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An Introduction to The Hydrologic Cycle. Hydrology focuses on the global hydrologic cycle and the processes involved in the land phase of that cycle. Hydrology describes and predicts:
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Hydrology focuses on the global hydrologic cycle and the processes involved in the land phase of that cycle. Hydrology describes and predicts: • The spatial and temporal variations of water substance in the terrestrial, oceanic, and atmospheric compartments of the global water system. • The movement of water on and under the earth’s surface, the physical, chemical, and biological processes accompanying that movement.
Historical Benchmarks • Monitoring of river flows started by Egyptians 3800 B. P. • Concept of hydrologic cycle, 3000 B. P., King Solomon in Ecclesiastes: All the rivers run into the sea; ….. unto the place from whence the rivers come, thither they return again. • Rainfall measuring, 2400 B. P. in India. . . . . • Formal recognition of the scientific status of hydrology, 1920’s and 1930’s, UGG, AGU…etc.
Atmosphere Biomass Glaciers Soil moisture Rivers Lakes Ground water Oceans
Changes in Water Storage Last glacial period: Sea level 400 ft lower than today Last inter-glacial period: Sea level 18 ft higher than today 3M years B.P.: Sea level 165 ft higher
Water Storage in the Atmosphere • 0.001% • Water vapor • Clouds (water vapor condensed on particulate)
What two processes change liquid water into vapor that can ascend into the atmosphere? • Evaporation 90% • Transpiration 10% What percent of the water in the atmosphere comes from evaporation?
Evaporation • Conversion of water from a liquid into a gas • Water transferred from surface to atmosphere through evaporation
Evaporation & the Sun The sun heats up water in the ocean, rivers, or lakes, turning it into vapor, which then goes into the air
Evaporation Cont. • 80% of evaporation from ocean • 20% of evaporation from inland water and vegetation • Winds transport evaporated water, influencing humidity around the world
Evaporation: As water is heated by the sun, its surface molecules become sufficiently energized to break free of the attractive force binding them together, and then evaporate and rise as invisible vapor in the atmosphere.
Evaporation • Primary mechanism for surface-to-atmosphere water transport • Energy breaks bonds that hold molecules together • Net evaporation occurs when the rate of evaporation exceeds the rate of condensation • Removes heat from the environment: Net Cooling
Transpiration The process of water loss from plants (plants sweating) through stomata. (Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues.) • passive process that depends on: • humidity of the atmosphere • the moisture content of the soil • only 1 % of the water transpired used for growth • transports nutrients from the soil into the roots and carries them to the various cells of the plant • keeps tissues from becoming overheated
Transpiration Cont. Environmental factors that affect transpiration: • Light • Temperature • Humidity • Wind • Soil water
Every day an actively growing plant transpires 5 to 10 times as much water as it can hold at one time.
Condensation • Condensation is the process of water changing from vapor to liquid. Water vapor in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warmer air rises, the water vapor will lose energy, causing its temperature to drop. The water vapor then has a change of state into liquid or ice.
Condensation & Convection • Warm air rises • Air cools and can no longer hold vapor • Process known as CONVECTION
Condensation & Cloud Formation • Cooling of water vapor forms CLOUDS • Other ways clouds are formed • Convergence • Lifting of air by fronts
Precipitation • Precipitation is water being released from clouds as rain, sleet, snow, or hail. Precipitation begins after water vapor, which has condensed in the atmosphere, becomes too heavy to remain in atmospheric air currents and falls. Under some circumstances precipitation actually evaporates before it reaches the surface.
Precipitation • The vapor that accumulates or freezes on condensation nuclei is acted on by gravity and falls to Earth’s surface. • Primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth rain, freezing rain, sleet, snow, or hail
Total Precipitable Water The total atmospheric water vapor contained in a vertical column of unit cross-sectional area from the Earth’s surface to the “top of the atmosphere”
Snow • Snow is a type of precipitation like rain but at cooler temperatures • Usually reaches the ground and stays • Eventually melts and becomes runoff in streams
Rain • Type of precipitation during warmer weather • Occurs when clouds become too full • Falls to earth in the form of water and runs off to streams & rivers
How many gallons of water fall when 1 inch of rain falls on 1 acre of land? About 27,154 gallons (102,800 liters) of water. On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches.
Evaporation v. Precipitation • About equal on a global scale • Evaporation more prevalent over the oceans than precipitation • Over land, precipitation exceeds evaporation • Most water evaporated from the oceans falls back into the ocean as precipitation • 10% of water evaporated from the ocean is transported over land and falls as precipitation • Once evaporated, a water molecule spends ~ 10 days airborne
Infiltration • Infiltration: A portion of the precipitation that reaches the Earth's surface seeps into the ground through the process called infiltration. The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water.
Groundwater begins as infiltration Precipitation falls and infiltrates into the subsurface soil and rock • Can remain in shallow soil layer • Might seep into a stream bank • May infiltrate deeper, recharging an aquifer • May travel long distances • May stay in storage as ground water
Factors affecting infiltration • Precipitation (greatest factor) • Magnitude, intensity, duration • Characteristics (rain, snow) • Soil Characteristics • Clay absorbs less water at a slower rate than sand • Soil Saturation • Higher saturation leads to more runoff instead • Land Cover • Slope of the Land • Hills enhance runoff velocity • Evapotranspiration • Plants use soil moisture to grow and transpire
-Rain water soaks into ground through soil and underlying rock layer. -Water cleaned as impurities filtered
Subsurface Water • As precipitation infiltrates subsurface soil, it forms two zones: • Unsaturated -- It includes the capillary fringe and pores in this zone contain both water and air • Saturated -- Water completely fills the voids between rocks and soil particles
Infiltration replenished aquifers • Slow process -- ground water moves slowly through the unsaturated zone • Recharge Rate determined by precipitation & depth • An aquifer in New Mexico, if emptied, would take centuries to refill whereas a shallow aquifer in south Georgia may be replenished almost immediately
Artificial Recharge • Enhanced Infiltration Spread water over the land in pits, furrows, ditches or build small dams in creeks and streams to deflect runoff • Ground water injection Construct recharge wells and inject water directly into aquifers
Surface Runoff • When precipitation rate exceeds infiltration rate, or when soil is saturated, water begins to move down slope on ground surface.
Runoff: When there is a lot of precipitation, soils become saturated with water. Additional rainfall can no longer enter it. Runoff will eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes.
Meteorological factors affecting surface (over soil) runoff - Type of precipitation - Rainfall intensity - Rainfall amount - Rainfall duration - Distribution of rainfall over the drainage basin - Direction of storm movement - Precipitation that occurred earlier and resulting soil moisture - Meteorological conditions that affect evapotranspiration
Physical characteristics affecting surface runoff - Land use - Vegetation - Soil type - Drainage area - Basin shape - Elevation - Topography, especially the slope of the land - Drainage network patterns - Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or delay runoff from continuing downstream
Runoff • Drains to a creek • To a stream • To a river • To ocean • Sometimes runoff drains to a closed lake • May be diverted for human uses
About 3% of precipitation will seep underground • About 31% will run off into rivers, streams and lakes • About 66% of the water returns to the atmosphere through evaporation and transpiration
A hydrologist is particularly interested in stream flow -- the 31% of precipitation which runs off into rivers, streams and lakes.
Quiz Watch the following Water Cycle movie (about 3 min.). Provide a one-page scientific (not too scientific) narrative describing what is seen in the movie i.e. describe the main processes (e.g. evaporation, transpiration, precipitation, infiltration, infiltration, runoff…etc).
