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.

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

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.

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

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.

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

• 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.

• 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

• 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.

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)

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