Emergence of the hydrologic cycle

An early record of the importance of water for human life can be found in Genesis, the first book of the Bible. In this account of creation, light is provided on day one by the Sun, Moon, and stars. Separation of waters below the sky from waters above it occurs on day two, and day three begins with the separation of land and oceans.

Contemporary thought recognizes that energy from the Sun warms the Earth and water dominates the distribution of heat over the planet (Langenberg, 2002), and the related energy and moisture transfers constitute the global hydrologic cycle. Consequently, the light and water present at the beginning of time represent the ingredients needed for the hydrologic cycle.

The full context of contemporary hydroclimatology emerges from the historical pursuit of knowledge to understand the Earth's atmosphere and the hydro-logic cycle. The early work was motivated by individual interests and curiosity, but over time the accumulation of information formed a coherent body of knowledge. False starts and imprecise ideas often related to mythology occurred, but these were identified and corrected or abandoned. An overview of the development of climatology and hydrology indicates the similarities and differences in how these two fields developed, the role of the hydrologic cycle in their development, and the ultimate emergence of hydroclimatology out of the two disciplines.

1.4.1 Speculation period

The rise of climatology as a science is closely related to developments in meteorology and to the human capacity to obtain more and improved atmospheric observations and measurements. The earliest evidence of human interest in the atmosphere was a concern for phenomena recognized in today's world as belonging to the field of meteorology. Climate is a more abstract concept than weather, and in these early days people did not travel extensively and were less likely to observe climatic differences between places (Linacre, 1992). However, interest in climate evolved as understanding of atmospheric processes improved, and a close coupling of climatology and meteorology characterizes much of their early history. Around 3000 BC, Mesopotamian astronomers and mathematicians studied clouds and thunder and were the first to identify winds according to the direction from which they blow. At about this same time, Egyptian astronomers and mathematicians recognized the seasonal position of the Sun in the sky which is a basic factor underlying climate differences. Climate was mentioned in the writings of the Xia dynasty in China (2100-1600 BC), and weather details were recorded in China as early as 1500 BC.

The earliest written record recognizing the global hydrologic cycle is attributed to the author of the book of Ecclesiastes around 1000 BC (Nace, 1974). Pre-800 BC texts in India maybe the earliest indication of human understanding of the atmospheric branch of the hydrologic cycle (Ward and Robinson, 2000). However, the human necessity for water required numerous responses that predate writings about the hydrologic cycle. As early as 3000 BC, occupants of the Indus Valley in India constructed water supply, irrigation, and drainage systems, and Egyptians constructed a rock-fill dam between 2950 and 2750 BC. A variety of water facilities were constructed in Assyria, Babylonia, Israel, Greece, Rome, and China before the Christian era.

1.4.2 Greek and Roman era

Early Greek philosophers were interested in weather phenomena, the atmosphere, and the hydrologic cycle. Herodotus (440 BC) compared the climate of places, and Hippocrates in 400 BC wrote about weather and health and the dangers of drinking polluted water. Aristotle wrote a comprehensive meteorological treatise in 334 BC that served as the basis for weather theory for the next 2000 years. Erastosthenes described climate in terms of the Sun's position in the sky in 200 BC. In general, early Greek philosophers embraced the basic idea of the hydrologic cycle and proposed a variety of explanations for the origins of rivers and springs. Some proposals portrayed reasonable constructs, but underground mechanisms were imaginary.

The sustained influence of Aristotle's meteorological treatise was partially due to Roman philosophers devoting little interest in the atmosphere. Roman philosophers were more concerned with hydrology and benefited from practical knowledge gained from construction of great hydraulic works. They expanded on the explanations of rivers and springs proposed by the Greeks, and Vitruvius, a Roman architect and engineer, in 100 BC conceived that groundwater is derived from rain and snow infiltrating from the surface. Many consider this theory to be the forerunner of modern hydrologic cycle concepts. Although Ptolemy (AD 130), a Greek astronomer living in Alexandria, created a map that divided the known world of the second century into seven roughly determined climatic zones, Roman philosophers, at the fall of the Roman Empire in AD 476, had contributed little toward understanding the atmosphere.

1.4.3 Middle Ages

With a few exceptions, advances in understanding the atmosphere and the hydrologic cycle by Western scholars languished between AD 400 and 1500 during the period known as the Middle Ages. The attention these topics received during this period came largely from other world regions. In the late tenth century, the Persian scholar Karaji described the basic principles of hydrology (Pazwash and Mavrigian, 1981). Norse poems of the ninth to twelfth centuries contained descriptions of the hydrologic cycle indicating recognition of the roles of ocean evaporation, condensation, cloud formation, and precipitation on the land (Ward and Robinson, 2000). Except for the introduction of the wind vane in AD 850, few advances in understanding the atmosphere were achieved, but Islamic scholars during the ninth to twelfth centuries translated and expanded on the work of the Greeks and Romans. By the middle of the fifteenth century, extended sea voyages opened new trading areas and the broadened knowledge of ocean winds acquired as a result of these voyages contributed to formulation of theories regarding global wind patterns.

1.4.4 Observation period

Early Greek and Roman theories of the hydrologic cycle remained dominant until the sixteenth century when Leonardo da Vinci in Italy and Palissy in France used field measurements to assert that the water in rivers comes from precipitation (Biswas, 1970). The observation-based approach to the hydrologic cycle was advanced in the late seventeenth century when Perrault and Mariotte in France and Halley in England provided a quantitative basis for the basic principles of the hydrologic cycle by showing that precipitation supplied the water in rivers and streams and moisture circulated between the land, oceans, and atmosphere. Measurements enabled scientists to draw correct conclusions on the observed hydrologic phenomena and the mass balance concept was established.

Scientific analysis of the atmosphere progressed in the sixteenth and seventeenth centuries with the development of weather instruments that provided data from which laws applicable to the atmosphere could be derived. The hygrometer and anemometer were designed by da Vinci in 1500, Galileo invented the thermometer in 1593, Torricelli introduced the barometer in 1643, and Pascal demonstrated the reduction in atmospheric pressure with increasing elevation up a mountain in 1647. Boyle discovered the fundamental relationship between pressure and volume in a gas in 1662, and Townley introduced the first European rain gauge in England around 1676. Halley recognized the relationship between the general atmospheric circulation and the Sun's heat over the Earth, and he constructed a comprehensive map of global winds in 1683 and a map of the trade winds and monsoon circulations in 1686.

1.4.5 Modernization era

Weather instruments were improved and standardized in the eighteenth century, and network measurements of precipitation began before 1800 in Europe. Ideas essential to understanding the atmosphere slowly evolved through the eighteenth century as contributions increased from a growing number of scholars and intellectual activity shifted westward out of Greece and Italy. Notable advances in understanding the atmosphere during this period included Hadley's treatise on tropical circulation published in 1735, Black's introduction of latent heat in 1760, and Erasmus Darwin's explanation of cloud formation by adiabatic expansion in 1788.

In hydrology, the eighteenth century was marked by advances in mathematical applications to fluid mechanics and hydraulics by European scientists that formed the foundation for understanding hydraulic principles. This experimental work formed the foundation for the modern science of hydrology. LeClerc described the Earth's hydrologic cycle in 1744, and the use of "hydrology" in approximately its current meaning began in 1750. Improvements in methods to quantify streamflow during this period complemented advances in measuring temperature and precipitation.

Around 1800, John Dalton established the nature of evaporation and the present concepts of the global hydrologic cycle. Mulvaney in 1851 described the time of concentration concept that is the basis for the rational method of runoff computation. In 1856, Darcy developed the law of flow through porous media that represented one of the final obstacles to understanding groundwater flow and the hydrologic cycle. The last half of the nineteenth century saw the beginning of the association of hydrology and civil engineering as scientists examined the relations between rainfall amounts and streamflow rates and estimated flood flows in designing bridges and other structures. Saint-Venant derived the equations of one-dimensional surface water flow in 1871, and Manning developed an equation for open channel velocity in 1891. These nineteenth century activities led to significant advancements in understanding the basic relations between precipitation and evaporation over an area and the runoff from that area (Mather,

1991). The fundamental principles and concepts produced by this early work emphasize water on the land. This perspective is now recognized as the terrestrial branch of the hydrologic cycle (Stricker et al., 1993).

In the nineteenth century, insights into the atmospheric branch of the hydrologic cycle were provided by improved understanding of the general circulation of the atmosphere. Redfield explained the circular nature of storms in 1831, and Gaspard Gustave de Coriolis formulated the "Coriolis force'' in 1835. Ferrel proposed his three-cell model of atmospheric motion in 1856, and Coffin prepared world wind charts in 1875. The systematic use of balloons to monitor the free atmosphere began in 1892. Weather instrument networks expanded beyond Europe early in the nineteenth century. Instrument networks were well established in India by 1820, and the Smithsonian Institute began gathering climatological data nationwide in the United States in 1847.

1.4.6 Twentieth century and beyond

In the first half of the twentieth century, more details of the character of the atmosphere were discovered. Systematic data collection in the atmosphere using aircraft began in 1925, and radiosondes were successfully deployed in 1928. The nature of the jet stream was first investigated in 1940. Although climatology became more quantitative in the early twentieth century, there was a decline in interest as climatology was perceived as being primarily descriptive, merely statistical meteorology, or only concerned with climate classification (Linacre,

1992). In contrast, hydrology gained formal recognition as a science in the first half of the twentieth century. Early in the period hydrology was characterized by empiricism and qualitative description, but this was replaced by more theoretical and quantitative approaches by the mid twentieth century. The first watershed-scale measurement of land use change effects on streamflow in the United States was undertaken at Wagon Wheel Gap, Colorado, in 1911 using two small watersheds. Other significant advances were the work of Sherman (1932) who introduced the unit hydrograph, and Horton's (1940) ideas on infiltration, soil moisture accounting, runoff, and that hydrologic processes operate at a variety of scales. Recognition of water quality as an aspect of hydrology developed during this period along with the formation of government agencies and international organizations concerned with various aspects of hydrology.

In the second half of the twentieth century, climatology joined other sciences undergoing rapid changes in response to new technology, data sources, and analytical tools. An especially important development for climatology was the increased availability of atmospheric data from ground sources and satellites and the development of coupled large-scale atmospheric and biosphere models. The development of radar, rockets, and satellites by the mid twentieth century supported expanded exploration of the atmosphere. The introduction of highspeed computers in the 1950s supported data storage and analysis and the solution of mathematical equations describing atmospheric behavior. The first meteorological satellite was launched by the United States on 1 April 1960, and the first geostationary satellite was launched in 1966. These developments stimulated a rapid increase in understanding meteorological processes and in focusing climatology on the solution of practical problems. Archived satellite imagery and data became sufficient to support studies of a variety of current and future climate questions. Advances in computer technology made possible the processing of equations of motion, thermodynamics, and conservation of mass for multiple-level atmospheric general circulation models and coupled atmosphere-ocean-biosphere models. However, model improvements continue to be sought for better physical representations of clouds, soil moisture, snowcover, and sea ice (Burroughs, 2001).

Detailed field studies designed to understand routes followed by water in reaching the stream channel expanded rapidly in hydrology during the last half of the twentieth century. Often these field studies incorporated subjecting rational hydrologic principles to mathematical analysis to gain understanding of natural processes. The temporal and spatial variability of natural processes presents a great challenge to achieving understanding of how water arrives at the stream channel.

The emergence of international and interdisciplinary programs late in the twentieth century was significant in forging the contemporary image of hydro-climatology and developing a more sophisticated understanding of the hydro-logic cycle. The Global Atmospheric Research Program (GARP) was conceived in the 1960s with over 140 nations participating. The GARP Atlantic Tropical Experiment (GATE) initiated in 1974 focused on understanding the relationship between tropical cloud clusters and the atmospheric general circulation. The Global Energy and Water Cycle Experiment (GEWEX) initiated in 1988 by the World Climate Research Program strives to improve understanding and prediction of precipitation and evaporation processes at the global scale by promoting improved understanding of the hydrologic cycle's role in the climate system. GEWEX goals are pursued through a series of related programs including the GEWEX Continental-Scale International Project (GCIP), the GEWEX Americas

Prediction Project (GAPP), the Climate Variability and Predictability (CLIVAR) Study, the Climate and Cryosphere (CliC) Project, and the GEWEX Cloud System Study (GCSS). The goal of GCSS organized in the early 1990s is to improve cloud parameterizations as a specific component of the hydrologic cycle for climate and numerical weather prediction models. The Coordinated Enhanced Observing Period (CEOP) is designed to collect a comprehensive dataset from observation, satellite, and model sources covering all aspects of the hydrologic cycle (Randall et al., 2003). The Global Precipitation Climatology Project (GPCP) is the GEWEX program devoted to producing community analysis of global precipitation (Adler et al., 2003). These programs involve scientists and engineers from numerous disciplines representing several nations, and their goal is to develop understanding of the hydrologic cycle that can be transferred to other regions of the world. Major GEWEX accomplishments include the creation of multiyear global data sets of clouds, precipitation, water vapor, surface radiation, and aerosols.

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