lecture 19 ground water (2) ground water storage porosity water retention and yield storage change
TRANSCRIPT
Lecture 19 Ground Water (2)
• Ground water storage
• Porosity
• Water retention and yield
• Storage change
Ground Water Storage
• Aquifers are reservoirs for water storage
• Water moves very slowly in aquifers acting as water pipelines
• The age of aquifers ranges from a few weeks to several tens of thousand years, depending on the depth and size.
Porosity
Figure 6.10, WR
Terms to RememberPorosity: percentage of the total volume of a rock or soil which is occupied by interstices or voids
Original interstices: pores which were created at the time of origin of the rock
Secondary interstices: pores results from subsequent actions of geological, climatic and biotic forces or factors upon the original rock, such as faults and joints enlarged by weathering and solution
Effective porosity: total porosity less the portion of interstices which are not interconnected with other and are hydrologically inert.
Porosity vs. soil/rock
type
Table 5.1, WR Ground Water
Specific yield and specific retention
Figure 5.3, WR Ground Water
Terms to Remember
Ground Water
Specific yield: the volume of water than can freely drain from a saturated rock or soil under the influence of gravity, usually expressed in percentage of the total aquifer volume
Specific retention: the percentage of the total aquifer volume which is retained by the surface tension as films around the individual grains and in capillary openings.
Ground Water Storage Change
• Infiltration of precipitation• Seepage from lakes, rivers, and even oceans• Inflow from adjacent aquitards and aquifers• Artificial recharge (irrigation, wells, pipelines, sewers, etc.)
ΔЅ=Qr-Qd ΔЅ = storage change per unit time Qr = recharge per unit time
Qd = discharge per unit time
Recharge:
Discharge:
• Evaporation• Spring flow and seepage into surface water bodies• Leakage to aquitards and other aquifers• Artificial extraction
Self reading
Characteristics of change in unconfined aquifers(WR Chapter 5.4.3)
Characteristics of change in confined aquifers (WR Chapter 5.4.3)