Water: Hydrologic Cycle and Human Use

Water: A Vital Resource

Polarity in H 2 O: The Water Molecule O HH covalent bond

Condensation O HH O HH O HH ++ --

Evaporation and Condensation Evaporation – liquid  gas  Purifies water Condensation – gas  liquid  Forms precipitation and clouds The process of evaporation and condensation provides all the fresh water on the planet

Condensation

Relative Humidity Low temperature holds less water than high temperature  Winter air is drier Amount of humidity that air can hold is relative to temperature

Effects of Temperature on Water Temperature (kinetic energy) counteracts the effect of hydrogen bonds  Gas: steam, water vapor Individual water molecules  Liquid: running water  Solid: ice 0°C, 32°F Why does ice float?

The Water Cycle

Precipitation Adiabatic cooling: warm air rises, expands and cools 100% relative humidity + cooling = clouds Increasing condensation = water droplets

Global Precipitation Global precipitation distribution depends on patterns of rising and falling air currents Two main factors:  global convection currents  rain shadow effect

Global Air Circulation

Rain Shadow Effect

Water Balance in the Hydrologic Cycle

A B G I F Ground water loop Surface runoff loop Evapotranspiration loop C E D H Pools and Fluxes in the Cycle

Human Impacts on the Water Cycle Changing the Earth’s surface Floods Climate change Atmospheric pollution Withdrawing water supplies

Groundwater Decreased infiltration Increased runoff Decreased groundwater recharge

Human Impacts on the Hydrologic Cycle

Surface Waters Dam impacts Ecological effects of changing salinity in estuaries  Increased salinity  Loss of native species  Invasive species increase

Dams Three Gorges Dam  Largest in world  Controls deadly floods  Displaced 1.2 million people including farms, cities, homes, and factories  370-mile-long reservoir

Dams: Disrupt Integrity of River System Above the dam  Flooding  Sediment deposition  Loss of functional floodplain Below the dam  Loss of normal river flow patterns  Loss of river biota  Loss of functional floodplain

Grand Coulee Dam Along Columbia River Washington

Hoover Dam Along Colorado River Border of Arizona and Nevada

Consequences of Overdrawing Groundwater Falling water tables Diminishing surface waters (wetlands) Land subsidence (sink holes) Saltwater intrusion

Aquifer Exploitation Groundwater use exceeds aquifer recharge Many remaining aquifers are heavily polluted Ogallala Aquifer

Saltwater Intrusion

Desalting Seawater Reverse osmosis Distillation

Using Less Water Irrigation – greatest use  Surge flow – computer controlled irrigation  Drip irrigation – very expensive to install Municipal uses  Incentives and regulations limiting water use  Gray water recycling