Home Water Resources Authority National Water Commission Rural Water Supply Company Water Allocation Water Resources Act Water History in Jamaica Water Glossary Water Resources Contact Maps from Zonu

Glossary Terms and their definitions This glossary contains terms and their definitions which may be unfamiliar to you. These terms are used throughout the web pages on this web site. Aquiclude An aquiclude is a saturated geologic unit that is incapable of transmitting significant quantities of water under ordinary hydraulic gradients. In Jamaica, there are three general types of aquiclude: Alluvium aquiclude is composed mainly of clay which is deposited by physical processes in river channels or on flood plains. Basement aquiclude is composed of volcanics and volcaniclastic sediments of Cretaceous ages and the overlying Yellow Limestone Group (calcarenites of Lower Eocene age). Coastal aquiclude is composed of soft marls and is patchily distributed along the coast of Jamaica. Aquifer An aquifer is a saturated permeable geologic unit that is capable of transmitting significant quantities of water under ordinary hydraulic gradients or is permeable enough to yield economic quantities of water to wells. In Jamaica, there are three general types of aquifer: Alluvium aquifer is composed mainly of gravel and sand with some silt and clay which are deposited by physical processes in river channels or on flood plains. Limestone aquifer is composed of members of the White Limestone Group and exhibits mature karstic features such as a very high infiltration capacity, predominant subsurface drainage, and highly compartmentalized subsurface conduit flow. Coastal aquifer is composed of raised reefs of the Falmouth Formation, which are highly karstified limestone aquifers with high permeability and low groundwater storage potential. Direct Runoff Direct runoff is the sum of overland flow and interflow (water flow in the top soil layer). Direct runoff reaches the stream shortly after it falls as rain and is discharged from the basin within one or two days. Drought Drought is viewed as a sustained and regionally extensive occurrence of below average natural water availability, either in the form of precipitation, river runoff, or groundwater. Droughts may be classified as meteorological or hydrological: A Meteorological Drought occurs when there is below average availability of precipitation. The National Meteorological Service (NMS) subdivides meteorological drought into three categories: drought, extreme drought, and severe drought. Drought is defined as less than 60% (range 41-60) of normal rainfall over 8 consecutive weeks. Extreme drought is defined as less than 40% (range 21-40) of the normal rainfall over 8 consecutive weeks. Severe drought is defined as less than 20% (range 0-20) of the normal rainfall over 8 consecutive weeks. The normal rainfall is the thirty year mean. A Hydrological Drought is marked by a deficit in the runoff of rivers where discharge (flow) remains below a low threshold or the rivers dry up completely and remain dry for a very long time and a significant natural depletion of aquifers (groundwater). The low threshold flow is the 10-year return period 7-day minimum mean flow. Environmental Impact Assessment (EIA) An environmental impact assessment determines the potential or actual impact(s) of an activity on the environment. Evapotranspiration Evapotranspiration is the combination of evaporation (the transfer of water from the liquid to the gaseous state) and transpiration (the process by which plants remove liquid water from the soil and release it to the air as gaseous water). Groundwater (a.k.a. Underground Water) Groundwater is that subsurface water contained in the interconnected pore spaces below the water table of an aquifer. Groundwater will occur where there is an impermeable subsurface barrier that allows water flowing through the unsaturated zone to be collected and stored within the interconnected pore spaces at depth. This impermeable barrier may be bedrock or an impermeable layer of rock. Once collected in the saturated zone, groundwater will flow from areas where the water table is highest toward areas where it is lowest. This percolation of water through the saturated zone is powered by gravity. Eventually, percolating water may leave the groundwater system and be discharged into streams or other surface water bodies. Groundwater Discharge The removal of water from the saturated zone is called groundwater discharge. The discharge area is simply the geographic area in which discharge occurs. Groundwater Recharge The addition of water to the saturated zone is called groundwater recharge. In another context, recharge may refer to the amount of water added per unit of time, e.g. cubic metres per year. The recharge area is simply the geographic area in which recharge occurs. Hydrological Basin A hydrological basin is a geographical area drained by a particular surface water and/or groundwater system. The basin boundaries are demarcated so that there is generally no flow from one basin into another. Hydrological Sub-basin A hydrological 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. Discharge can take place from one sub-basin to another within the same basin via a surface channel or a groundwater flow path. Intermittent Streams Streams which go dry if much time elapses between rains. Perennial Streams Steams with continuous flow. Pollution Risk Pollution risk is the probability that resource contamination will occur beyond acceptable limits. Reliable Yield The reliable yield or exploitable surface water is the daily water flow that is exceeded 90 percent of the time. This level of reliability is sufficient for irrigation; domestic and industrial water supplies usually need a higher level of reliability. Rock Formation A rock formation is a body of rock of considerable thickness that has characteristics which make it distinguishable from an adjacent rock unit. Safe Yield The safe yield or exploitable groundwater is the quantity of groundwater which can be withdrawn over a long period without impairing the aquifer as a water source or causing contamination by intrusion of seawater into the aquifer. Sewage Effluent Treated waste water from a sewage treatment plant. Surface Water That part of the rainfall that travels across or collects at the ground surface. Surface Water Runoff That part of the rainfall that travels across the ground surface to the nearest channel. Well A well is a borehole, adit tunnel, or any other excavation constructed or used for the abstraction of water.