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Cape Verde - Groundwater Resources Assessment

Project Chief: Victor Heilweil, USGS Salt Lake City, Utah
Cooperator: Millennium Challenge Corporation
Period of Project: 2005-2011

PURPOSE

The objective of the USGS Cape Verde Groundwater Resources Assessment is to evaluate groundwater availability in three watersheds of this island nation located off the coast of West Africa. This work was funded by the Millennium Challenge Account in support of their evaluation for potential groundwater development for additional agriculture.  The study was conducted in cooperation with the Instituto Nacional de Gestão dos Recursos Hídricos and the Millennium Challenge Account – Cape Verde.

DESCRIPTION OF THE STUDY AREA

Figure 1. Location of study basins within Cape Verde, West Africa

Cape Verde is a volcanic island archipelago located about 750 km off the west coast of Africa (fig. 1). Its 400,000 inhabitants live on nine separate islands with a combined area of 4,000 square kilometers, resulting in one of the highest population densities among the West African nations. The study area included watersheds on three different Cape Verde islands: Fogo (Mosteiros Basin), Santo Antão (Ribeira Paul Basin), and Sao Nicolau (Ribeira Fajã Basin). The three watersheds all have steep topography. Mosteiros is the largest basin (42 km2) with a maximum altitude of 2,400 m; the Ribeira Paul and Ribeira Fajã Basins are smaller with lower maximum altitudes of 1,500 m and 1,300 m, respectively.
Rainfall in Cape Verde is extremely variable from year to year. Moreover, total annual rainfall ranges from 0 to 50 mm along the populated coastal areas to between 400 and 800 mm in the highlands, with the higher-altitude areas of each basin generally receiving the most rainfall. Most of the population resides in rural areas and derives its livelihood from rain-fed agriculture; this irregular rainfall makes farming extremely challenging in all but the wettest areas. The Ribeira Paul Basin receives about 500 to 800 mm/yr of rainfall, whereas the Mosteiros and Ribeira Fajã Basins only receive about half of this amount. There are very few perennial streams because most rainfall rapidly runs to the ocean, evaporates, or is used by plants. The remaining rainfall infiltrates the ground through permeable rocks and soil to recharge the underlying groundwater aquifers. The groundwater slowly moves downgradient from the upper elevations to the lower parts of each watershed, where it is either pumped from production wells or discharged as gravity-flow to springs, streams and gallery tunnels- or ultimately as submarine outflow. A lens of fresh groundwater typically “floats” above a layer of brackish water at the freshwater/saltwater boundary in smaller ocean island aquifers (fig. 2)

Figure 2. Generalized conceptual model of volcanic island hydrology

APPROACH

This study has collected and synthesized hydrogeological data including environmental tracers, hydraulic properties, meteorological data, and water-level data to develop a conceptual understanding of groundwater movement and groundwater resources within these aquifer systems.

PRODUCTS

This is a seven-year (2005-2011) study of groundwater resources in the Cape Verde Islands. Products include:

• USGS Scientific Investigations Report 2006-5207, entitled “Evaluation of Baseline Ground-Water Conditions in the Mosteiros, Ribeira Paul, and Ribeira Faja Basins, Republic of Cape Verde, West Africa, 2005-2006
• USGS Fact Sheet 2010-3069, entitled “Groundwater Resources of Mosteiros Basin, Island of Fogo, Cape Verde, West Africa
• USGS Fact Sheet 2010-3070, entitled “Groundwater Resources of Ribeira Paúl Basin, Island of Santo Antão, Cape Verde, West Africa
• USGS Fact Sheet 2010-3071, entitled “Groundwater Resources of Ribeira Fajã Basin, Island of São Nicolau, Cape Verde, West Africa
• Hydrogeology Journal article (2009 v. 17, no. 5) entitled “Oxygen, Hydrogen, and Helium Isotopes for Investigating Groundwater Systems of the Cape Verde Islands, West Africa
• Planned publication of a journal article constraining volcanic island interior water-table depth with dissolved gases and coupled flow/thermal modeling