ORIGINAL PAPER

Ahmed A. Ibrahim Æ C.J. Stigter Æ Hussein S. AdamAli M. Adeeb

Water-use efficiency of sorghum and groundnut under traditionaland current irrigation in the Gezira scheme, Sudan

Received: 12 May 2001 /Accepted: 1 February 2002 / Published online: 9 May 2002� Springer-Verlag 2002

Abstract In the Gezira irrigation scheme in central Su-dan, serious symptoms of water waste have been iden-tified in the last two decades, especially in sorghum andgroundnut fields. To quantify losses, water-use efficien-cies and related parameters were obtained for these twofood crops under the traditional attended daytime waterapplication and the newly evolved unattended continu-ous watering method. In this on-farm research, theneutron scattering method was used to determine theactual soil water deficits of the two crops. A simplePenman equation was used for approximating referencecrop evapotranspiration and evaporation losses fromstanding water and wet soil surface. An updated ap-proach using the Penman-Monteith equation was addi-tionally applied. The study revealed wastage ofirrigation water in both irrigation methods but at dif-ferent rates and also differently for each crop. In theattended field, the average seasonal over-irrigation,which is the difference between average applicationdepth Q and average soil moisture deficit SWD, wasobserved to range between 0.4 and 1.5 of SWD (0.3 and0.6 of Q) and the corresponding values in the unattendedfield were 0.6 and 3.2 of SWD (0.4 and 0.8 of Q). Highervalues are shown by the groundnut subplots, which cropalso suffers from excess water, and by the drier year aswell as in the unattended fields. A first approximation isgiven, still including readily available water at harvest,of minimum water requirements in attended watering

for maximum yields. In the drier year, when more irri-gation water was applied, an amount equal to 30–50%of these minimum water requirements was lost inevaporation from standing water/wet surface, which isthe main unproductive water. More frequent land lev-elling aiming at minimum standing water in better at-tended irrigation and farm management (e.g. weeding)are priority measures proposed. The quantitativeon-farm water waste determinations represent theinnovative content of this paper. Knowing precisely howlarge the problem is and being able to quantify itscomponents will contribute much to the arguments ofthose who wish to take the proposed measures.

Introduction

The Gezira irrigation scheme of the Sudan (0.9 millionha, see Fig. 1) is fed by gravity from Sennar dam in theblue Nile, about 110 km south of Wad Medani town. Itconsumes one third of Sudan’s share in Nile waters asdetermined under the Nile Waters Agreement with Egyptin 1959 and is considered of paramount importance toSudanese agriculture, as the backbone of the country’seconomy. The irrigation system consists of two maincanals, branches, majors (major canals) and minors(minor canals) that feed the field outlet pipes (FOP).Ideally, under the now abandoned night storage system(NS) application method, in which only closely watchedattended daytime (0600–1800 hours) watering was car-ried out, each FOP should equally satisfy 18 fields(2.1 ha each). Due to ageing of the system, siltation,weed growth, poor water management and the increaseddemand from the intensification and diversificationwhich were introduced around 1975, water delivery atthe farm level has become unreliable. Consequently thefarmers tended to use water during the night for theirprivate crops, dura (Sorghum bicolor) and groundnut(Arachis hypogaea), from which the yields are notcompulsorily sold to the government. In this way they

Irrig Sci (2002) 21: 115–125DOI 10.1007/s00271-002-0057-z

A.A. IbrahimTTMI-Project, Hydraulics Research Station,PO Box 318, Wad Medani, Sudan

C.J. Stigter (&)TTMI-Project, Department of Environmental Sciences,Wageningen University, Duivendaal 2,6701 AP Wageningen, The NetherlandsE-mail: kees.stigter@user.metair.wau.nlFax: +31-317-482811

H.S. Adam Æ A.M. AdeebTTMI-Project, Institute of Water Management and Irrigation,University of Gezira, Wad Medani, Sudan

interrupted the flow of the NS system. Maximum com-mand water levels (the levels of about 50 cm above thehighest ground to be served that are mandatory for the

NS system) are thus not maintained and dischargesthrough the FOPs are reduced. Accordingly irrigationtimes are increased, further encouraging the farmers topractice unattended watering, which is believed to wastewater.

The objectives of this study were to quantify (1) howmuch water farmers are actually wasting by applyingunattended irrigation, (2) whether differences with at-tended daytime watering justify the advice to return tothe labour-intensive traditional method or some aspectsof it, and (3) a first approximation of the actual mini-mum water requirements of food crops grown in thecracking heavy clay soils of the Gezira. Various aspectsof this work have been published recently (Ibrahim et al.1999, 2000).

Sites and methods

Sites and on-farm practices

The on-farm research was conducted near Hamza minor canal ofthe Tibub Block (Central Division of the Gezira, see Fig. 1). Twofields of 2.1 ha were selected (one for each application method) onthe basis that they received water from the same FOP, had goodand identical access to water (i.e. not tail-end fields) and hadcomparable yields in the period 1975–1985 (i.e. after the intro-duction of intensification and diversification). The FOPs are 35 cmdiameter concrete tubes drawing water from a minor canal at rightangles to them (Fig. 2). When the water head across the FOP isabove 15 cm, these pipes discharge about 115 l/s. Each FOP feeds awater course, an abu ishrin (abu xx) that itself feeds a number of280 m-long secondary water courses, abu sittas (abu vis), that lay75 m apart, parallel to the minor canal and parallel to the furrowsof the 5 feddan (1 hawasha, 2.1 ha) of agricultural fields of whichthey form the upstream and downstream borders (Fig. 2). Each abuxx feeds 17 abu vis. From each abu vi twice seven jedwals departperpendicularly, each of which feed an area 1/7th of an hawasha,

Fig. 1. Location of the Gezira scheme in central Sudan. Black dotindicates location of the experimental site at Tibub Block, centralGezira

Fig. 2. The canalization systemin a 5 feddan (2.1 ha) hawasha(75·280 m2) in the Gezirascheme. Arrows show the flowdirection. FOP field outlet pipe,ABU xx feeder channel, ABU vifield channel

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called an angaya (=3,000 m2). The approximately 90 furrows ofeach angaya are watered by one jedwal. In the middle of eachangaya, the irrigation borders are closed by a low dyke, called atagnet (Fig. 2). Furrows are spaced 80 cm apart and are about50 cm wide and 30 cm deep. The crops are grown on the approx-imately 30 cm wide ridges. Slopes of the land are on average only5 cm/km and the agricultural land may be considered flat, withploughing determining differences in elevation.

The most important practices that influence on-farm water-useefficiencies in the scheme are the following:

1. In the attended method, the farmer irrigates one angaya at atime (angaya-by-angaya application method). In the unattendedmethod, the farmer opens at least two angayas at a time and heoften breaks the borders between two angayas or more, wateringthem as one plot.

2. Dura and groundnut are grown separately. Dura subfields arefurther divided into two varieties: the common short-seasongrowing type (Fitareeta) and a relatively longer growing seasonhybrid (Hageen). The Hageen variety requires one more irriga-tion after the maturation of Fitareeta.

3. Intercropping is commonly practised. Such crops (okra, pi-geonpea, karkade and kenaf, all till about 1.5 m high) are grownin isolated rows in the groundnut subfields (height £ 30 cm),almost identically for both fields.

4. Both farmers were applying the common practices of the Gez-ira: (a) two seeds of groundnut per hole; the holes were manu-ally dug on the top of the ridges at about 30 cm distance.Because sowing was done in each field by several people, thesedistances varied; (b) dura was also grown on ridges at about25 cm distance and thinned to three, sometimes four plants perhole, and (c) estimated standing water in the furrows (notexactly measured) was used as a yardstick for completion of theirrigation.

5. The number of water applications was four for Fitareeta duraand five for Hageen dura and groundnut in the 1988/1989season and just like this for dura, but four for attendedgroundnut and five for unattended groundnut in the 1989/1990season.

6. Additional irrigation is neither necessary nor possible to preventsalination of the soils. Nachtergaele, cited by Fadul (2000), re-ported in the 1970s that after many (sometimes 40) years ofirrigation in the Gezira the weighted average of the electricalconductivity in the top 30 cm never exceeded 4 mS/cm and inthe majority of cases was below 2 mS/cm. Fadul (2000) also citesevidence that local indications for salt accumulation in theGezira are at much greater depths than reached by roots andthat fortunately so-called secondary salination processes are notactive in the Gezira soils, because the groundwater is too deepand the irrigation water is of such excellent quality. Fadul (2000)finally quotes evidence that the Gezira vertisols are very oftensodic, to an extent that does not affect crop growth, and thatgenerally such soils are non-saline.

Methods applied

Data collected included soil samples, neutron-probe soil moisturereadings, data on irrigation discharges, rainfall, climate, surface ofwet soil/standing water, the number of days of waterlogging, andrelevant agricultural practices. Details on the methodology of datacollection and interpretation are given by Ibrahim (1992). Themeasuring accuracy needed was determined by that of the leastaccurate field quantification, that of actual water discharge to thecropped fields by a vane flow meter (development of HydraulicsResearch Laboratories (HRL), Wallingford, UK), in particular tothe unattended fields. According to our own calibration in thelaboratories of the Hydraulics Research Station (HRS) of theMinistry of Irrigation, Sudan, the accuracy of the device was ±5%when measuring a discharge of 0.02 m3/s or higher. The accuracyreduced with decreasing discharge to become about ±10% at0.01 m3/s. For still lower flows (down to 0.005 m3/s) the accuracy

became as low as ±25%. The contribution to the total seasonalirrigation depth of such low flows, that of flows too low to measure(<0.005 m3/s), and that of unmeasured flows in occasionalbreakages, decreased water application accuracy in the unattendedfield to the order of between 10% and 15%.

In each field of 2.1 ha, 20 access tubes were installed. Twoneutron soil moisture gauges (3321 series; Troxler ElectronicLaboratories, Raleigh, N.C. and 503 series; CPN Corporation,Martinez, Calif.) were used for soil moisture readings at 30, 50, 70,90 and 110 cm depths. Particularities of neutron-probe calibrationsin cracking clay soils were dealt with by Ibrahim et al. (1999).Gravimetric soil samples were collected from 10±5 cm depth.Eq. 1 (e.g. Kristensen 1975) was used to obtain the integrated soilmoisture content of the profile 0–120 cm depth:

ht ¼ d � h10 þ h30 þ :::::þ h110ð Þ mm ð1Þ

where ht (mm) is the total water depth available in the profile, h10,h30 etc. are volumetric moisture ratios (mm/cm) at the depthsconcerned and d (cm) is the depth interval. Soil moisture mea-surements were only taken when the fields could be entered; that is,at moisture values at the ‘‘post-irrigation soil moisture stage(POSTISM)’’. POSTISM is a condition which can be clearly de-fined, and is reproducible within close limits over a wide range ofsites on Gezira clay (Ibrahim et al. 2000). POSTISM is determinedat least 3 days after irrigation but with a drying soil surface(completely dry ridges). For dry soil surfaces, differences betweenconsecutive readings determine actual evapotranspiration (ETa;mm/day).

For the evaporation rates from standing water and a dark wetsoil surface (E0; mm/day) a revised Penman equation for openwater was used (Doorenbos and Pruitt 1977; Stigter 1980), withPiche evaporation for the aerodynamic term using previously de-termined correlations for three different seasons (Ibrahim et al.1989; Abdulai et al. 1990). Using an above-crop wind function, asproposed by Stigter (1979, 1983), in these correlations, referencecrop evaporation (ET0; mm/day) was computed (see Doorenbosand Pruitt 1977; Stigter 1978).

When working on a revision of this paper, new guidelines ap-peared for computing reference crop evapotranspiration (Allen etal. 1998; Smith 2000) and it was necessary to study the conse-quences for our calculations. Early data summarized in Smith(2000) show that in arid regions the 1977 Doorenbos and Pruittcalculations overestimate those from the Penman-Monteith equa-tion now adopted by FAO by 13%. From abundant calculations,Shahin (1998) concludes for the Arabian peninsula, where the cli-mate is very close to ours, ‘‘that the results estimated from theFAO-Penman-Monteith equation are smaller by about 10% thanthose estimated by the FAO-Penman equation for all stations in-vestigated in this study’’. The overestimation may be somewhatlower due to the adapted wind function that we originally applied(Stigter 1978). This does not justify for our purposes an attempt ofrecalculation of ET0, which, in any case, is complicated by our useof Piche data for the aerodynamic term. The approaches nowsuggested for such sites with missing basic data (Allen et al. 1998)would certainly not be more accurate. We have therefore also usedcalculations in which the consequences are shown of a 10% lowerET0, for example in determining crop factors.

Crop coefficients (kc) from the literature (Doorenbos andKassam 1979) were used to compute the maximum evapotranspi-ration rates (ETm=kc*ET0 mm/day) in and immediately after rainsand in the period between the days of irrigation and the first cor-responding post-irrigation soil moisture readings (periods ofstanding water/wet soil). It may be noted that generally conserva-tive KC/kc data after the new calculations due to the appearance ofAllen et al. (1998) made recalculation unnecessary here. Because ofa lower evaporative demand under the well developed crops,evaporation losses from standing water in these periods were takento be 50% of E0 (see Hanato et al. 1988).

The data were used to calculate:

1. Soil moisture changes with time and contributions from thedifferent soil layers to the total depletion rates of the fields.

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2. On-farm actual evapotranspiration rates and total on-farm cropwater use, including (1) above and evapotranspiration duringperiods of standing water/wet soil (from 1 day up to more than5 days).

3. A first approximation of total minimum on-farm water re-quirements of the two private crops in the Gezira.

4. Water-use efficiencies in the form of a yield response factor, theresponse of yield to water supply (Ky) as defined in Doorenbosand Kassam (1979) and of a yield/water application relationship(AE; yield/area per unit water applied) as defined in Kanemasuet al. (1984) as on-farm water-use efficiency.

Crop factors (KC), defined as ETa/ET0, were computed as inEq. 2. ETa values include actual depletion from the soil under drysurface conditions (difference in T days between first post- and lastpre-irrigation soil moisture readings (dh)), rainfall P in the T daysbetween two measurements and the estimated ETm summed overthe period and the surface of standing water/wet soil conditions(ETaw mm):

KC ¼ ETa=ET0 ¼ ETaw þ dh þ Pð Þ=Tf g=ET0 ð2Þ

KC obtained this way includes other factors that might have pre-vented the crop from attaining its maximum transpiration rateunder the prevailing climatological conditions (e.g. Saxton et al.1974; Hanato et al. 1988). These were therefore compared with thecrop coefficient kc, which does not contain such factors.

For kc, values from the literature (Doorenbos and Kassam1979) were originally used. When the period under consideration[measurement period(MP) in days after sowing (DAS)], determinedby the period over which soil moisture was measured and KC wascalculated, was covering parts of two growing stages as suggested inDoorenbos and Kassam, a weighted mean kc for the two stageswas used. For comparison, kc(56) was also determined, calculatingupdated kc values along the lines given in Allen et al. (1998) andusing the procedure of constructing a kc curve as described andexemplified there.

In these additional calculations, the first new emphasis is onusing a distribution of growth stages (GS) as actually observed inthe field and characteristic for the varieties and growing conditionsconcerned. Over the past 20 years so many varieties with differentgrowth habits have been introduced that this is an essential newcondition for the determination of crop coefficients. This means forour case a difference between Hageen and Fitareeta of 10 days (orone irrigation) and shorter seasons (95 and 105 days) than used inthe original calculations. This was also shorter than for ground-nuts, where only the distribution of GS had to be adjusted, but notthe length of the growing season (130 days).

For average KC/kc and KC(56)/kc(56) data over the growingseason, another weighted mean was subsequently determined,taking the new lengths of each of the GS of the growing seasonsinto account for the latter, and the gaps before, between and afterthe MPs of KC for both cases.

When no factor other than water availability is influencing thecrop production, crop yield is most commonly related to ETa as inEq. 3 (see Doorenbos and Kassam 1979; Kanemasu et al. 1984;Stanhill 1987; Allen et al. 1998):

1� Ya=Ym ¼ Ky � 1� ETa=ETmð Þ ð3Þ

where Ya and Ym are the actual and potential yields (for example intonnes per unit area), Ky is the yield response factor, and ETa andETm are as defined above. Eq. 3 can also be written as:

Ky ¼ 1� Ya=Ymð Þ= 1�KC=kcð Þ ð4Þ

The theory behind Eq. 3 assumes a continuous and relativelyequal water deficit to be experienced throughout the growing sea-son or one deficit that occurs during a specific growth period(Doorenbos and Kassam 1979; Kanemasu et al. 1984). Effects fromsuccessive deficits and watering, as could be the case in our on-farmirrigation, have not actually been determined. Multiple stresses,resulting from (additional) waterlogging, nutrient deficiency,

diseases, and so on, can easily invalidate a comparison with the Ky

values given in the literature.For the application of Eq. 4 in this experiment we assumed

that: (1) the crop is water stressed if ETa/ETm<0.5 (Doorenbosand Kassam 1979); (2) potential yields (Ym) for groundnut aretaken as 3.5 t/ha, as given by Ishag et al. (1985) from a well-con-trolled irrigation experiment in Gezira and confirmed by theFarmers’ Council of Production; and (3) a rough approximation ofthe contribution of the low-density intercrops to the evapotran-spiration in the groundnut subfields could be included as correc-tion.

A serious problem was that, for Fitareeta and Hageen hybriddura varieties, only the maximum yield figures of 3.0 and 3.5 t/hacould be obtained from farmers’ sources, but these high yields wereobtained with fertilizers of which the use was reduced considerablyin the course of time.

The intercrops which were planted in isolated rows across theridges of the groundnut plants represented rows of high plants(150 cm height) towering above the continuous groundnut cropwhich does not exceed 25–30 cm height. As a result, the twoevaporating surfaces (groundnut and intercropped plants) are ex-posed to two different microclimates with respect to interceptedsolar radiation, wind speed and advection of warmer and drier air(see Stigter and Baldy 1995; Baldy and Stigter 1997). It is knownthat such ‘‘clothesline effects’’ can increase evaporation consider-ably. The number of intercropped plants was estimated to be about8–10% of the total. Their evaporation was therefore set at a con-tribution of 15±5%. This inaccuracy only contributes ±1% to theoverall inaccuracy of water use.

Very relevant to the theme of the study was the yield/waterapplication relationship (see Kanemasu et al. 1984), which definesthe actual farm water-use efficiency (Eq. 5):

AE ¼ Ya=Qt t=ha per metre ð5Þ

with AE application efficiency or actual farm water-use efficiencyand Qt total water applied to the crop, including rainfall.

Results and discussion

On-farm crop densities

Densities of groundnuts after establishment in the at-tended field were 27,760/ha in 1988/1989 and 30,000/hain 1989/1990. For the groundnut crop in the unattendedfield they were 88% and 81% of those in the attendedfield in the 1988/1989 and 1989/1990 growing seasons,respectively, due to sowing being done manually bydifferent people. These density differences fortunatelywould not influence evapotranspiration when such fieldswere been identically watered (Fischer 1980). Inter-cropped plants in both fields and both seasons wereobserved to be at the rate of 60±3 plants per ridge,2,570/ha. Although it is difficult to calculate their con-tributions to the total evapotranspiration with a highdegree of accuracy, they did not contribute to differencesin evapotranspiration between the attended and unat-tended fields, which therefore remain due to the methodof application. Dura was grown at 36,000 plants/ha.

Application rates and Gezira soil water behaviour

The most striking observations from soil moisturereadings are:

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1. Figure 3 suggests but does not fully prove that nodeep percolation is occurring between subsequent ir-rigations in the Gezira. Transport of water is drivenby a gradient in the hydraulic head, which is notnecessarily reflected in a fall in water content. How-ever, there is circumstantial evidence, like that of thesmall gradients shown in the soil water head at 90 and110 cm in Fig. 3 and the low value of the hydraulicconductivity, that argue in favour of the assumptionthat deep percolation may be neglected in these ex-periments. As illustrated in Fig. 3, the difference be-tween highest and lowest soil moisture contentsdecreases with depth, to reach almost insignificantvalues from 70 cm soil depth downwards. Close tofull watering at these layers is attained in the firstirrigation through the shrinkage cracks that form inthe dry summer (e.g. Ibrahim et al. 1999). An ex-ample of seasonal soil moisture fluctuation under agrowing crop is given for the attended dura of 1988/1989 in Fig. 4. From the minimum in the earlygrowing stage (12 DAS) to close to crop maturity(100 DAS) the moisture range at the 70 cm horizon isno more than 2% moisture content by volume.

2. The highest soil moisture contents at POSTISM, inmillimetres, for the 1988/1989 season, were deter-mined as 37 (10 cm); 45 (30 cm); 40 (50 cm); 36(70 cm) and 35 (90 and 110 cm). For the 1989/1990season these data were 31 (10 cm); 40 (30 cm); 40(50 cm); 37 (70 and 90 cm) and 38 (110 cm). Aspresented in Table 1, about 60–85% of average pre-irrigation soil water deficit (SWD), calculated as thedifference between highest contents at POSTISM andaverage pre-irrigation soil moisture contents, iscaused by the top 40 cm layer. Retention at deeperlayers has considerably less contribution to cropwater use.

Fig. 3. Ranges of volumetric moisture content (%) betweenmaximum and minimum values under sorghum (dura) andgroundnut in the Gezira heavy clay soils in season 1988/1989, forthe attended fields

Fig. 4. Seasonal volumetricmoisture (%) fluctuation underirrigated sorghum for the sea-son 1988/1989 in the Geziraheavy clay soils (attended field).At 90 and 110 cm depths, thedata were close to those of70 cm depth. See also Fig. 3

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One should compare the highest constant valuesbeyond 50 DAS in Fig. 4 with the data for highestsoil water contents at POSTISM given above.Where such values do not exist, as for 30 cm and10 cm, repetitive highest values can be used instead,keeping in mind that these must be underestima-tions. These values are more than 35% for 70 cm,near 39% for 50 cm, just below 42% for 30 cm andaround 35% for 10 cm depth. These data are only1% different (70 cm, 50 cm) from the highest POS-TISM data given above for 1988/1989, or com-pletely within each other’s accuracy margins of±2%. The somewhat lower values at 10–30 DASshown in Fig. 3: 1% at 70 cm; 2.5% at 50 cm; 2% at30 cm and less than 2% at 10 cm, are only 2%(70 cm), 3.5% (50 cm) and as much as 5% (30 cm)and less than 4% (10 cm) lower than those given forthe POSTISM. This may have two reasons. Firstly,the lateral water movement in these soils is ex-tremely slow, so the soil may not yet have been fullysaturated after the first irrigation in Fig. 4. This mayexplain 2±1% throughout the profile and explainsmost of the difference for 50 and 70 cm. Moreover,in the data given in Fig. 4 there is some consump-tion by roots in the topsoil, diminishing from 10 to30 cm. This may explain another 2±1% at 10 cmand 1±1% at 30 cm and brings all the data easilywithin each other’s accuracy margins. It should fi-nally be noted that the relatively low POSTISMvalues at 10 cm depth may be due to the fact thatsoil depth is measured from the ridges and the firstmeasurements are taken after the fields became ac-cessible. This may also be the reason for part of thedifference in highest POSTISM values between thetwo years, with much lower values in 1989/1990 at10 and 30 cm. Another reason could be differencesin the volume of cracks, which is the single mostimportant factor in POSTISM values.

3. Assuming that readily available water (RAW: valuesat highest POSTISM minus those at the permanentwilting point (PWP)) of the top 120 cm profile of the

Gezira soil is about 120 mm depth, the farmers ap-peared to decide to irrigate at widely varying deple-tions. They irrigated dura between 30% and 40%depletion in the drier year (1989/1990) and between40% and 60% in 1988/1989. For groundnut thesevalues were 20–40% depletion for the drier year and20–50% for the wetter one. Higher depletions wereshown in the attended fields, with the exception ofdura in the wetter year, for which they were the same(40–60%). For dura in 1989/1990 the range was 30–40% in the unattended field and 40±5% in the at-tended field, only a small difference. Observations ofmidday wilting of plants at the edges are used in durato decide when to irrigate. For groundnut, whereappearance of cracks and time since the previous ir-rigation are used to decide on irrigation, these dif-ferences were much larger: 20–30% (in both years) inthe unattended and 30–40% (1989/1990) and 40–50%(1988/1989) in the attended fields. In line with this, itwas observed that cracks in the attended groundnutfields were larger at the moment of irrigation than inthe unattended fields. When 50% depletion is takenas the stress limit (Doorenbos and Kassam 1979),very mild stress occurred at only one irrigation fordura in the wetter year (1988/1989) in both applica-tion modes.

The above factors determined the application effi-ciencies of both watering methods. As can be seen fromTable 1, in the attended field, the average seasonaloverirrigation, which is the difference between Q andSWD, was observed to range between 0.4 and 1.5 ofSWD (0.3 and 0.6 of Q) and the corresponding values inthe unattended field were 0.6 and 3.2 of SWD (0.4 and0.8 of Q). Higher values are indeed shown by thegroundnut subfields and by the drier year, as well as forthe unattended fields.

Prolonged standing water was observed in the un-attended fields. Because of the lower evaporative de-mand beneath a well-developed crop (Hanato et al.1988), this must have negatively influenced yields from

Table 1. Average application depths (Q mm), average soil mois-ture deficit (SWD mm) for sorghum (dura) (Du) and groundnut(Gn) in the top 120 cm profile, and the average percentage contri-bution to the SWD of the different horizons at the time the irri-gations were applied. Higher average Q was applied in the drier

season, rainfall being 148 mm in 1989/1990 and 213 mm in 1988/1989. Four irrigations were applied to Fitareeta dura for all fields inboth seasons, while groundnut received five irrigations with theexception of attended groundnuts in the 1989/1990 season, whichreceived four

1988/1989 season 1989/1990 season

Field Attended Unattended Attended Unattended

Crop Du Gn Du Gn Du Gn Du Gn

Q (mm) 85 70 105 100 100 100 120 125SWD (mm) 60 50 65 30 50 40 40 30

0–20 40 37 45 40 34 33 34 3020–40 37 33 42 28 26 31 31 2840–60 11 8 10 0 14 16 15 1360–80 6 10 2 8 11 9 10 1080–100 6 6 1 11 8 7 6 10100–120 1 6 0 13 7 4 4 9

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the unattended groundnut that cannot tolerate water-logging. As shown in Ibrahim (1992) and Ibrahim et al.(2000), in the year of lower rainfall (1989/1990) in theunattended field, evaporation losses from standingwater reached 200 and 230 mm depths for dura andgroundnut, respectively, against 180 and 170 mm inthe attended fields, all many times the SWD valuesshown in Table 1.

The differences in Q (Table 1) were mainly due to thewatering methods. As direct result of ploughing, tenants’fields have lost their design slope. With the yardstick thefarmers use for irrigation satisfaction, they over-irrigatethe lower parts of the fields more when larger areas areto be satisfied at one time, as is the case in the unat-tended application method.

In addition to the above, about 50–80 mm in theform of readily available water was observed to be stillremaining in the soil profile at harvesting time. Thisamount, which is one to three times an irrigation thatwould satisfy SWD (standing water evaporation ex-cluded), is kept in the profile to be lost through soilevaporation the following summer. This type of losscould be reduced if the final irrigation date and depthwere adjusted in such a way that PWP is targetedtowards maturity of the crop. Some farmers prefer amoisture content above PWP for harvesting groundnutsbut some harvest, more laboriously, under drier condi-tions. With an average potential evapotranspiration of6.5 mm/day in October and November (Ibrahim 1992),the last irrigation that is expected to bring the soil undereach crop to its soil moisture content at POSTISM canbe applied 3–4 weeks before full maturity of both crops.

All the above leads to the conclusion that irrigationwater is wasted in both application methods, but atdifferent rates and differently for each crop. The waste isdefinitely higher in unattended irrigation of the two foodcrops. Between them more water is wasted by irrigatinggroundnuts.

On-farm crop water use

Results from all eight different fields are shown inTable 2. The kc(56) values were higher than the previouskc values early and late in the season but lower in themiddle of the season for dura in both years, with theexception of Fitareeta in 1989/1990 at the end of theseason. The cause of these differences is the other choiceof the lengths of the growing season and the GS in thenew calculations. This resulted, in general, in about 10%or less lower KC/kc over the season for Hageen dura,with the exception of the attended dura of 1988/1989,where the smaller difference goes in the other direction.Fitareeta had, in general, about 5% higher average KC/kc than Hageen. For groundnut in 1988/1989, differ-ences in kc values were generally small, from similarvalues to slightly larger kc(56) and only one case of 35%larger kc(56) early in the season. Average KC/kc valuestherefore differ little between methods and the same

applies to groundnut in 1989/1990, where lower kc(56)early in the season is compensated for by higher valuesin the second half of the season.

It should be recalled that KCs for the crops (Table 2)were originally directly computed using Eq. 2. It may benoted that the general trend in KC data over the seasonsis low values in the beginning and at the end of theseason and higher ones mid-season. There were twoexceptions to this rule, attended groundnut in 1988/1989and attended dura in 1989/1990, where the trends werethe opposite. We observed that in the middle of theseseasons ETa values were less than half the ET0 valuesand these are indeed the seasons with the largest SWDs,for groundnut in general and for dura in the year oflowest rainfall, in Table 1.

Comparisons made after applying the new cropcoefficients calculated from crop coefficient curvesdetermined following the new very detailed instructionspublished by FAO (Allen at al. 1998) revealed that fordura KC/kc ranged from 0.79 to 0.98 and KC(56)/kc(56)from 0.71 to 0.97. For groundnuts this was 0.75–0.95 forKC/kc and 0.73–0.97 for KC(56)/kc(56). The ranges ofKC(56)/kc(56) were slightly larger but, given the largeerror margins, they did not differ in principle. AverageKC/kc for unattended groundnut was always lower thanfor attended groundnut. For dura this trend was gen-erally the opposite. It is suggested that different stressesand their sequences and combinations and different cropreactions, together with differences between the fields,must be behind the above results, including the higherror ranges.

Including the good yield of the attended groundnutof 1989/1990 (Table 3), Ky values in this experimentwere observed to range between 1.4 and 30 (using Eq. 4).The range for groundnut was much smaller, from 2.0 to6.7. Ky and Ky(56) differed relatively little for groundnut.They generally differed less for dura in the drier 1989/1990 and least for attended sorghum, Ky(56) alwaysbeing smaller, but considerably so in 1988/1989 and inthe opposite direction for attended and unattended dura.These values are not comparable with the values cited byDoorenbos and Kassam (1979) and kept by Allen et al.(1998), which are 0.7 and 0.9 for groundnut and duracrops, respectively. As has been mentioned above, littlewater stress was suffered by the crops in this experiment.Therefore, water stress will not have influenced theyields very much. The reason why Eq. 4 appears not tobe directly applicable to the data in this experiment mustbe in other explanations of lower yields. For groundnutthe lowest Ky value of 2 still shows the positive influenceof the on-station management of the experiment ofIshag et al. (1985), in which no nutrients were appliedalso. The low groundnut yields in the wet year and thelower yields in the unattended fields are due to periodsof waterlogging. After a correction for termite damage,the Ya/Ym value for the attended field in 1988/1989 forgroundnut becomes 0.4. This still low value may be dueto a combination of insufficient farm management, nu-trient stress and waterlogging, while the even much

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Table 2. Crop factors (KC), crop coefficients (kc) and KC/kc ra-tios for different measuring periods (MP) averaged over thegrowing seasons with weighting methods, as explained in the text,using FAO(33) and FAO(56) determination methods, respectively,for both growth stages and kc. KC(56) values, calculated with the

Penman-Monteith approach of Allen et al. (1998), were taken 10%higher than our earlier KCs, determined using an adapted Penmanapproach. Averages given are weighted averages (WA) over theduration of the measuring periods (DAS days after sowing)

MP DAS KC kc KC/kc KC(56) kc(56)a KC(56)/kc(56)

Attended sorghum (dura) 1988/1989a

Gap 1 1.0 1.01 14–45 0.55 0.55 1.0 0.61 0.61 1.0Gap 2 0.98 1.01 (1.02)2 59–74 0.96 1.0 0.96 1.06 1.04 (1.03) 1.02 (1.03)3 75–106 0.65 1.0 0.65 0.72 0.86(0.73) 0.84 (0.99)4 107–119 0.41 0.52 0.79 0.45 0.62 0.73WA 0.88±0.08 0.93(0.97)±0.07Unattended sorghum (dura) 1988/1989a

Gap 1 1.0 0.851 12–59 0.55 0.55 1.0 0.61 0.72 0.85Gap 2 0.95 0.9 (1.0)2 80–82 0.88 1.0 0.88 0.97 1.02 (0.85) 0.95 (1.14)3 not enoughdata (Gap 3)

0.8 0.92 (1.04)

4 96–118 0.53 0.52 1.02 0.58 0.65 (0.62) 0.89 (0.94)WA 0.98±0.08 0.88(0.92)±0.07Attended sorghum (dura) 1989/1990a

Gap 1 1.6 1.41 5–20 0.55 0.35 1.57 0.61 0.46 1.332 21–49 0.51 0.7 0.73 0.56 0.74 0.763 50–61 0.31 1.1 0.28 0.34 1.04 0.33Gap 2 0.35 0.37 (0.39)4 71–79 0.45 1.1 0.41 0.43 1.04 (0.96) 0.41 (0.45)Gap 3 0.42 0.40 (0.47)5 85–94 0.32 0.75 0.43 0.35 0.89 (0.72) 0.39 (0.49)Gap 4 1.0 0.39 (1.0)WA 0.79±0.10 0.71(0.75)±0.08Unattended sorghum (dura) 1989/1990a

Gap 1 1.5 1.31 7–20 0.52 0.35 1.49 0.57 0.46 1.24Gap 2 1.09 0.922 41–60 0.50 0.72 0.69 0.55 0.93 0.59Gap 3 0.70 0.73 (0.8)3 79–80 0.82 1.1 0.75 0.9 1.04 (0.89) 0.87 (1.01)Gap 4 0.58 0.64 (0.75)4 89–93 0.31 0.75 0.41 0.34 0.86 (0.69) 0.4 (0.49)Gap 5 1.0 0.4 (1.0)WA 0.94±0.12 0.83(0.88)±0.10Attended groundnut 1988/1989Gap 1 1.0 1.11 42–44 0.65 1.1 0.59 0.72 1.1 0.66Gap 2 0.51 0.562 48–64 0.47 1.1 0.43 0.52 1.15 0.45Gap 3 0.37 0.393 75–88 0.34 1.1 0.31 0.38 1.15 0.33Gap 4 0.7 0.754 99–101 1.16 1.1 1.06 1.28 1.15 1.11Gap 5 1.06 1.11WA 0.77±0.12 0.83±0.09Unattended groundnut 1988/1989Gap 1 1.0 1.01 33–51 0.47 0.75 0.63 0.52 1.05 0.5Gap 2 0.56 0.482 54–68 0.48 1.0 0.48 0.53 1.15 0.46Gap 3 0.75 0.753 73–76 1.07 1.1 0.97 1.18 1.15 1.03Gap 4 0.91 0.974 85–86 0.94 1.1 0.85 1.05 1.15 0.91Gap 5 0.67 0.75 101–107 0.5 1.05 0.48 0.55 1.14 0.48Gap 6 0.7 0.7WA 0.75±0.12 0.73±0.9

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lower value for the unattended watering must be due tothe more serious waterlogging observed. As to dura, theapplication of Ym values under different inputs spoil thefit of Eq. 4. For dura in 1989/1990, watering methodhad little or no influence on yields. The differences fordura are small and within each other’s error margins ofabout ±0.1 t/ha (estimated as >5% but <10%) orwith touching margins. In practice, groundnut is verysensitive to overwatering, but dura is not. In 1988/1989the unattended field did better and our experiment musthave been spoiled here by differences in nutrient statusresulting from previous cropping. The colour of theleaves showed this. The Ky(56) values express this betterthan the original Ky values, confirming the new distri-butions of growth stages to be more realistic. Fortu-nately our water waste data are not influenced by this.

Table 3 also shows the total application depth (Qt

mm, including rainfall) and farm water applicationefficiency, which is the yield per unit area per unit waterapplied (AE t/ha per metre), for the Fitareeta (F),

Hageen (H) and groundnut crop (G) for the two appli-cation methods in the 1988/1989 and 1989/1990 growingseasons. These parts of Table 3 show that:

1. Return from Hageen per unit land and unit water ishigher than the corresponding return from Fitareetain both application methods. The ratio of farm water-use efficiency AE for Hageen to that for Fitareetaranged between 1.1 and 1.8. This is despite the factthat one more irrigation was required by the Hageen.The reason is clearly the small use of fertilizer on theHageen crop only.

2. The maximum AE values of dura observed werearound 50% of the minimum AE values shown byKanemasu et al. (1984) for a similar range of appli-cations (0.44 m £ Qt £ 0.72 m of water) and similarsoil water contents (irrigation at 50% readily avail-able water). From only 0.38 m of water in twoirrigations they show an AE of 6.0 t/ha per metre.This comparison indicates that much of the water

Table 2. (Contd.)

MP DAS KC kc KC/kc KC(56) kc(56)a KC(56)/kc(56)

Attended groundnut 1989/1990Gap 1 1.0 1.261 2–20 0.44 0.54 0.82 0.48 0.38 1.262 21–63 0.85 0.89 0.96 0.94 0.88 1.073 64–77 0.86 1.0 0.86 0.95 1.15 0.83Gap 2 0.86 0.834 82–117 0.74 0.69 1.06 0.81 1.07 0.76Gap 3 0.8 1.0WA 0.95±0.11 0.97±0.08Unattended groundnut 1989/1990Gap 1 0.67 1.051 9–22 0.36 0.54 0.67 0.40 0.38 1.052 23–62 0.67 0.89 0.75 0.74 0.90 0.823 63–82 0.74 1.0 0.74 0.81 1.15 0.70Gap 2 0.74 0.704 101–118 0.51 0.57 0.90 0.56 1.05 0.535 119–130 0.44 0.57 0.79 0.48 0.76 0.63WA 0.75±0.09 0.77±0.07

aFor Hageen sorghum, with data for Fitareeta sorghum in brackets when different

Table 3. Farm water-use efficiencies (AE t/ha per metre) of Fit-areeta sorghum (F), Hageen sorghum (H) and groundnut (G) cropsunder the attended and unattended watering methods. Qt (mm) isthe total application including rainfall, Ym and Ya (t/ha) are the

maximum and actual yields per unit area, Ky is the yield responsefactor, using KC/kc values from Table 2. Ky(56) is the same factorfor KC/kc(56) values in Table 2

Season 1988/1989 1989/1990

Irrigationmethod

Attended Unattended Attended Unattended

Crop F H G F H G F H G F H G

Ym 3.0 3.5 3.5 3.0 3.5 3.5 3.0 3.5 3.5 3.0 3.5 3.5Qt 510 570 610 560 560 650 470 580 570 650 650 750Ya 0.8 1.3 1.2 1.1 2.1 0.6 1.5 2.0 2.8 1.3 1.9 1.8AE 1.6 2.3 2.0 2.0 3.8 0.9 3.2 3.4 4.9 2.0 2.9 2.4Ya/Ym 0.3 0.4 0.3 0.4 0.6 0.2 0.5 0.6 0.8 0.4 0.5 0.5Ky 5.8 5.0 3.0 30 20 4.7 2.4 1.9 4.0 10 8.3 2.0Ky(56) 23 8.6 2.8 7.5 3.3 3.0 2.0 1.4 6.7 5.0 2.9 2.2

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application in the Gezira is not being economicallyinvested, due to the lack of other inputs, mainly nu-trients.

3. AE values of the attended groundnut were appre-ciably higher in both seasons than in the unattendedfield (ratio of about 2:1), while the applications to theunattended groundnut in both seasons were higherthan the corresponding ones to the attendedgroundnut. The reasons are the same as those givenfor yield differences above.

4. The highest AE of groundnut (attended field of 1989/1990), of which Ya is only 20% less than the potentialyield in the Gezira, shows that 570 mm is likely to be agood first approximation of the minimum require-ments of the crop under attended irrigation in theGezira farming system studied. Higher water applica-tions reduce groundnut yields. Such a first approxi-mation is also the 470 mm depth of water which gavethe highest Ya and AE of the Fitareeta dura (attendedfield of 1989/1990), that received one fewer irrigation.As indicated by the unattended Hageen dura of 1988/1989 and the attended Hageen dura of 1989/1990, thevalue for Hageen may not be far from 560 mm depth.These values are for the present field topography andother farm management conditions. These values alsostill include a too-high soil moisture value at maturity,which was estimated as 50–80 mm readily availablewater in this case. Fischer (1980), for example, has alsopointed out the existence of this waste.

Because no surface runoff occurs from the fields, theGezira free surface evaporation losses, which for thedrier year amounted to the highest values (170–230 mm)for groundnut in the attended and the unattended fields,respectively, which were 30–50% of the above men-tioned minimum requirements of the two crops, willremain the main type of non-productive water reachingthe present fields. It should be recalled that these valueswere 180–200 mm for dura in that same year and 70–130 mm (attended versus unattended for groundnut)and 70–80 mm (idem for dura) for the wetter year ofdata-taking (Ibrahim et al. 2000).

Conclusions and recommendations

The experiments reported on above strongly suggest thatirrigation water is wasted in both application methods,but at different rates and differently for each crop. Thewaste was higher in unattended irrigation of both duraand groundnut, and the waste was larger on groundnuts.It also had greater consequences because groundnutyields drop with excess water applied. Even much of theconsumptive use is economically ill invested in non-fer-tilized dura, because with higher inputs the sameamounts of water would give higher returns. The ap-plication differences were mainly due to the wateringmethods, causing different amounts of standing water,

and the methods of determining the moment of irriga-tion. Another type of non-productive water is the readilyavailable water retained in the soil profile at the end ofeach growing season.

With this last waste still included and present fieldtopography, 470, 560 and 570 mm depths are first ap-proximations of the minimum water requirements forFitareeta, Hageen and groundnut crops, respectively, formaximum yields under the present Gezira inputs andfarm management conditions and attended watering.However, yields as well as farm water-use efficiencies areseriously reduced by the fact that no fertilizers are usedon dura, apart from low amounts on the hybrid Hageenvariety, which immediately increased the yields. Farmmanagement and, even more so, periods of waterlog-ging, perhaps in combination with nutrient stresses,greatly influenced groundnut yields.

Because Sudan is approaching the limit of depletingits quota in Nile waters and the Gezira scheme consumesone-third of this share, more efficient water and farmmanagement (e.g. weeding) in the scheme is crucial forobtaining the same or somewhat higher yields with otherexternal inputs remaining at the present low level. Themost important measure in this respect would be toadopt a land-levelling programme to the practical limitspossible and to apply partly or fully attended wateringon small areas, as was recommended in the traditionalnight storage system. Minimum practical standing waterin the furrows during and immediately after each irri-gation must be targeted. Economic measures related tothe payment and prize of irrigation water should also betaken (Ibrahim et al. 1999).

Our research contained participative on-farm inves-tigations (Ibrahim et al. 1999). The water waste has beenquantified on-farm to the extent that the problem cannow not only be better understood but also better as-sessed. This belongs to the innovative contents of thispaper. Knowing precisely how large the problem is andbeing able to quantify its components will contributemuch to the arguments of those who wish to take theproposed measures.

More on-farm scientific research is recommended.This should aim at using accurate estimations of evap-oration and evapotranspiration (e.g. Allen et al. 1998;Smith 2000) and determination of on-farm maximumyields and highest water-use efficiencies under actual andimproved water and farm management by farmers in thescheme. To determine improved application efficiencieson farm, studies should cover (1) influence of multiplestresses (in time and in kind) on on-farm yields, makingit possible to distinguish influences on Ky from differentsources; (2) actual on-farm improved (maximum) yieldsunder various increases of inputs, again to make Ky

values more realistic, and (3) as far as this is possible on-farm, the influence of soil conditions on root develop-ment and soil moisture distribution and redistribution.Results presently obtained in agroforestry research andother research where root competition is crucial show

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the potential and limitations of this approach (seeUmaya et al. 1999).

Acknowledgements The authors are highly indebted to the Direc-torate General of International Cooperation research focal pro-gram (DGIS/DST/SO) of the Ministry of Foreign Affairs, TheNetherlands, which funded all necessary equipment and Dutch co-supervision for the research through the Traditional Techniques ofMicroclimate Improvement (TTMI) project. Thanks are also dueto the technical staff of the Hydraulics Research Station of theMinistry of Irrigation and the University of Gezira, Wad Medani,Sudan, and of Wageningen Agricultural University, The Nether-lands, for their technical support.

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