Water as a unifying concept

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Water is an essential resource for humans and for natural ecosystems. Satellite images of the Earth show convincing evidence of an abundance of water on the planet. Unfortunately, only a small percentage of the total water volume is available as freshwater suitable for humans and many natural ecosystems. The relatively small volume of freshwater is further constrained by an uneven distribution over the globe that is paradoxical to the image of Earth as a water planet. Approximately one-third of the world's population lives in countries where the freshwater supply is less than the recommended per capita minimum, and 70 percent of all freshwater withdrawals from lakes, rivers, and groundwater is for crop irrigation to provide food (Entekhabi et al., 1999). Such disparities in water supply and water demand require understanding the underlying physical processes that account for spatial and temporal differences in the occurrence and magnitude of the water supply.

The physical characteristics of water are significant in accounting for how the freshwater supply is sustained. A combination of natural processes collectively recognized as the hydrologic cycle provides the mechanism for the natural redistribution of water among the land, oceans, and atmosphere. Water is the only chemical compound that occurs in natural conditions as a solid, liquid, and gas. The transformation of water from one physical state, or phase, to another is a critical factor in the transportability of water. Water's phase changes and transportability in each of its phases are the foundations of the hydrologic cycle which constantly replenishes and redistributes the relatively small volume of freshwater.

Expanding knowledge of land-atmosphere interactions in the closing decades of the twentieth century heightened awareness of the strong coupling between climate and land surface hydrological processes embracing the hydrologic cycle. The long-term interest in the total hydrologic cycle shared by the disciplines of hydrology and climatology was magnified as the twenty-first century began with the emergence of non-traditional datasets and new investigative techniques applied to an expanding array of water-related problems. Accelerated interest in the hydrologic cycle was especially evident in the field of hydroclimatology that overarches the disciplines of hydrology and climatology.

1.1.1 Hydrology

Modern hydrology is broadly defined as the science that studies the occurrence and movement of water on and under the Earth's surface, water's chemical and physical properties, water's relationship to biotic and abiotic environmental components, and human effects on water (Ward and Robinson, 2000). Hydrology has a predominant land surface orientation and emphasizes processes involved in the land phase of the hydrologic cycle. Hydrology employs the sciences of biology, chemistry, ecology, mathematics, and physics to focus on solving water resource and water management problems concerned with water use, water control, and water quality. A watershed is often the most convenient spatial unit for integrative and synthesizing studies of hydrologic problems, but spatial scale variations are ultimately determined by the nature of the problem being examined.

1.1.2 Climatology

Climatology is an applied science that examines the fluxes of energy, mass, and momentum among the land and ocean surfaces and the atmosphere. These fluxes are integral parts of the climate system modulated by both external and internal factors (Peixoto, 1995). The vertical and horizontal fluxes of energy and mass that are central to climatology are components of physical and dynamical phenomena operating at various scales as an integrated and interactive spatial system that links the land and ocean surfaces with atmospheric circulation. Collectively these components embrace a broad spectrum of thermodynamic and hydrodynamic processes that display identifiable seasonal variations. The atmospheric general circulation is an expression of these seasonal variations, and the general circulation determines the concurrent array of weather patterns (Bryson, 1997). Therefore, the atmospheric phase of the hydrologic cycle is a climate-related phenomenon and one that is expected to display seasonal variability. Climatology requires knowledge of chemistry, mathematics, and physics and a thorough understanding of the atmosphere's physical and chemical interactions with the ocean and land surfaces. Climatologists apply this knowledge to understand atmospheric circulation and dynamics and the atmospheric role in climate variability and climate change. Modern climatology has a strong relationship with computers because simulation is an important aspect of climate science through its serving as the platform for climate experimentation (Petersen, 2000). Climate system models strive to reveal global energy and circulation conditions, flood and drought recurrence, the influence of land surface changes on climate, and climate's role in a variety of social, economic, and environmental problems.

1.1.3 A broadened climate perspective

The traditional concept of climate as the mean atmospheric condition expressed as average temperature, precipitation, and other weather variables and possibly higher moment statistics of these variables for a specified period (Peixoto, 1995), such as a month, a season, or a year, has a limited role in contemporary hydroclimatology. The climate variables are the same as those variables relevant to meteorology, but they are applied at different time and space scales. Climate expressed as the average weather or the average state of the atmosphere is almost synonymous with "statistical meteorology", and it is easy to see why meteorologists have some propriety about the province of climatology (Bryson, 1997). The movement of the atmosphere is the dynamic of interest behind the quantitative distributions used to depict climatic fields, but climate variables owe their importance to forecasting purposes in meteorology. Averaging is what makes the focus climatic. Individual weather events are explained by the incursion of air masses and fronts and by the vertical arrangement of the atmosphere. Consequently, averages and normals make climate appear statistical and useful as a descriptive tool. Probability estimates of extreme events are a logical ancillary feature of averages, and probabilities are widely used in construction and engineering professions.

A definition of climate exclusively concerned with the atmosphere ignores the coupling that exists between the atmosphere and the land and ocean surfaces. Therefore, the traditional view of climate as a static or constant entity does not fulfill the requirements of a contemporary construct of the climate system. The modern climate system (Fig. 1.1) is depicted as five subsystems linked by exchanges of energy, mass, and momentum among the subsystems. The coupling of the subsystems results in a dynamic climate undergoing constant change. This is a more accurate depiction of the actual natural processes involved in the continuous redistribution of energy, moisture, and momentum accomplished by the atmosphere's close interaction with the land, oceans, vegetation, and snow and ice at the Earth's surface. Climate is a direct response

Fig. 1.1. The climate system. Arrows depict generalized fluxes of energy, mass, and momentum linking the five subsystems.
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Fig. 1.2. GOES visible spectrum image of Earth on 19 April 2006. (Image courtesy of NOAA and the National Environmental Satellite, Data, and Information Service from their website at http://www.goes.noaa.gov/ goesfull.html.)

to vertical and horizontal fluxes resulting from the coupling of the subsystems, and the atmosphere has an important transportation role. This dynamic climate perspective includes the more restrictive traditional concept of climate based on the mean atmospheric condition at the Earth's surface, but it emphasizes climate expressed as a physical system (Peixoto, 1995). Consequently, modern hydroclimatology is best viewed within the context of a thermodynamic and hydrodynamic system driving energy and moisture exchanges at the land and ocean surfaces while being influenced by these same energy and moisture fluxes. The resulting transport of energy and mass is evident in the structure and distribution of clouds revealed by satellite imagery (Fig. 1.2).

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