Assessment of land degradation indicators in an insular sahelian mountain region – case study of the“Ribeira Seca” watershed

J. Tavares; I. Baptista; A.Ferreira; L.Varela; J. Bentub and COA Coelho

Content• Introduction (background and justification

and objectives)• Country data• Factors contributing to land degradation• Soil and water conservation techniques• Materials and method

– The study site– Methodology

• Results and discussion• Conclusion

Introduction • Cape Verde, as a sahelian

country, has been severely affected by the desertification phenomenon, although extreme efforts are being made to reverse the process

• Desertification is a major issue in Cape Verde and directly influences the livelihoods of local farmers

• Several techniques were implemented throughout the last decades to halt the desertification process– Efficiency has hardly been

accessed.

Introduction • To combat desertification,

integrated approaches are to be sought, combining social and biophysical viewpoints, and linking local and scientific knowledge

• The first step is to assess the problem using an integrated approach, to identify variability and worst cases, as basis of strategy and actions to be defined

• Indicators are very useful in this context

Indicators• Environmental and socio-economic indicators play

an increasingly important role in supporting development policies for combating desertification

• An indicator is a measurement that reflects the status of social, economic or environmental systems

• They can be used to track changes in complex systems and monitor progress towards pre-determined goals

• Their role is to simplify the complex interrelations of the socio-economic and environmental reality and convey them in an unambiguous way

Some country data

• Location: West African coast• Surface area: 4033 km2

• Composed of 10 islands and 5 islets• Population: 430 000 • Arable land: 10% of total land area (42000 ha)• Dryland agriculture: 38 000 ha (>90%)• Irrigated land: 3000-5000 ha (<5.0%)• Climate: dry tropical w/2 distinct seasons: dry season

(November to June) and the rainy season (July to October)• Mean annual rainfall: 230 mm with great spatial-temporal

variability• Terrain: steep, rugged, rocky and volcanic

• Contribution of agriculture to GDP: 12 %

Objectives• Goal: To improve the knowledge on

desertification risks related with land use

• Specific objectives:1. assess the indicators applied in the

various land use type

2. analyze the main drivers of desertification and their impact on social, economic, natural capital, ecological and economic functions

Factors contributing to land degradation

• Heavy rainfall events ⇒ Flash floods (Soil erosion by water)

• Low, insufficient and irregular rainfall (space and time)

• Frequent droughts• Soil fertility decline (no nutrient replenishment)• Low soil OM content• Cultivation of very steep slopes• Inadequate rain-fed farming practices• Overexploitation of ground water ⇒ Water and soil

salinization Land degradation and desertification

Soil and water conservation techniques

• To deal with land degradation (erosion and drought), CV governments have implemented several SWC techniques

– in field (mechanical and biological structures)• Terraces, stone walls, contour ridges• Vegetation live barriers (green belts)

– off field • water harvesting (large dams, reservoirs, etc)• check dams, etc.)

Contour ridges combined with leucaena leucocephala

Contour stone wall combined w/ agroforestry species

Contour stone walls combined w/ Furcraea gigantean

Stone walls combined with aloe vera

• Contour stone walls in corn/bean production system (left)

• Terraces built on slope, planted with rainfed crops - corn & beans (right)

Contour ridges on very steep slope for peanut production

in-field (contour stone wall w/ vegetation barrier) and off-field (check dams) SWC techniques

contour stone wall

Aloe vera barriers

Off-field SWC techniques

Gabion check damsMasonry check dam

Water harvesting

Large Dam construction for rain water retention (Poilão Dam) Large Reservoir (São

Jorge)

Materials and Methods

Study site characteristics• Location: East-central side of

the Santiago island• Population: 14 343• Main activity: agriculture • Drainage area: 71.50km2

• Land use: 83% rainfed agriculture; 5 % irrigated land; 4% forest; 1% rock outcrop; 7% other

• 4 bioclimatic zones: 11% humid; 20% sub humid; 49% semiarid; 20% arid

• Main dryland crops: maize and beans

• Irrigated crops: banana, sugarcane, roots and tubers and vegetables

• Average slope: 8.6%

The study site: Physiographic indices

Sub basin / basin

Area Length

river

Alt. max

Alt. min Gravelius

Index (Kc)

Shape form (Kf)

River slope

Basin slope

Km2 Km m m - - % %

Longueira 4,18 3,45 1126 239 1,31 0,35 26 33,0Grande 4,86 4,98 750 130 1,33 0,20 12,4 15,9Godim 5,01 5,30 570 145 1,37 0,20 8,5 24,0Seca 24,94 18,0 642 0 2,47 0,07 7,2 6,3Ribeira Seca 71,50 18 1394 0 1,31 0,22 7,20 8,60

Methodology

• 103 points were surveyed in the watershed following WB2 methodology

• The indicators used are based in the DPSIR framework

• Classes were assigned using existing classification systems such as the European geo-referenced soil data base, or existing research data

• The various classes of indicators were organized according to the importance to land degradation and desertification risk

DRIVERS

Efficiency/Descr iptive Indicators

Performance Indicators

RESPONSES

Risk Assessment

PRESSURES

Descriptive Indicators

STATE

IMPACTS

Descriptive Indicators

DPSIR elements and related indicators

Flow chart of the process

Results and Discussion

Physiographic characteristics

Desertification risk

Mitigation techniques

Desertification risk map

• Almost 90 % of the study area is sensitive to desertification

• This situation reflects the particular geo-morphological characteristics of the watershed: low lands, high population density, and lack of soil cover

Conclusion • The indicators used to assess the desertification

risk at the Ribeira Seca catchment revealed to be a very practical and useful tool:– Allowing direct comparison between different

realities, rank the desertification risk, – Identifying the more detrimental practices and – Pinpointing the locations at higher risk of

desertification • They allow perception of different realities within a

same area, and to evaluate the performance of different conservation strategies, practices and techniques.

Thank you for your attention and inputs