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The Global Water Cycle

The Global Water Cycle. Main Processes involved in the Water Cycle. Condensation: process in which water molecules in the atmosphere change from a gas phase to a liquid phase Precipitation: process in which water falls on earth’s surface in the form of rain, snow, hail, and sleet

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The Global Water Cycle

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  1. The Global Water Cycle

  2. Main Processes involved in the Water Cycle • Condensation: process in which water molecules in the atmosphere change from a gas phase to a liquid phase • Precipitation: process in which water falls on earth’s surface in the form of rain, snow, hail, and sleet • Evapotranspiration = Evaporation + Transpiration: process where liquid water either from land surface, soil, or ocean goes back to the atmosphere as water vapor • Runoff: process of water running off the land surface, occurs when precipitation exceeds evapotranspiration • Infiltration: process of water seeping into the ground • Groundwater flow: process that facilitates ground water coming back to the land surface

  3. Reservoirs of Available Water (liquid equivalent) on Earth Reservoir Volume (106 km3) Percent of Total Residence Time Oceans 1350 97.3 103 — 104 yr Glaciers 29 2.1 101 — 103 yr Aquifers 8 0.6 2 wks — 104 yr Lakes 0.1 0.01 10 yr Soil Moisture 0.1 0.01 52 days Atmosphere 0.013 (~0.3 cm rainfall) 0.001 10 days Rivers 0.002 0.0002 2 wks Biosphere 0.001 0.0001 6 days Based on Graedel, T. E., and P. J. Crutzen, 1993: Atmospheric Change. An Earth System Perspective. W. H. Freeman and Company. 446 pp.

  4. Water Cycle and Vegetation • Actual evapotranspiration (AET) is the amount of water delivered to the air from evaporation and transpiration (water use). • Potential Evapotranspiration (PET) is the maximum evapotranspiration that is expected to occur under the climatic conditions of a particular site, assuming that water is always present in the soil and plant cover is 100% (water demand) • Tropical rainforests: AET = PET • Deserts: AET < < PET • AET is a useful predictor of net primary productivity • Annual variability in AET greatest for ecosystem with low AET • Transpiration of land plants important factor in determining the movement of water in the water cycle and the Earth’s climate

  5. Regional Differences in the Water Cycle • Evaporation decreases from the tropics to the poles • Tropics a source of water vapor in the atmosphere resulting in high salinity in tropical oceans • Relative balance of precipitation and evaporation differs strongly between region on land • Precipitation > evaporation in tropical rainforests  runoff • Precipitation = evaporation in deserts  no runoff • Sources of water contributing to precipitation differ in different regions • Rainfall in maritime and monsoonal climates derived from evaporation from the sea • Rainfall in Amazon basin derived from evapotranspiration and long-distance atmospheric-transport

  6. Total Precipitable Water Vapor Derived From The GOES-8 And GOES-10 Sounders • Precipitable water is the amount of liquid water, in millimeters, if all the atmospheric water vapor in the column is condensed. • This information is particularly valuable for the short-term forecasting of fog, thunderstorms and precipitation. Its accuracy is +/-10%.

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