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Geography Study Guide- Exam 2
The Hydrologic Cycle:
Transpiration- the movement of water vapor out through the pores in leaves; the
water is drawn by the plant roots from soil-moisture storage
Precipitation- rain, snow, sleet, and hail—the moisture supply (78% falls on the
ocean and 22% falls on land)
Evaporation- a weather instrument consisting of a standardized pan from which
evaporation occurs, with water automatically replaced and measured
Percolation- the process by which water permeates the soil or porous rock into the
Infiltration- water access to subsurface regions of soil moisture storage through
penetration of soil surface
The Soil-Water-Balance Equation:
A soil-water-budget can be established for any area of earth’s surface by
measuring the precipitation input and the output of various water demands in the
Groundwater is the largest potential freshwater source in the hydrologic cycle and
is tied to surface supplies
The moisture supply to earth’s surface is precipitation arriving as rain, sleet, snow
and hail. Precipitation is measured with the rain gauge. Evaporation is the net
movement of free water molecules away from a wet surface into air. Transpiration
in the movement of water through plants and back into the atmosphere; it is a
cooling mechanism fro plants. Evaporation and transpiration are combined into
one term—evapotranspiration. The ultimate demand for moisture is potential
evapotranspiration, the amount of water that would evaporate and transpire under
optimum moisture conditions. Evapotranspiration is measured with an
evaporation pan or the more elaborate lysimeter.
Unsatisfied potential evapotranspiration is deficit. By subtracting deficit from
potential evapotranspiration, we determine actual evapotranspiration. Ideally,
potential evapotranspiration and actual evapotranspiration are about the same, so
that plants have sufficient water. If potential evapotranspiration is satisfied and
the soil is full of moisture, then additional water input becomes surplus, which
may puddle on the surface, flow across the surface toward stream channels, or
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percolate underground through the soil. The overland flow to streams includes
precipitation and groundwater flows into river channels to make up the total
runoff from the area.
A “saving account” of water that receives deposits and provides withdrawals as
water-balance conditions change is the soil-moisture storage. This is the volume
of water stored in the soil that is accessible to plant roots. In soil, hygroscopic
water is inaccessible because it is a molecule-thin layer that is tightly bound to
each soil particle by hydrogen bonding. As available water is utilized, soil reaches
the wilting point. Capillary water is generally accessible to plant roots because it
is held in the soil by surface tension and hydrogen bonding between water and
soil. Almost all capillary water that remains in the soil is available water in soil-
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This note was uploaded on 10/11/2011 for the course PSYC, PSYC 2076, 2060 taught by Professor Briganti,gustan,perlis,namikas,wheeler during the Spring '10 term at LSU.