ATMOS 397G The Global Water Cycle

ATMOS 397G Main Processes involved in the Water Cycle nCondensation: process in which water molecules in the atmosphere change from a gas phase to a liquid phase nPrecipitation: process in which water falls on earth’s surface in the form of rain, snow, hail, and sleet nEvapotranspiration = Evaporation + Transpiration: process where liquid water either from land surface, soil, or ocean goes back to the atmosphere as water vapor nRunoff: process of water running off the land surface, occurs when precipitation exceeds evapotranspiration nInfiltration: process of water seeping into the ground nGroundwater flow: process that facilitates ground water coming back to the land surface

ATMOS 397G Reservoirs of Available Water (liquid equivalent) on Earth Reservoir Volume (10 6 km 3 ) Percent of Total Residence Time Oceans — 10 4 yr Glaciers — 10 3 yr Aquifers wks — 10 4 yr Lakes yr Soil Moisture days Atmosphere (~0.3 cm rainfall) days Rivers wks Biosphere days Based on Graedel, T. E., and P. J. Crutzen, 1993: Atmospheric Change. An Earth System Perspective. W. H. Freeman and Company. 446 pp.

ATMOS 397G Water Cycle and Vegetation Actual evapotranspiration (AET) is the amount of water delivered to the air from evaporation and transpiration (water use). nPotential 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) lTropical rainforests: AET = PET lDeserts: AET < < PET nAET is a useful predictor of net primary productivity nAnnual variability in AET greatest for ecosystem with low AET nTranspiration of land plants important factor in determining the movement of water in the water cycle and the Earth’s climate

ATMOS 397G Regional Differences in the Water Cycle nEvaporation decreases from the tropics to the poles nTropics a source of water vapor in the atmosphere resulting in high salinity in tropical oceans nRelative balance of precipitation and evaporation differs strongly between region on land lPrecipitation > evaporation in tropical rainforests  runoff lPrecipitation = evaporation in deserts  no runoff nSources of water contributing to precipitation differ in different regions lRainfall in maritime and monsoonal climates derived from evaporation from the sea lRainfall in Amazon basin derived from evapotranspiration and long- distance atmospheric-transport

ATMOS 397G Total Precipitable Water Vapor Derived From The GOES-8 And GOES-10 Sounders nPrecipitable water is the amount of liquid water, in millimeters, if all the atmospheric water vapor in the column is condensed. nThis information is particularly valuable for the short-term forecasting of fog, thunderstorms and precipitation. Its accuracy is +/-10%.