Sedimentation in Small Dams - 1

Sedimentation in small damsImpacts on the incomes of poor rural communities

P LawrenceN Hasnip

Report OD TN 118Rev 0.0January 2004

Sedimentation in small dams - impacts on the incomes of poor rural communities

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Document Information

Project Uptake of Tools for Mitigating Sedimentation

Report title Sedimentation in small dams – Impacts on the incomes of poor ruralcommunities

Client DFIDClient Representative Mr. M EdwardsProject No. DFID Project R7391 HR Project MDS0533Report No. OD TN 118Doc. ref. OD TN 118 - Sedimentation in small dams.docProject Manager P LawrenceProject Sponsor J Skutsch

Document HistoryDate Revision Prepared Approved Authorised Notes14/01/04 0.0

Contract

This report is an output of the Department for International Development’s (DFID) Knowledgeand Research contract R 7391, Uptake of tools for mitigating sedimentation, carried out byHR Wallingford Ltd. The HR Wallingford job No. was MDS 0533. The views expressed arenot necessarily those of DFID. The DFID KAR project details are:

Theme: W5 Improved availability of water for sustainable food productionand rural development

Project title Uptake of tools for mitigating sedimentationProject number R 7391Start date 31 August 1999End Date 31 March 2003

Prepared

Approved

Authorised

© HR Wallingford Limited

This report is a contribution to research generally and it would be imprudent for third parties to rely on it inspecific applications without first checking its suitability. Various sections of this report rely on data supplied by ordrawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client orthird parties as a result of errors or inaccuracies in such third party data. HR Wallingford will only acceptresponsibility for the use of its material in specific projects where it has been engaged to advise upon a specificcommission and given the opportunity to express a view on the reliability of the material for the particularapplications.


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Executive Summary

Sedimentation in small dams

Impacts on the incomes of poor rural communities

P LawrenceN Hasnip

Report OD TN 118January 2004

This report describes the link between sedimentation in small dams and the incomes of ruralcommunities living in semi-arid regions of Southern Africa. It is an intermediate output from aDFID KAR project R7391, “Uptake of Tools for Mitigating Sedimentation”.

A literature review (HR Wallingford (2000), attached as annex 2), highlights the strong linkbetween the availability of water for agriculture and livestock production, and incomes of therural poor. Household income data for rainfed subsistence farmers in Zimbabwe are presentedthat show average household incomes reducing by two thirds in a drought year. One means ofincreasing resilience to the shocks produced by rainfall variations, particularly droughts, is tostore water in small dams to irrigate crops and water cattle. Water from dams is used to irrigategarden crops and water cattle through the dry season, while some water is stored to provideinsurance against a failure of the following year’s rains.

The effect of sedimentation on the livelihoods of communities using small dams wasinvestigated using socio-economic data from a study of the impact of a DFID-funded small damrehabilitation programme in Zimbabwe. A wide range of benefits was identified, but by far thelargest benefits are derived from irrigated gardens growing vegetables for home consumptionand sale. For this study information obtained from communities on the benefits of a small damwas used with estimates of water yield reductions over time due to sedimentation, to estimatereductions in benefits as a dam silts up.

The table below shows the predicted loss in community benefits due to siltation for a typicalsmall dam. The losses are expressed in US $, and are the sum of the benefits lost over 20 yearsof siltation.

ScenarioPredicted loss in community benefitsover 20 years due to sedimentation

(US$)1

Typical (4.5m high) dam with no dry season rechargeand average (2% per annum) sedimentation rate

171, 000

Note 1 At the conversion rate of 1US$ = 50 Z$ adopted in CARE (2000). The accumulatedbenefit over 20 years without reductions due to siltation would have been 552, 000 US$.


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Executive Summary continued

After twenty years of siltation the water yield, and hence the annual benefit derived from thedam, is reduced to 34 % of the initial benefit. (The reduction in water yield over the period issignificantly larger than the loss in storage capacity attributable to siltation, due to the increasingproportion of water that is evaporated as a dam becomes shallower.)

For a typical dam the average annual loss in benefit per household (of those benefiting from thedam) is estimated as 82 US$. This is equivalent to 50 % of the average household cash incomes(among the dam using communities). As reliance on irrigated vegetable production representsan increasingly large proportion of household benefits as household incomes reduce, the poorestfamilies suffer the most as dams silt up. The annual monetary loss in benefits due to siltation forthe poorest dam using resource group, as identified in a wealth ranking study, exceeds theaverage annual household cash income.

These estimates are for a “typical” dam; much larger losses in benefits will occur in shallowerdams where siltation has a greater impact on water yield, and in dams with larger than averagesiltation rates.

It is concluded that a strong link is demonstrated between sedimentation in dams and reductionin livelihood benefits of the poorest sectors of the rural communities. This intuitively obviousresult justifies investment in technical studies designed to provide tools that enable futuresedimentation rates in small dams to be predicted by local engineers, using the limited data thatare available. Sedimentation is a natural and inevitable process. It needs to be consideredproperly at the site selection and design stage of dam rehabilitation and construction projects ifthe benefits of small dams are not to be rapidly lost, with potentially devastating impacts on thepoorest sections of rural communities.


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ContentsTitle page iDocument Information iiExecutive Summary iiiContents v

1. Introduction ...................................................................................................................... 1

2. Climate variability and rural communities’ perceptions of risk ....................................... 2

3. Impacts of rainfall variations on agricultural and livestock production ........................... 3

4. Links between rainfall, incomes and poverty ................................................................... 4

5. Role of irrigation and water storage ................................................................................. 5

6. Impact of sedimentation on benefits from small dams..................................................... 66.1 Monetary benefits of dams identified by Communities....................................... 66.2 Benefit Cost Analysis .......................................................................................... 66.3 Water yield reductions due to sedimentation....................................................... 76.4 Impact of sedimentation on community benefits ................................................. 7

7. Conclusion...................................................................................................................... 10

TablesTable 1 Percentage contributions of net benefits, including indirect benefits, to project

impact (from CARE 2000b)........................................................................................ 6Table 2 Water yield reductions due to siltation........................................................................ 7Table 3 Loss in community benefits due to siltation for 3 scenarios ....................................... 8Table 4 Summary statistics showing mean parameters for each of 4 resource group

categories .................................................................................................................... 8

FiguresFigure 1 Subjective assessment of risk to livelihood factors reported by rural communities

in semi- arid Tanzania (Based on data reported in Quinn (2001)).............................. 3Figure 2 Zimbabwean Maize and Sorghum yields as a function of annual rainfall .................. 4Figure 3 Household incomes as a function of rainfall – Zimbabwe.......................................... 5

AnnexesAnnex 1 Investment by CARE and communities – Small dam rehabilitation projectAnnex 2 Literature review


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1. IntroductionThis report is an intermediate output from the DFID-funded KAR project R7391“Uptake of Tools for Mitigating Sedimentation”. It describes the quantitative linkbetween sediment related reductions in the availability of water for irrigation or stockwatering, and the incomes of rural communities living in a semi-arid region of SouthernAfrica. The study was carried out as part of a component of the project which isdeveloping improved methods for planning and designing small community dams insemi-arid areas so as to reduce the impacts of excessive sedimentation on wateravailability1. The series of reports describing the outputs from the small damscomponent of the project is listed below:

Report Title ReportNumber

Guidelines for predicting and minimising sedimentation in smalldams

OD 152

Sedimentation in small dams – impacts on the incomes of poorrural communities

OD TN 118

Sedimentation in small dams – hydrology and drawdowncomputations

OD TN 119

Sedimentation in small dams – development of catchmentcharacterisation and sediment yield prediction procedures

OD TN 120

Sedimentation in small dams – the potential for catchmentconservation, check dams and sediment bypassing to reduce damsiltation rates

OD TN 121

At the start of the project a literature review was carried out on poverty and water use insemi-arid zones (in the context of small dams) (HR Wallingford (2000), attached asannex 2). The conclusions were that while there is a large body of information onpoverty, livelihoods, livestock, dams, irrigation and soil and water conservation, there isno information directly linking the impacts of sedimentation on livelihoods and poverty.However there are strong links between the availability of water for agriculture andlivestock production, and incomes of the rural poor. Reductions in the availability ofwater, due to sedimentation in irrigation canals and small dams, will have significantnegative impacts on the rural communities reliant on these resources. An attempt toquantify this intuitively obvious linkage for the case of small communal dams isdescribed in this report.

Information used to demonstrate the link between sedimentation and rural incomes isdrawn from Zimbabwe and Tanzania, the countries where sedimentation in small damswas studied by this project. Socio-economic data were obtained from a project in

1 The work reported here was carried out following a request from DFID that a “literature reviewon poverty and water use in semi-arid areas in the context of the use of small dams”, wasincluded in the work programme. (See Appendix 1 of the project inception report, HRWallingford (2000)).


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Zimbabwe which examined the impact on livelihoods of a small dam rehabilitationproject funded by DFID (reported in CARE 2000a and 2000b). Information on damsiltation rates and sediment related reductions in dry season water yields were derivedfrom the studies reported in HR Wallingford (2003a and 2003b). References cited inboth the report and the literature review are listed in annex 2.

2. Climate variability and rural communities’perceptions of riskSemi-arid regions in Southern and Eastern Africa are characterised by highly variablerainfall concentrated in one or two rainy seasons, separated by relatively long dryseasons. Ellis (1996) notes that variability in rainfall is one of the most pervasive andunalterable sources of uncertainty impinging on African pastoral and agro-pastoralsystems. The importance of this variability to the livelihoods of the poor in arid andsemi-arid regions is obvious in drought years, and access to water ranks very high incommunities’ own perceptions of livelihood risks.

This was demonstrated by a DFID funded study of risks to livelihoods, as perceived byrural communities in a semi-arid region in Tanzania. The study was carried out toidentify and understand variability in risk, in the context of projects designed to alleviatepoverty (Quinn, 2001). Rapid Rural Appraisal methods were applied as part of a surveyof twelve villages, in six districts. (Some of these villages are located in the same areasas the small dams surveyed as part of this project.)

An overall ranking of communities’ perceptions of risk to their livelihoods is developedin Quinn (2001) by combining indices of severity of risk and incidence of risk, afterresponses to open questions concerning risk factors had been grouped into twenty-onecategories. Results are shown in Figure 1, which is derived from the data reported inQuinn (2001). Risk to livelihoods associated with access to water has by far the highestranking, particularly when considered in conjunction with the high perception of riskalso associated with “weather” and “irrigation”.


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SUBJECTIVE RANKING OF RISK

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

WATER

HOSPITAL

TRANSPORT

DISEASE

HUNGER

WEATHER

IRRIGATION

PESTS

LIVESTOCK DISEASE

SCHOOL

LAND

AGRI-INPUTS

FINANCE

AGE

MARKETS

SUPPORT

FOREST

SHOPS

ELECTRICITY

THEFT

WITCHCRAFT

RISK INDEX

Figure 1 Subjective assessment of risk to livelihood factors reported by rural communitiesin semi-arid Tanzania (Based on data reported in Quinn (2001))

3. Impacts of rainfall variations on agricultural andlivestock productionIn the SADAC (South African Development and Co-operation) region about 70 % of thepopulation are subsistence farmers who are working on communal land in semi-aridzones, where dryland farming is a risky occupation due to low and erratic rainfall. Theeffect of rainfall variations on yields of some subsistence crops grown in Zimbabwe isshown in Figure 2, which illustrates the dramatic reduction in yields that occur duringdrought years. Similar relationships are observed for the production of dry matter grazedby cattle and smaller ruminates. While livestock numbers are affected by several otherfactors, periods of lower than average rainfall are strongly associated with reductions inboth livestock and small stock numbers in communal farming areas (see for exampleScoones et al. (1996)).


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4. Links between rainfall, incomes and povertyA study of incomes and poverty in the Chivi communal area of Zimbabwe (Cavendish,1999) noted that a considerable portion of poor families’ income variation over time isdue to rainfall variations, rather than reflecting any long-term trend. Supportingevidence comes from one of the few long-term studies of panel household data fromZimbabwe. Owens and Hoddinott (1998) report income data collected over four yearsfrom a random sample of 370 households located in three resettlement schemes inZimbabwe. The schemes were chosen to represent each of the major agro-ecologicalzones in Zimbabwe suited to rainfed cropping. Between 1992 and 1996, they found thatincome from crop production comprised between 70 and 79 percent of householdincomes in normal rainfall years, but dropped to 31 percent in the mini drought of 94/95.The strong correlation between annual rainfall and household incomes is shown inFigure 3.

Zimbabwe Maize Yields 1970 to 1993 as a function of mean annual rainfall

0

500

1000

1500

2000

2500

3000

0 200 400 600 800 1000 1200

Mean annual rainfall, mm

Mea

n m

aize

yie

ld k

g/ha

MaizeYield, kg/haSorghum Yield kg/ha

Figure 2 Zimbabwean Maize and Sorghum yields as a function of annual rainfall


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0

1000

2000

3000

4000

5000

6000

7000

8000

300 400 500 600 700 800 900

Mean Annual Rainfall, mm

Aver

age

inco

me,

Zim

$

Figure 3 Household incomes as a function of rainfall – Zimbabwe

5. Role of irrigation and water storageOne means of increasing communities’ resilience to the shocks produced by rainfallvariations is to store water in small dams for use in the dry season to irrigate crops andwater cattle. Some of the stored water is used in the dry season, but water is also storedin the dam in case the following year’s rains fail. Irrigation systems fed directly fromperennial and seasonal rivers also reduce the effects of local rainfall variations, andallow some crops to be grown in water short years when rainfed agriculture would beimpossible.

CARE International has carried out a large programme of small dam rehabilitation inZimbabwe with financial support from DFID and several other donors, based on thepremise that a lack of adequate water in semi-arid areas prevents many households fromdeveloping activities that can sustain their livelihoods. When water becomes available itcan be put to a wide variety of uses including irrigation, cattle watering, drinking,washing, brick making, etc. While the CARE projects were nominally rehabilitationprogrammes, in all but a few cases dam heights were increased to recover storage lostby past siltation, providing on average a 60% increase in capacity. In some cases damsand spillways have been completely re-built, essentially providing a new dam.

An impact assessment carried out for DFID (CARE, 2000b) showed that a wide range ofbenefits are reported by communities, the most important being:

• Irrigated vegetable gardens provide fresh vegetables for home consumption andsale;

• Cash is generated to start new projects;• The security of having stored water nearby.

A summary of the wide range of activities and benefits associated with the small damrehabilitation programme, copied from Care (2000b), is presented in Annex 1.


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6. Impact of sedimentation on benefits from smalldams

6.1 MONETARY BENEFITS OF DAMS IDENTIFIED BY COMMUNITIESFor this study the effect of sedimentation was quantified by considering the effect it has,in monetary terms, on the benefits identified by communities. These are listed for theGwitma dam, regarded as a typical small dam, in CARE (2000b), which also describesthe participatory methodologies used to collect the information.

Irrigated production and improved nutrition and heath provide the largest proportion ofthe benefits, while in total 80% of the benefits would be affected by reductions in wateravailability. 2 The initial (before siltation) annual benefit, including indirect benefits, isestimated at 80 % of 1.73 million Z$, i.e. 1.38 million Z$, from the informationpresented in CARE (2000b).

Table 1 Percentage contributions of net benefits, including indirect benefits, to the projectimpact (from CARE 2000b)

Component %Irrigated production 52Fishing 0Livestock watering 6Dryland farming note 1 2Individual fruit trees 13Common property 6Fodder for livestock 0Poultry 1Increased food security 5Improved nutrition and health 17Total 100

(Note 1 Excludes contribution to reducing siltation.)

6.2 BENEFIT COST ANALYSISBenefit cost analysis, reported in CARE (2000b), shows that small dam rehabilitationprojects are likely to have a high Internal Rate of Return, with a Benefit Cost ratio ofaround 2. Although the CARE analysis considered the impact of sedimentation, theresults are insensitive to assumptions about future sedimentation rates. Virtually thesame Benefit Cost ratio was obtained for with and without sedimentation scenarios. Thisis due to the discounting of future costs and benefits, which is a feature of conventionalbenefit cost analysis3. Discounting has the effect of minimising the impact of costs andbenefits occurring towards the end of the economic life of a project, and makes theresults very dependent on the discount rate that is used. It should not be applied toinvestigate sustainability issues like sedimentation in dams, as the negative effects ofsedimentation are largest at the end of a project, when they have only a small effect onbenefit cost ratio, unless an artificially low “social” discount rate is selected. For

2 Some small dam project activities, for example planting fruit trees in the dam catchment, arenot affected by sedimentation in a dam.3 A base rate of 12 % was adopted for the CARE analysis.


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example investment in soil conservation at the start of a dam project does not providesignificant benefits until the middle and end of the project life, and in spite of theobvious sustainability and environmental benefits is unattractive when analysed usingconventional benefit cost calculations. An alternative approach was adopted in thisreport to analyse the impacts of sedimentation.

6.3 WATER YIELD REDUCTIONS DUE TO SEDIMENTATIONHydrographic surveys carried out at nine CARE dams in Zimbabwe showed annualsiltation rates ranging between 1% and 3% of original dam capacities, with an averageof around 2% per annum (HR Wallingford, 2003b). Thus for a “typical” dam thevolume of water that can be stored is reduced to 60 % of the original dam storagevolume (over the twenty year economic life for dams assumed by CARE). In shallowdams the loss in water yield is significantly larger than the loss in gross storage capacity.This is due to the effects of evaporation, which account for an increasingly largerproportion of the water lost from dams as they silt up and become shallower.

The effects of evaporation in reducing water yields from shallow dams are illustrated inTable 2, which is derived from “drawdown” simulations to determine the dry seasonwater yields4 for typical CARE small dams. It shows the expected water yield as aproportion of the original yield (i.e. before siltation) for a dam silted by 40 %, and for arange of pre-silted dam spillway heights. It demonstrates the dramatic impact ofsedimentation on water yields from shallow dams that are not recharged by perennialbase flows or groundwater flows during the dry season.

Table 2 Water yield reductions due to siltation

Silted Water yield(As a proportion of the original water yield)

Silted Capacity(as a proportion of theoriginal capacity) Spillway

height = 3 mSpillway




height = 8 m

0.60 0.13 0.34 0.41 0.45 0.49

6.4 IMPACT OF SEDIMENTATION ON COMMUNITY BENEFITSMaking the reasonable assumption that the benefits derived from a small dam reduceover time in proportion to reductions in water yield, and with the information on themonetary value of benefits presented in section 6.1, it is possible to quantify the effectsof sedimentation in monetary terms.

Table 3 below shows the loss in community benefits due to siltation over twenty years,for three scenarios. The monetary losses are expressed in Z$ at constant 2000 prices,and are the sum of benefits lost over 20 years. (From section 6.1 the annual benefitwithout siltation is 1.38 Z$.)

4 Defined here as the maximum volume of water abstracted from a dam over the dry season whenthe pattern of irrigation abstractions follows crop water demand.


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Table 3 Loss in community benefits due to siltation for 3 scenarios

Scenario

Loss in communitybenefits over 20 yearsdue to sedimentation,

Z$ (2000 prices)

% loss

(benefit lost /benefitwith no

sedimentation)

Deep (8.0m) dam with some dry season rechargeand low (1 % per annum ) sedimentation rate

1.65 million 6%

Typical (4.5m) dam with average (2% perannum) sedimentation rate

8.55 million 31%

Shallow (3.0m) dam with high (3% per annum)sedimentation rate

16.84 million 61%

For the “typical” case (a 4.5 metre high dam with an average 2 % per annumsedimentation rate) the benefit lost over 20 years would be 8.55 million Z$, or 82200 Z$dollars per household for the 104 households that benefit from irrigation (CARE,2000b). The average annual loss in benefit per household over twenty years is 4110 Z$,at 2000 prices.

To place these totals in context, losses in individual household benefits can be comparedto the household cash incomes of dam using communities. Data derived from a numberof participatory wealth ranking exercises carried out in Masvingo Province and otherareas covered by the small dams project are reported in CARE (2000a). Informationdescribing the four resource groups adopted for the CARE analysis, and derived fromTable 3 in CARE (2000a), is summarised in Table 4.

Table 4 Summary statistics showing mean parameters for each of 4 resource group5

categories

RG1(n=166)

RG2(n=281)

RG3(n=192)

RG4(n=111)

% of farmers in each resourcecategory 22% 38% 25% 15%

Average household size 11.8 9.7 9.1 6.3

Average income levels (Z$ in1998) 7364 4236 3107 1956

Average income levels in 2000calculated by adjusting 1998incomes for inflation (Z$ 1

13733 7900 5794 3648

Note 1 Conversion rate based on average exchange rates for Z$ to US $ in 1998 and 2000

5 Participatory wealth ranking exercises were carried out in Masvingo Province and other areascovered by the small dams project. Seven main indicators of resource categories were identifiedby participants: livestock and implement ownership; use of crop inputs; yields achieved; type ofhomestead; education level of the head of household and sources of income. Participants in theresource ranking exercises classified themselves into one of the four categories (RG or resourcegroups) that were considered to provide sufficient differentiation (CARE, 2000a).


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The average household income for all resource groups is 8022 Zim $, at 2000 prices.Thus for an average household and a typical dam the average annual benefit lost due tosedimentation is equivalent to 50 % of the average annual household cash income. Moredetailed data reported in CARE (2000a) show that reliance on irrigated vegetableproduction represents an increasingly larger proportion of household benefits for thepoorer resource groups. The poorest families suffer the most as dams silt up, and thevolumes of water available for irrigation and cattle watering reduce. For a typical damand the poorest resource group, defined in Table 4 above, the loss in benefit due tosiltation over twenty years exceeds household cash incomes over the same period.

In dams that are significantly shallower, or have a significantly larger sediment inputthan the “typical” case considered above, in most cases there will be no useful waterabstraction and no benefit to communities by the end of a dam’s twenty-year design life.Unlike run of river irrigation systems benefits cannot in general be sustained byrehabilitation of existing dams or by constructing new dams. This is due to the limitednumber of feasible dam sites, and a limit imposed by topography on the possibility ofincreasing existing dam heights to regain lost capacity at an economic cost.


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7. ConclusionIt is concluded that a strong link is demonstrated between sedimentation in dams and thelivelihoods of the poorest sectors of the rural dam using communities. This intuitivelyobvious result justifies investment in technical studies designed to provide tools thatenable future sedimentation in small dams to be predicted using simple methods andlimited data, and the impacts of remedial measures to be quantified. Sedimentation is anatural and inevitable process. It needs to be considered properly at the site selectionstage of dam rehabilitation and construction projects, if the benefits of small dams arenot to be rapidly lost, with potentially devastating impacts on the poorest sections ofrural communities.


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Annexes


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Annex 1 Investment by CARE and communities –Small dam rehabilitation project

Both CARE and individual communities have made a number of investments inincreasing community resources (or capitals). These include investments in improvingphysical, natural, social and human capital.

Physical capitalEach dam has a number of inter-linked components requiring development ofphysical capital. This includes:� Dam rehabilitation, which involves repairing or raising the dam wall, or

raising or repairing the dam spillway.� Fencing the micro-catchment, being the area immediately surrounding each

dam.� Irrigation and garden development which includes fencing the irrigated area,

water-conveyancing and storage, toilet construction, conservation works andgully reclamation in the irrigated area.

� Construction of livestock watering points, usually below the dam, adjacent tothe irrigated area.

� Construction of shallow wells below the dam for drawing potable drinkingwater.

Natural capitalProtection initially of the geographical catchment through a process ofparticipatory catchment planning and management, which seeks to conserve andimprove the management of both individual and common property resourceswithin the catchment. This comprises:- On individual property: establishing runoff orchards, improved dryland

cropping, soil conservation works and small woodlots.- On common property, usually grazing areas: gully reclamation, silt traps

(stone or vegetative), grazing management and establishment of woodlots.

Social and human capitalTo support investments in physical and natural capital CARE has provided fundsfor environmental awareness training, facilitating farmer exchange visits andfocus group discussions, as well as training sessions with each community, whichhas increased their knowledge, technical and management skills. This has beendesigned to increase local communities' social and human capital. It has led togreater empowerment and social cohesion. It has helped to strengthen localinstitutions, specifically those which have enabled the project to proceed. Thishas included, not only dam, irrigation and agronomy or conservation committees,but others which have been established as a result of increased incomes andtraining. This has included savings clubs, credit groups, poultry clubs, sewingclubs and even socially orientated institutions such as sports clubs.

Decreased vulnerability and an increased sense of security are factors oftenmentioned by beneficiary communities as being major benefits to them.


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Financial capitalBuilding these assets requires an initial investment over a three-four year periodand thereafter on-going maintenance. Such investments have also resulted in anincrease in financial capital through increased sales, improved access to credit andother farming inputs and improved marketing.


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Annex 2 Literature review

A2.1 Introduction

This review was carried out as part of a DFID-funded KAR project R 7391 “Uptake ofTools for Mitigating Sedimentation”. The objective is to investigate the link betweensediment related reductions in the availability of water for irrigation or stock watering,and the livelihoods of the communities reliant on these activities.

One of the most pervasive, powerful and unalterable sources of uncertainty impingingon African pastoral and agro-pastoral systems is climate variability (Ellis, 1996).Scoones et al. (1996) point out that there is no better way to reduce rural vulnerabilityand ensure the viability of people’s livelihoods than to increase the productive base.Proofing the system against drought means strategic investment, which in semi-aridareas should obviously be in water management. Irrigation and the water storageprovided by small dams reduce the vulnerability of rural communities to periods ofdrought.

There is a large body of literature describing the benefits of land husbandry and soilconservation, where downstream impacts such as reduced dam sedimentation are oftenclaimed. However, no references which directly link sedimentation in small dams orirrigation networks with poverty in developing countries have been found. There aremany references, however, which link access to water with decreased poverty levels andmore secure livelihoods6. The approach taken has thus been to summarise the benefitsfrom increased access to water, and then to estimate the impact of sedimentation insmall dams and irrigation networks in reducing these benefits. Some examples and casehistories follow this. More work is required if the impacts of sedimentation on poverty,and hence the benefits of improved sediment management, are to be quantified.

A2.2 Benefits of increased access to water

It is widely recognised that water is vital for multiple and universally recognised aspectsof well-being, health, incomes, safety and freedom from drudgery (World Water Vision– Gender Mainstreaming Project, 2000). DFID point out in their consultative document“Addressing the Water Crisis” (2000), that if people have access to greater quantities ofwater they can use that water to improve their livelihood security and reduce theirvulnerability. They also state that building up secure livelihoods for poor households isthe only effective long-term way of eliminating poverty.

In semi-arid zones with erratic rainfall, access to water is directly linked to the incomesof the rural poor. An example from Zimbabwe (Chapter 2, Box 1) shows that there is astrong correlation between rainfall and income, with rainfall variation resulting insubstantial variation in household real incomes. The direct linkage between rainfallvariations, stock numbers and incomes of pastoralists in the semi-arid regions of sub-Saharan Africa is also well researched (see for example Scoones (1995) and Scoones etal. (1996)). Cattle provide a variety of functions in the communal areas, ranging fromdraught power for ploughing and transport, to milk and manure production, to capital

6 IDS’s definition of livelihood is: A livelihood comprises the capabilities, assets (including bothmaterial and social resources) and activities required for a means of living. A livelihood issustainable when it can cope with and recover from stresses and shocks, maintain or enhance itscapabilities and assets, while not undermining the natural resource base.


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growth or sale. A number of researchers have made calculations of the value of thesefunctions (Dankwerts, 1974; Barrett, 1992; Scoones, 1992) and they conclude that thevalue of cattle in the communal lands is high.

Small dams

The water collected in small dams has many uses in rural areas, for example:• Livestock;• Irrigation (cash and food crop cultivation);• Domestic use;• Home gardens, trees and other permanent vegetation;• Fish;• Other enterprises such as brick making and pottery.

(CARE, 2000; DFID, 2000; World Water Vision, 2000)

Thus, an increase in available water, through the provision and maintenance of smalldams, can have a positive impact on rural households, the rural economy and theenvironment in a number of ways, including:

• Improved food security at all levels (FAO, 1996; Pinstrup-Anderson and Pandya-Lorch, 1999);

• Improved health and nutrition (Barker and van Koppen, 1999);• Increased cash incomes resulting from the sale of produce (Cavendish, 1999);• Time saving – fewer trips to water collection points for domestic water and

livestock water (usually a task undertaken by women) (Rathgeber, 1996; CARE,2000);

• Reduced risk due to the ability of a community to withstand shocks such as drought(von Braun et al., 1999; World Water Vision, 2000);

• Increase in livestock numbers (Sweet, 1988);• Groundwater recharge.

Irrigation

There is substantial evidence that irrigation reduces poverty (Chapter 2, Box 2). TheFAO (1996) states that, in developing countries, irrigation can increase yields for mostcrops by 100 to 400%, whilst also allowing farmers to reap the economic benefits ofgrowing higher-value cash crops. Shah (1993) points out that the artificial supply ofwater brings a range of benefits to individuals and households, which can bedistinguished as ‘primary’ and ‘spillover’ benefits.

Primary Benefits• Increased intensity of cropping.• Improved yields.• Increased and more stable flow of income from farming.• A more secure supply of fodder for livestock.

Spillover Benefits• Increased and more evenly spread farm labour opportunities.• Reduced out-migration.• Improved security against impoverishment.• More water for non-agricultural uses including domestic uses that improve health.


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• Ensure some production output even in drought years.• Lower food prices and better nutrition throughout the year.

A2.3 Impacts of Sedimentation

Sediment runoff from catchments in semi-arid zones in Africa, south of the Sahara,range from a few hundred to several thousand tons/km2/year, with large inter-annualvariations between wet and drought periods, corresponding to wet and dry years(Walling, 1983 and 1988). Sediment concentrations carried by rivers show widevariations, but typically range between a few hundred to several thousand ppm byweight. For example, data for the rivers draining to the south and east of Zimbabweindicated mean sediment concentrations ranging between 1000ppm and 10,000ppm(Bake, 1986).

Virtually all the sediment in the water entering a dam is trapped, and as dams are oftendesigned with capacities that are much smaller than the annual runoff (to ensure thatthey fill in drier than average years) sedimentation rates in small dams can be veryrapid. For example, Elwell (1985) reports that over 50% of the 132 dams surveyed inMasvingo Province in Zimbabwe were severely silted. The Takavarasha dam in Chiviwas completed in 1985 and, according to local information, lost 100% of its storagecapacity over the next two years (Hart-Frost, 1999). However, excessive siltation is notinevitable, and some of the older dams in the Masvingo region have functioned formany decades. If the benefits of small dams are to be sustained, they must be located,sized, and operated so that excessive sedimentation is avoided. (CARE Zimbabweassumes a twenty-year economic life for the dams included in their small damrehabilitation programme – which seems a reasonable minimum requirement).

The sediment transporting capacity of irrigation channels is usually limited to a fewhundred ppm of bed material sediment. Canals silt up when larger sedimentconcentrations are diverted from rivers. This not only imposes an excessivemaintenance burden on farmers or scheme operators, but the reduced conveyancecapacities reduce the areas that can be irrigated. Specific data on desilting inputs orsediment related irrigated area reductions are not widely available outside the records inAgency-managed schemes. Information collected in the Philippines by HRWallingford2, showed that, in spite of substantial expenditure on canal desilting, theareas actually irrigated in sediment affected systems ranged between 60% and 75% ofthe original design area. This indicates the scale of sediment related reductions on theavailability of water.

2 Where river sediment loads are of the same order to those in parts of sub-Saharan Africa.


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A2.4 Case Studies and examples

Income Levels

The example below cites the correlation between rainfall and income, but can also beapplied to available water in dams and income levels. If the capacity of a dam decreasesdue to sedimentation, then the water available for production will therefore also fall,thus having a similar impact on production and incomes as drought.

Water collection, irrigation and agricultural production

The World Water Council (2000) stated in its World Water Vision that it is imperativethat ways are found to develop water supplies – that is, store water for later use, withlower economic, social and environmental costs. These could be small dams,groundwater recharge and traditional small-scale water storage techniques and rainwaterharvesting.

The FAO (1996) found that collecting runoff and using it to irrigate crops, pastures andtrees significantly improves both yields and the reliability of agricultural production.Experience from Burkino Faso, the Sudan and Kenya shows that rain harvested fromone hectare, for supplementary irrigation of another, can triple or even quadrupleproduction.

Box 1A study of Incomes and Poverty in the Chivi communal area of Zimbabwe(Cavendish, 1999) noted that a considerable portion of income variation over time isdue to rainfall variations, rather than reflecting any long-term trend. Supportingevidence comes from one of the few long-term studies of panel household data fromZimbabwe. Looking at the years 1992/93 to 1995/96, they found that income fromcrop production comprised between 70 and 79 percent of household incomes innormal rainfall years but dropped to 31 percent in the mini drought of 94/95. Duringthe same period, average real per capita incomes ranged between Z$800 and Z$1,000in normal rainfall years, and fell to approximately Z$500 in 94/95, leading to theconclusion that rainfall variation results in substantial variation in household realincomes. There is a strong correlation between rainfall and income.


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Many countries, when investigating the causes of poverty, have found that agriculture(in particular, a better supply of irrigation water) is a key strategy for reducing poverty.

Box 2Studies in India in 1988 demonstrated the inverse relationship between the incidenceof poverty and the extent of irrigation development. For districts where less than10% of gross cropped area was irrigated, 69% of the population had incomes belowthe poverty line, while in districts where irrigation covered more than 50% of the croparea, the poverty incidence was only 26% (World Bank, 1991).

McCully (1999) points out that small villages in India are showing how small dams,built by the people and for the people with the help of non-governmentalorganisations, have helped to improve the lives of all those living and farming in thesurrounding areas.

The Matam Agricultural Development Project in Senegal restored and built villageirrigation systems and provided watering points for pastoralists. The projectincreased food security. Rice production in the restored areas increased from 2-3 to5.5 tonnes per hectare (IFAD, 1998).

In the 1970s, two highly ecologically degraded and economically destitute villages inIndia – Realegan Siddhi in Maharashtra and Sukhomarjri in Haryana, startedrainwater harvesting. With more water available, these villages slowly improved andstabilised their agricultural and animal husbandry outputs and are today foodexporters rather than food importers (World Water Council, 2000).

Whiteside (1997) highlights that small dams providing water for human consumption,small stock and small-scale fruit and vegetable production have the potential toencourage more sustainable smallholder agriculture in Botswana.


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Water and Livestock

Livestock are central to the livelihoods of the rural poor in developing countries in atleast six ways (Livestock in Development, 1998):

1) They are an important source of cash income;2) They are one of the few assets available to the poor, especially poor women;3) Livestock manure and draught power are vital for the preservation of soil fertility

and the sustainable intensification of farming systems in many developing areasfacing increasing population density;

4) Livestock allow the poor to exploit common property resources, such as opengrazing areas, in order to earn income;

5) Livestock products enable farmers to diversify incomes, helping to reduce incomevariability, especially in semi-arid systems characterised by one cropping season peryear;

6) Livestock provide a vital and often the only source of income for the poorest andmost marginal of the rural poor, such as pastoralists, sharecroppers and widows.

However, livestock productivity is constrained by water stress. McDonald et al. (1990)state that calves require around 9 litres of water per day during their faster growingperiods and sheep around 5 litres per day. During major periods of water stress(drought) it is the poorest households who lose the largest share of their herds due totheir inability to secure adequate water and feed (von Braun et al., 1999).

Fluctuations in livestock numbers are likely to be similar to the ones described in theexample below, if access to water from small dams is diminished due to sedimentation.Health, incomes and livelihoods in general will therefore be affected.

Box 3In Mozambique, where about 70% of Mozambicans live in absolute poverty, aNational Household living standards survey was carried out and reported on byIFPRI (International Food Policy Research Institute), the Ministry of Planning andFinance, and Eduardo Mondlane University in Maputo. The study highlighted sixelements of a prospective poverty alleviation strategy for Mozambique, whichincluded as one of its points – raising agricultural productivity as well as improvingrural infrastructure. The report highlights that the relatively high levels of povertyin the agricultural sector reflect currently low levels of production in that sector. Theresults also indicate that increasing the size of land held by small landholders will notreduce poverty unless productivity-improving investments are made in irrigation(Datt and Jolliffe, 1999).

Similarly in Egypt, a series of studies undertaken by IFPRI in conjunction with theEgyptian Ministry of Agriculture and Land Reclamation (MALR) and the Ministryof Trade and Supply (MOTS), provides a profile of poverty and examines thecomplex interaction among poverty-related variables. This report also concludes thatbetter supply of irrigation has a positive effect on livelihoods and living standards(Datt et al., 2000).


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Women, domestic water and health

In many countries, water scarcity represents a critical constraint to food production anda major cause of poverty and hunger. Women are often the major suppliers ofhousehold subsistence. When their access to productive resources (such as water)declines, more people suffer from poverty and its related effects, including hunger,malnutrition and illness. Improving women’s access to resources and services increasesfarm productivity, provides a more efficient use of resources, and ultimately yieldshigher profitability, as the time spent fetching water would be decreased.

Box 4Sweet (1998) presents a case study from Namibia entitled: Livestock – coping withdrought. He found that the numbers of cattle and small stock fluctuate considerablyin response to high and low rainfall years. It was also found that mortality rates weregreater in the communal areas than the commercial areas. This is because thecommercial farmers have higher cash reserves to buy water and feed. During the lowrainfall period in 1992, communal area households lost approximately a quarter oftheir average monthly incomes due to crop losses, livestock mortality and reducedemployment opportunities (Deveraux et al., 1993).

A study on drought contingency planning to support pastoralist livelihoods in Ethiopiafound that a lack of water initially leads to a lack of feed for livestock (UNDP, 1997).Under-nourishment then results in a decline in milk yields, an increase in disease anddeath of the more vulnerable animals. Livestock prices then fall, subsequentlyresulting in further poverty and more deaths. FAO (2000) state that the most negativeimpact of cattle mortality and poor body condition (lack of water) is the lack of milkfor human consumption, particularly for children. When animals die, it leads tosevere food security problems in communities dependent on livestock. Livestockproduction is crucial for the survival and well-being of pastoral communities.Livestock are less resistant to disease after being weakened by the lack of water andfeed (FAO, 2000).

A study on livelihoods and food security in Ethiopia’s Somali Region, showed thatincreasing investment in burkas (cement-lined water catchments) have allowedpreviously nomadic households to settle permanently, maintain herds and even engagein agriculture in areas that had previously been too dry for these purposes. Thesechanges also increased the herders’ incomes (USAID, 1998).

In Egypt in the mid-1970’s, Fitch and Soliman (1983) found that an average of 63%of the income of landless or near landless households came from livestock. Only 14%of the income of large landowners came from livestock.

Von Braun and Pandya-lorch (1991) identify four countries where the malnourishedobtain a greater part of their incomes from livestock than those who are notmalnourished.


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Water scarcity has also been linked to a decline in water quality, which has an especiallyadverse impact on the poor. Many of the poorest people in developing countries, wherewater is scarce, are forced to drink water that is unfit for human consumption, oftenleading to a range of skin and internal diseases and health problems (Barker and vanKoppen, 1999).

A2.5 Conclusions

The review has shown that while there is a large body of information on poverty,sustainable livelihoods, livestock and dams, small-scale irrigation and soil and waterconservation, there is very little specific information directly linking the impacts ofsedimentation on livelihoods and poverty. However there are strong links between theavailability of water and incomes of the poor. Reductions in the availability of waterdue to sedimentation in irrigation channels or small dams thus have significant negativeimpacts on communities reliant on these resources for their livelihoods.

The review has illustrated these linkages but has not quantified them. This task wouldrequire a substantial study, requiring additional support if it is to be carried out withinthe existing KAR project. An alternative would be to use the information to becollected on the benefits arising from the CARE small dam rehabilitation programme inZimbabwe to develop a specific case study quantifying the links between sedimentationin dams and reduced benefits to the poor.

Box 5Coppock (1994) reports that in the Sidamo regions of Ethopia, women spend around12% of their waking time fetching water to meet household needs. Their source isnormally a well or spring that lies an average of 14km from home. Nearer,alternative sources of water would therefore have a large impact on the livelihoodsof these women.

Von Wijk-Sijbesa (1985) states that water collection is not only energy consuming,but may also have detrimental consequences. Carrying heavy water pots, forinstance, is mentioned as a primary cause of pelvic distortion, which in turn maylead to death in child-birth.

Seager and Olson (1986) highlighted that it is women who collect, cook with andwash family and home using local water. If the water store is far away, unclean, orin short supply, it is primarily women who suffer from the resulting fatigue anddisease. And it is the women who are held responsible for the poor health of theirfamilies when polluted water and inadequate sanitation make the practice of goodhygiene either difficult or well-nigh impossible.

Investment in water supplies for villages improves health and nutrition in thecommunity as a whole. A secure water supply improves the lives of women andchildren who usually bear the brunt of collecting and carrying water. It creates newopportunities for income, such as rearing and fattening small ruminants, growingvegetables and small-scale food processing (IFAD, 1998).


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Sedimentation in Small Dams - 1

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