1. 28.1 The Hydrologic Cycle
Hydrological cycle: natural circulation of water from the oceans to the air, then to the ground, then to the oceans and then back to the atmosphere.

2. Balance The total amount of water vapor in the atmosphere is constant
This can occur if evaporation and precipitation balance each other
Evaporation and precipitation is greater over oceans
85% of atmosphere’s water vapor is water evaporated from oceans and 75% of the water vapor is precipitated back to oceans
On the continents, precipitation (25%) exceeds evaporation (15%)
In this case balance is achieved (85+15 =75+25)
The rain or snow that falls on the continents is the earth’s only natural supply of fresh water.
More than ¾ of the earth’s fresh water is in polar ice caps and glaciers.
Most of the freely flowing fresh water is beneath the surface of the earth (ground water)
Rain falls and sinks into the ground, it percolates dopwnward

4. 28.2 Ground water
98.5% of Earth fresh water that is not in the form of ice is in the porous region under earth
Beneath earth: either as groundwater or soil moisture.
Ground water is in saturated zones: the underground region were water has completely filled all open pores spaces
Soil moisture in un-saturated zones: pore spaces are no completely filled with water but contain also air
Pressure in the underground water increases with depth
We can pump groundwater from ground
In un-saturated zone, the air prevents us from pumping water.

6. Porosity and Permeability
Some soils, such as sand, soak up water easily. Other soils like clay do not. Rocky surfaces are the poorest absorbers of water
The amount of water that can be contained in a location depend on the porosity.
Porosity is the ratio of open space to the volume of solids plus voids
It is a measure of open space
It depends on the size and shape of the particles
It represent the maximum amount o ground water.
Hydraulic conductivity: measure of permeability. It tells the degree to which geological materials can transmit water
It pores are small and poorly connected water barely move
Also it is difficult to move through clay permeability (hydraulic conductivity is almost zero), although porosity is very high.
Sand and gravel have high permeability

7. The water table
Just below the surface of earth we find the unsaturated zone where pore spaces are partially filled with water
As we descend further we reach the saturated zone, where pores are filled with water
The boundary between the zones is called the water table.

8. Aquifers and Springs
Aquifer: any water-bearing underground region through which water can flow. Reservoir.
Unconfined aquifers: the soil or sediment above the water table is permeable, which allow the recharge of water

9. Confined aquifers: sandwiched between continuous low-permeability layers. Recharge does not come from directly above . It comes from unconfied portions of the aquifer at higher elevations.
Artesian systems: groundwater in the confined aquifers (under pressure) flows out through openings at lower elevations.
Artesian spring: If the opening is natural and water flows out of the ground.
Artesian well: if the opening is drilled.

10. Discontinuous low-permeability layers in an unconfined aquifers can intercept downward-percolating water above the water level. When this happened a perched water table is created

11. When the water table intercepts the land surface, groundwater emerges from an aquifer as either a spring, a steam or a lake. Springs are usually found where the water table intercepts the surface abruptly, such as on a hillside or on a coastal cliff. They are always associated with faults.

12. Ground water movement
Hydraulic head: The elevation of water above a sea level. It is the same elevation to which water in an unconfined aquifer rises in a well.
The higher the hydraulic head in a region the greater is the water pressure
The downward slope is called the hydraulic gradient (figure)

13. Darcey’s law
Groundwater flows because of pressure differences, from high to low head.
Ground water flow is proportional to hydraulic gradient
It also depends on the hydraulic conductivity and the cross-sectional area.
Darcy’s law
Groundwater flow rate=
Hydraulic conductivity x cross-sectional area x hydraulic gradient