THE WATER CYCLE AND HUMAN USE

THE SUN DRIVES EVAPORATION AND CONVECTION. ONCE WATER IS EVAPORATED, AIR CURRENTS MOVE THE MOISTURE. AS AIR RISES, IT COOLS, AND PRECIPITATION CAN OCCUR. WATER CONDENSES ON VERY FINE PARTICLES OF DUST. AS THE PARTICLES GROW, THEY CAN BECOME HEAVIER THAN AIR, AND RAIN CAN OCCUR. OR, IF TEMPERATURES ARE LOW ENOUGH, SNOW CAN OCCUR.

ONCE, RAIN OR SNOW FALLS, GRAVITY STARTS THE FLOW OF WATER BACK TO THE OCEAN. THE AVERAGE SURFACE HEIGHT OF LAND IS 875 METERS, WHICH IS ABOUT 2700 FEET. THIS FLOW OF WATER CAN CONTRIBUTE TO THE PROCCES OF WEATHERING AND EROSION. MOST GOES INTO GROUND WATER, AND THE REMAINING FLOWS BACK TO THE OCEAN IN THE FORM OF SURFACE WATER.

SUPPLIES OF WATER EXIST IN DIFFERENT PARTS OF THE WORLD AS SURFACE WATER AND GROUND WATER. THESE SUPPLIES ARE BASED ON CLIMATE AND NATURAL GEOGRAPHY. THESE SUPPLIES CAN CHANGE BASED ON CHANGES IN CLIMATE AND HUMAN INTERVENTION. FOR EXAMPLE, DURING THE LAST ICE AGE MUCH OF AFRICA WAS UNDER A DROUGHT. AREAS THAT WERE RAINFOREST BECAME SAVANAH.

A LARGE PERCENTAGE OF FRESH WATER IS TIED UP AS ICE. ALMOST 90% OF THIS IS PART OF THE ANTARTIC ICE CAP. 10% IS TIED UP IN THE GREENLAND ICE SHEET. THE REMAINDER IS IN GLACIERS. GLACIERS AND SNOW CAPS PLAY A MAJOR ROLE IN WATER SUPPLIES IN MANY PARTS OF THE WORLD, ESPECIALLY ASIA AND NORTH AMERICA. FOR EXAMPLE, MUCH OF THE WATER IN WESTERN U.S. RIVERS DEPENDS ON MELT RUNOFF DURING SUMMER MONTHS.

THE AMOUNT OF WATER TIED UP AS ICE HAS CHANGED OVER GEOLOGICAL TIME. RIGHT NOW, 2.5% OF THE WATER, WHICH IS 80% OF THE FRESH WATER IS ICE. IF ALL THE ICE MELTED, THE SEA LEVEL WOULD CHANGE BY ABOUT 75 METERS (230 FEET). THIS WOULD PUT THE MOUTH OF THE MISSISSIPPI RIVER NEAR SOUTHERN ILLINOIS. 20,000 YEARS AGO, SEA LEVEL WAS 400 FEET LOWER.

IN TURN, HUMAN INTERVENTION CAN CHANGE WATER AVAILABILITY. HUMANS HAVE HARVESTED SO MUCH FIREWOOD IN SUB SAHARAN AFRICA THAT THE DESERT HAS REPLACED MUCH OF WHAT WAS ONCE GRASSLAND. DEFORESTATION IS OCCURING IN THE AMAZON BASIN. 50% OF THE MOISTURE IN THIS AREA COMES FROM TRANSPIRATION OF MOISTURE FROM TREES. DEFORESTATION COULD RESULT IN THE AMAZON BECOMING GRASSLAND IN A SHORT PERIOD OF TIME.

IN MANY AREAS OF THE WORLD, WATER IS BEING PUMPED OUT OF AQUIFERS FASTER THAN IT CAN BE REPLINISHED BY RAINFALL AND INFILTRATION. THIS CAN QUICKLY RESULT IN WATER NOT BEING AVAILABLE FOR ANY HUMAN USE IN THOSE AREAS. ANOTHER EXAMPLE IS THE ARAL SEA. SO MUCH WATER IS TAKEN FROM RIVERS FEEDING THE ARAL SEA THAT THE VOLUME HAS DRASTICALLY BEEN REDUCED. THE OLD SEA BED IS DESERT, AND THE LAND AROUND THE ARAL IS BECOMING COOLER AND DRYER (MICRO CLIMATE CHANGE) DUE TO LOSS OF WATER.

GROUNDWATER: THE WATER HELD IN THE PORES OF SOIL AND ROCK. AS YOU DID DEEPER, THE SOIL BECOMES MOISTER. THE WATER TABLE IS THE TOP OF THE SATURATED ZONE. THE WATER TABLE TENDS TO FOLLOW THE TOPOGRAPHY OF THE LAND.

HOW DOES WATER BECOME GROUNDWATER? FIRST, IT HAS TO SOAK INTO THE GROUND. THIS HAPPENS WHEN IT RAINS. SOME OF THE WATER SOAKS INTO THE GROUND, AND SOME OF THE WATER BECOMES RUNOFF.

RUNOFF IS DETERMINED BY THREE FACTORS: 1. AMOUNT OF RAIN 2. TYPE OF SURFACE 3. SLOPE OF LAND THESE SAME FACTORS WILL DETERMINE HOW MUCH WATER SOAKS INTO THE GROUND. SOME SURFACES WILL BE MUCH MORE PERMEABLE THAN OTHERS. A PERMEABLE SOIL HAS PORES THAT ARE CONNECTED THAT WATER CAN FLOW THROUGH. A MATERIAL LIKE GRANITE WITH NO CRACKS WOULD BE IMPERMEABLE.

IF YOU HAD PLENTY OF TIME AND DUG DOWN INTO THE GROUND, THE SOIL WOULD GET WETTER AND WETTER THE DEEPER YOU DUG. IF YOU DUG DEEP ENOUGH, THE SOIL WOULD BE SO WET THAT IT WOULD NOT HOLD ANY MORE WATER. WE SAY THAT THIS SOIL IS SATURATED. THE TOP OF THE SATURATED ZONE IS THE WATER TABLE. IF THE GROUNDWATER IN THE SATURATED ZONE COVERS A LARGE AREA, WE CALL THIS AN AQUIFER. SOME AQUIFERS ARE HUGE.

NOTE: IF THE WATER TABLE ON THE SURROUNDING LAND IS HIGHER THAT A STREAM, FLOW OF GROUNDWATER INTO THE STREAM WILL KEEP THE STREAM FLOWING BETWEEN RAINS.

IDEALLY, IF YOU ARE DEPENDING ON GROUND WATER FOR PERSONAL WATER SUPPLY, YOU WOULD LIKE TO HAVE A WELL DRILLED IN A VERY TRANSMISSIVE GEOLOGIC FORMATION. IF THE FORMATION IS NOT VERY TRANSMISSIVE, A MUCH DEEPER WELL WOULD HAVE TO BE DRILLED. WATER HAS A VERY LONG RESIDENCE TIME IN AQUIFERS. WATER CAN REMAIN IN AQUIFERS FROM THOUSANDS OF YEARS TO HUNDREDS OF THOUSANDS OF YEARS.

WHAT THIS MEANS IS THAT POLLUTION AND WASTES THAT HAVE BEEN DUMPED ON THE GROUND IN THE LAST CENTURY MAY JUST BE REACHING AQUIFERS. A GOOD EXAMPLE OF AQUIFER CONTAMINATION IS THE USE OF MTBE IN GASOLINE AS A MEANS OF REDUCING HYDROCARBON PARTICULATES AND SMOG. PARTS OF CALIFORNIA ENDED UP WITH GROUND WATER CONTAMINATION AS A RESULT.

THE OGALLALA AQUIFER COVERS MOST OF THE GREAT PLAINS AND EXTENDS FROM SOUTH DAKOTA TO TEXAS. THERE IS A LARGE AQUIFER UNDER MEXICO CITY. AQUIFERS SUPPLY WATER TO MANY PEOPLE IN THE WORLD. THE PROBLEM IS THAT IF YOU TAKE OUT WATER FASTER THAN WATER GOES IN, YOU CAN DRAIN THE AQUIFER. THIS IS HAPPENING IN MEXICO CITY AND WITH THE OGALLALA AQUIFER. IN FLORIDA, SOME OF THE GROUND WATER FLOWS THROUGH UNDERGROUND CAVES IN THE LIMESTONE.

INCREASE IN SALT LEVELS IN WATER SUPPLIES CAN ALSO BE A PROBLEM. IN ARID REGIONS, SUCH AS THE DESERT SOUTH WEST, WATER USED FOR IRRIGATION CAN EVAPORATE, LEAVING SALT DEPOSITS BEHIND. HIGH SALT LEVELS CAN KILL VEGETATION AND CAUSE SOILS TO BECOME HADR AND MORE COMPACT. GROUNDWATER IN COASTAL AREAS CAN BECOME SALINE WHEN THE FRESH WATER IS PUMPED OUT FASTER THAN IT CAN BE REPLENISHED.

THIS CAUSES SALT WATER TO INTRUDE FROM THE OCEAN AND FROM ESTUARIES. THIS IS A PROBLEM IN EASTERN NORTH CAROLINA. Concern about aquifer depletion and salt water intrusion.

REVIEW 1) HOW CAN HUMAN INTERVENTION CHANGE WATER SUPPLIES?

REVIEW 1)HOW CAN HUMAN INTERVENTION CHANGE WATER SUPPLIES? A. PUMPING AQUIFERS DRY B. CONTAMINATING GROUND WATER C. USING UP SURFACE WATER FOR IRRIGATION D. POLLUTING RIVERS AND STREAMS E. CHANGING THE CLIMATE BY CUTTING DOWN TREES

2) WHAT COULD CAUSE SEA LEVEL CHANGE?

HOW MUCH WATER IS TIED UP AS ICE. THIS IS DETERMINED BY CLIMATE (AVERAGE GLOBAL SURFACE TEMPERATURE).

3) WHAT DRIVES THE WATER CYCLE?

ENERGY FROM THE SUN

4) WHAT IS AN AQUIFER?

A LARGE COLLECTION OF GROUND WATER

5) WHAT IS THE WATER TABLE?

THE TOP OF THE SATURATED ZONE

6) WHAT DETERMINES THE AMOUNT OF RUNOFF AFTER A RAIN?

A.HOW MUCH IT RAINED B.THE SLOPE OF THE LAND C.THE TYPE OF SURFACE