Water : Hydrologic Cycle

⇧Figure 7 Systems diagram of the impacts of human activities on streamflow (from Ward, 1990). (Reproduced with the permission of McGraw-Hill Publishing Company)

Hydrologic Cycle 

The importance of water on Earth cannot be underestimated. Water is transported endlessly throughout the various components of the Earth’s climate system, affecting every component along the way. Clouds and water vapor in the atmosphere influence the energy balance of the Earth,and snow and ice-covered surfaces reflect a significant amount of the Sun’s radiation back to space. Water at and under the land’s surface, in the form of stream-flow or groundwater, plays an important role in the maintenance of living organisms and human societies. However, while water is a benefit, if it arrives at the wrong time, in the wrong quantity, or is of poor quality, it can be a severe hazard. To mitigate these hardships, humans have significantly altered the hydrologic cycle with construction of dams, cultivation of farmland, urbanization, draining of swamp lands, etc. The local consequences of these changes can be dramatic, leading to environmental changes and, in some cases,local degradation. The global impacts are difficult to ascertain, however. With the increasing demand for freshwater resources and increased societal vulnerability to climate  extremes, the effects on humans by water-related global environmental change remain an interesting but as of yet unresolved question. 

INTRODUCTION
The hydrologic cycle is the perpetual movement of water throughout the various components of the Earth’s climate system. Water is stored in the oceans, in the atmosphere,as well as on and under the land surface. The transport of water between these reservoirs in various phases plays a central role in the Earth’s climate. Water evaporates from the oceans and the land surface into the atmosphere, where it is advect*ed across the face of the Earth in the form of water vapor. [ advection (noun):the transfer of heat or matter by the flow of a fluid, especially horizontally in the atmosphere or the sea. ] Eventually, this water vapor condenses within clouds and precipitates in the forms of rain, snow,sleet, or hail back to the Earth’s surface. This precipitation can fall on open bodies of water, be intercepted and transpired by vegetation, and become surface runoff and/or recharge groundwater. Water that infiltrates into the ground surface can percolate into deeper zones to become a part of groundwater storage to eventually reappear as stream-flow or become mixed with saline groundwater in coastal zones. In this final step, water re-enters the ocean from which it will eventually evaporate again, completing the hydrologic cycle. The hydrologic cycle qualitatively, quantitatively, and conceptually is depicted in Figure 1 to Figure 3.

The important reservoirs within the hydrologic cycle include:

Ocean

This vast body of salt water covers 70% of the Earth’s surface; it stores and circulates enormous amounts of water and energy. In addition, patterns of ocean surface temperatures can exert a strong influence on circulation patterns in the atmosphere. Frequently, the ocean is divided into two parts, an upper and lower zone. The upper zone is considerably warmer and less saline than the lower zone, and the two are separated by a relatively sharp thermocline*. The depth from the ocean surface to the thermocline can be as much as 400 m, but is generally less than 150 m. [*thermocline (noun) an abrupt temperature gradient in a body of water such as a lake, marked by a layer above and below which the water is at different temperatures.]

Atmosphere

Water can be stored in the atmosphere as liquid in clouds or as water vapor. Water vapor content of the atmosphere is described by its humidity. Specific humidity is a measure of the water content per unit of dry atmosphere (typical values are 1– 20 g kg1); relative humidity is the amount of water vapor present relative to the amount of water vapor that would saturate the air at a particular temperature. The presence of water in the atmosphere alters the radiation budget of the atmosphere, directly through latent heat and indirectly as both a reflector and absorber of radiation.Water in the atmosphere is the most significant contributor to the natural greenhouse effect.

Cryosphere

The largest stores of fresh water on the Earth are contained in glaciers and icecaps, primarily at high latitudes. The cryosphere has a significant impact on the climate of the Earth because snow and ice-covered surfaces have a very high albedo* (comparable to that of clouds). [ *Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth's surface. Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space. ] The large volume of runoff from northern high-latitude rivers also influences the Arctic and Atlantic Ocean circulation, which impacts the climate in those regions. Despite the importance of the cryosphere to the hydrologic cycle, relatively little is understood about this part of the climate system, partially because of the lack of adequate data in these often remote and difficult to access areas. Groundwater Water beneath the land surface can be classified in a variety of ways. Water closest to the surface (within a few meters) is considered soil moisture, and this water influences the evapo-transpiration rate of water from the surface.Soil moisture that is frozen year-round is called permafrost. Deeper below the surface is the aquifer, where the water 



⇧ Figure 1. A schematic diagram of various fluxes within the hydrologic cycle
concentration in the rock and soil is sufficient for withdrawal by pumping. Groundwater for human activities is contained primarily in the aquifer. In this saturated zone, all available spaces within the rock and soil are filled with water. Between the aquifer and soil moisture lays an unsaturated intermediate (vadose*) zone that has a lesser influence on the atmosphere than soil moisture.Despite the great societal importance of groundwater sup-plies, quality spatially distributed subsurface data are elusive. [ *vadose
(adjective) use in GEOLOGY, relating to or denoting underground water above the water table. ] ⇩



Land Surface

Water on land can be contained in lakes and marshes as well as rivers and within living organisms (biological water).The volume of water stored on land is relatively small,but the flux of water throughout these systems is relatively high. The relevance of this water to human activities is paramount.

In the broadest sense, the major fluxes between reservoirs are:

Precipitation

Precipitation is the fall of solid or liquid water over land and oceans, and is the major driver of the hydrologic cycle over land. Hydrologists have traditionally recognized precipitation as the start of the hydrologic cycle because all other hydrologic phenomena (e.g., evaporation, runoff, recharge) result from it. The importance of precipitation to the hydrologic cycle cannot be overstated.

Evapo-transpiration

Evaporation is the return of water from bare soil or open bodies of water (mainly the ocean surface) to the atmosphere. Transpiration is the transfer of water to the atmosphere through the stomata of vegetation. Collectively, they are considered evapo-transpiration.



Runoff 

Runoff is the transport of liquid water across the surface of the Earth. Excess water in saturated soils flows into rivers to the ocean, to terminal lakes or swamps. Groundwater can interact with stream-flow in rivers if the water table is near the surface.

Water Vapor Transport 

Atmospheric water vapor transport is the redistribution of atmospheric water vapor. Globally, there is a net transfer from over ocean to over land. This process is known as advection, and this flux is the major source of water vapor for precipitation over land, aside from recycling.



⇧ Figure 3 A conceptual diagram of the hydrologic cycle (after Wisler and Brater, 1959). (Reproduced by permission ofJohn Wiley & Sons)

DESCRIPTIONS OF THE HYDROLOGIC CYCLE

Mathematical Models 

Mathematically, the movement of water throughout the hydrologic cycle can be described using the hydrologic continuity equation:

I  - O  ＝  △ S ／△t

where input (I) and output (O) depend on the reservoir inquestion (e.g., evapotranspiration is an input to the atmo-sphere, whereas precipitation is an output). The change instorage (S) in time describes the removal from or additionto present supplies to make up for the imbalance betweeninput and output (in the case of the atmosphere, change instorage would signify a change in specific humidity). 





⇧Figure 4 Mathematical schematic of a water balance for: (a) the land surface.


⇧Figure 4 Mathematical schematic of a water balance for: (a) the land surface; (b) the atmosphere; and (c) the combined atmosphere and surface (from Oki, 1999). (Reprinted with permission of Cambridge University Press) 

In contrast to the atmosphere, the water balance of a surface portion of a river basin is considerably more complex. Water is input into this system through precipitation,surface runoff, and groundwater inflow from other parts of the basin. Water is lost through surface runoff, groundwater outflow, and evapo-transpiration. The change in storage is reflected in changes in soil moisture content. A graphical depiction of a water balance for a portion of the atmosphere and land surface is shown in Figure 4.On a global basis, the Earth is effectively a closed system,and the amount of water present remains relatively constant(i.e., S/t ³ 0). However, input and output rates of the hydrologic cycle vary regionally and on a wide range of time scales. Describing, quantifying, and predicting these variations are, in essence, major tasks in contemporary hydrology.To describe and predict variations within the hydrologic cycle, considerable effort has been invested in developing computer-based numerical models of hydrology and climate. Every component of the climate system has its own models (from groundwater to oceanography and the atmosphere to the land surface) and, within disciplines, there are too many different models to be completely described here.For example, surface hydrologic models range from simple statistics, to regression models (such as the antecedent precipitation index and the soil conservation service curve number model) to more complex conceptual rainfall-runoff models (like the Sacramento model) and finally to the physically based distributed models such as HEC-1 and KINEROS. The spatial and temporal scales of their applications vary from model to model, but ranges from tens to thousands of square kilometers and from minutes and hours to days and years, respectively (Sorooshian et al., 1996).The most complex models available are general circulation models (GCMs; see General Circulation Models (GCMs),Volume 1), which have full global representations of the ocean, atmosphere, cryosphere, and land surface. Most of the early work on GCMs related to refining the treatment of the ocean–atmosphere interface. Recently, increasing emphasis has been put upon the land surface– atmosphere interface,improving such models as the Biosphere Atmosphere Trans-fer Scheme and the Simple Biosphere model (see Land Surface, Volume 1). Lau et al. (1995) compared the ability of 29 GCMs in simulating various aspects of regional hydrologic processes and found them insufficient for use in climate studies related to continental scale water balance.Regardless, this is an area of very active research and as computing power rapidly increases in the near future, one can expect these models to improve.

Data

To date, there are several definitive works providing quantitative descriptions of the global hydrologic cycle(for example, Korzun, 1978; Piexoto and Oort, 1992;Oki, 1999). The most comprehensive review of fresh water resources (supply and use) can be found in Shiklomanov (1999) http://espejo.unesco.org.uy/ and Shiklomanov (1997) http://pangea.upc.es/orgs/unesco/webpc/worldwater resources.html. Earlier works quantifying the complete water cycle have attempted analysis using sparse measurements; the creation of re-analyzed data sets by the European Center for Medium-Range Weather Fore-casts and the National Center for Atmospheric Research