Climate And Precipitation
Jamaica Has A Tropical Maritime Climate. Mean Daily Temperature Ranges From A Seasonal Low Of 26° C In February To A High Of 30° C In August . On Average, The Temperature Changes By 2 ° C With Every 300 M Change In Altitude.
Precipitation In Jamaica Vary Seasonally And Spatially. Long Term Mean Annual Rainfall Is 1,981 Mm. The Heaviest Rainfall Is Concentrated Over The Blue Mountains. They Receive Over 5,080 Mm Of Rain Annually Whereas The City Of Kingston Receives Less Than 762 Mm. Kingston And Most Of The Southern Coast Is Located In The Rain Shadow Of The Blue Mountains And Receive Much Less Rain Than The Northern Coast. Islandwide Long Term Mean Annual Rainfall Exhibit A Marked Bi-modal Pattern With The Primary Maximum In October And Secondary In May. The Drier Months Are January, February, March And July. The Island Regularly Comes Under The Influence Of Tropical Storms And Hurricanes During The Period July To November, Primarily Resulting In Flood Producing Rainfal Of High Intensity And Magnitude.
Basins And Sub-basins
A Basin Is A Hydrogeological Unit, Which Refers To A Geographical Area Drained By A Particular Surface Water And/Or Groundwater System. The Boundaries Are Demarcated So That There Is Generally No Flow From One Basin To Another. The Island Is Divided Into Ten (10) Hydrological Basin. The Basin Boundaries Are The Main Surface Water Divides, But In Some Cases Groundwater Divides Have Been Used For The Basin Division.
For Planning Purposes It Was Found Convenient To Subdivide The Basins Into Sub-basins. A Sub-basin Is A Discrete Hydrologic Sub-unit Of A Basin, The Water Resources Of Which Can Be Assessed, Developed And Managed In A Near Independent Manner From The Rest Of The Basin. Impermeable Rocks Will Not Allow Water To Pass Through, Instead It Will Run Off Over The Surface To Form Rivers.
Aquifer - Rock Or Group Of Rocks That Allow For The Movement And Storage Of Goundwater. Aquiclude - Rock Or Groups Of Rocks That Does Not Allow For The Movement Or Storage Groundwater.
Rainfall Interacts With The Surface Of The Eath On Impact. The Nature Of The Interactionis Dependent On The Nature Of The Rocks That Exist At The Particular Location. Permeable Rocks Will Allow Water To Penetrate The Ground And Become A Part Of The Groundwater Reservoir. Aquifer Types In Jamaica Are Predominantly White Limestones Rocks And Alluvium Which Constitutes 50% Of The Land Surface. Limestone Aquifers Are Enhanced With Secondary Porosity In The Form Of Channels, Caves And Conduits In The Subsurface. These Areas Are Characterized By The Absence Of Surface Streams And Have Well Developed Subsurface Drainage. Another Aquifer Type Is Allluvium Which Is Made Up Of Sediments (Sand, Silt, Clay, Gravel). Aquicludes Comprised Of Volcanic Formations And Yellow Limestone Group. The Areas Where The Aquiclude Occupy Are Characterized By A Dense Network Of Suface Streams.
The Wra Monitors Groundwater Levels At 390 Index Wells Throughout The Island To Provide Information On The Availability Of Groundwater And To Facilitate The Prediction Of Trends In The Quantity Of The Resource
In Order To Use Groundwater It Has To Be Brought To The Surface. Groundwater May Be Brought To The Surface By Natural And Artificial Means
Springs Are The Natural Means By Which Grounwater Gets To The Surface. They Occur Where Geologic Conditions Are Such That The Groundwater Is Forced To Flow To The Surfcace. Blue Holes And Contact Springs Are Two Of The Spring Types That Occur In Jamaica.
Tube Wells Drilled Into The Water Table Is One Artificial Methods By Which Groundwater Is Brought To The Surface. Under Special Conditions Geology Creates Conditions Where Water In A Well Will Naturally Flow At The Surface. This Removes The Need To Pump. Such Wells Are Known As Flowing Artesian Wells.
Rivers And Their Origin

Water Flowing In Discrete Channes On Its Way To The Sea Is Known As Surface Water And The Channels Are Known As Rivers. The Source Area From Which A River Gathers Its Flow Is Known As The Watershed. The Watershed Effectively Stretches From The River Source To Its End At The Coast. At Source There Are Three Main Mechanisms By Which A River May Derive
(A)Rainfall Runoff
(B)Shallow Sub-surface Flow
On Impermeable Rocks Most Of The Rainfall Runs Directly Off The Surface And Into Channels To Form Rivers. Flow Of The Water In Rivers Is Dependent On Rainfall. During Periods Of Low Rainfall The River Channels Often Go Dry And Carry Water Only During And Immediately After The Occurrence Of Rain. These Types Of Rivers Are Known As Seasonal River And Gullies Geological Processes Act Upon Rocks Over Time. In Jamaica This Has Resulted In The Creation Of A Weathered Zone Overlying The Impermeable Rocks. The Rivers That Originate On These Rocks Are More Reliable Than The Rivers That Originate On The Impermeable Rocks. The Permeable Zones Act As A Reservoir For Rainwater, And After Rain This Storage Is Gradually Released Into Rivers. A Portion Of The Rainfall Penetrates The Pemeable Rocks To Become A Part Of The Groundwater Reservoir. Geological Conditions Force This Groundwater Reservoir To Discharge At The Surface Into Rivers. This Point Of Discharge Of Groundwater At The Surface Is Known As A Spring. The Rivers Sourced By Groundwater Discharge Are Very Reliable And They Maintain Their Flow Even In The Dry Season.

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