Page 73 - Introduction to Agriculture by: Aqleem Abbas

Introduction to Agriculture Notes prepared by: Aqleem Abbas

Water Cycle

Introduction

Water Cycle or Hydrologic Cycle, series of movements of water above, on, and below the surface of the earth. The water cycle
consists of four distinct stages: storage, evaporation, precipitation, and runoff. Water may be stored temporarily in the ground; in
oceans, lakes, and rivers; and in ice caps and glaciers. It evaporates from the earth’s surface, condenses in clouds, falls back to
the earth as precipitation (rain or snow), and eventually either runs into the seas or reevaporates into the atmosphere. Almost all
the water on the earth has passed through the water cycle countless times. Very little water has been created or lost over the past
billion years.

Storage

Enormous volumes of water are involved in the water cycle. There are about 1.4 billion cu km (about 340 million cu mi) of water
on the earth, enough to cover the United States with water 147 km (92 mi) deep. Slightly more than 97 percent of this amount is
ocean water and is therefore salty. However, because the water that evaporates from the ocean is almost free of salt, the rain and
snow that fall on the earth are relatively fresh. Fresh water is stored in glaciers, lakes, and rivers. It is also stored as groundwater
in the soil and rocks. There are about 36 million cu km (about 8.6 million cu mi) of fresh water on the earth.

The atmosphere holds about 12,000 cu km (about 2,900 cu mi) of water at any time, while all the world’s rivers and freshwater
lakes hold about 120,000 cu km (about 29,000 cu mi). The world’s two main reservoirs of fresh water are the great polar ice caps,
which contain about 28 million cu km (about 6.7 million cu mi), and the ground, which contains about 8 million cu km (about 2
million cu mi).

Almost all of the world’s fresh ice is found in the ice caps of Antarctica and Greenland. These ice caps cover more than 17
million sq km (more than 6.6 million sq mi) of land to an average depth of more than 1.5 km (more than 0.93 mi). Most other
glaciers, formed in mountain valleys at high latitudes, are tiny compared to the ice caps. If all of the ice in the ice caps and other
glaciers melted, it would raise the sea level by about 80 m (about 260 ft).

The amount of water stored as ice on the land varies with climate. At the peak of the last ice age, about 22,000 years ago, an
additional 20 million sq km (8 million sq mi) of land—including almost all of Canada, the northern fringe of the United States,
northern Europe, and large tracts in Siberia—were covered with ice about 1.5 km (about 0.93 mi) thick. Because this water came
from the oceans, sea level was about 120 m (about 390 ft) lower than it is today. Most water in the ice caps remains frozen for
centuries and is not readily accessible.

Most groundwater is more accessible and supplies much of people’s water needs in many regions of the earth. Permafrost,
ground that is always frozen, forms an impermeable barrier to the flow of groundwater. Permafrost occurs in places such as
northern Canada and Siberia where the annual average temperature is below 0° C (below 32° F).