The Hydrologic Cycle

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Alice Nash
6 years ago
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1 The Hydrologic Cycle Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 The hydrologic cycle is something that will be relevant as we discuss several of the remaining topics, but it is most important in understanding streams. In its simplest form it can be understood with this diagram: Water Vapor in Atmosphere Precipitation Evapotranspiration Liquid Water and Ice on Earth s Surface This cycling of water into and out of the atmosphere ensures that there is always fresh water (and/or ice) in lakes and streams, underground, and in snow-pack and glaciers. We will look at the components of this cycle in more detail in the following slides.

3 Evapotranspiration adds water vapor to Atmosphere Evaporation is a familiar process. Water comes in small molecules that are fairly easy to separate from the liquid phase. It evaporates from any place it occurs at or near the Earth s surface. Transpiration is similar, but involves a more complex route to the atmosphere. Evaporation from Soil Transpiration from Plants Evaporation from Lakes Evaporation from Streams Evaporation from Oceans

4 Evaporation During the summer, the man-made lakes in the desert southwest of the US can loose the equivalent of several feet of water from their surfaces each day. Indeed, the evaporation rate is so high that their levels drop even though more water comes in from upstream! They would do so even if water were not extracted for irrigation and municipal uses. The dams make more water readily available, but actually decrease the total amount available in these desert regions because of how much extra evaporation they allow. Because the Earth s surface is over 70% ocean, most of the vapor in the air comes from there. The same latitudes as the Earth s major deserts is desert even over the ocean less that 10cm of rain falls per year. Imagine the loss from desert lakes (which are very small areally) magnified by a continuous water surface, as in the oceans. The amount of vapor evaporating here is astronomical feet per day over more than half the area of the Atlantic, Indian, and Pacific Oceans!

5 Earth s rainforest and Taiga (green) lie at latitudes of 0 (equator) and near 30 N&S. Its desert belts (brown) lie near 30 and 90 (poles) N&S, as the map shows. DRY 60 WET 30 DRY 0 WET 30 DRY 60 WET Image National Geographic Society. Used under fair use clause of copyright law. DRY

6 Transpiration VAPOR TO AIR 2 Liquid water enters petiole and is distributed through leaf veins to thousands of stomata on lower surface. 1 Liquid water moves via its cohesion through the xylem toward the high salt concentration caused by the loss of water through stomata. 3 Water Molecules Evaporate Through Each Stoma stoma 4 Water remaining in leaf tissue has a higher salinity ( salt content) than water farther from the stoma. More water osmoses into the stoma cells to compensate, and this gradient propagates all the way back to the roots!

7 A single large broad-leaf tree can transpire thousands of gallons of water into the atmosphere on a warm day! The Great Smoky Mountains are called that for the transpired water vapor from their forest. On many days there is so much of it that it condenses as fog or mist, even at fairly low elevations. Between evaporation and transpiration a huge amount of water vapor is shunted into the atmosphere every day. Bodies of water and transpiring leaves do not go dry because water is added to them at the same rate, on average, that it is removed.

8 So now we have a better idea of how the evapotranspiration side of this system works. Water Vapor in Atmosphere Precipitation Evapotranspiration Liquid water on Earth s Surface It remains to examine how the vapor is precipitated and how the surface water behaves.

9 Precipitation Our atmosphere is made of fluid gasses, which move in response to the forces that result from local differences in pressure. These motions occur both horizontally (which we perceive as wind) and vertically (which we can measure as atmospheric pressure with a barometer). Ultimately, the primary cause of the motions is differential heating of the air by sunlight (insolation). In places with more insolation the warmer air is necessarily less dense that the air beside it. Consequently it rises as the adjacent, denser and heavier air, flows beneath it. As it rises, two things result: 1. Because there is progressively less air-weight above it, it expands more and its pressure decreases further. This is (within limits that result from the force of gravity) a positive feedback system, driving its own perpetuation. When a weatherman talks about a low this is what he means. 2. Also because of the progressively lower pressure the temperature of the air drops, just as the stuff coming out of a spray can (of, say, underarm deodorant) is noticeably colder that the air around it. Cooling air with water vapor in it drives that vapor to condense, if there is enough of it, as rain. (It also raises the air pressure, or, at least, keeps the lowering in check, partly offsetting its tendency to keep rising.) This is called adiabatic cooling.

10 Colder air sinking. (Adjacent Convection Cell) No Rain 5. Eventually the air is cool and dense enough that it can (and will) sink beneath the warmer air below it, displacing that air laterally. 6. The subsiding air is warmed adiabatically as the pressure on it increases in the lower atmosphere. Its relative humidity goes down. 7. The air s descent is stopped at the surface and it is forced to spread laterally. Cooler air sinking. (Atmospheric high) 4. Gravity limits the air s upward motion causing it to spread sideways in the upper atmosphere. The winds aloft will thus move away from an atmospheric low. These winds continue to lose heat to space at this elevation. Relatively still air ONE CONVECTION CELL 8. Surface winds always blow toward an atmospheric low. (Ground) 3. As air cools its relative humidity rises. It doesn t gain any extra vapor, but what is there is more likely to condense. If it does, we get rain. 2. It is displaced upward by cooler, heavier air coming in to replace it. As it rises, it cools adiabatically. 1. Vapor-laden air is warmed as it approaches an area of high insolation. Rain (Adjacent Convection Cell) Warmer air rising. (Atmospheric low)

We can now revisit an earlier slide and tweak our interpretation of it. The rainy and desert zones are (mostly) where air is very likely to rise and sink respectively.

11 We can now revisit an earlier slide and tweak our interpretation of it. The rainy and desert zones are (mostly) where air is very likely to rise and sink respectively. We will look into this more deeply later this term and next. HIGH - Sinking 60 LOW - Rising 30 HIGH - Sinking 0 LOW - Rising 30 HIGH - Sinking 60 LOW - Rising Image National Geographic Society. Used under fair use clause of copyright law. HIGH - Sinking

12 Whether in a latitude belt that favors long-term climatic raininess (previous slides), or under the effects of random, temporary differential heating (a winter storm or a summer shower), rising air always sees an increase in relative humidity. If the RH reaches 100%, precipitation will occur. There are several possible types of precipitation. Most common, of course, is rain, the liquid type. Rain may reach the ground as liquid water or, in some cases (particularly in deserts), may re-evaporate before reaching the ground. (This is called virga ). Alternately, the water may condense in air below its freezing point and fall as ice snow or hail or may freeze on its way to the ground as sleet. It may also freeze after it hits the ground. In most (but not all) of these cases the ice subsequently melts and the water behaves just as it would had it fallen as liquid water. At high latitudes and at high altitudes the ice may persist and accumulate.

13 Snow/Sleet Snow/Sleet Rain/Hail Water can be stored as glacial ice In the highest mountain ranges (high altitude) and near the poles (high latitude) the precipitated ice is stored in glaciers. When glaciers expand, sea level drops markedly because of this stored water. When they thaw it will rise in proportion. We will get to glaciers later in the term.

14 Part of the liquid water that falls will soak into the pore spaces of the soil or rocks they land on. This will go on until those pores are saturated. The process is called infiltration and it recharges the groundwater. We will come back to groundwater as our next topic. Snow/Sleet Snow/Sleet Rain/Hail Water can infiltrate and recharge the groundwater

15 Snow/Sleet Snow/Sleet Rain Rain and snowmelt that does not infiltrate runs off. Once the ground is saturated, liquid precipitation will flow downhill and join the nearest stream. This we call runoff and is, of course, the part of the hydrologic cycle that interests us as we consider streams. melting

16 Before ending we should point out that runoff is not just that water that initially flows at the surface. It can be joined by meltwater from glaciers (some of which might also infiltrate and recharge groundwater) and by groundwater returning to the surface as springs. melting infiltration melting By the time it reaches the Gulf of Mexico, over half the water in the Suwannee River of northern Florida is unaccounted for by surface tributaries. That water comes from springs, most of them tiny and unnoticed. The Suwannee is a gaining stream. In contrast, by the time the Colorado River reaches the Gulf of California (Sea of Cortez) there is no water left. This is largely because of withdrawal for irrigation and municipal water supplies, but the Colorado has always been a losing stream losing water to infiltration and evaporation below the Grand Canyon. melting springs infiltration

Thermal / Solar. When air is warmed it... Rises. Solar Energy. Evaporation. Condensation Forms Clouds

Thermal / Solar. When air is warmed it... Rises. Solar Energy. Evaporation. Condensation Forms Clouds Thermal / Solar Light from the Sun is transformed into what type of energy when it hits Earth's surface? Rises When air is warmed it... Solar Energy Water moves through the water cycle using what type