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 hight. Sand and gravel have high permeability

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