WATER CYCLE
· Earth is the only planet with water in all three forms (solid, liquid, gas)
· Heated by the sun, water moves on the Earth in hydrologic cycle
· Oceans contain 97% of water
· Less than 3% of water on earth is freshwater
· Most of the freshwater on Earth is held in icecaps and glaciers (77% of freshwater)
· Groundwater holds 22% of the Earth's freshwater
· Freshwater lakes and streams hold less then 1/3 % of Earth's freshwater
· Evaporation (84% water vapor from ocean evaporation) brings water vapor into the atmosphere. Salts are left behind in the ocean during evaporation è produces freshwater precipitation
· Precipitation over the land surface causes runoff into river systems and infiltration into groundwater systems
· Surface water and groundwater flow to hydrologic base levels
· Most water flows to the ultimate base level (sea level) and returns to the oceans

EVAPORATION AND CONDENSATION
· Sources of water vapor in the atmosphere are:
o Evaporation (most from oceans as have larger surface areas)
o Transpiration = release from plants. Water is taken up by plants with the roots, moves upward through trunk/stems as sap and released by small holes on the undersides of leaves (stomata). Examples: 11,000 to 15,000 liters each day for an acre of corn; 150,000 liters per year for a large oak tree.
· Transpiration often produces more vapor from land surfaces, lakes and streams than does evaporation. However, most of the water vapor present is from oceanic evaporation and subsequent movement inland
· The maximum amount of water vapor the air can carry without loss by condensation (saturation level) depends on the air temperature
· Higher temperature air can hold more water vapor
· Air can become oversaturated with water if it is cooled resulting in condensation of water vapor to liquid water droplets
· When moist air cools condensation generally occurs (water on outside of ice-filled glass)

PRECIPITATION
· Cooling of atmosphere with moist air can result in precipitation (temperature falls below the dew point)
· Principal cause of cooling is the lifting of warm air to higher and cooler altitudes
· As air is lifted from ground level upward, pressure on the air lessens and it expands. Molecules in air will spread farther apart (reducing collisions of molecules) and air cools. Example: metal cans containing compressed gas (i.e., bug bombs) get cold when the gas and pressure are released
· If cooled sufficiently, vapor will condense into droplets of water (rain) or ice (snow)
· What will cause air to lift upward?
o Orographic effect = winds blowing air towards mountain. Air is forced upward along mountainsides. Windward side of mountain is rainy, lee side is dry and, if dry enough, has a rainshadow desert
o Atmospheric fronts = mass of warm or light air meets a cold and heavy mass of air. Lighter air rises over the heavier air.
o Ground surface heating = warm ground surface on hot midsummer days heat air from below. Heated air expands, becomes lighter, and rises.
· Clouds are composed of droplets of ice or condensed water
o Wispy clouds at high levels = small ice crystals
o Dark storm clouds = water droplets
o Fleecy clouds = water droplets

GLOBAL ATMOSPHERIC CIRCULATION
· Atmospheric motion is driven by the uneven distribution of solar energy
· Amount of solar radiation absorbed by Earth decreases with distance from the equator
o Angle at which solar rays hit spherical Earth ranges from nearly vertical (at equator) to nearly horizontal (at poles)
o At near horizontal angle, less solar energy is received per unit surface area of Earth - same amount of solar energy spread over a larger area at the poles vs. equator
o At poles, low angle of solar rays results in passage through greater thickness of atmosphere
o Length of days (controlled by Earth's spin axis tilted at an angle with respect to the plane of orbit) varies with seasons. In Winter, spin axis is tilted away from the Sun resulting in shorter days. At poles during winter, no sun falls on surface for weeks. Equator seasonal variations are much less.
· Global atmospheric circulation results as the temperature differences at poles and equator try to equalize (reach equilibrium)
o At equator, warm air expands and rises. At high altitudes, air cools è rain, becomes denser and descends at the subtropical latitudes (30 degrees N and S also called Tropics of Cancer and Capricorn)
o At subtropical latitudes, descending air is warmed (increasing evaporation > precipitation) è deserts found between 15 and 30 deg latitude. At surface, air returns to equator deflected by Earth's rotation (Coriolis effect) forming the trade winds
o Similar circulation cells are produced between 30 degrees and 60 degrees latitude (temperate cells). Warmer air rises along the cold polar fronts (at 60 deg latitude), cools and returns to surface at subtropical latitude (30 deg latitude). Surface winds returning air to 60 deg latitude produce westerly winds of Northern Hemisphere

GLOBAL CLIMATES
· Tropical climates - Near the equator, annual average temperature exceeds 20 deg C. Precipitation rates as high as 2 m/yr. Most rain usually falls in a wet summer season.
· Desert climates - precipitation is less than the evaporation rate. Occurs globally at subtropical latitudes. Prevailing winds and mountain occurrence control the location of rainshadow deserts.
· Temperate climates - between latitudes of 35 and 60 deg, annual temperatures range from 0 to 25 deg C. Precipitation falls throughout the year.
· Polar climates - above 60 deg latitude, average temperature is less than 10 deg C.

RAINFALL PATTERNS
· Rainfall is described in units of length (mm or inches) per time. 1 mm/hr = One millimeter of rain falling on a unit area of ground surface in one hour = water standing in a pan 1 mm deep after 1 hr
· Eastern US states receive from 1,000 to 1,270 mm annually
· Midwestern US states receive from 750 to 1,000 mm annually
· Western US states (area west of the 100th meridian)
o Nonmountainous but east of California Sierra Nevada range receives 500 mm or less annually
o Mountain tops can receive as much as 900 mm annually
o Valley areas range from 380 to 500 mm annually
o Sierra Nevada mountain tops receive from 1,500 to 1,750 mm annually
o Coastal California receives from 500 to more than 2,500 mm annually
· Differences in the season of greatest precipitation
o California has a Mediterranean climate with the rainy season in the winter and a dry summer
o Eastern Arizona, New Mexico and west Texas have 2 rainy seasons = frontal winter storms and summer thunderstorms
o Northern states rainfall is distributed fairly uniformly through spring, summer and fall, with less rain and more snow in the winter

RAINFALL INTENSITY
· Intensity of rains will affect floods and erosion
· Maximum 24-hr rainfalls range from 100 to 500 mm in northern CA, western WA and OR
· Eastern US states average from 150 to 200 mm in 24 hrs
· Maximum 24-hr rainfalls of 250 to 500 mm found in Texas, Iowa, western Carolinas and Florida
· 1-hr rainfalls ranging from 75 to 130 mm found in Texas, Florida and eastern N.C.
· Great flood of 1993 in Mississippi broke most rainfall records
o 177 mm in 24-hr in Minnesota
o 165 mm in 15-min in New London, Iowa

INFILTRATION
· Compare ground surface to a sieve. Water infiltrating into the ground is controlled by the pore sizes (sieve opening size) and interconnection. For a given rainfall rate, large size pore spaces will allow more infiltration and smaller size pore spaces will cause more surface runoff
· Large amount of rainfall on a sand or gravel surface will usually infiltrate into the ground and little surface runoff
· Large amount of rainfall on silt or clay surface will usually have limited infiltration and more surface runoff
· Presence of organic matter (loam) and vegetation will affect infiltration and runoff. Generally will increase infiltration.
· Infiltration rates are stated in units of length (mm or inches) per time.
o Rainfall = 1 mm/hr and Infiltration rate = 0.25 mm/hr è remaining 0.75 mm/hr becomes surface runoff
· Infiltration rates are variable with time. Rates are highest when the soil is dry and lower after it is wetted è infiltration rates decrease with time during a rainstorm and finally assume a uniform and minimum value
o Wetting soil causes clay or silt to expand and closes some of the pore spaces
o The film of water that surround each grain is more or less continuous and interconnected. Water moves downward through this water network but permeability is limited to a maximum value once the soil zone is saturated with water.

EVAPORATION RATES
· Evaporation from open water bodies can be great
o Northcentral New Mexico annual evaporation from reservoirs is 1,200 to 1,500 mm
o Southern Texas annual evaporation from reservoirs is 1,750 to 2,030 mm
o Eastern US = 750 to 1,000 mm annually

WATER CYCLE
· Precipitation on land surface is divided into 3 pathways
o Evapotranspiration (70% returned to atmosphere)
o Infiltration into ground è groundwater and in time, rivers
o Surface runoff è rivers