Our Island Jamaica

Land of Wood And Water

 

The geographic scope of the activities of the Water Resources Authority (WRA) is defined by the coastline of the island of Jamaica.The island of Jamaica is located in the northwestern Caribbean Sea and is centered on latitude 18 degrees 15 minutes north and longitude 77 degrees 20 minutes west.  It is elongated, along westnorthwest-eastsoutheast alignment, roughly 130 kilometres long by 80 kilometres wide at its broadest; its area totalling 10,991 square kilometres.The Island is divided into three counties (Cornwall, Middlesex, and Surrey) and is further subdivided into fourteen parishes.The population of Jamaica is approximately 2.5 million.

 

 

 

Physiography
Jamaica's physiography closely reflects the three major rock types of which it is composed.  These three major rock types are, in chronological order:

 

  i.      Quaternary alluviums of generally moderate permeability, which occupy about 15 percent of the land area - mainly in the coastal plains and in the floors of interior valleys.
ii. Tertiary limestones with variably developed karstification and moderate to high permeabilities, which occupy about 60 percent of the land area.
iii. Cretaceous volcaniclastics of low permeability, which occupy about 25 percent of the land area - mainly within inliers along the upland axis.

 

There are a series of mountain ranges along the major westnorthwest-eastsoutheast axis of the Island.  In the eastern third of the Island these mountains generally exceed elevations of 1,000 metres above sea level with the highest peak rising to 2,257 metres.  Major alluvial lowlands occur in the southern half of the Island, where they are often associated with coastal swamps/wetlands.

The central mountain ranges form the main watershed for rivers which drain either to the north or to the south coasts.  The Island is subdivided into ten major hydrologic basins.  Surface runoff predominates on the outcrops of basement rocks and interior valley alluviums, whereas groundwater is the dominant water resource associated with the karstic limestones and coastal alluviums.  The surface water resources are characterized by a marked seasonal variability in flow.

Streams flowing northward originate mainly in the Tertiary limestones.  These are mostly perennial rivers, like the Martha Brea and White Rivers, with baseflow components and low seasonal flow variability.  Exceptions are the Great River and several rivers in the Blue Mountains (North) Basin, which, like many of the south draining rivers, are characterized by widely varying seasonal flows and comparatively low baseflow (Chin, 1977).  Some of their catchments consist of cretaceous volcaniclastics of low permeability.  The Black River drains a predominantly limestone catchment.

 

Climate & Rainfall
Jamaica has a tropical maritime climate.  Mean daily temperatures range form a seasonal low of 26 degrees centigrade in February to a high of 28 degrees centigrade in August.  Daily sunshine hours are fairly constant throughout the year, averaging about 8.2 hours in the southern plains (Evans, 1973).Long term mean annual rainfall over the Island is about 1,980 mm (Evans, 1973).  Much of the rainfall results from the northeasterly trade winds, which deposit most of their moisture on the northern slopes of the axial mountain ranges, with the consequence that the southern half of the Island is a rain shadow.  Rainfall on the northeastern slopes of the Blue Mountain range is generally 3,000 to 5,000 mm/year, whereas in the south coastal plains of St. Catherine and Clarendon it is generally less than 1,500 mm/year.  Islandwide long term mean annual rainfall exhibits a characteristic pattern, with the primary maximum in October and the secondary in May.  The main dry season lasts from December to April.

Jamaica regularly comes under the influence of tropical storms and hurricanes during the period July to November, characterized by flood producing rainfall of high intensity and magnitude.Mean monthly relative humidity on the south coast is near constant throughout the year, ranging from a low of 71 percent in August to a high of 77 percent in October and averaging 74 percent (Evans, 1973).

 

Hydrogeology
The geology of the Island, that is, the various rock formations and their interrelationships, strongly influences the occurrence and availability of water resources.Based on its hydrologic character, each rock formation is classified as an aquifer or aquiclude.  Groundwater is the main water resource associated with an aquifer whereas surface water is the main water resource related to an aquiclude.
Jamaica's rock formations are grouped into six hydrostratigraphic units:

 

i. Basement aquiclude.  This unit comprises volcaniclastic and limestone rocks on which the other units are deposited.  It occupies about 25 percent of the Island's surface area outcropping mainly in the Blue Mountains to the east and along the westnorthwest-eastsoutheast central spine of central and western Jamaica.  The Basement Aquiclude outcrops are related to dense surface stream networks which have high peak flows in the wet season and low peak flows in the dry season.
   
ii. Limestone aquifer.  This unit comprises highly karstified limestone.  The Limestone Aquifer rests unconformably on the Basement Aquiclude and thickens towards the coast.  It occupies about 50 percent of the Island's surface area.  The Limestone Aquifer outcrops are related to an absence of surface streams and to well developed subsurface drainage systems.  This relationship is due to the high infiltration capacity of the Limestone Aquifer.
   
iii. Limestone aquiclude.  This unit comprises fine grained chalk and occurs as a coastal band.  The Limestone Aquiclude functions as a subsurface barrier and creates subsurface groundwater reservoirs within the Limestone Aquifer.  This accounts for the high stream flows in the dry season of rivers draining Limestone Aquifer areas.
   
iv. Coastal aquiclude.  This unit comprises soft marls which are patchily distributed along the coast.  The Coastal Aquiclude functions as a subsurface barrier to pond groundwater within the Limestone Aquifer.
   
v. Coastal aquifer.  This unit comprises raised reefs patchily distributed along the north coast.
   
vi. Alluvium aquifer.  This unit comprises the upper sequences of coastal alluviums in the Rio Minho, Rio Cobre, Kingston, and Blue Mountain Basins.  The Alluvium Aquiclude comprises interior valley clays.