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Surface water and groundwater resources
About 52 percent of the average annual rainfall is lost to evapotranspiration. The internal renewable water resources (IRWR) are 10 823 million m3/year with 9 111 and 5 472 million m3/year for surface water and groundwater respectively and the overlap between surface water and groundwater (baseflow) being 3 760 million m3/year (Table 2).
The central mountain ranges divide the catchment areas for rivers which drain either to the north or to the south coasts. Surface runoff predominates on outcrops of basement rocks and interior valley alluviums, whereas groundwater is the dominant water resource associated with the karstic limestone and coastal alluviums. The surface water resources are characterized by a marked seasonal variability in flow. Streams flowing northward originate mainly in the tertiary limestone. These are mostly perennial rivers, like the Martha Brae and White rivers, with significant baseflow components and low seasonal flow variability. Exceptions are the Great river and several rivers in the Blue Mountains basin which, like many of the south draining rivers, are characterized by widely varying seasonal flows and comparatively low baseflow. Some of the catchments consist of cretaceous volcanoclastic of low permeability.
Jamaica is subdivided into ten major hydrological basins. The basins are further subdivided into 26 watershed management units (WMU). The WMUs and basins are presented in Figure 1.
Almost 40 percent of the IRWR are considered exploitable or reliable, defined as daily flow exceeded during 90 percent of the time for surface water and quantity of water which can be withdrawn over a long period without impairing the limestone and alluvial aquifers as a water source or causing contamination by seawater intrusion for groundwater. About 20 percent from the limestone aquifer are developed through wells, mainly in the Río Cobre and Rio Minho. However, in other basins, the water is generally available as base flow and is exploitable through run-of-river developments.
In 2000, desalinated water produced was estimated at 0.5 million m3.
The National Water Commission (NWC) collects wastewater and sewage from over 600 000 customers across the country. Wastewater is treated and used for irrigation, but this data is not currently being collected.
Approximately 23 percent of the rural population relies on rainwater harvesting via roof collection. The NWC and Local Parish Councils manage 353 public rainwater harvesting catchment tanks. In 2003, 15.3 percent of the population used rainwater harvesting as a water source.
Lakes, dams and wetlands
There are two major raw water storage facilities, both located in Saint Andrew. The Mona reservoir, with intakes at the Hope and Yallahs rivers, has a storage capacity of 3.67 million m3. The Hermitage Reservoir, with intakes at Ginger river and Wag/Morsham river, has a storage capacity of 1.78 million m3.
Jamaica is party to the Convention on Wetlands of International Importance especially as Waterfowl Habitat (Ramsar Convention). Three sites in Jamaica have been designated as Ramsar sites: Black River Lower Morass, Palisadoes Port Royal, and Portland Bight Wetlands and Cays.