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Newsletter and Technical Publications

Lakes and Reservoirs vol. 2

The Watershed: Water from the Mountains into the Sea

The Hydrologic Cycle: How Water Moves Around the World

Comprised simply of two atoms of hydrogen and one of oxygen, water is a remarkable substance in many ways. Water covers nearly three-fourths of the surface of our planet, and it is present in the Earth ’s atmosphere and in its crust. It also comprises a large part of all plant and animal matter. It is the only natural resource that exists naturally in three forms: liquid, solid (snow, ice) and gas (clouds). Unlike most mineral resources, it is renewable — it exists in an endless cycle, , moving between its gaseous, liquid and solid forms. This “hydrologic cycle ” comprises nature ’s method of replenishing, redistributing and purifying the world' s natural water resources.

To understand how water moves in our world, attention must be directed to the hydrologic cycle. The hydrologic cycle is essentially a water continuum, representing the different paths through which water circulates and is transformed in the natural environment (Fig. 1). Being a cycle, it has no specific beginning or ending. Rather, liquid water from the Earth ’s surface, particularly the oceans, is evaporated into a gaseous form and enters the atmosphere as water vapor (clouds). The atmospheric moisture is eventually returned to the Earth ’s surface in the form of rain or snow. It is estimated that approximately 100, 000 cubic kilometres (about 20% of the total global annual precipitation) falls onto the land surface of the continents. The liquid fresh water moves over the land surface on its journey back to the ocean. During its overland journey, it creates rivers, lakes, wetlands and/or groundwater aquifers. Further, a portion (so-called endorheic water-bodies) have no direct access to the oceans. As discussed in a later section, examples of the latter include the Caspian Sea in eastern Europe, the Aral Sea in south-central Asia, and Pyramid Lake in the western United States.

Fig. 1: The hydrologic cycle. detail

It is estimated that approximately 42,000 cubic kilometres of precipitation flows back to the oceans through the world’s rivers each year. Some of the precipitation will seep down into the Earth’s surface and become groundwater. Some of it will be taken up by plants and subsequently released in gaseous form back into the atmosphere via a process called transpiration. A substantial quantity of water is returned to the atmosphere in this manner, thereby short-circuiting the full hydrologic cycle. It is estimated, for example, that a hectare of corn can transpire about 30-40 cubic metres of water back into the atmosphere each day. Nevertheless, because of their enormous surface area, the most important source of water in the atmosphere is evaporation from the oceans, which comprises nearly 90% of the total global evaporation. Indeed, it is the fact that more water evaporates from the oceans than is directly precipitated back, thus creating the driving force for the hydrologic cycle.

Photo 2: Floating ice on Lake Argentino

A substantial input of heat energy is required to melt ice into liquid water, and to boil it into water vapor. A substantial uptake of heat energy also is required to freeze it. These properties give water a considerable capacity to resist freezing or boiling in response to temperature changes. Another unusual property is that water is most dense at 4¾C, when still in liquid form. Because ice (solid water at 0¾C) is less dense than liquid water at 4¾C, it will float on the top of a water-body, with the liquid water below it (Photo 2). Otherwise, lakes would freeze from the bottom up, freezing all life contained within it. Further, water can dissolve materials in its flow over the land surface and carry the materials as it moves. If the water volume and velocity are sufficiently large, flowing water also can pick up and carry materials that it cannot readily dissolve. A small mountain stream, for example, will not be able to carry large rocks along the stream channel, while a large, swiftly- flowing river can readily carry rocks, soil and other materials in its flow. A third factor is that humans can control the movement of water, including such measures as pumping it upstream against the force of gravity, pooling or storing it in different locations, and even moving it over long distances.

Except for that portion evaporated or transpired directly back into the atmosphere, most of the water reaching the land surface in the form of rain or snow (plus the dissolved and particulate matter it accumulates during the course of its overland journey) will eventually find its way back to the oceans via transport in streams, rivers, lakes, reservoirs, wetlands and groundwater aquifers, to begin its cycle anew.

Fig. 2: Fresh water availability on Earth. detail

Although it is impossible to get an exact figure, it is estimated that the oceans contain approximately 1.3-1.4 billion cubic kilometres of water, comprising about 97% of all the water on Earth. Of this global total, the volume of fresh water is estimated to be about 35 million cubic kilometres. It is estimated that about 75% of the world’s freshwater volume is locked up in frozen form in polar ice caps and glaciers, 14% is located deep underground beyond easy human reach, 11% is in groundwater at depths accessible to human use, 0.35% is in lakes, 0.03% is in rivers, 0.06% exists as soil moisture, and 0.035% is in the atmosphere (Fig. 2). To illustrate the relative volumes, if all the water on Earth could be put into a gallon jug (about 3.85 liters), the quantity readily available for human use would be equal to about one tablespoon.

Photo 3: Mojave Desert, an arid region, USA

Atmospheric deposition is the primary means by which water is distributed over the Earth’s surface. Ironically, it is estimated that there is a sufficient quantity of fresh water to supply all present and foreseeable human water needs if the water were distributed evenly around the world. Unfortunately, however, although nature is bountiful in supplying fresh water, it also displays a confounding fickleness in not distributing the water equitably around the world. This observation applies to both location and timing. Some areas receive large quantities of precipitation each year. In contrast, semi-arid and arid regions are characterized by a limited or scarce water supply (Photo 3). The world ’s water resources are also unequally distributed in regard to the water flows in different regions. The Parama River (Photo 4), is the second largest river in South America after the Amazon River. The latter carries 16% of the world ’s water runoff (Photo 5). In contrast, arid and semi-arid regions only account for about 2% of the total global runoff, even though they comprise about 40% of the Earth’s land surface.

Photo 4:Parana River and city of Posadas, Argentina

Nature is also inconsistent with regard to the timing of the precipitation. Some regions receive the bulk of their annual precipitation at one time of the year, while the primary human water needs may occur at a different time during the year. This is a major impetus for the construction of artificial lakes or impoundments, thereby allowing humans to store water for use at a time of their choosing (Photo 6).

Photo 5: River Ucayali which gives birth to the Amazon River, Peru.

Whatever its distribution and timing, adequate supplies of fresh water are an obvious and fundamental requirement for socioeconomic development. This is readily evident in the limited economic development that characterizes most semi-arid and arid regions in the absence of major human intervention to overcome the water deficit.

Rivers are a very most important component of the hydrologic cycle, being a primary determinant of human settlement patterns and economic development. They also represent the major portion of the world ’s water withdrawals and water consumption. Water runoff over the land surface is the primary interface between human activities and their impacts on water supplies. Thus, human actions in a watershed are the primary factor determining the quantity and quality of water available for human water uses, as well as for maintaining natural ecosystems. One can even characterize human water use as a type of “anthropogenic hydrologic cycle”, in that humans typically extract water, use and degrade it, discharge it, extract and treat it when needed again, and then re-use it in a continuing cycle. Unfortunately, the cycle can become increasingly expensive and difficult to maintain as humans continue to pollute and/or otherwise abuse their water resources.

Photo 6: Takayama Dam on the Nabari River, Japan

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