1. Lecture 19 Ground Water (2)
Ground water storage
Porosity
Water retention and yield
Storage change

2. 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.

3. Porosity
Figure 6.10, WR

4. Terms to Remember
Porosity: 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.

5. Porosity vs. soil/rock type
Table 5.1, WR Ground Water

6. Specific yield and specific retention
Figure 5.3, WR Ground Water

7. Terms to Remember
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

8. Ground Water Storage Change
ΔЅ=Qr-Qd
ΔЅ = storage change per unit time
Qr = recharge per unit time
Qd = discharge per unit time
Recharge:
Infiltration of precipitation
Seepage from lakes, rivers, and even oceans
Inflow from adjacent aquitards and aquifers
Artificial recharge (irrigation, wells, pipelines, sewers, etc.)
Evaporation
Spring flow and seepage into surface water bodies
Leakage to aquitards and other aquifers
Artificial extraction
Discharge:

9. Self reading Characteristics of change in unconfined aquifers
(WR Chapter 5.4.3)
Characteristics of change in confined aquifers