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

Hartmann’s 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

Chahine’s focus: clouds and radiation, atmospheric moisture, precipitation, ocean fluxes, land surface processes -- hydrologic components in GCM modeling.

http://www.physicalgeography.net/fundamentals/8b.html

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 land.

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/bdgt.rxml

Linkages between the 3 major water reservoirs (not to scale!)

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

 

Most of the Earth’s 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.

Figure is inexact

• 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.

Typical residence times of water in various reservoirsReservoir Average Residence Time

glaciers 20-100 yearsseasonal snow cover 2-6 monthssoil moisture 1 - 2 monthsgroundwater- shallow 100-200 yearsgroundwater - deep 10,000 yearslakes 50-100 yearsrivers 2 weeks -6 monthsatmosphere 8-10 days

Oceans avg. 3,000 years!

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.

Latitudinal distribution of the surface hydrologic balance between precipitation (P), evaporation (E), and runoff (Δf)

From Hartmann 1994

Δf = P - E

Atmospheric Moisture Balance - Horizontal Transport

• 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 > E (these are “sinks” for atmospheric moisture)

• In the subtropics: E > P – (these are “sources” for atmospheric moisture - these regions export water vapor to regions where P > 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.

http://earthobservatory.nasa.gov/Library/Water/water_3.html

Another hypothesized change will lead to wetter regions getting wetter and drier regions getting drier.

How Much More Rain Will Global Warming Bring?, Wentz et al. 2007, Science 13 July 2007: Vol. 317. no. 5835, pp. 233 - 235

Near surface warm currents are red; deep cold currents blue

http://www.physicalgeography.net/fundamentals/8q.html; http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html

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.

Deep Ocean Thermohaline Circulation – the Result of Balancing Salinity and Temperature

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).

http://aquarius.nasa.gov/education-datatools.php

More on Salinity and the Hydrologic Cycle

• Tropical regions reflect greater precipitation than evaporation • Pacific Ocean is fresh relative to the Atlantic Ocean • 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

Average Global Sea Surface Salinity Map

Ocean circulation acts to move lower salinity seawater into evaporative regions, and more saline water into humid regions.

The oceans store and transport heat.

http://aquarius.nasa.gov/education-salinity.php

Annual Average Global Precipitation Distribution, mm

Patterns of monthly average precipitation

Precipitation and Evaporation

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 land.

EvaporationDepends 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

Evapotranspiration (ET) = Evaporation + Transpiration

• 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 water).

• 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.

http://geochange.er.usgs.gov/sw/changes/natural/et/

Terrestrial Water Balance – precipitation and evapotranspiration

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 EvapotranspirationA 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.

Hartmann 1994(if P is not shown, it’s the same as E)

Surface Water Storage

CPC Soil Moisture and Runoff

http://www.publicaffairs.water.ca.gov/newsreleases/2006/strategicgrowthplan/1-26-07sgp-flood.pdf

SnowpackIn 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 accurate?

• What do you think about Chahine’s 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 (NSF?)?

• 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, difficult.

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 precipitation.

•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 IIGEWEX 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 changing?

• How do processes contribute to feedback and causes of natural variability?

•Can we predict these changes on up to seasonal to interannual?

•What are the impacts of these changes on water resources?

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 modelsA 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 http://www.bio.vu.nl/thb/course/tb/tb/node28.html

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 tocapture 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://www.tufts.edu/~rvogel/Editorial.pdf --good discussion on this

Incoming solar radiation at the top of the atmosphere

http://www.ucar.edu/learn/1_3_1.htm

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 effectCloud 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