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Review

A review of in situ rainwater harvesting (RWH) practices modifying landscapefunctions in African drylands

Katrin Vohland a,*, Boubacar Barry b

a Potsdam Institute for Climate Impact Research (PIK), Telegraphenberg A 62, D-14473 Potsdam, Germanyb International Water Management Institute (IWMI), West Africa Office, PMB CT 112, Cantonments, Accra, Ghana

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

1.1. Vulnerability of African drylands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

1.2. Climate change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

1.3. Landscape functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

1.4. Expectations from RWH practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

2. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

3. Results and observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.1. What is assigned to in situ rainwater harvesting practices? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.2. How landscape functions in semi-arid areas are modified. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.2.1. Hydrological functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

3.3. Soil fertility and biomass production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

3.3.1. Soil fertility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

3.3.2. Crop yields and biomass production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

3.3.3. Biodiversity conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

3.3.4. Sustainable livelihoods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

4.1. How do in situ RWH practices affect resilience of dryland ecosystems? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Agriculture, Ecosystems and Environment 131 (2009) 119–127

A R T I C L E I N F O

Article history:

Received 22 September 2008

Received in revised form 13 January 2009

Accepted 20 January 2009

Available online 20 February 2009

Keywords:

Ecosystem service

Climate change

Biodiversity

Soil and water conservation

Downstream effects

Ecohydrology

A B S T R A C T

In situ rainwater harvesting (RWH) belong to the promising practices to support sustainable

development in sub-Saharan Africa facing climate change impacts. However, appropriate indicators

for their long-term sustainability are missing. Here, impacts for different aspects of sustainability are

reviewed: in situ RWH practices improve hydrological indicators such as infiltration and groundwater

recharge. Soil nutrients are enriched. Biomass production increases, with subsequent higher yields.

Higher biomass supports a higher number of plants and animals, although native species might be

replaced by crops as the landscape might change as a whole. This might strengthen conflicts between a

nomadic and a sedentary population. Farmers applying in situ RWH practices profit from higher food

security and higher income. However, some aspects are only poorly covered within the scientific

literature. More integrative research concepts are needed.

� 2009 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +49 331 288 2518; fax: +49 331 288 2695.

E-mail addresses: [email protected] (K. Vohland), [email protected] (B. Barry).

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

journal homepage: www.e lsev ier .com/ locate /agee

0167-8809/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2009.01.010


1. Introduction

1.1. Vulnerability of African drylands

Drylands, defined as areas with low and irregular precipitation,cover 41% of the Earth’s terrestrial surface, of which 10–20% of thisland type are already degraded due to overuse (MillenniumEcosystem Assessment, 2005). Estimates show that the two billionof the world’s inhabitants of drylands have the lowest incomes andhighest infant mortality of all global population groups.

The arid and semi-arid zones (SARZ) of sub-Saharan Africa (SSA)cover about 41% of SSA and are characterized by low, erraticrainfall (300–600 mm per annum), and infertile deplenished soils(Sanchez, 2002). In the semi-arid zone of sub-Saharan Africa,especially the rural savannah zone, poverty and food insecurity areinterlinked and widespread and are strongly linked to the naturalresource endowment (water, soils and vegetation). Land and waterdegradation, overgrazing, and slash and burn agricultural produc-tion practices have led to significant environmental degradationand food shortages. The capacity of most African countries tomanage climate change is limited, due to widespread poverty,recurring droughts, inequitable land distribution, the dependenceon rain-fed agriculture and other factors which are described in theIPCC 2001 (McCarthy et al., 2001). The combination of highexposure and low adaptive capacity makes Africa’s ecosystemsvulnerable to loss of ecosystem functions and services such asbiomass production. In comparison to all other regions of theworld, the agricultural productivity per unit of water (‘‘crop perdrop’’) in the semi-arid zones is the lowest worldwide (Rockstromet al., 2004). The increasing vulnerability of societies andecosystems leads to a downward spiral of ecological and socialdegradation and consequently to an increase in disasters.

1.2. Climate change

African drylands are affected by climate change. In the IPCC, itwas confirmed that most published climate change scenariosindicate temperature increases for most of Africa, while expectedrainfall trends vary (Christensen et al., 2007). There is a generalconsensus that climatic variability will increase, leading to anincrease in droughts and floods and to growing uncertainty aboutthe onset of the rainy season. Climate change thus affects thehydrological cycle, water resources, agriculture and ecosystemperformance and services. This resource degradation enhancesproneness to conflicts, and makes Africa even more vulnerable(Nyong, 2005a).

A simulation of cropping boundaries for the year 2050 impliesthat large areas at the margins of current arable lands will nolonger be suitable for cropping (Thornton et al., 2002). Since about65% of sub-Saharan Africa’s population live in rural areas and aremostly dependent on rain-fed agriculture (World Bank, 2000), theneed to improve food security and livelihoods becomes a matter ofutmost importance and urgency. The current level of dependencyon irrigated land is very low (less than 2% of the cultivable land),therefore rain-fed agriculture increasingly plays central role insustaining rural livelihoods and meeting food requirements. Thechallenge in this region is to optimize crop production per drop of

rain.

1.3. Landscape functions

According to the Millennium Ecosystem Assessment (2005),landscape functions comprise ecosystem functions, and theprovision of goods and services. Ecosystem functions such asphotosynthesis, litter turnover or water cycling are naturalprocesses inherent to ecosystems. Ecosystem goods such as crop

biomass or clean water can be derived from ecosystems. Ecosystemservices denote processes such as purifying water or maintainingsoil fertility and biodiversity. In dryland ecosystems water is themajor limiting factor in agricultural production systems, and theperformance of landscape functions relies heavily on the avail-ability of water.

1.4. Expectations from RWH practices

In the context of agricultural production in African drylands,soil and water conservation (SWC) practices such as rainwaterharvesting (RWH) provide an opportunity to stabilize agriculturallandscapes in semiarid regions and to make them more productiveand more resilient towards climate change (Wallace, 2000; Lal,2001). Stabilization of the agricultural landscape includes therestoration of degraded cultivated and/or natural grazing lands.There are many marginal water sources that could be used moreefficiently such as road and land runoffs that are normally lostthrough erosion processes (Prinz and Malik, 2002).

Among the most common soil and water conservationtechniques, rainwater harvesting is massively promoted byNGOs, national agricultural extension services and governmentagencies in African countries (Stroosnijder, 2003), as well as inIndia (Batchelor et al., 2002) where RWH practices already have along tradition (Pandey et al., 2003). Rainwater harvesting is alsoone of the practices recommended by UNCCD to combatdesertification.

RWH practices are generally considered to be only beneficialin this respect but the main problems are low rates of adoption(e.g. Tabor, 1995; Nji and Fonteh, 2002; Bodnar and de Graaff,2003; Woyessa et al., 2005) or failed adoption processes due toinsufficient participation by farmers (Aberra, 2004). Never-theless, some experts warn about the unreflected and inap-propriate use of RWH which might lead to severe side effects asshown for erosive events in Kenya (Ngigi, 2003a), competitionbetween neighbours, or unreliable drinking water supply forparts of the community in India (Batchelor et al., 2002). In thesecases, RWH practices do not fulfil all the landscape functionsdescribed above.

The overall aim of this paper is to present a general overview ofdifferent, partly contradictionary effects of small scale, the so-called, in situ rain water harvesting practices. Recognition of thetrade-offs between different landscape functions might supportthe implementation of measures that should increase resilienceagainst climate change impacts.

2. Methodology

An extensive literature survey was performed using standardliterature on rain water harvesting as well as key words relatedsearch in ISI Web databases as well as free search in the internet.The objective of the literature search was to identify, quantify andanalyze the impact of RWH on different ecosystem servicesranging from hydrology functions such as groundwater rechargeand maintenance of aquatic and wetland ecosystems, nutrientcycling, biomass production, and biodiversity conservation tofood security, water availability and income generation. The focuswas on landscape scale, because landscapes are increasinglyrecognized as useful entities for the investigation of thecombination of ecological and spatial processes (Wagner andFortin, 2005).

To assess the relationships between precipitation and biomass,information and data obtained were divided into practices withand without applying in situ RWH practices. The results, expressedas grain harvest against mean precipitation, were plotted, and theresults of a nonlinear regression, second order are shown.

K. Vohland, B. Barry / Agriculture, Ecosystems and Environment 131 (2009) 119–127120


3. Results and observations

3.1. What is assigned to in situ rainwater harvesting practices?

Semi-arid areas in Africa face climatic variability at differenttemporal scales. High natural inter-annual climatic variability isexpressed as droughts and floods; a high seasonal variability leadsto dry spells (Usman and Reason, 2004). Farmers in the semi-aridzones have therefore developed strategies, including RWH, to copewith this uncertain and erratic rainfall patterns. RWH practicesrefer to all practices whereby rainwater is collected artificially tomake it available for cropping or domestic purposes (Ngigi, 2003a).Rainwater is collected from fields, house roofs and streets, and canbe stored in underground tanks or open ponds. In situ RWHpractices refer to micro-catchments at field level (Prinz and Malik,2002). In situ RWH practices mainly help to overcome dry spells, asthe soil, which is the main storage site of in situ RHW practices,serves only some days to weeks as a storage system (Falkenmarket al., 2001). By manipulating the soil surface structure andvegetation cover and density, evaporation from the soil surface andsurface runoff can be potentially reduced, infiltration is enhancedand thereby the availability of water in the root zone increased. In

situ RWH practices need low technical efforts compared to larger-scale water harvesting schemes but show only comparativelyshort-term effects on soil water availability compared totemporally (storage) or spatially (spate irrigation) extendedsystems (Falkenmark et al., 2001).

A vast variety of traditional as well as innovative in situ RWHpractices exist (Kriegl, 2001; Critchley and Mutunga, 2003; Ngigi,2003a; IWMI, 2005). Typical in situ RWH structures are linearstructures (e.g. embankment of stone or earth as contour bunds, orgrass strips) which are sometimes sophisticated, such as the Terasystem in the Sudan (van Dijk and Ahmed, 1993) and terracing aspracticed in East Africa. Semi-circular bunds are common in thesemi-arid zones of Western Africa (e.g. half moons or Demi lunes

(Barry and Sonou, 2003)). Pitting cultivation also takes place in theform of Zai in Burkina Faso (Kabore, 1995; Kassogue et al., 1996;Ouedraogo and Kabore, 1996; Fatondji et al., 2001; Kabore and Reij,2004), Tassa in Niger (Hassan, 1996), or the Chololo pits and Ngoropits of the Matengo people in East Africa (Mutunga and Critchley,2001; Kato, 2001; Mati and Lange, 2003; Malley et al., 2004)(Fig. 1). The different systems differ mainly in the size of the pits. Ingeneral, biomass production is improved by applying mulch in thepit before planting. Other approaches, such as conservation tillage,improve infiltration ability on the field scale (Stroosnijder, 2003).

The natural preconditions for RWH practices such as slope andmean precipitation are well known (Tauer, 1992; Prinz et al., 1994,1998; van der Marck, 1999). However, the high variability of

rainfall means that a high degree of uncertainty is still intrinsic tofarming in dryland areas.

Socio-economic preconditions are considered to be as leastimportant as specific improvements of the RWH practices,although integrated assessments are only being initiated (Kunze,2000; Drechsel et al., 2005). The implementation of policy plans,but also market parameters such as an improved infrastructure orhigh world market prices for coffee were shown to be the mainfactors responsible for ecological restoration and social stabiliza-tion into a success story, as shown for the Machakos, example inKenya (Zaal, 2002).

3.2. How landscape functions in semi-arid areas are modified

Arid and semi-arid landscapes fulfil a range of functions,ranging from ecological processes such as water and nutrientcycling, through biomass production and biodiversity conservationto socio-economic services such as providing the basis forsustainable rural livelihoods. Landscape functions processes differin their importance with regard to their spatial scale, e.g. biomassproduction at the plot scale up to carbon sequestration at theglobal scale (Hein et al., 2006). The introduction of RWH structuresmodifies landscape functions at different spatial scales. In thefollowing sections of this paper, the effects of RWH practices willbe reviewed and evaluated with respect to the appropriatelandscape scale. The major issues are presented according to theprocesses affected by RWH, indicators as well as the spatial scaleare considered and conclusions are made about the impact of RWHpractices on landscape functions (Table 1).

3.2.1. Hydrological functions

3.2.1.1. Infiltration. RWH structures modify water flows in thelandscape mainly by enhancing water infiltration at plot scale(Wakindiki and Ben-Hur, 2004).The velocityofrunoff isreduced,andthe water is collected behind the structures. Soil moisture increasessignificantly below semi-circular bunds in Burkina Faso (Zougmoreet al., 2003) and in run-on basins in South Africa (van Rensburg et al.,2004).Furthermore, the greaternumberofsoilmacroporesenhancesinfiltration due to increased biological activity at vegetation strips, asshown for Australian drylands (Ludwig et al., 2005).

Infiltration at RWH structures might require enhanced runoffup slope. Soil physical characteristics are therefore important(Prinz and Malik, 2002), but biological crusts might also be crucialin arid ecosystems to extend the runoff area in order that sufficientrun-on collects at RWH structures (Eldridge et al., 2002). Anegative effect might occur under poor drainage conditions thatmight lead to water logging (Mohamed, 2002).

Fig. 1. Examples of in situ RWH structures; left: demi lunes in West Africa; right: Zai-holes at a plot experimenting with the number per area. Photos courtesy of Abdou Hassan

et al. (April 2000), IFAD’s soil and water conservation project in Illela District Niger.

K. Vohland, B. Barry / Agriculture, Ecosystems and Environment 131 (2009) 119–127 121


3.2.1.2. Groundwater recharge. Very few direct field measurementsof groundwater recharge are available. When local people wereasked directly about the impact of in situ RWH on groundwaterrecharge, they stated that in situ RWH practices enhanced wateravailability (e.g. Woldearegay, 2002; Mutekwa and Kusangaya,2006).

3.2.1.3. Water competition. RWH practices not only trap surfacewater but also reduce runoff. Aquatic and wetland ecosystemsdownstream of the RWH structures might not receive enoughsurface runoff to maintain and sustain their functionality. Surfacerunoff might not always reach the next stream (Falkenmark et al.,2001; Woyessa et al., 2005). Very little is known about the effectsof in situ RWH practices on the hydrology, or on downstreamfreshwater ecology and biodiversity (Oweis et al., 1999), althoughearly studies, including hydrological modelling, indicate a positiveimpact on ground water (Kongo and Jewitt, 2006 in South Africa).

To take account of the competing demands for water, adaptiveresearch increasingly integrates agronomic, legal, socio-economic

and ecological aspects at the watershed scale, for example insouthern Africa (Rockstrom et al., 2004) or in the water-scarceEwaso Ngi’ro River basin, Kenya (Ngigi, 2003b), as well as forcompensating mechanisms as with the Green Water CreditsProject (Dent and Kauffman, 2007).

3.3. Soil fertility and biomass production

3.3.1. Soil fertility

Depending on slope, precipitation, and farming practices, aswell as on the quality of design and maintenance, soil erosion isreduced under RWH (Herweg and Ludi, 1999; de Graaff, 2000;Schiettecatte et al., 2005). RWH structures act as sediment traps,and therefore can enhance nutrient availability at the structures, aswas shown for traditional Zai in Niger (Fatondji, 2002). Althoughanother study on soil under half moons in Burkina Faso did notshow significant differences in P, K, Ca or N between the controland the various treatments consisting of stone rows, grass stripsand manure. The most significant effect of RWH practices on

Table 1The impact of RWH practices on important landscape functions in African drylands. The data basis is evaluated by us concerning the number of publications related to it as

well as the intensity the function was investigated.

Function Indicators Mechanisms Spatial scale Impact of

RWH

practice on

function

Sources Data basis

Groundwater recharge Fill of wells Improved infiltration Catchment area + Mohamed (2002), Woldearegay

(2002), Zougmore et al. (2003),

Ngigi (2003a,b), Botha et al. (2004),

Wakindiki and Ben-Hur (2004)

Fair

Wetland/aquatic ecosystem

maintenance

Plant species and

freshwater zoonoeses

Competition on superficial

water/improved infiltration

River, wetland �/+ Oweis et al. (1999), Falkenmark

et al. (2001), Ngigi (2003a,b),

Woyessa et al. (2005)

Fair

Biomass production Harvests Rain water use efficiency Plant – field –

landscape

+ Roose et al. (1993), Tabor (1995),

Singh et al. (1998), Ellis-Jones and

Tengberg (2000), Herweg and

Steiner (2002a,b), Barron (2004),

Kayombo et al. (2004), Kabore and

Reij (2004), Rockstrom

(2004),Wakindiki and

Ben-Hur (2004), Pretorius

et al. (2005)

Good

Nutrient cycling Soil nutrients Sediment trapping Field + Herweg and Ludi (1999),

de Graaff (2000) Fatondji (2002),

Zougmore et al. (2003),

Schiettecatte et al. (2005)

Good

Nutrient cycling Biological activity Attraction of e.g. termites Field Fatondji et al. (2001) Fair

Floral diversity Species diversity Plant growth on bare soils Field +/� Ouedraogo and Kabore (1996),

Bangoura (2002)

Fair

Floral diversity Endemic/local species Crop species instead of

savanna matrix

Community � Warren (1995) Fair

Structural heterogeneity Patch parameters

(size, distribution,

composition)

RHW induced water

availability combined

with agronomic measures

Field, landscape +/� Very poor

Animal diversity More species Enhanced biomass for

food and shelter

Field, landscape + Pandey (2001), Herweg and

Steiner (2002a,b)

Poor

Animal diversity Higher abundance Enhanced biomass for

food and shelter

Field, landscape + Pandey (2001), Herweg and

Steiner (2002a,b)

Poor

Animal diversity Less rare species Conversion of matrix

savanna into cropland,

lack of host plants

Landscape ?� Very poor

Food security Yields in bad years Bridging of dry spells Landscape + van Dijk and Ahmed (1993),

Ellis-Jones and Tengberg (2000),

Rockstrom et al. (2002),

Rockstrom et al. (2004)

Good

Water availability

for society

Time spent water

collecting

Improved groundwater

recharge

Community + Falkenmark et al. (2001) Fair

Water availability

for society

Community rules Upstream–downstream

competition on water

Community � Ngigi (2003a,b), Rockstrom

et al. (2004)

Fair

Income Money Enhanced yields Household + Ellis-Jones and Tengberg (2000),

Kunze (2000), Zaal (2002),

Cofie et al. (2004)

Good

Income Money Adoption rate Community +/� depends Bangoura (2002) Fair



biomass and grain yield was obtained with mulching (Zougmoreet al., 2003).

Irrigation has often been emphasized as a regional planningmeasure because it has appeared to be the best way of makingfarmers less dependent on erratic rainfall. However, this policy hadlimited success because of the build-up of salt in irrigated soils, aproblem which does not usually occur in runoff farming because ofthe better quality of runoff water from very small catchments(FAO, 2003). However, the quality of irrigation water varies greatlybetween different sources, locations, and seasons.

3.3.2. Crop yields and biomass production

In some cases, only RWH enhances the production of biomass ashas been shown for Zai in Burkina Faso (Roose et al., 1993; Kaboreand Reij, 2004). In the Central Plateau of Burkina Faso, applicationof Zai contributed to the restoration of large degraded encrustedsites and reduced the impact of dry spells (Rockstrom, 2004).

Generally, biomass production is enhanced by RWH structures(Fig. 2). Studies have mainly been conducted on cereal crops (Singhet al., 1998) such as sorghum (Tabor, 1995; Ellis-Jones andTengberg, 2000; Kabore and Reij, 2004) and maize (Kayombo et al.,2004; Wakindiki and Ben-Hur, 2004; Pretorius et al., 2005).Biomass production generally starts with low precipitation, withmore biomass produced per mm precipitation.

RWH structures do not lead to increases in crop yields under allconditions. In a one-field experiment with Fanya juu in Kenya,yields were stabilized but increased only at locations with lowerprecipitation (Herweg and Steiner, 2002a).

As far as we know, there have so far been no studies consideringtotal biomass changes, including non-crop plants and pastures, dueto RWH practices in Africa.

3.3.3. Biodiversity conservation

Landscapes are a repository of genetic diversity for currentecosystem processes and for future demands on ecosystemprocesses. The impact of RWH practices on three aspects ofbiodiversity, namely, floral diversity, structural heterogeneity, andanimal diversity are now considered in the following sections ofthis paper.

3.3.3.1. Changes in floral diversity. Though mainly adopted for cropand fodder purposes, RWH practices are known, in principle, toenhance floral diversity, by the fact that plants start to grow inplaces where there were bare degraded soils. It has been

anecdotally reported that native trees and bushes increase inabundance as RWH practices supported the replenishment ofaquifers (Bangoura, 2002; Kongo and Jewitt, 2006). But anecological conflict might arise because RWH allows cropping inareas that were formerly exclusively used by nomads/herders andwild animals. Transformation of land is often carried out withoutthe necessary knowledge of RWH practices and might negativelyimpact soils and the productivity of the land. In many arid or semi-arid areas of Africa, a change from pastures to cropland is notsustainable in the long term, especially where the rural populationis heavily dependent on livestock production systems (FAO, 2006).

3.3.3.2. Changes in structural heterogeneity/patchiness. RWH prac-tices enhance biomass production per unit area (c.f. section above),and therefore modify the spatial structure of the ecosystems invarious ways. RWH practices can completely change the characterof an arid landscape as reported in the ‘‘Savannization’’ project inthe northern Negev, where trees have been planted in a largelytree-less semi-arid landscape (Warren, 1995). The relationshipbetween RWH practices and landscape heterogeneity refers to thenumber of plant patches, the spatial and temporal structure of thepatches as well as their composition:

The impact of RWH practices on the number of plant patchesper unit area has so far scarcely been investigated. In Burkina Faso,the number of plant patches certainly increased, as plants weregrown on totally degraded soil following the introduction of RWH(Ouedraogo and Kabore, 1996). The height of plants and thereforethe three-dimensional size of plant patches are also enhanced.Plants grow to larger sizes, as data for crop plants, fodder grassesand trees indicate (cf. crop yields).

In dryland areas the distribution of plants is more stronglyclumped in drier environments because water and nutrients areconcentrated on micro-sites (Ludwig and Tongway, 1995). RWHstructures improve water availability locally by creating artificialpatches, i.e. at crop plant sites. Therefore, as in any agro-ecosystem,the spatial distribution of crops is strongly man-made, as farmersdecide the spacing between plants. In general, crops are plantedmore regularly than natural vegetation occurs, which means that arandom dispersal of patches is displaced by more equally dispersedpatches, which might impact dispersal success of different plantsand animals (Wiens, 1976).

Most RWH structures are installed for crop plants, and muchless frequently for rangeland improvement or ecosystem con-servation. Consequently, the special structure of indigenousgrasses and herbs are replaced by crops such as maize and millet.Further, weeding is carried out around these crops. Thus managedplant patches differ from natural ones in their composition, notonly due to enhanced water availability induced by RWH but alsoto agronomic measures.

In some cases, multiple crop species are planted within a patchcreated by RWH structures. In Kenya, the edge of the RWHstructure is planted with grasses surrounding the (cash) crop(Mutunga and Critchley, 2001). In the Zai structures of West Africa,in spiny bushes are planted to protect the crop against livestock.

3.3.3.3. Changes in animal diversity. Animal diversity should beenhanced as more biomass becomes available for food and shelter,as shown for trees in Rajasthan, India (Pandey, 2001). For most taxaoccurring in drylands, a positive relation exists between biomassand species diversity and abundance. More biomass also allowsmore complex trophic chains. Several field studies, including somefor Africa, show that the numbers of animal species and individualsin an area increase due to more and larger vegetation patches (e.g.Belsky et al., 1993; Dean et al., 1999). Other studies report theopposite situation that decreases in animal species and abundanceare due to missing plant patches (Hoffman, 2000; Nangula and

Fig. 2. Grain production increases with higher precipitation but with RWH

structures grain production per area is higher than without. For areas with annual

precipitation above 800 mm no records were found. The absolute biomass is

depending also on other factors such as soil type. Original data are listed in

Appendix B.



Oba, 2004). In the context of RWH structures an increased speciesbiodiversity is anecdotally reported for increased rodent numbersin the Fanya juu terracing systems (Herweg and Steiner, 2002b).This is a matter that farmers and nature conservationists mightconsider differently.

If RWH structures are not suitably designed, standing watermay lead to increases in vector borne parasitic diseases such asmalaria or bilharzias through insect vectors (Bangoura, 2002).

Changes in the composition of animal communities may occuras natural vegetation is partly replaced by crop plants. Generally,natural vegetation hosts more animal species than (monospecific)crops, so this probably is true for transformed rangelands, too. Evenmore, it should be considered that if combined with only few cropsRWH practices might attract pests.

3.3.4. Sustainable livelihoods

3.3.4.1. Food security. Increased and sustained crop yields are vitalfor food security in rural communities. Several studies have shownthat in many cases, crop yields are higher when RWH practices areapplied (e.g. Ellis-Jones and Tengberg, 2000). Other studies showedthat RWH structures such as tera (van Dijk and Ahmed, 1993)mainly serve to reduce crop failure during dry spells and droughtsand thus help to enhance food security. Valuable mechanisms inthis context are a prolonged vegetation period caused by earlyplanting as well as an improved ability of crops to survive dry spells(Falkenmark et al., 2001). Increased efficiency of rainwater use canbe reached by supplementary irrigation (Rockstrom et al., 2002,2004).

Over and above a site specific amount of precipitation, RWHstructures do not necessarily lead to increased crop yields andimproved food security because RWH structures lend to occupyprecious cropping areas that are subsequently lost for cropping.Weeding might be complicated and unsatisfactory, which mighteven permit rodents to become established within the fields(Herweg and Ludi, 1999).

3.3.4.2. Conflicts concerning water resources. Enhanced infiltrationfavours groundwater recharge and refilling of wells. Nevertheless,due to reduced downstream runoff, conflicts might arise betweenneighbours competing for available water resources. In denselypopulated areas of Kenya, such communal conflicts have beenminimized through the adoption of a sophisticated technique ofcollecting street runoff and distributing the rainwater (Mutungaand Critchley, 2001; Ngigi, 2003a).

Furthermore, social conflicts might arise, mainly concerningland and water rights when RWH systems facilitate cropping inareas formerly used exclusively by nomads (Tesfay and Tafere,2004).

3.3.4.3. Income/social balance. The adoption of RWH proved tohave a positive effect on incomes, measured in return to labour(Cofie et al., 2004). However, RWH adoption rates are still low.Farmers hesitate to invest time and money in setting up RWHstructures, as they often have no security of land ownership and/orlimited access to local markets where they could sell surpluses offood crops or cash crops (Drechsel et al., 2005). An increase inmarket access, measured as travel time to the capital, wasidentified as a driving force in improving agricultural productionand productivity in densely populated areas in Kenya (Zaal, 2002),and this could improve investments in RWH practices such asterracing. The Machakos and Kitui regions of Kenya are nowadaysfrequently cited as areas where successful RWH managementpractices have been put into practice.

Case studies show that under specific conditions, application ofRWH practices is not profitable as yields are not high enough to

justify the investments in labour and materials (see e.g, van derMarck, 1999), although in the long term, gross margins aregenerally higher with RWH (Ellis-Jones and Tengberg, 2000; vanRensburg et al., 2004).

4. Discussion

In situ RWH practices have an overall positive effect onlandscape functions. Hydrologic improvements concern therecharge of aquifers and increase in soil water. But competitionmay arise in communities between different water users, betweendifferent ecosystems (including natural and agro-ecosystems), andbetween ecosystems and humans (c.f. Falkenmark and Rockstrom,2004). The data basis needs to be improved, and research effortsshould aim at reducing unproductive evaporation.

Different scales of spatial diversity (heterogeneity) have to beconsidered. At the scale of the individual plant, spatial diversity isenhanced as larger plants provide more three-dimensionalstructures. At the field scale, spatial diversity is enhanced as morebiomass is present, but species diversity could be reduced as plantsmight belong to only one or a few crop species, to the exclusion ofthe natural plant community. The same is true for the landscapescale.

RWH systems aim to minimize seasonal variation in wateravailability such as droughts and dry spells (Rockstrom et al.,2002). Consequently temporal diversity is modified as naturalpatch dynamics are replaced by an anthropogenic patch system.These considerations beg the vital question: Are in situ RWH

practices sustainable?

Under erratic rainfall conditions in the semi-arid zone of sub-Saharan Africa, a major contribution to improving crop productioncan be anticipated from improved and up-scaled SWC and RWHconservation practices. Fostering and improving these technolo-gies in a sustainable way, while taking biodiversity aspects intoaccount, offers a means of minimising the risk of drought, cropfailure and ecological refugees. The social and economic sustain-ability of RWH practices depend largely on the extent ofinvolvement by farmers and the general communities. This mightbe the weakest link in the chain of sustainability issues (Bothaet al., 2004). The more local communities are involved in planning,the higher the possibility that RWH structures will be maintainedand benefits are shared (Bangoura, 2002).

The general conclusions of this review focusing on Africa areconsistent with results obtained from research on the effect ofrainwater harvesting practices in other parts of the world. Forexample, in Rajasthan in India, where RWH practices have an evenlonger tradition, Pandey (2003) concluded that RWH practicespromote landscape diversity by an increased growth of trees andtheir associated fauna and thereby serve to provide humanrequirements, though he concedes that natural ecosystems stillmight harbour more biodiversity resources.

Possible ‘‘off site’’ effects are competition for water betweennatural and agro-ecosystems as well as between different users, i.e.between upstream and downstream users. A particularly impor-tant example is the competition and conflicts that RWH practicescould potentially cause between pastoralists and sedentaryfarmers. RWH is related to large-scale changes of land usepatterns, turning rangeland and natural vegetation into cropland.This might strengthen conflicts such as in Darfur (Nyong, 2005b).

4.1. How do in situ RWH practices affect resilience of dryland

ecosystems?

Dryland ecosystems are described as non-equilibrium ecosys-tems (Behnke et al., 1993). The concept of resilience such as theability to swing back to a stable state is therefore not applicable.



We accept the resilience concept of Folke et al. (2004) thatdescribes resilience as the ability to buffer sudden and largeexternal impacts in that the major functions of landscapes asdescribed above are maintained by socio-economic as well asecological adaptability.

RWH practices improve resilience in enhancing socio-economicas well as ecological adaptability. Socio-economic adaptabilitymay be improved by increased food security, extra income andconsequently enhanced and sustainable livelihoods. However, thiscertainly is not enough to enable achievement of the MillenniumDevelopment Goals (MDGs) although it may reduce pressure onland and natural resources.

RWH practices used to restore degraded areas improveresilience by improving productivity (Lal, 2001, 2004; UNCCD,2001; Rockstrom, 2004). Humans therefore are not only a cause ofdegradation but can be the driving force for restoration processesdue to their efforts (Cofie et al., 2004).

5. Conclusion

In situ RWH practices are mostly simple and do not leave muchspace for technical improvement. However, major challenges lie inimproving nutrient management, through mulching, and instronger mechanisation, with animal tracking. Further, bestpractices and traditional methods may be combined.

Another challenge remains to assess the potential and impact ofRWH practices with regard to future climate and other global andregional changes. Thornton et al. (2002) combined populationscenarios and climate until 2050 to outline cropping boundariestogether with the expected range of livestock and other productionsystems. Similar studies for the potentials RWH have beenfragmentary, covering, e.g. the potential of up-scaling Zai (e.g.Freeman, 1999), and studies on larger spatial scales are only nowemerging (e.g. Senay and Verdin, 2004).

More in-depth research is also required at local scales. The socio-economic and political conditions are qualitatively known (Critchleyet al., 1992; Oweis et al., 1999) but not applied quantitatively tounderstand and support the individual decisions of farmers.

Crop and risk assessment models and approaches developed sofar for RWH practices (e.g. Cohen et al., 1995; Young et al., 2002)rely mainly on ecological and technical information. Economic,social, political or cultural factors are neglected, the spatialanalyses of bio-physical factors are not combined with socio-economic analyses, e.g. for access to markets, available labour andmicrocredit systems.

The scanty literature on the impact of RWH practices onbiodiversity conservation shows that the effects of RWH onlandscape functions are still poorly understood (Table 1). Researchaimed at improving agronomic practices does not necessarily focuson biodiversity-conserving landscape functional aspects such asanimal biodiversity, or on the spatial landscape structure. This goesbeyond issues of food security and poverty alleviation. Despitesome attention gained, e.g. through the TEEB-process (Sukhdevet al., 2008), biodiversity conservation is not deeply embedded inagricultural research. Studies on landscape ecology and conserva-tion biology consider humans less as acting and shaping entities,rather than as a disturbance against which the natural ecosystem isdefined and defended. More integrated adaptive research tocombine societal and ecological demands is needed to minimisetrade-offs and escape the trap of combined poverty and landdegradation.

Acknowledgements

We thank the Institute for Geography of the HumboldtUniversity, Berlin, and especially Ludwig Ellenberg for constructive

discussion and support of this review by providing access toliterature. Useful comments and contributions were received fromseveral colleagues especially from the Chair for Vegetation Ecologyand Nature Conservation of the University Potsdam, fromWolfgang Cramer from the Potsdam Institute for Climate ImpactResearch, from Anthony Youdeowei and Pay Drechsel, IWMIGhana, as well as by two anonymous referees. We are grateful forthese contributions to this paper.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.agee.2009.01.010.

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