Groundwater

Water drains vertically below the root zone under the force of gravity until it reaches impermeable or non-conductive strata. This deep percolating water accumulates as perched groundwater in a temporarily saturated zone above less permeable material or it enters the saturated porous rock or uncon-solidated materials that form the regional groundwater system. A geologic unit that stores and transmits enough water to be hydrologically significant is known as an aquifer (Hillel, 2004). The fluctuating upper boundary of the saturated zone is the water table. In many parts of the world, groundwater recharge is mainly a seasonal phenomenon during the rainy season or the snowmelt season. In either case, groundwater recharge is a residual quantity remaining after precipitation has been allocated to surface runoff, soil moisture, and evapotranspiration. Under most natural conditions, groundwater discharges into rivers, lakes, or directly into the ocean, but in some instances groundwater is drawn upward by capillary forces (Dingman, 1994). Water at depth commonly moves very slowly, and the outflow of groundwater into streams may lag the occurrence of precipitation by days to years. The slow movement of groundwater accounts for it being considered water in storage in the same way a lake or reservoir is viewed as a storage mechanism.

Groundwater flow tends to be very regular, but groundwater flow patterns are controlled by the elevation and location of recharge and discharge areas, the heterogeneity of the geologic materials, the thickness of the strata, and the configuration of the water table (Bair, 1995). Two major types of idealized groundwater flow are unconfined and confined flow. In unconfined flow, the pressure at the water table is atmospheric and the hydraulic head is equal to the water table elevation above some specified datum. Recharge to unconfined aquifers typically occurs from water percolating vertically from the surface above the aquifer. The elevation of the water table varies as the flow through the aquifer changes. Consequently, flow in unconfined aquifers is analogous to surface flow in streams.

A confined aquifer is bounded by strata with significantly lower hydraulic conductivity than the strata forming the aquifer. Recharge for a confined aquifer typically occurs from water infiltrating at the highest elevation end of the strata where the aquifer is not confined and a water table is present. The flow in confined aquifers is analogous to flow in pipes because the boundary of the flow for a confined aquifer does not change (Hillel, 2004).

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Hypothetical runoff hydrograph for a drainage area of about 4 km2 common hydrograph components and the hyetograph.

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The direction and rates of groundwater movement are controlled by the geology of a watershed that may not be readily apparent from the surface. The groundwater storage volume is a function of the specific yield and depth of the saturated layer. Specific yield is the volume of water that will drain from an aquifer relative to the total volume of the aquifer or the water released from storage per unit surface area of aquifer per unit change in the water table. Average specific yield values for unconfined aquifers typically range from 0.01 to 0.40 (Bair, 1995). Differences in the extent of the aquifer system are important in determining the volume of water actually retained in the regional groundwater system. Groundwater discharge into the surface stream system contributes water identified as base flow that sustains streamflow during dry periods and represents the main long-term component of total runoff (Ward and Robinson, 2000).

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