The Water Cycle

Until the 16th century, oceans were thought to be the source of rivers and springs via underground seepage. Palissy and others suggested that storage of rainwater was the real source, based on several lines of reasoning. It was noted that springs would not dry up in summer if oceans were the source, since the oceans do not decrease noticeably. Springs should be more common at low elevations if they derive from oceanic water. However springs often do dry up in summer, they are more common on mountain slopes, and finally, springs are fresh. In 1674, measurements by Perrault showed that precipitation into the Seine basin was six times greater than discharge (Morisawa 1968). This finding changed the focus from whether rainfall is sufficient to provision rivers, to where does the rest of the rainfall go.

The hydrologic cycle describes the continuous cycling of water from atmosphere to earth and oceans, and back to the atmosphere (Figure 2.2). Conceptually this cycle can be viewed as a series of storage places and transfer processes, although water in rivers is both a storage place, however temporary, and a transfer between land and sea. The hydrologic cycle is powered by solar energy. This drives evaporation, transferring water from the surface of the land and especially from the oceans, into the atmosphere, and also the water loss that plants experience as a consequence of gas exchange necessary for photosynthesis. Together these are referred to as evapotranspiration (ET). Precipitation, primarily as rain and snow, transfers water from the atmosphere to the land surface. These inputs immediately run off as surface water, or follow a number of alternative subsurface pathways, some of which (e.g., groundwater) release to the stream channel much more slowly and so are, in effect, storage places as well.

Despite the enormous significance of rivers in the development of civilizations and the shaping of land masses, the amount of water in rivers


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