Irrigation and Drainage Systems12: 35–48, 1998.c 1998Kluwer Academic Publishers. Printed in the Netherlands.

Aflaj irrigation and on-farm water management innorthern Oman

W.R. NORMAN1, W.H. SHAYYA�, A.S. AL-GHAFRI & I.R. McCANNDepartment of Bioresource and Agricultural Engineering, Sultan Qaboos University,Al-Khod 123, Sultanate of Oman;1African Development Bank, Abidjan, Ivory Coast(�Corresponding author)

Accepted 22 October 1997

Abstract. This paper reports on results from a case study on water management within atraditional,falaj irrigation system in northern Oman. In the planning and design of regionalirrigation development programs, generalized assumptions are frequently made as to the effi-ciency of traditional surface irrigation systems. Although qualitative accounts abound, verylittle quantitative research has been conducted on on-farm water management withinfalajsystems. Daily irrigation applications and crop water use was monitored during an 11-monthperiod among 6 farm holdings at Falaj Hageer in Wilayat Al-Awabi. Contrary to the frequentassumptions that all surface irrigation systems incur unnecessarily high water losses, on-farmratios of crop water demand to irrigation supply were found to be relatively high. Based onactual crop water use, irrigation demand/supply ratios among monitored farms varied from0.60 to 0.98, with a mean of 0.79. Examination of the soil moisture budget indicates thatduring most irrigations of wheat (cultivated in the low evapotranspiration months of October–March) sufficient water is applied for the shallow root zone to attain field capacity. With theexception of temporary periods of highfalaj delivery flows or periods of rainfall, field capac-ity is usually not attained during irrigations within the more extensive root zones of date palmfarms. The data presented in this paper should provide a better understanding of water use per-formance by farmers within traditionalfalaj systems. Moreover, these data should also serveto facilitate more effective development planning for irrigation water conservation programsin the region.

Key words: irrigation, on-farm water management, efficiency, traditional systems, aflaj, Oman

Introduction

Most crop producing areas in the Sultanate of Oman receive only 100–200 mm of rainfall annually. Virtually all crop production is therefore depen-dent on irrigation. About half of the 62,000 ha cultivated in the Sultanateis irrigated from wells, while the remaining half is irrigated by traditionalfalaj systems (Abdel-Rahman & Omezzine 1996; MAF 1995). These indige-nous, community-managed systems access ground water by gravity flowfrom underground galleries or surface springs on neighbouring mountainslopes. Theaflaj of Oman (aflaj is the plural rendering in Arabic, whilefalajis the singular), whose origins date back several centuries and more, have his-

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torically provided a secure and stable means of agricultural production in theregion’s dry, desert environment (Wilkinson 1983). They have also served asthe foundation for settlement, community development and social structureamong most interior communities of northern Oman (Wilkinson 1977).

The rapid social and economic changes which have occurred in Omanduring the past 25 years have had a profound impact on the viability offalajsystems. Today, lower-cost (although often environmentally unsound) accessto water is available from wells and boreholes in many areas. Among manyaflaj communities, greater returns to labour are now available through non-agricultural and/or urban employment. These factors, among others, havecontributed to labour shortages, decreased local investment in system main-tenance and lower water tables (and, therefore, lower flow rates) among anincreasing number ofaflaj in the country (Dutton 1995).

Recent years have also seen a significant expansion in the cultivated area– most notably, areas irrigated from wells. With agriculture’s present use of80%–90% of the nation’s fresh water, water conservation in the agriculturalsector has now become a priority for the government (George 1996; Adbel-Rahman and Abdel-Magid 1993). Measures are presently being sought bywhich irrigation water can be used more efficiently. Particular attention hasbeen given to the improvement of “traditional”, on-farm surface irrigation,since this method is employed in the majority of farms with wells and withinall falaj systems. In many parts of the world, including the countries of theGulf Countries Council (GCC), the assumption is often made that high waterlosses are inherent in all forms of surface irrigation, with efficiencies oftenestimated at 50% or less (e.g. Ahmad 1996). Unfortunately, little quantita-tive work has been done to evaluate actual, on-farm water use among tra-ditional systems in Oman. Amongaflaj in particular, performance levels ofon-farm water management have remained unknown. This form of baselineinformation is crucial in the search for (and evaluation of) solutions to thecrisis facing many of Oman’saflaj and in the attempt to ameliorate waterconservation within irrigated systems in the Sultanate. This paper reports onfindings from a water management case study conducted at Falaj Hageer inWadi Bani Kharus, Wilayat Al-Awabi, in northern Oman. The objective ofthis paper is to provide an assessment offalaj on-farm water managementthrough the evaluation of soil water budgets and the efficiency of water use.Seasonal, on-farm water use data from 6 farm plots within thefalaj systemare examined and presented.

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Methodology

Site selection

The selection of Falaj Hageer for study was based on several criteria. It is amedium- to small-sizedfalaj system (under 10 ha) which supports 30 par-ticipant farmers and is sustained by year-around spring (ayn) flow. It sup-ports both date palm production and significant production of wheat inawabiland.Awabi land is “excess” land where perennial date palms are not culti-vated. Varying portions of it are used in the low evaportranspiration months(October–March) of most years when excess water allows for the supplemen-tary cultivation of a variety of non-perennial crops. The system is still man-aged according to traditional methods offalaj management (including thesundial and star system for irrigation scheduling). Another reason for select-ing this site is that the majority of farm labour is provided by Omani farmers(greater than 95%), rather than by expatriate labour. Finally, the site is withinreasonable access to university research staff (1.5 hours driving time). Theselection of farm plots within the system included: a) the identification offarmers willing to have their plots monitored on a long-term basis, and b) theselection of plots well distributed within different sectors of the system (e.g.at the head-end versus the tail-end of the channel delivery system).

Water use monitoring

During the period from October 1995 to March 1996, four wheat plots inawabiland were monitored for water use throughout the entire growing sea-son. Water use within two date palm plots was also monitored during theperiod from October 1995 to October 1996. Direct water use monitoringand informal farmer interviews were employed in the assessment of on-farmwater management. Flow measuring flumes were placed at the channel waterdelivery outlet at each plot, and water delivery times and flow rates wererecorded during each irrigation. Soil characteristics and root zone develop-ment were also measured within each plot. Soil moisture was monitoredusing both gravimetric and time-domain reflectometry (TDR) methods. Dur-ing the same period a small climatological station was maintained on-site.Daily maximum and minimum temperatures, relative humidity, solar radia-tion and precipitation were recorded for the purpose of estimating crop wateruse.

Soil water budget

The SCS-Microcomputer Irrigation Scheduling Package (SCS-Scheduler)was employed for evaluating soil moisture budgets. SCS-Scheduler was

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developed for use by the Soil Conservation Service of United States Depart-ment of Agriculture and is widely used in the U.S.A. (Shayya & Bralts 1994).It is a versatile and user friendly general irrigation scheduling package formicrocomputers. The program is suited for both on-farm irrigation schedul-ing and regional analysis. It can schedule irrigation based on either real-timeor historical weather data for an unlimited number of fields simultaneously.The package is not specific to an irrigation system, crop, climate, or soil type.

SCS-Scheduler employs the computed root zone water balance methodalong with field-specific characteristics and local weather data for water bud-get updates and irrigation scheduling. The software can accommodate a widevariety of soil types and is applicable to any number of crops once crop-specific growth data are established.

The theory used as the basis for the development of SCS-Scheduler wasdiscussed in several publications (Shayya et al. 1990; Shayya et al. 1991;Shayya & Bralts 1994). Consequently, only a brief review will be presentedhere.

The soil water content is determined daily by adding rain and irriga-tion amounts and subtracting evapotranspiration as follows (Shayya & Bralts1994):

SMi+1 = SMi +RAINi+1 + IRRi+r �ETi+1�DPi+1 (1)

whereSMi is the soil moisture in inches on dayI, SMi+1 is the soil waterin inches on dayi+1, RAINi+1 is the effective rainfall in inches on dayi+1,IRRi+1 is the irrigation in inches on dayi+1, ETi+1 is the evapotranspira-tion in inches on dayi+1, andDPi+1 is the amount of deep percolation ininches on dayi+1. TheET is defined as the total amount of water lost tothe atmosphere through transpiration, or removal of water through the planttissue, and evaporation of water from the surrounding soil surface.

Soil water characteristics required by the program are site-specific andmust be provided by the user. Weather data may be entered manually into thecomputer or transferred directly from a local weather station. Rainfall andirrigation measurements must also be supplied by the user.

To predict soil water for a given field, SCS-Scheduler requires an initialestimate of the available water capacity (AWC) of the soil, daily climatic datato determine crop evapotranspiration (ET), and daily precipitation records.These parameters are discussed below (Shayya & Bralts 1994).

1. Soil waterThe principal measure of soil water used in SCS-Scheduler is the availablewater capacity, AWC, which is the difference between the amount of watercontained in the soil at field capacity and the amount of water held in the soil

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at the permanent wilting point. This quantity may be expressed in terms oftotal depth of water in a column of soil or as a percent of the maximum AWC.

2. Root growthSCS-Scheduler calculates soil water available in the root zone. Rates of rootgrowth are established for each crop to be scheduled. As the rooting depthincreases, SCS-Scheduler uses the available water in each added soil incre-ment. The available water capacities of these soil increments are defined bythe user.

3. Evapotranspiration estimationMost methods for computing crop ET involve the basic equation:

ET = Kd(Kc ET0) (2)

where ET0 is the reference evapotranspiration,Kd is a coefficient thatdepends on the soil water depletion,Kc is the crop coefficient that varieswith the growth stage of the crop, andET is the crop evapotranspiration.

Daily weather data are used by SCS-Scheduler to estimate crop ET. FutureET predictions are based on historical or estimated weather data.

4. Precipitation recordsIrrigation and rainfall events are incorporated into the farm file for a givenfield at each scheduling session.

Water use performance

Irrigation water supply and the respective crop demand for water consti-tute two important water measures in irrigation water use evaluation. Vari-ous combinations of these measures are often employed as water use per-formance variables. For the purpose of this study, the ratio of (actual) cropwater demand to irrigation supply is used as a performance indictor and isexpressed as

Da=S =(ETa ��S � Pe)

IRR(3)

whereETa is the actual crop evapotranspiration (mm),�S is the change inroot zone moisture storage between the beginning and the end of the cropcycle,IRR is the total irrigation supply (mm), andPe is the effective precipi-tation (mm). In this study, it was assumed that all precipitation was effectivedue to the depth of rainfall events and the lack of runoff. Conveyance seepage

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losses are assumed to be negligible within the plot. When applied to an indi-vidual plot,Da/Sprovides a measure of actual farmer water use performance(Norman & Walter 1993).

Demand/supply (D/S) values frequently used to compare performanceamong systems and/or between geographic regions are often based on a the-oretical or design evapotranspiration value for crop water needs (as opposedto actual evapotranspiration,ETa). Generally, a maximum evapotranspirationvalue (ETmax) is used to establish this design limit or requirement for eachcrop type, and it is usually obtained by applying crop coefficients to ref-erence evapotranspiration (ET0) for various stages of crop development (i.e.assuming Kd = 1 in equation 2). A “design” demand/ supply ratio (Dd/S) can,therefore, be obtained by replacingETa in equation 3 with a value forETmax.AlthoughDd/Svalues are also derived in this paper, it should be understoodthat design demand values (ETmax) are frequently used in system planningand for (generalized) comparative purposes among irrigation systems. Butthey may be less useful in measuring farmer irrigation performance, particu-larly in cases where farmers may view optimal production at evapotranspira-tion rates below design crop demand (Norman & Walter 1993).

Application of the “irrigation efficiency” term to the “design”demand/supply ratio is avoided sinceDd/Svalues may exceed 1.0 (i.e. 100%)when the seasonal supply of irrigation water is less than the potential demand(i.e. ETmax).

Results and discussion

Figures 1–6 depict the available soil moisture regimes for all monitored plots,as generated by the soil moisture budget model. It should be noted that Fig-ures 1–4 depict the entire cropping cycle for wheat, while Figures 5 and 6depict 12 months of soil water management for perennial date palms. Soilwater content as predicted by the model was compared with the measuredwater content in the field to determine whether the model satisfactorily pre-dicted the major features of the soil water regime during the period of inter-est. The model agreed exceptionally well with the measurements consideringthat it predicted average soil water content over an area while the measure-ments were made at specific points. No calibration of the model for a specificfield was necessary, and all trends in soil water content over time that themodel predicted were supported by the field measurements. All water withinfalaj systems is owned by participant farmers in the form of water shares.These shares are usually divided into half-hour time units (athars) and farm-ers have access to their shares on a fixed rotation interval (thedawran). AtFalaj Hageer, thedawranis 7 days. Thus, all plot irrigations are spaced at 7

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Figure 1. Soil moisture budget for wheat in plot W1.

Figure 2. Soil moisture budget for wheat in plot W2.

or 14 day intervals, according to farmer choice or crop need. The sequenceof water shares among farmers will not change when a given farmer does notuse his allotted water during the rotation.

The period of highest reference ET (ET0) occurs during the 7 months fromApril to October. Average dailyET0 during the remaining 5 months from

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Figure 3. Soil moisture budget for wheat in plot W3.

Figure 4. Soil moisture budget for wheat in plot W4.

November to March may be as low as half of that of the highET0 months.During this period, the cultivation of non-perennial crops (usually wheat) ispractised inawabi land. In most years, this occupies 20%–30% of the totalcultivated area of thefalaj. The extent of cultivated area inaflaj is generallya function of the base flow of the mainfalaj delivery channel. At Hageer,

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Figure 5. Soil moisture budget for date palms in plot D1.

Figure 6. Soil moisture budget for date palms in plot D2.

rainfall within the system’s catchment watershed may result in a significant,yet short-lived, increase of delivery flow of the source spring (usually last-ing a week or less). When this occurs, farmers may have no choice but toapply excess amounts of water within their plots during their weekly watershare delivery period. This is because farmers have no means for storing

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Table 1. Soil moisture balance components, irrigation efficiencies and yields from moni-tored plots

Plot numberW1 W2 W3 W4 D1 D2 Mean

Crop wheat wheat wheat wheat dates dates –Area (m2) 419 520 365 249 465 299 386ETmax (mm) 344 358 411 377 1940 1943 –ETa (mm) 305 332 358 345 1779 1611 –�S(mm) 41 29 23 17 44 52 –Pe (mm) 48 28 24 10 262 337 –IRR(mm) 272 333 406 531 1882 1245 –Dd/S(%) 0.94 0.90 0.90 0.66 0.87 1.25 0.92Da/S(%) 0.79 0.83 0.77 0.60 0.78 0.98 0.79Yields (T/ha) 3.7 3.7 4.3 3.9 25 29 –

Note: The lowerDa/S of 0.61 for W4 is due primarily to a single “excess” irrigation on18/11 in which more than twice the usual irrigation dose was applied. Had a normal irri-gation depth been applied on this day,Da/S and Dd/S would have been 0.65 and 0.74,respectively.

large, excess volumes of water. Irrigations for date palms occurring on 13/3,20/3, 5/6, 12/6 and 18/7 in Figure 5 are exemplary of this practice. Excesswater is applied directly to the field plot, even if it will eventually be lost asexcess subsurface drainage or as surface runoff. Traditionally, it is consideredunsuitable practice among farmers at Falaj Hageer to allow water (including“excess water”) to discharge directly from thefalaj channel into uncultivatedareas or outside the system. One finds that among plots whose owners have anumber of other holdings within the system, these excessive irrigation appli-cations are less pronounced because farmers can divide the “excess” amonga greater number of plots. Figure 6 is an example of such a plot where thefarmer’s water shares must be (or can be) divided among four or more plotswithin the system.

At Falaj Hageer, most rainfall events within the system’s contributingwatershed occur just after the growing season for wheat. This is evident whenone compares rainfall and high irrigations that occur before and after March,1996 in Figures 5 and 6. For this reason, the evaluation of irrigation watermanagement practice among wheat plots may provide a better assessment offarmer water use performance since periods of “excess”falaj delivery flowsare infrequent.

Table 1 provides a summary of the components of seasonal water use formonitored plots as well as measures of the irrigation demand/supply (D/S)ratio and yield. Values ofDa/S range from 0.60–0.98, with a mean of 0.79.

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Dd/S values are approximately 12% higher. There is probably a significantvariation ofDa/S among plots throughout the system, due to the aforemen-tioned variance in the extent of demand (i.e. the total number of system hold-ings per farmer) on the farmer’s water shares. Nevertheless, the mean on-farmDa/S for the system as a whole is probably relatively high (around 0.8).

Plots W1 and W2 had the highestD/Sratios for wheat, with W1 approach-ing a deficit (i.e.Dd/Sin excess of 1.0) with respect to potential crop demand.For both plots, water may be a limiting factor which resulted in the slightlylower yields than plots W3 and W4. Where water delivery to the root zone isnot as limiting (e.g. W3 and W4, with lowerDd/Svalues) other factors mayhave a greater effect on yields (e.g. fertilizer levels, etc.). A comparison ofFigures 1–4 indicates that W3 performed better in the first half of the cropcycle than the other wheat plots. With the exception of W3, which has thehighest yield, most deficits beyond the 50% MADL (management alloweddepletion level) occurred before plant maturity (i.e. before early January).However, some depletion of readily available soil moisture during the post-maturity period does not seem to have adversely affected crop yields.

As indicated earlier, excess irrigations may occur more frequently amongperennial date palm plots than for non-perennial wheat, cultivated when rain-fall events are less frequent. However, the extent of the occurrence of excessirrigations may be dependent on the water demand (of other plots) on thewater shares of the plot owner. This is particularly evident between D1 (Da/S= 0.78) and D2 (Da/S= 0.98), where D2 has considerably more demand (interms of the number of other plots) on allotted water shares than D1 (alsocompare Figures 5 and 6). TheDa/S ratio of 0.98 in D2 also indicates thatsoil moisture is usually being maintained at a level which results in somewater savings. This may not be optimal for obtaining maximum potentialyield, but D2 nevertheless had a higher yield than D1 – which indicates thatfactors other than water may be involved (e.g. disease, pests, tree spacing,date variety, etc.).

A review of the soil moisture regimes for dates in Figures 5 and 6 indi-cates that soil moisture levels are brought up to field capacity only when arainfall event occurs which affects the cropped area both directly and indi-rectly. While the direct effect is obvious due to an additional precipitationevent in the field, the indirect effect of a rainfall event results in a signifi-cant increase (although temporarily) offalaj delivery flow and a subsequentincrease in irrigation water. The indication is that in a dry year it is possi-ble that soil moisture in many date palm plots may be maintained beyondMADL’s of 50% when there is little rainfall in the cropped area or in thefalaj watershed. This is demonstrated when the soil moisture budget is gen-erated for D2withoutrainfall. Plot D2 was used to demonstrate this due to the

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Figure 7. Hypothetical soil moisture budget for date palms wheat in plot D2, with supplemen-tary rainfall removed.

absence of “excess” irrigation applications during temporary periods of highfalaj flows. Figure 7 depicts this soil moisture regime where it is evident thatsoil moisture would not exceed the 70% depletion level during most periods.

Although it is beyond the immediate scope of the field research programat Falaj Hageer, it should be added that the relative cost or value of water tothe farmer has an impact on the way he uses it. Data from a related studyamong farmers using wells in northern Oman indicate that there is an inverserelationship between the cost of irrigation water and the way in which it ismanaged (Norman et al. 1996). Among surface irrigation systems (in whichwells are used) it was found that, when the cost to the farmer for irrigationwater is low, there is a tendency for it to be used excessively. When watercosts are relatively high and the supply is limited, water is used more con-servatively. For most farms, only when the volumetric cost of water exceedsabout 0.04 RO/m3 (1 RO=2.6 USD) would irrigation efficiencies attain a levelof 80% or better (Norman et al. 1996). Indigenous, traditional irrigation sys-tems, such as those found in parts of Asia and Africa, are often very effectiveat minimizing risks, providing equitable access of water to users and assuringthe efficient use of water (Coward 1977; Norman & Walter 1993; Norman1995). Theaflaj of Oman, in their historical traditional setting, apparentlyhad most of these characteristics. And, in spite of the present-day threats toaflaj sustainability, these inherent strengths warrant closer study. Among theaflaj of Wadi Bani Kharus, the cost of water in 1995–1996 was about 0.10–

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0.15 RO/m3 (when considering water share, system maintenance and labourcosts). Given the data presented in Table 1, it is evident that meanDd/Sratiosare 0.90 or higher. Furthermore, due to the traditional importance ofaflaj oncommunity structure and organization, a better assessment of thesocialval-ue ofaflaj water among rural communities would perhaps also be warranted.It is probable that the monetary value ofaflaj irrigation water, alone, doesnot adequately reflect its full value within and among rural communities. Forregional development planning, the relative value of water to the user shouldperhaps be given equal (if not greater) consideration to the methods(s) ortechnology used for on-farm irrigation.

Conclusions

This study has demonstrated that on-farm ratios of crop water demand toirrigation supply (D/S) are relatively high in Falaj Hageer. Mean “actual”demand/supply ratios (Da/S) which reflect farmer practices are in excess of0.80, for both wheat and date palm crops. When “design” demand/supplyratios (Dd/S) are considered, the values are even higher (considerably high-er than the often-quoted efficiencies of 50% or less for traditional surfaceirrigation methods). It is likely that an important portion of surface-irrigatedfarms in the region do operate at low irrigationD/Svalues, particularly thosewhich have been developed in more recent years. But the case study at FalajHageer indicates that ruralfalaj systems, in which traditional Omani man-agement methods continue to be employed, tend to use less water since thesupply is limited and may be operated atD/Svalues considerably higher thanpresumed.

The data indicate that farmers understand how to manage water carefullywhen using traditional surface irrigation methods, in so long as flow rates (inparticular, base flow rates) can be anticipated and remain within their control.Brief periods of “excess” flow resulting from periodic rains are, in a sense,beyond the control of the farmer. Consequently, high irrigation applicationsdo not necessarily reflect a lack of management ability or a poor grasp of cropwater need by farmers. More accurately, such “excesses” reflect a techni-cal problem brought on by natural, environmental processes which are quitebeyond local farmer capacity to control.

DisclaimerThe views expressed in this paper are those of the authors and not necessarily thoseof their respective institutions.

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