an unrestricted, on-demand irrigation service gives the highest yields per hectare, lowest waterdemand to meet full crop demand, and lowest deep percolation.

In areas that are currently, or eventually will be, water short, deficit irrigation combined with fixedscheduling offers significant advantages:

• the productivity of water (kg m−3) is at least 47% higher when water supplies are restricted than whenwater is supplied to meet full crop demand

• management requirements are minimised—schedules are fixed in advance in terms of timing and quantity• infrastructure requirements are simplified• drainage requirements are minimised

Table I. Performance indicators as influenced by a fixed, flexible andon-demand water delivery schedules with restricted water supply

Indicators Fixed Flexible On-demand

Irrigation (mm) 845 760 780Tact/Tpot 0.95 1.0 1.0Salt storage change (SSC ) −0.30 −0.21 −0.25Deep percolation (mm) 246 94 110Yact/Irr (kg m3) 0.61 0.72 0.74

Table II. Performance indicators as influenced by a fixed, flexible andon-demand water delivery schedules with restricted water supply

Indicators Fixed Flexible On-demand

Irrigation (mm) 600 600 600Tact/Tpot 0.90 0.91 0.92Salt storage change (SSC ) +0.11 +0.13 +0.12Deep percolation (mm) 20 5 15Yact/Irr (kg m−3) 1.11 1.09 1.12



These results apply to large parts of Pakistan, and to north-west India: elsewhere, local drainage, soil andclimatic conditions may change the quantitative conclusions; the qualitative conclusions, that deficit irrigationand fixed schedules offer significant potential improvements to the productivity of water, are likely to berelevant.

The issue of reliability of service is particularly important. Other studies have shown (Perry and Narayana-murthy, 1998) that a critical aspect in persuading farmers to adopt deficit irrigation practices, which maximisethe productivity of water, is reliability of supply. Fortunately, precisely the same approaches (simple irriga-tion rules, reliable, easy-to-manage infrastructure) that underlie the restricted water allocation system analysedabove are also most likely to result in reliability of supply.

NOTE

1. The issue of high-value crops is often raised when a relatively simple irrigation service is proposed. Wenote that in many countries a large proportion of the irrigated area is served by groundwater, which is ideallysuited to providing a high degree of control. Adequate quantities of vegetables and the like can readily beproduced in these areas without the expense of constructing large surface systems to the necessary levels ofsophistication to meet their irrigation scheduling demands.

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increasing water productivity through deficit irrigation: evidence from the indus plains of pakistan

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