The availability of water in sub-Saharan Africa varies dramaticallybetween the high-rainfall central tropi-cal zone to the semi-arid zones to thenorth and south, and this divergencehas increased as a result of recentclimatic variations. Similarly the needfor irrigation also varies from zone tozone, but some areas cannot be devel-oped unless further water resourcesbecome available to them. Because ofthe difficulty and expense of collectingflow data in remote areas, someschemes can, and have, been based onflow records that cover too short aperiod to deal adequately with variabil-ity in flow volumes. As a result theyare vulnerable to low flows that couldnot have been predicted from therecords. In these circumstances justone crop failure is enough to putfarmers into a difficult economic situ-ation, limiting their ability to purchasesufficient inputs for subsequent crops.

stances. Any proposed irrigation devel-opment needs to be examined firstagainst more conventional rain-fedagriculture. Since the further expan-sion of farming is becoming moredifficult as suitable areas becomeharder to find - particularly aroundpopulation centres - and as interna-tional funding for projects that involvethe clearance of natural vegetation isless easily secured, the intensificationof farming existing agricultural areasis a good, and for some countries theonly, way to increase production. h isin this context that the need forirrigation has to be judged.

The question of how agricultural out-put in African countries can be in-creased has become more importantas populations have grown and re-sources have dwindled. The recentseries of droughts that has affectedcertain regions has focused the world'sattention on short-term problems, suchas crop failure and insufficient foodreserves. The challenge facing plannersis a long-term problem: how to matchfood supply with projected require-ments. Irrigation is often promoted asa means of meeting these requirements,but it is not the only choice, and it canbe inappropriate in many circum-

The role of small irrigation schemes inAfricaby Geoff PearceResearch from small irrigation schemes inZimbabwe and Kenya has identified performanceparameters which have resulted in the MIDASdesign software package.

A data-logger is installed to measure the water flow at a division box at the Mathina irrigation scheme in Kenya.

WATERLINES VOL.12 NO.1 JULY 1993 19

A flume is built to measure the flow in an unlined distribution channel in the Mutunyi irrigation scheme.

Although there is a large amount ofland suitable for irrigation develop-ment in Africa, the water resourcesavailable are generally less than thosein other regions of the world, and arevery unevenly distributed.' The highevaporation rates in Africa cause fur-ther problems. In drought-prone re-gions, where irrigation could providelarge benefits, the flows in the majorrivers are very seasonal. Any benefitsfrom irrigation are thus counterbal-anced by the high cost of buildingbarrages and dams to regulate the flow,and the risk of them having short lifespans because of sedimentation prob-lems. To illustrate the problem ofuneven distribution, the Zaire basin has55 per cent of the mean annual waterdischarge of sub-Saharan Africa, butcovers only 10 per cent of the totalarea. It has plenty of water, but thefarmers do not require widespreadirrigation, since there is sufficientprecipitation for them to practise rain-fed agriculture.'

To complete the comparison be-tween irrigated and rain-fed agricul-ture, recent figures show that 45 percent of Africa's land area is too dry forrain-fed crops, 8 per cent is too

variable, and 16 per cent is too wet.Only 30 per cent is climatically well-suited for rain-fed staple crops. Thequestions for planners are how muchof the rest is both suitable for irrigationand has sufficient water with which tocarry it out. There is evidence that thesoils suitable for irrigation tend to bein rain-fed agriculture areas, I but cur-rent estimates indicate that between30 and 140 million hectares could bedeveloped for irrigation; this is fromthree to fifteen times the area currentlyirrigated.

Other limitations constraining theuse of irrigation are the:o lack of capital;o lack of trained people;o inappropriate economic analysis for

assessing viability;o high construction costs because of

the lack of infrastructure (such asroads);

o the need for expensive water storagestructures;

o the remoteness of suitable sites; ando inadequate data about sites and

water sources.None of these constraints are serious

for small irrigation though.

Small irrigation schemesIn choosing to develop irrigation in anyarea, it is important to consider whattype of irrigation is most suitable. Thechoices of whether the projects shouldbe large or small, and whether it isbetter to rehabilitate existing schemesthat are no longer fully functional orto create new ones, depend verycritically on each specific situation.As pointed out by FAa Paper 42,1however, experience in Africa so farstrongly supports the value of small-scale irrigation and the importance ofrehabilitating older schemes.

Small-scale irrigation makes upabout 33 per cent of Africa's presentirrigated area. The remaining 67 percent is large formal schemes, thoughalmost half of this is the Nile irrigationsystems in Egypt and Sudan. In otherareas of Africa the performance ofmany large-scale schemes has beendisappointing, largely because of cum-bersome administration and high op-eration costs. The difficulties involvedwere not so much a function of schemesize, however, as a gap between theintentions of the designer and theaspirations of the farmers involved.

20 WATERLINES VOL. 12 NO.1 mLY 1993

An example of a basin irrigation scheme, a type which is common in Kenya.

There has been a marked increasein interest in recent years in thealternative strategy, developing small-scale irrigation. These systems consistmainly of the use of traditional meth-ods, such as simple river diversions,water lifting from fadamas (shallowgroundwater, in Nigeria), and pumpingfrom lakes. As far as investment indevelopment is concerned, smallschemes are much favoured by plan-ners, and this development is sup-ported by the self-help approach pro-moted by NGOs. The preference forsmall schemes has emerged becausethey are perceived to have a goodrecord of success and sustainability,and because they can be adapted easilyto suit local conditions. They alsorequire relatively low capital invest-ment and can be constructed in areaswhere the tracts of irrigable land andthe amount of water available aresmall.

From a designer's point of view,there are constraints to the successfuldevelopment of new formal smallirrigation schemes. Information aboutthe performance of small schemes israre, as few small schemes have beenmonitored technically or have had theirperformance analysed. There is there-fore little information from existingsmall schemes for irrigation designersto use in the planning of new schemes.Further, there has been a shortage ofreference works specifically for thedesigner of a small irrigation schemein Africa, someone who has to carryout, on his own, a wide range ofactivities which, on a larger scheme,would be performed by a multi-disciplinary team.

Recent work addressing these con-straints has focused on assessing theperformance of a number of smallirrigation schemes in Zimbabwe andKenya. This has included both physicaland engineering assessments as wellas socio-economic analysis. Anothermajor focus has been the developmentof appropriate and practical computer-aided design procedures for use bydesign engineers in African irrigationdepartments. This work has been car-ried out by the Overseas DevelopmentUnit of HR Wallingford, with fundingprovided by the British Overseas De-velopment Administration. All thework involved has been carried outwith the full support and participationof the Zimbabwean Agritex Depart-ment and "the Irrigation and DrainageBranch of the Kenyan Ministry ofAgriculture. The following case stud-ies describe some of the main pointsthat have come out of the work.

Nyanyadzi, Eastern Highlands,ZimbabweIrrigation efficiencies below the nightstorage dam on this 414ha, run-of-riverscheme were found to be 40 per cent,but varied from year-to-year and atdifferent places within the scheme.Irrigation was by siphon, usually intoborder strips. Top-end farmers re-ceived twice as much water as thoseat the lower end, despite a formalsystem for distributing water that wasestablished by the scheme's manage-ment. This created serious shortagesat the bottom end of the scheme, whichwere very damaging for the farmersthere. Most of the losses were relatedto water management practice at andbelow the field distribution channel,such as the tardy readjustment of theflow after a farmer had irrigated hisfield. If top-end farmers could reducetheir losses, the savings passed onwould enable bottom-end farmers toreceive enough water for their needs.The run-of-river nature of the schemehas made it very vulnerable duringtimes of water shortage to abstractionsfrom the river by other irrigationschemes upstream. Another major con-straint on performance was the verylarge seepage loss from the canal thatcame from the river offtake, but thecosts of lining the canal and ofrehabilitating its offtake structurewould be an expensive undertaking.

Exchange, Midlands, ZimbabweIrrigation efficiencies on this remote,dam-backed, 165ha scheme appeared,

in contrast, to be much higher. The useof siphon irrigation into furrows meantthat farmers were necessarily veryefficient irrigators, especially sincetheir holdings were small and irrigationrotation was practised in a tight se-quential rota that was, as far as thefarmers were concerned, reliable. Thesocio-economic situation of the farm-ers reflected the difficulty of growingand selling cash crops in such a remotearea. The holdings allocated to themwere small, only 0.1 ha, and wereintended to provide support during dryyears to farmers with larger drylandholdings elsewhere. Although watersupplies were matching demand, andcrop yields were generally very high,the scheme was beginning to haveproblems with rising water tables.

Gem Rai, Nyanza, KenyaThis small, rice scheme near LakeVictoria was found to be an efficientuser of water, reflecting the fact thatwater flowed from field to field. Themajor problem faced by the localfarmers was the need to carry out amajor desilting of the scheme's chan-nel network before the fields areprepared. The situation had recentlygot worse and is a result of soil erosionproblems in the upper catchment of thesmall river supplying the scheme.

Kwa Chai, Machokos, KenyaIrrigation using small basins, typicalin Kenya, is practised on this smallscheme which receives water from anearby spring and grows vegetables

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A furrow infiltration test being carried out at the Exchange irrigation scheme,Zimbabwe.

throughout the year. The scheme func-tioned relatively well at the top end,but the supply of water along the singledistributary channel was insufficientto support the farmer's crops at thebottom end. Preparations are beingmade to provide more water to thelower area by means of a secondchannel.

Kamleza, Taveta, KenyaThis scheme was dependent on adja-cent commercial schemes not to dis-rupt supply from the shared spring.Difficulties in distributing the water tofanners were caused by complicatedfield channel layouts. Water bailiffshelped to run a rota, but the schemewas hampered by a lack of resources,and was in need of rehabilitation. Otherschemes close-by had experiencedhigh water-tables, and although therewas no evidence that this was aboutto happen, it was of concern to allfarmers there.

New study areasIn May 1993 performance assessmentstudies were carried out at four irriga-tion schemes in Kenya's central prov-ince. These were at Mathina (smallbasin irrigation), Kangocho (small ba-sins), Mutunyi (small basins), andKiguru (piped distribution with sprin-klers). These will be monitored for atleast one year like the previous studies,with data being collected on watersupply, water distribution within thescheme, and other physical factors, andalso for socio-economic situation.

Schistosomiasis control study,Mushandike, ZimbabweThe aim of this study has been todevelop and test practical methods ofcontrolling the transmission of thewater-related disease schistosomiasisin smallholder irrigation schemes.Measures to inhibit aquatic snail popu-lations, the vector for the disease, weredesigned and built into four parts of

the Mushandike Irrigation Scheme.Detailed monitoring of both the snailpopulation and the prevalence of thedisease in the local human populationhave been carried out. The mainmeasures used are structures and pro-cedures that ensure that the irrigationblocks are completely drained anddried out on a regular basis, therebydenying the snails a consistent habitat.

MIDAS Design AidSoftwareThe MIDAS design package has beendeveloped to help local irrigationdepartments in the design and develop-ment of small irrigation schemes. Thepackage has been adapted for condi-tions in Zimbabwe and more recentlyin Kenya, where it is being used bygovernment engineers.

MIDAS includes a digital groundmodel, DGM, designed by LM Techni-cal, to process raw survey data andproduce a contoured representation ofthe ground surface. The results arepassed to AutoCAD, where alternativelayouts are prepared, and detaileddesign processes are carried out usingstandard structures and simple rulesderived from the field studies. Thisenables the consequences of alternativeoperation patterns on the system to beinvestigated, and is able to produceworking drawings.

HR Wallingford's project has so farused in-depth research from two smallschemes in Zimbabwe, and more re-cently from seven small schemes inKenya, to identify scheme perform-ance parameters. These results havebeen published in a series of reports2•3.4so that the information is widelyavailable, and have been fed back intothe MIDAS design software. TheMIDAS design software package hasbeen the main outcome of this work,and a design manual for conditions inZimbabwe has been produced .•

References1. FAO, 'Consultation on irrigation in Africa',

FAO Irrigation and Drainage Paper 42.Rome. 1987.

2. Pearce, G.R., 'Small Irrigation SchemeDesign: Nyanyadzi, Zimbabwe', Report0098. HR Wallingford, UK, 1990.

3. Tiffen, M., 'Socio-economic parameters indesigning small irrigation schemes for small-scale farmers: Nyanyadzi case study', Re-ports 00 114, 115, 116, 117. HRWallingford, UK, 1990.

4. Chancellor, F., 'Socio-economic parametersin designing small irrigation schemes forsmall-scale farmers: The Exchange casestudy', Report 00 121. HR Wallingford,UK,1990.

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