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Groundwater hydrology may be defined as the science of the occurrence distribution, and movement of water below the surface of the earth. Geochydrology has an identical connotation, and hydrogeology differs only by its greater emphasis on geology. Utilization of groundwater dates from ancient times, although an understanding of the occurrence and movement of subsurface water as part of the hydrologic cycle has come only relatively recently.
SCOPE: Groundwater referred to without specification is commonly understood to mean water occupying all the voids within geologic stratum. This saturated zone should be distinguished from an unsaturated, or earation zone where voids are filled with water and air. Water contained in saturated zone is important for engineering work, geologic studies and water supply development consequently, the occurrence of water in this zones will be emphasized here. Unsaturated zone a re usually found above saturated zones and extend uquad to the ground surface. Because this water includes soil masture within the root zone, it is a major concern for agriculture, binary, and soil science. No rigid demarcation of water between the two zones is possible, for they possess an interdependent boundary and water can move from owe zone to the other in either direction. The interrelationships are described more in some higher hydrogeology texts.

Groundwater development dates from ancient times the Old Testament contains numerous references to groundwater, springs, and wells, other that dug wells, groundwater in ancient times we supplied from horizontal wells known as QAUNATS. These persist to the present day and can be found in a band across the regions of the South Western Asia and North Africa extending from Aghanistan to Morocco. A cross section a long a qanat ie shown in fig 1.1 typically, a gently sloping tunnel dug through alluvial material leads water by gravity flow beneath the water table at its upper end to a ground.

FIG 1.1

A vertical cross section along a qanat surface outlet and irrigation canal at its lower end. Vertical shafts dug at closely s paced intervals provide access to the tunnel. Qanats are laboriously hand constructed by skilld workers employing techniques that date back 3000 years.
Iran possesses the greatest concentration of qanats; here some 22,000 qanats supply 75 percent of all water used in the country. Lengths of qanats extend up to 30km, but most are less than 5km. The depth of the qanat mother well (see fig 1.1) ie normally less than 50m, but instances of depth exceeding 250m have been reported. Discharge of Qantas varies. Seasonally with water table fluctuations and seldom exceed 100m3/hr.

Utilization of groundwater greatly preceded understanding of its origin, occurrence, and movement. The writing of Greek philosophers to explain origins of springs and groundwater contain theories ranging from fantasy to nearly correct accounts. As late as the seventeenth century it was generally assumed that water emerging from springs could not be derived from rainfall, for it was believed that the quantity was in adequate and the earth too impervious to permit penetration of rain water for below the surface. Thus, early Greek philosophers such as Homer, Thates and Plato hypothesized that springs were formed by seawater. Conducted through subterranean channels below the mountains, then Aristotle suggested that air enters cold dark caverns under the mountains where it condenses into water and contributes to springs.
The Roman philosophers, including Seneca Pliny, followed the Greek ideas and contributed little to the subject. An important step forward, however was made by the Roman architect Vitnvius he explained the now accepted infiltration theory that the mountains receive large amounts of rain that percolate through the rock strata and emerge at their base to form streams.
The Greek theories persisted through the Middle Ages with no advances until the end of the Renaissance. The French Poffer and Philosopher Bernard Palissy (1510 – 1589) reiterated the infiltration theory in 1580, but his teachings were generally ignored. The German astronomer Johannes Kepler (1571 – 1630) was a man of strong imagination, who likened the earth to a huge animal that takes in water of the ocean, digests and assimilates it, and discharges the end products of these physiological processes as groundwater and springs. The seawater theory of the Greeks, supplemented by ideas of vapourizaton and condensation processes within the earth, was restated by the French Philosopher Rene’ Descarfes (1596 – 1650).
A clear understanding of the hydrologic cycle was achieved by the latter part of the seventeenth century. For the first time theories were based on observations and quantitative data. Three European countries made notable contributions, although others contributed to and supported these advances. Pierre Perrault (1611 – 1680) and estimated runoff of the upper sein drainage basin. He reported in 1674 that precipitation on the basin was about six times the river discharge, thereby demonstrating false the early assumption of inadequate rainfall. The French Physicist Edme Mariotte (1620 – 1684) made measurements of the same of paris and confirmed paraults work. His publication appeared in 1686, after his death, and contained factual data strongly supported the infiltration theory. Meinzer once stated. Mariotte probably deserves more than any other man the distinction of being regarded as the founder of groundwater hydrology, perhaps I should say the entire science of hydrology”. The third contribution came from the English astronomer Edmund Halley (1656 – 1742), who reported in 1693 on measurements of evaporation demonstrating that sea evaporation was sufficient to account for all springs and stream flow.

During the eighteenth century fundamentals in geologic were established that provided a basis for understanding the occurrence and movement of groundwater. During the first half of the nineteenth century many artesian wells were drilled in France stimulation interest in groundwater. The French hydraulic engineer Henry Darcy (1803 – 1858). Studied the movement of water through sand. His treatise of 1856 defined the relation, now known as Darcy’s law, governing groundwater flow in most alluvial and sedimentary formations. Later European contributions of the nineteenth century emphasized the hydraulics of groundwater development. Significantly contributions were made by J. Boussineq, G.A. Daubree, J. Dupuru and A. Thiem. In the twentieth century, increased activity in all phases of groundwater hydrology has occurred. Many Europeans have participated with publications of either specialized or comprehensive works. There are too many people to mention them all, but R. Dachler, E. Imbeaux, K. Keihack are best known in the United States. 
American contributions to groundwater hydrology date from near the end of the nineteenth century. In the past 90 years, tremendous advances have been made. Important early theoretical contributions were made by A. Hazem, F.H. King while detailed field investigations were begun by men such as T.C Chamberline, N.H Darton through his consuming interest in groundwater and his dynamic leadership of groundwater activities of the U.S geological survey, stimulated many individuals in the quest for groundwater knowledge. In recent decadeds the publications of M.S. Hantush, C.E. Jacob. Within the lat 20 years the surge in university research on groundwater problems, the establishment of professional consulting firms specializing in water resources, and the advent of the digital computer have jointly produced a competence for development and management of groundwater resources that was nonexistent hereto fore.

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