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Daniel Marques
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2322597
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Parent(s):
1e98b17
feat: add epi info dataset
Browse files- SOURCE_DOCUMENTS/dataset.txt +268 -1
SOURCE_DOCUMENTS/dataset.txt
CHANGED
@@ -25414,7 +25414,274 @@ and precipitate. Precipitation and evaporation vary with latitude and their rela
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25414 |
to the global wind belts. The trade winds, for example, are initially cool,
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25415 |
but they warm up as they blow toward the Equator. These winds pick up moisture
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25416 |
from the ocean, increasing ocean surface salinity and causing seawater at the surface to sink.
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25417 |
-
When the trade winds reach the Equator, they rise, and the water vapour in them condenses and forms clouds.
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25418 |
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25419 |
The EPI - Environmental Performance Index EPI provides a data-driven
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25420 |
summary of the state of sustainability around the world. Using 40 performance
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25414 |
to the global wind belts. The trade winds, for example, are initially cool,
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25415 |
but they warm up as they blow toward the Equator. These winds pick up moisture
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25416 |
from the ocean, increasing ocean surface salinity and causing seawater at the surface to sink.
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25417 |
+
When the trade winds reach the Equator, they rise, and the water vapour in them condenses and forms clouds.
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25418 |
+
Net precipitation is high near the Equator and also in the belts of the prevailing westerlies, where there is
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25419 |
+
frequent storm activity. Evaporation exceeds precipitation in the subtropics, where the air is stable,
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25420 |
+
and near the poles, where the air is both stable and has a low water vapour content because of the cold.
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25421 |
+
The Greenland Ice Sheet and the Antarctic Ice Sheet formed because the very low evaporation rates at the
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25422 |
+
poles resulted in precipitation exceeding evaporation in these local regions.
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25423 |
+
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25424 |
+
Water vapour and precipitation
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25425 |
+
As noted above, water exists in the atmosphere in gaseous form. Its liquid form,
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25426 |
+
either as water droplets in clouds or as rain, and its solid form, as ice crystals in clouds,
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25427 |
+
snowflakes, or hail, occur only momentarily and locally.
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25428 |
+
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25429 |
+
Water vapour performs two major functions: (1) it is important to the radiation balance of
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25430 |
+
Earth, as its presence keeps the planetary surface warmer than would otherwise be the case,
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25431 |
+
and (2) it is the principal phase of the ascending part of the water cycle.
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25432 |
+
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25433 |
+
The mass of water vapour in the atmosphere, which represents only 0.001 percent of the
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25434 |
+
hydrosphere, is highest in the tropics and decreases toward the poles. At a mean temperature
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25435 |
+
of Earth’s surface of 15 °C (59 °F), the partial pressure of water vapour at equilibrium with
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25436 |
+
pure water is 0.017 atmosphere. The addition of salts to pure water lowers its vapour pressure.
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25437 |
+
The equilibrium, or saturation, water vapour pressure of a saturated solution of sodium chloride
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25438 |
+
is 22 percent lower than that of pure water. Precipitable water vapour has, on the average,
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25439 |
+
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25440 |
+
a vapour pressure of 0.0025 atmosphere, which amounts to 15 percent of the saturation vapour
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25441 |
+
pressure. The ratio of observed water vapour pressure to the saturation vapour pressure at
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25442 |
+
the same temperature is the relative humidity of the air. Thus, the mean relative humidity
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25443 |
+
of the atmosphere is only 15 percent, a value that is low in human terms, in which
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25444 |
+
levels of 50 to 60 percent are preferred for maximum comfort. The relative humidity
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25445 |
+
of the air, however, varies greatly from one geographic region to another and also
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25446 |
+
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25447 |
+
vertically in the atmosphere. Atmospheric water vapour decreases rapidly with increasing
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25448 |
+
altitude relative to its surface value. The amount of water required to saturate a volume
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25449 |
+
of air depends on the temperature of the air. Air at high temperature can hold more water
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25450 |
+
vapour at saturation than can air at low temperature. Because the temperature of the lower
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25451 |
+
atmosphere (the troposphere) decreases rapidly with increasing altitude to about 15 km (about 9 miles),
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25452 |
+
the upper levels of the troposphere contain little water vapour; most of the vapour is
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25453 |
+
found within a few kilometres of Earth’s surface. The average relative humidity of tropospheric
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25454 |
+
air is about 50 percent. Above 15 km, water vapour is essentially frozen out of the atmosphere,
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25455 |
+
amounting to less than 0.1 percent of its concentration at Earth’s surface.
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25456 |
+
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25457 |
+
Aside from temperature, other factors determine the water vapour content of the air and
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25458 |
+
are particularly important in the lower troposphere. These factors include local evaporation
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25459 |
+
and the horizontal atmospheric transportation of moisture, which varies with altitude, latitude,
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25460 |
+
season, and topography. During a period of 10 days (i.e., the residence time of water in the atmosphere),
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25461 |
+
horizontal eddy turbulence may disperse vapour over distances up to 1,000 km (600 miles).
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25462 |
+
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25463 |
+
When a mass of air at Earth’s surface is exposed to a body of water, it gains water by evaporation or
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25464 |
+
loses water by precipitation, depending on its relative humidity. If the air is undersaturated, with a
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25465 |
+
relative humidity of less than 100 percent, it gains water vapour because the rate of
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25466 |
+
evaporation exceeds the rate of condensation. If the air is supersaturated, with a relative
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25467 |
+
humidity greater than 100 percent, the air mass loses water vapour because the rate of precipitation
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25468 |
+
exceeds that of evaporation. This interaction between air masses and surface water bodies drives the
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25469 |
+
atmosphere toward a state of saturation, which is not achieved for the entire atmosphere because of
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25470 |
+
the variability in weather and because not all air masses are in contact with water bodies. In general,
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25471 |
+
the level of atmospheric water vapour is higher in the summer, since temperatures are higher at this
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25472 |
+
time of year. Also, atmospheric water vapour content is higher near the source of moisture than in
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25473 |
+
distant regions. Over the oceans, the air is almost always near saturation, whereas over the deserts,
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25474 |
+
where the supply of moisture is limited, the air is far below water vapour saturation values. In most
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25475 |
+
cases, atmospheric water vapour content decreases inland over continents, but this decrease
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25476 |
+
is modified by rainfall conditions, by the presence or absence of high mountains, large lakes, extensive
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25477 |
+
forests, and swamps, and by the prevailing wind directions. Horizontal winds and air mass movements
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25478 |
+
transfer water vapour from the ocean to the land. Although the processes are not completely separable,
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25479 |
+
the horizontal transfer of water vapour seldom causes the vapour to undergo condensation, whereas vertical
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25480 |
+
movements are most important in the condensation process.
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25481 |
+
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25482 |
+
Condensation depends strongly on the average temperature of Earth’s surface because the water
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25483 |
+
vapour content of the air is strongly dependent on temperature. In figures that show the states of
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25484 |
+
water as a function of the variables of pressure and temperature, the slope of the phase boundary
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25485 |
+
between liquid water and water vapour is positive, implying that with increasing temperature the
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25486 |
+
air at equilibrium will hold increasing amounts of water vapour. Cooling or mixing of this air
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25487 |
+
results in condensation of the vapour and precipitation as water droplets or as ice crystals if
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25488 |
+
the air temperature is below 0 °C (32 °F). When first formed, the water droplets or ice crystals
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25489 |
+
are very small, on the order of 10−2 to 10−3 cm (0.004 to 0.0004 inch) in diameter, and they
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25490 |
+
float freely in the atmosphere. In large quantities, these water droplets and ice crystals
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25491 |
+
produce a cloud. All clouds are formed as a result of cooling below the dew point,
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25492 |
+
the temperature at which condensation begins when air is cooled at constant pressure and constant
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25493 |
+
water vapour content. When the droplets or crystals coalesce to a size of about 10−2 cm (0.004 inch)
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25494 |
+
in diameter, they become heavy enough to fall as raindrops or snowflakes. Hailstones measure about
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25495 |
+
10−1 cm (0.04 inch) in diameter or much larger. Water vapour condensing in the atmosphere contains
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25496 |
+
strongly soluble salts (mostly of oceanic origin), weakly soluble or insoluble solids (dust),
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25497 |
+
and dissolved gases. The dust and sea salt aerosol particles in the air may act as sites of
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25498 |
+
condensation by serving as nuclei for bringing initially a few water molecules together and inducing condensation from supersaturated air.
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25499 |
+
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25500 |
+
Distribution of precipitation
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25501 |
+
Precipitation falling toward Earth’s surface may suffer several fates. It may be evaporated
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25502 |
+
during its fall or after it reaches the ground surface. If the surface is covered with dense
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25503 |
+
vegetation, much of the precipitation may be held on leaves and plant limbs and stems. This
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25504 |
+
process is termed interception and may result in little water reaching the ground because
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25505 |
+
the water may be directly evaporated from plant surfaces back into the atmosphere. If
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25506 |
+
precipitation reaches the ground in the form of snow, it may remain there for some time.
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25507 |
+
On the other hand, if precipitation falls as rain, it may evaporate, infiltrate the soil,
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25508 |
+
be detained in small catchment areas, or become overland flow—a form of runoff. Overland
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25509 |
+
flow (Ro) may be expressed in terms of intensity units, water depth per unit of time (e.g.,
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25510 |
+
centimetres per hour, or inches per hour), as Chemical equation.
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25511 |
+
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25512 |
+
where P is precipitation rate and I is infiltration rate (rate of entry and downward movement
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25513 |
+
of water into the soil profile). Infiltration rate will equal precipitation rate until the
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25514 |
+
limit of the infiltration rate, or infiltration capacity, is reached. Soil infiltration rates
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25515 |
+
are usually high at the beginning of a rain preceded by a dry spell and decrease as the rainfall
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25516 |
+
continues. This change in rate is due to the clogging of soil pores by particles brought from
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25517 |
+
above by the infiltrating rain and to the swelling of colloidal soil particles as they absorb
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25518 |
+
water. Thus, rapid decreases in infiltration rates during a rain are more likely to occur in
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25519 |
+
clay-rich soils than in sandy soils.
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25520 |
+
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25521 |
+
Between rainfall periods, water held in the soil as moisture is gradually lost by direct
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25522 |
+
evaporation or by withdrawal by plants. Evaporation into the open atmosphere occurs at the
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25523 |
+
surface of the soil, and the soil dries progressively downward with time. Water vapour in
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25524 |
+
the soil diffuses upward, replenishing the evaporated water, and in turn is evaporated.
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25525 |
+
The pumping of air into and out of the soil by atmospheric pressure changes enhances the
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25526 |
+
movement of soil moisture upward. It has been shown that evaporation of a water droplet
|
25527 |
+
in the free atmosphere, and to a first approximation in various soil atmospheres, is
|
25528 |
+
proportional to the droplet surface area 4πr2 (square centimetres, where r is the radius
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25529 |
+
of the droplet), the diffusional flux of water at the droplet surface, and the transfer
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25530 |
+
of heat as the droplet evaporates. The equation for the rate of shrinkage of a water
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25531 |
+
droplet due to evaporation is Chemical equation.
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25532 |
+
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25533 |
+
where dr/dt is the rate of change in the radius of the water droplet (centimetres per second),
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25534 |
+
D is the diffusion coefficient of water vapour in air (cubic centimetres per second), ρvo is
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25535 |
+
the equilibrium vapour concentration at the droplet surface, Sp is the degree of undersaturation
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25536 |
+
of water vapour in the environment, ρL is the density of liquid water (grams per cubic centimetre),
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25537 |
+
and X is a dimensionless parameter depending on D, ρvo, temperature, the heat of evaporation of
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25538 |
+
water vapour, the coefficient of thermal conductivity of air, and the spherical coordinate system
|
25539 |
+
necessary to define processes occurring to a spherical water droplet. Water droplets
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25540 |
+
shrink—dr/dt < 0, evaporate—when the water vapour concentration in the environment
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25541 |
+
(atmosphere or soil atmosphere) is less than the saturation water vapour concentration
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25542 |
+
at the droplet surface. They grow—dr/dt > 0, condense—when the converse is true in the
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25543 |
+
free atmosphere. The term dr/dt has negative values for evaporation and positive ones
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25544 |
+
for condensation. Use of this equation shows, as an example, that it would take 23 minutes
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25545 |
+
for a water droplet to shrink (evaporate) in size from 50 to 5 micrometres (0.002 to 0.0002 inch)
|
25546 |
+
in air at 10 °C (50 °F) and a water vapour undersaturation of 1 percent.
|
25547 |
+
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25548 |
+
Besides simple evaporation of water from soils, water is also returned to the atmosphere by
|
25549 |
+
transpiration in plants. Plants draw water from soil moisture through their vast network of
|
25550 |
+
root hairs and rootlets. This water is carried upward through the plant trunk and branches
|
25551 |
+
into the leaves, where it is discharged as water vapour. The term evapotranspiration is used
|
25552 |
+
in climatic and hydrologic studies to include the combined water loss from Earth’s surface
|
25553 |
+
resulting from evaporation and transpiration. The maximum possible evapotranspiration is
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25554 |
+
termed potential evapotranspiration and is governed by the available heat energy. It is
|
25555 |
+
taken as equal to evaporation from a large water surface and is generally much less than
|
25556 |
+
actual evapotranspiration. Actual evapotranspiration is never greater than precipitation
|
25557 |
+
except on irrigated land because of percolation of water into groundwater bodies and surface runoff.
|
25558 |
+
|
25559 |
+
The soil moisture zone gains water by precipitation and infiltration and loses water by
|
25560 |
+
evapotranspiration, overland flow, and percolation of water downward due to gravity
|
25561 |
+
into the groundwater zone. The contact between the groundwater zone (phreatic zone) and
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25562 |
+
the overlying unsaturated zone (vadose zone) is called the groundwater table. The water
|
25563 |
+
balance equation for change of moisture storage in a soil is given as Chemical equation.
|
25564 |
+
|
25565 |
+
where S is storage, P is precipitation, E is evaporation, and R is surface runoff plus
|
25566 |
+
percolation rate into the groundwater zone; all terms are in units of length per unit
|
25567 |
+
of time (e.g., millimetres per day, centimetres per month). In humid midlatitude climates
|
25568 |
+
where a strong contrast between winter and summer temperatures exists, there is an annual
|
25569 |
+
cycle of the water content of soils. The annual cycle of moisture in soil in Ohio, U.S.,
|
25570 |
+
for example, demonstrates the processes controlling soil moisture. Of special importance is
|
25571 |
+
the fact that the soils are saturated in this temperate climate in the spring, and the
|
25572 |
+
evaporation rate is low because of the low input of radiant energy from the Sun. By contrast,
|
25573 |
+
in the summer, evaporation increases because of increasing solar radiation, and with the growth
|
25574 |
+
of plants so does transpiration. Soil moisture is reduced to very low levels at this time of year.
|
25575 |
+
|
25576 |
+
Groundwaters and river runoff
|
25577 |
+
|
25578 |
+
The term R in the water balance equation for change of soil moisture storage above represents
|
25579 |
+
groundwater and river runoff losses from the soil moisture zone. Water percolates from the soil
|
25580 |
+
moisture zone through the unsaturated (vadose) zone to the water table. Flow through the
|
25581 |
+
unsaturated zone is complicated. After a rainfall, water may form a nearly continuous phase
|
25582 |
+
in pores in this zone, but, with drying, the last amount of water is held in clusters at points
|
25583 |
+
of contact of solid grains and as thin films on solid surfaces. The flow paths of water become
|
25584 |
+
more tortuous, and the water-conducting properties decrease rapidly. Structured soils and fractured
|
25585 |
+
rock in the vadose zone may act as conduits for fluids to reach the water table. Because of the complex
|
25586 |
+
geometry of water contained in the unsaturated zone, the properties of water are expressed by means of
|
25587 |
+
empirical relationships. Darcy’s law, derived in 1856 from experimentation by the French engineer Henri
|
25588 |
+
Darcy, permits quantification of water flow through porous media. The law states that the rate of
|
25589 |
+
flow Q of a fluid through a porous layer of medium (e.g., a sand bed) is directly proportional to the
|
25590 |
+
area A of the layer and to the difference Δh between the fluid heads at the inlet and outlet faces of
|
25591 |
+
the layer and is inversely proportional to the thickness L of the layer. Expressed analytically, Chemical equation.
|
25592 |
+
|
25593 |
+
where K is a constant characteristic of the medium.
|
25594 |
+
The term K for a porous rock medium is the volume of fluid of unit viscosity passing
|
25595 |
+
through a unit cross section of the rock in unit time under the action of a unit pressure
|
25596 |
+
gradient; this characteristic is called permeability. The permeability of a rock is
|
25597 |
+
dependent on the geometric properties of the rock, such as porosity, shape and size
|
25598 |
+
distribution of constituent rock grains, and degree of cementation of the rock.
|
25599 |
+
Permeabilities of rocks vary greatly. Unconsolidated sands may have permeabilities
|
25600 |
+
measured in hundreds of darcys, whereas consolidated sands that will transmit reasonable
|
25601 |
+
amounts of fluid have permeabilities of 0.01 to 1 darcy. A rough idea of the meaning of
|
25602 |
+
one darcy of permeability (which equals 9.869 × 10−12 square metre [1.261 × 10−11 square foot])
|
25603 |
+
can be obtained by imagining a cube of sand one foot on a side. If the sand has a permeability
|
25604 |
+
of one darcy, approximately one barrel of water per day will pass through the one-foot cube with
|
25605 |
+
a one-pound pressure head. The general equation of Darcy can be modified to express flow in both
|
25606 |
+
the unsaturated zone and the saturated groundwater zone.
|
25607 |
+
|
25608 |
+
Groundwater is constantly in motion. When a lake or stream intersects the groundwater table,
|
25609 |
+
groundwater communicates directly with these bodies of water. If the groundwater table is higher
|
25610 |
+
than the stream or lake level, a pressure head will develop such that the groundwater flows
|
25611 |
+
into the water body; conversely, if the groundwater table is lower than the river or lake level,
|
25612 |
+
the pressure gradient induces flow into the groundwater. Most groundwater ultimately reaches the
|
25613 |
+
channels of surface streams and rivers and flows to the sea. On the average, groundwater contributes
|
25614 |
+
to total river runoff about 30 percent of its water on a global basis.
|
25615 |
+
|
25616 |
+
Water runoff from the land surface is that part of precipitation which eventually appears in
|
25617 |
+
perennial or intermittent surface streams. Streamflow-generation mechanisms have been studied
|
25618 |
+
for several decades, and there is now considerable knowledge regarding rainfall runoff processes
|
25619 |
+
and their controls. This understanding is the result of both careful observations from field
|
25620 |
+
experiments and the heuristic simulations of hypothetical realities with rigorous mathematical models.
|
25621 |
+
The discharge measured at the downstream end of a channel reach is supplied by channel inflow at the
|
25622 |
+
upstream end of the reach and by the lateral inflows that enter the channel from the hillslope along
|
25623 |
+
the reach. The lateral inflows may arrive at the stream in one of three forms: (1) groundwater flow,
|
25624 |
+
(2) subsurface storm flow, or (3) overland flow.
|
25625 |
+
|
25626 |
+
Groundwater flow provides the base flow component of streams that sustains their flow between
|
25627 |
+
storms. The “flashy” response of streamflow to individual precipitation events may be ascribed
|
25628 |
+
to either subsurface storm flow or overland flow. Under intense rainfall events during which
|
25629 |
+
the surface soil layer becomes saturated to some depth, water is able to migrate through “preferred pathways”
|
25630 |
+
rapidly enough to deliver contributions to the stream during the peak runoff period. The conditions for
|
25631 |
+
subsurface storm flow are quite restrictive. The mechanism is most likely to be operative on steep, humid,
|
25632 |
+
forested hillslopes with very permeable surface soils.
|
25633 |
+
|
25634 |
+
Overland flow is generated at a point on a hillslope only after surface ponding takes place.
|
25635 |
+
Ponding cannot occur until the surface soil layers become saturated. It is now widely recognized
|
25636 |
+
that surface saturation can occur because of two quite distinct mechanisms—specifically, Horton overland
|
25637 |
+
flow (named for American hydraulic engineer and hydrologist Robert E. Horton) and Dunne overland flow
|
25638 |
+
(named for British hydrologist Thomas Dunne).
|
25639 |
+
|
25640 |
+
The former classic mechanism is for a precipitation rate that exceeds the saturated hydraulic
|
25641 |
+
conductivity of the surface soil. A moisture content versus depth profile during such a rainfall
|
25642 |
+
event will show moisture contents that increase at the surface as a function of time. At some point
|
25643 |
+
in time the surface becomes saturated, and an inverted zone of saturation begins to propagate downward
|
25644 |
+
into the soil. It is at this time that the infiltration rate drops below the rainfall rate and overland
|
25645 |
+
flow is generated. The time is called the ponding time. The necessary conditions for the generation of
|
25646 |
+
overland flow by the Horton mechanism are (1) a rainfall rate greater than the saturated hydraulic
|
25647 |
+
conductivity of the soil and (2) a rainfall duration longer than the required ponding time for a given
|
25648 |
+
initial moisture profile. Horton overland flow is generated from partial areas of the hillslope where
|
25649 |
+
surface hydraulic conductivities are lowest.
|
25650 |
+
|
25651 |
+
In Dunne overland flow, the precipitation rate is less than the saturated hydraulic conductivity,
|
25652 |
+
and the initial water table is shallow or there is a shallow impeding layer. Surface saturation occurs
|
25653 |
+
because of a rising water table; ponding and overland flow occur at a time when no further soil moisture
|
25654 |
+
storage is available. The Dunne mechanism is more common to near-channel areas. Dunne overland flow is
|
25655 |
+
generated from partial areas of the hillslope where water tables are shallowest. Both Horton and Dunne
|
25656 |
+
mechanisms result in variable source areas that expand and contract through wet and dry periods.
|
25657 |
+
|
25658 |
+
Total river discharge and the chemistry of the discharge vary from continent to continent; some
|
25659 |
+
continents are wetter and some drier than the world average, but the deviations are not extreme.
|
25660 |
+
The runoff per unit area from Asia and Europe is almost exactly equal to the world average; it is a
|
25661 |
+
little lower in Africa and North America; and it is considerably higher in South America. Antarctica
|
25662 |
+
is frozen and Australia is arid, and so they contribute little runoff. Also, since their areas are
|
25663 |
+
relatively small, they do not affect the global runoff average significantly. The waters draining
|
25664 |
+
the continents have quite different chemistries; those from Europe are very rich in calcium and
|
25665 |
+
bicarbonates, whereas those from Africa and South America are not. North American and Asian rivers
|
25666 |
+
re somewhat intermediate in their concentrations of these dissolved constituents. Such differences
|
25667 |
+
in composition reflect a variety of factors, including runoff, temperature, and relief, but certainly
|
25668 |
+
the bulk composition of the continental rocks in contact with these waters and their underground sources
|
25669 |
+
play a major role. The surface rocks of Europe are rich in carbonates, and those of South America are not;
|
25670 |
+
the latter are dominated by sediments rich in silicate minerals.
|
25671 |
+
|
25672 |
+
The chemistry of groundwater and river runoff is being modified by human activities on a global
|
25673 |
+
scale. The natural dissolved riverine input of major constituents to the oceans already has been
|
25674 |
+
increased by more than 10 percent because of human activities. In the case of sodium, chlorine,
|
25675 |
+
and sulfate, the increases are as high as 30 percent. In the United States alone, total water use
|
25676 |
+
is equivalent to one-third of total runoff, with about 2 percent of the water used coming from
|
25677 |
+
underground wells. In the southwestern region of the country, water supplies have been tapped heavily
|
25678 |
+
and in some areas have been exhausted with no hope of replacement. This extensive use of fresh waters in
|
25679 |
+
the United States and throughout the globe makes them particularly susceptible to pollution. Leachates
|
25680 |
+
from fertilizers, herbicides, and pesticides are found in some freshwater bodies; toxins or excessive
|
25681 |
+
amounts of certain inorganic or organic chemicals are present; radioactive elements have been detected;
|
25682 |
+
and some surface water bodies have had their salinities increased dramatically, rendering them useless
|
25683 |
+
for human consumption. It is therefore imperative that countries closely monitor the use of freshwater
|
25684 |
+
systems and promote their conservation.
|
25685 |
|
25686 |
The EPI - Environmental Performance Index EPI provides a data-driven
|
25687 |
summary of the state of sustainability around the world. Using 40 performance
|