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Title: The%20Hydrologic%20Cycle

The Hydrologic Cycle Chihine, M.T. 1992. The
hydrologic cycle and its influence on climate.
Nature 359, 373-380. PDF Hartmann, D.L. 1994.
Global Physical Climatology, Chapter 5, The
Hydrologic Cycle.  Academic Press, pp. 115-135.
PDF Climate and Water, Jan. 23, 2008
Hartmanns focus surface water storage and
runoff, precipitation and dewfall, evaporation
and transpiration (measurement, calculation and
modeling, potential evaporation, modeling land
surface water balance, and the terrestrial water
balance Chahines focus clouds and radiation,
atmospheric moisture, precipitation, ocean
fluxes, land surface processes -- hydrologic
components in GCM modeling.
The Hydrologic Cycle a conceptual model to
describe the storage and movement of water
between the biosphere, atmosphere, lithosphere,
and the hydrosphere.
Another way of looking at the Hydrologic Cycle a
cycle, powered by solar radiation, of continuous
moisture exchange between oceans, atmosphere, and
Linkages between the 3 major water reservoirs
(not to scale!)
Most of the Earths water, 97, resides in the
ocean system, with about 2.5 on land. The
atmosphere holds less than .001, in spite of the
fact that atmospheric water is so important to
weather and climate. The annual precipitation for
the earth is more than 30 times the atmosphere's
total capacity to hold water. This fact indicates
the rapid recycling of water that must occur
between the earth's surface and the atmosphere.
Reservoir Volume (cubic km x 1,000,000) Percent of Total
Oceans 1370 97.25
Ice Caps and Glaciers 29 2.05
Groundwater 9.5 0.68
Lakes 0.125 0.01
Soil Moisture 0.065 0.005
Atmosphere 0.013 0.001
Streams and Rivers 0.0017 0.0001
Biosphere 0.0006 0.00004
Figure is inexact
Typical residence times of water in various
Oceans avg. 3,000
  • Water in the atmosphere is completely replaced
    once every 8 days.
  • Replacement in large lakes, glaciers, ocean
    bodies and groundwater reservoirs can take from
    hundreds to thousands of years.
  • Processes related to days-to-weeks residence
    times determine amplitudes and regional patterns
    of climate
  • Processes at decades to centuries modulate
    transient responses, lagged responses.

  • The global fresh water budget a balance between
    evaporation, precipitation and runoff
  • The oceans supply most of the evaporated water
    found in the atmosphere.
  • Of this evaporated water, 91 of it is returned
    to the ocean basins by way of precipitation.
  • The remaining 9 is transported to areas over
    landmasses where climatological factors induce
    the formation of precipitation.
  • The resulting imbalance between
    rates of
    evaporation and precipitation
    over land and
    ocean is corrected by
    runoff and groundwater
    flow to
    the oceans.

?f P - E
Latitudinal distribution of the surface
hydrologic balance between precipitation (P),
evaporation (E), and runoff (?f)
From Hartmann 1994
  • Atmospheric Moisture Balance - Horizontal
  • The atmosphere contains a 10-day supply for
    rainfall over the earth.
  • Intense horizontal flux of moisture into air
    makes high short-term totals possible.
  • There is a net transfer from oceans to land
  • There is meridional (south-north) transport to
    balance moisture at a given latitude
  • At low and middle latitudes P gt E (these are
    sinks for atmospheric moisture)
  • In the subtropics E gt P (these are sources
    for atmospheric moisture - these regions export
    water vapor to regions where P gt E)
  • Global P E animation
  • Atmospheric moisture is transported equator-ward
    into low latitudes by easterly trade winds and
    pole-ward in middle latitudes by westerlies
  • local evaporation is not a major source of local
    precipitation e.g., 32 of summer precipitation
    over the Mississippi River basin is local the
    rest is transported from more distant locations

Water vapor, clouds, precipitation and climate
change Agreement is widespread that these
changes in climate may profoundly affect
atmospheric water vapor concentrations, clouds,
and precipitation patterns. For example, climate
models and satellite observations both indicate
that the total amount of water in the atmosphere
will increase 7 per kelvin of surface warming,
but climate models predict that global
precipitation will increase at a much slower rate
of 1 to 3 per kelvin. However, A recent
analysis of satellite observations does not
support this prediction of a muted response of
precipitation to global warming. Rather, the
observations suggest that precipitation and total
atmospheric water have increased at about the
same rate over the past two decades.
Another hypothesized change will lead to wetter
regions getting wetter and drier regions getting
How Much More Rain Will Global Warming Bring?,
Wentz et al. 2007, Science 13 July 2007 Vol.
317. no. 5835, pp. 233 - 235
Deep Ocean Thermohaline Circulation the Result
of Balancing Salinity and Temperature
Near surface warm currents are red deep cold
currents blue
In contrast the deep Pacific is lower in
salinity its surface layer is too fresh and
buoyant to sink. Pacific deep water is derived
from the lower salinity water column of the
southern ocean.
tml http//
  • Ocean Circulation and Salinity
  • The range of salt concentration in the ocean
    varies from about 3.2 to 3.8
  • The more saline, the denser the seawater
  • In the evaporation process, freshwater is
    evaporated into the atmosphere, and the salt
    remains in the sea water
  • Areas of great evaporation (subtropical oceans)
    are salty areas where precipitation is greater
    are fresher.
  • Sea ice formation also removes freshwater from
    the ocean, concentrating salt in the remaining
    body of seawater (a small amount of salt is in
    sea ice).

  • More on Salinity and the Hydrologic Cycle
  • Tropical regions reflect greater precipitation
    than evaporation
  • Pacific Ocean is fresh relative to the Atlantic
  • Arctic Sea is fresh due to the enormous amount
    of river water that drains into it from the
    northern continents
  • Antarctic margins are saltier (relative to the
    Arctic) due to sea ice formation

Ocean circulation acts to move lower salinity
seawater into evaporative regions, and more
saline water into humid regions. The oceans
store and transport heat.
Average Global Sea Surface Salinity Map
Precipitation and Evaporation
Annual Average Global Precipitation Distribution,
Patterns of monthly average precipitation
Besides being responsible for water supplies,
floods, drought, etc., global precipitation is
also a key feature in the energy balance (latent
heat of evaporation, condensation) and
interactions between the oceans, atmosphere and
Evaporation Depends on the difference between
vapor pressure at the water surface and the vapor
pressure of the air, and an energy
supply Evaporation is more intense in the
presence of warmer temperatures.
  • Evaporation distribution
  • Highest over regions of high solar radiation,
    water, and tropical vegetation
  • Seasonally, ocean evaporative losses are
    greatest in winter, due to outflows of cold, dry
    continental air over warm currents

Terrestrial Water Balance precipitation and
  • Evapotranspiration (ET) Evaporation
  • Combined process of water loss from plants
    (intercepted and transpired) and evaporation from
    open bodies of water, wetlands, snow cover, and
    bare soil (although sometimes limited to soil
  • Constrained by surface water supply, energy for
    latent heat of vaporization (solar radiation),
    and ability of air to accommodate water vapor
    (wind can help in this regard).
  • In the conterminous United States,
    evapotranspiration averages about 67 percent of
    the average annual precipitation
  • Evapotranspiration within the conterminous
    United States ranges from about 10 inches per
    year in the semiarid Southwest to about 35 inches
    per year in the humid Southeast.

Potential Evapotranspiration (PE) The rate of
evapotranspiration that would occur if the
surface is wet (i.e., yielding the greatest
amount of evapotranspiration possible). If the
potential evapotranspiration exceeds the actual
evapotranspiration, a moisture deficit exists.
  • Calculated using a variety of theoretical (e.g.,
    inferred from energy balance) and empirical
    approaches (lysimeters, eddy flux towers)

  • Terrestrial Water Balance based on Precipitation,
    Evapotranspiration, and Potential
  • A basis for climate classification
  • Seattle P is greater than E in winter, but
    April- Oct, E is greater. PE is greater than
    actual E from May to September, resulting in a
    moisture deficit.
  • San Francisco is similar, with a shorter wet
    season, and longer period of deficit.
  • Los Angeles P equal E in all months, and there
    is a moisture deficit almost all year long.
  • Acapulco summers are wet, exceeding E, with PE
    remaining high and constant in all months because
    of its tropical location. Winters are very dry.
  • Denver warm dry air in summer makes PE high,
    greatly exceeding precipitation. P equals E in
    all months.

(if P is not shown, its the same as E)
Hartmann 1994
Surface Water Storage CPC Soil Moisture and
Snowpack In the western United States, winter
snowfall in the mountains provides 50 to 80
percent of our water supply.
  • Discussion Questions
  • In section 5.3 on page 120 it mentions that the
    dynamics of water storage from snow is different
    from that of rain, with snow apparently much more
    simple. How much does this impact the modeling of
    future water supplies in the western US? I know
    there are questions as to how precipitation will
    change seasonally, but are the models that put a
    higher emphasis on snowpack more likely to be
  • What do you think about Chahines point about
    the need for a more integrated program of
    hydrologic studies (now dispersed among many
    disciplines)? How is this addressed at the
    university (such as at the UA?) or national level
  • The difference between deterministic and
    stochastic models?
  • The Chahine paper is admittedly out-of-date with
    respect to the various data collections and
    modeling efforts mentioned. What has changed
    since 1992?

  • Land Surface Modeling Update IPCC 2007
  • Major advance since the TAR is the inclusion of
    carbon cycle dynamics including vegetation and
    soil carbon cycling
  • Other improvements root parametrization,
    higher-resolution river routing, cold land
    processes with multi-layer snowpack models,
    inclusion of soil freezing and thawing,
    snow-vegetation interactions, wind redistribution
    of snow are more commonly considered.
  • Coupling of groundwater models into land surface
    schemes (currently only evaluated locally but may
    be adaptable to global scales).
  • Little work has been done to assess the
    capability of the land surface models in coupled
    climate models, but upgrading of the land surface
    models is gradually taking place and the
    inclusion of carbon in these models is a major
    conceptual advance.
  • Many models represent the continental-scale land
    surface more realistically than the standard
    bucket hydrology scheme, and include spatially
    variable water-holding capacity, canopy
    conductance, etc.
  • Soil moisture modelling is challenging
    (naturally varies at small scales, linked to
    landscape characteristics, soil processes,
    groundwater recharge, vegetation type, etc.), It
    is not clear how to compare climate-model
    simulated soil moisture with point-based or
    remotely sensed soil moisture. This makes
    assessing how well climate models simulate soil
    moisture, or the change in soil moisture,

  • Phase I Results Summarized
  • 10-25 year global data sets of clouds,
    precipitation, water vapor, surface radiation,
    and aerosols--indicating no large global trends,
    but with evidence of regional variability.
  • Implementation of the land surface and cloud
    parameterization upgrades suggested for most
    regional and global models--showing improved
  • Initial results from the GEWEX Continental-Scale
    Experiments--approaching closure of the regional
    water and energy budgets and determining the
    importance of recycling and diurnal processes for
    regional predictions.
  • Phase II
  • GEWEX is in Phase II (2003-2012), which in the
    context of the original objectives, is addressing
    the following principal scientific questions.
  • Are the Earth's energy budget and water cycle
  • How do processes contribute to feedback and
    causes of natural variability?
  • Can we predict these changes on up to seasonal to
  • What are the impacts of these changes on water
  • In Phase II  there will also be increasing
    interaction with the water resource and
    applications communities to ensure the usefulness
    of GEWEX results. 

Stochastic versus deterministic models A complex
deterministic model can (in principle) predict
the outcome, when the forces, the trajectory in
the air, the tumbling and bouncing is modeled in
great detail, including the many imperfection's
of dice and table. A very simple stochastic model
(with the six possible outcomes having equal
probability) usually works better become most
parameters of the deterministic model are not
known, and the process of throwing cannot be
controlled in sufficient detail. From
Deterministic models are not necessarily
physically based, but they often contain
empirical components. A model is classified as
deterministic if the internal structure of the
model at least attempts to capture some physical
processes. E.g. a streamflow models represent
watershed processes such as infiltration and
channel routing in addition to predicting
watershed discharge. A stochastic streamflow
model is that its structure is derived to assure
that certain statistical properties of the
generated streamflows, such as such as its
probability distribution or perhaps its mean,
variance, skewness and serial correlation are
preserved. Stochastic models are usually not
derived from physical processes as in a
deterministic model. From http//
vogel/Editorial.pdf --good discussion on this
Incoming solar radiation at the top of the
  • Three main atmospheric processes modify the solar
    radiation passing through the atmosphere to the
    Earth's surface as well as clouds
  • scattering
  • absorption
  • reflectance

Cloud albedo and greenhouse effect Cloud
reflectivity varies from less than 10 to more
than 90 of the insolation and depends on drop
sizes, liquid water content, water vapor content,
thickness of the cloud, and the sun's zenith
angle. The smaller the drops and the greater the
liquid water content, the greater the cloud
albedo, if all other factors are the same. High
clouds (cirrus) thin and transparent, so low
albedo, but because they are high, and therefore
cold, they readily absorb the outgoing longwave
radiation, so net effect is warming Low clouds
(stratocumulus) lower, thicker, not as
transparent so block more shortwave radiation and
also reflect more (high albedo) they emit
longwave radiation but at almost the same
temperature as the surface, so greenhouse effect
is not great, and net effect is cooling.
Convective clouds high albedo and high
greenhouse effect, so net effect is neutral with
regard to warming.
annual, global, mean effect of clouds is cooling