The American hydrologist Walter Langbein (1967) defined hydroclimatology as the study of the influence of climate upon the waters of the land. He identified precipitation and evapotranspiration and the imbalance of these climatic elements as the focus of hydroclimate. However, subsequent advances in understanding natural processes complemented by development of contemporary measurement techniques, data acquisition, and analytical tools suggest this perspective is too restrictive for modern science. Modern hydroclimatology requires a more holistic view that emphasizes a process orientation and a role in a variety of environmental systems ranging from water quantity and quality to stream habitats. Although the significance of the imbalance between precipitation and evapotranspiration remains valid, additional conceptual elements are needed to account for an expanding range of concerns.
The perspective adopted in this book is that hydroclimatology is an approach to studying moisture in its three phases in the atmosphere and on the Earth's surface. This realm is the intersection of climatology and hydrology, and it includes energy and moisture exchanges between the atmosphere and the Earth's surface and energy and moisture transport by the atmosphere. Emerging from this array of moisture fluxes and storages is a conceptual framework defining the occurrence of hydrologic events within their climatological context. The climatological context is a specific array or pattern of atmospheric pressure and circulation identifiable with a particular hydrologic event for a given location. For example, extreme events like floods and drought tend to have well-defined atmospheric features related to the land surface event. This approach is consistent with the hydroclimatology perspective suggested by Kilmartin (1980) and Hirschboeck (1988). Such a robust concept of hydroclimatology includes hydro-meteorology and knowledge of evaporation, runoff, interception, groundwater recharge and other surface or near surface water relations (Mather, 1991). It does not attempt to define hydroclimatology as dealing with problems of the borderline between climatology and hydrology or the width of that borderline which varies according to the view of the individual investigator. Perhaps most importantly, this approach to hydroclimatology does not attempt to set the breadth of the field but recognizes the opportunities for continual expansion with advances in understanding natural processes.
Hydroclimatology viewed as hydrologic events driven by climatically related energy and moisture fluxes and storages requires distinguishing between climatology and meteorology and hydroclimatology and hydrometeorology. Meteorology is the study of the weather or the day-to-day state of the atmosphere emphasizing variations in temperature, precipitation, pressure, wind, cloudiness, and humidity for a specific location. Meteorology employs physics and mathematics to explain short-term atmospheric motion and related phenomena. Hydrometeorology is the application of meteorology to problems involving the hydrologic cycle, the water balance, and the rainfall statistics of storms. In practice, hydrometeorology is concerned with measurement and analysis of precipitation data involving extrapolation of point data to spatial units, determination of rainfall probabilities, computing the frequency of intense storms, evaluating flood hazards, and design of local hydraulic structures. The boundaries of hydrometeorology are not distinct and the problems explored often overlap those of climatologists, hydrologists, cloud physicists, and weather forecasters.
One of the strengths of the hydroclimate concept is that it is robust. It is applicable to the study of a broad range of natural processes. It is equally useful for examining human modification of the climate system or the hydrologic cycle. Hydroclimate emphasizes study of the precipitation-evapotranspiration difference and the consequences of the imbalance. Precipitation and evapotranspiration are due to different meteorological, physical, and biological causes. For any given location they are not often the same in either amount or distribution through the year. The character of this imbalance is the basis for defining the hydroclimatic significance of an event or set of conditions. Hydroclimatic significance provides the structure for examining a series of important questions. What is the nature of the imbalance? What accounts for the imbalance? What are the ramifications of the imbalance in terms of how water is processed at the land-atmosphere interface?
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