Page 74 - Introduction to Agriculture by: Aqleem Abbas

Introduction to Agriculture Notes prepared by: Aqleem Abbas

Almost all groundwater fills the tiny pores and cracks in the soil and rocks. Very little is stored in subterranean caverns. Near the
earth’s surface, most soils and sedimentary rocks are so porous that water can occupy from 20 to 40 percent of their volume. As
depth increases, the pores and open spaces in the rocks are squeezed shut. As a result, almost all groundwater is found in the top
8 to 16 km (5 to 10 mi) of the earth. Water below this depth is chemically bound in the rocks and minerals and is not readily
available, but it can be released as a result of geologic processes such as volcanic eruptions (see volcano).

Evaporation

Evaporation is the process by which liquid water changes to water vapor and enters the atmosphere as a gas. Evaporation of ice is
called sublimation. Evaporation from the leaf pores, or stomata, of plants is called transpiration. Every day about 1,200 cu km
(about 290 cu mi) of water evaporates from the ocean, land, plants, and ice caps, while an equal amount of precipitation falls
back on the earth. If evaporation did not replenish the water lost by precipitation, the atmosphere would dry out in ten days.

The evaporation rate increases with temperature, sunlight intensity, wind speed, plant cover, and ground moisture, and it
decreases as the humidity of the air increases. The evaporation rate on the earth varies from almost zero on the polar ice caps to
as much as 4 m (as much as 13 ft) per year over the Gulf Stream. The average is about 1 m (about 3.3 ft) per year. At this rate,
evaporation would lower sea level about 1 m per year if the water were not replenished by precipitation and runoff.

Precipitation

Precipitation occurs when water vapor in the atmosphere condenses into clouds and falls to the earth. Precipitation can take a
variety of forms, including rain, snow, ice pellets, and hail. About 300 cu km (about 70 cu mi) of precipitation falls on the land
each day. Almost two-thirds of this precipitation reevaporates into the atmosphere, while the rest flows down rivers to the oceans.
Individual storms can produce enormous amounts of precipitation. For example, an average winter low-pressure system drops
about 100 cu km (about 24 cu mi) of water on the earth during its lifetime of several days, and a severe thunderstorm can drop
0.1 cu km (0.02 cu mi) of water in a few hours over a small area.

Runoff

Water that flows down streams and rivers is called surface runoff. Every day about 100 cu km (about 24 cu mi) of water flows
into the seas from the world’s rivers. The Amazon River, the world’s largest river, provides about 15 percent of this water.
Runoff is not constant. It decreases during periods of drought or dry seasons and increases during rainy seasons, storms, and
periods of rapid melting of snow and ice.

Water reaches rivers in the form of either overland flow or groundwater flow and then flows downstream. Overland flow occurs
during and shortly after intense rainstorms or periods of rapid melting of snow and ice. It can raise river levels rapidly and
produce floods. In severe floods, river levels can rise more than 10 m (more than 33 ft) and inundate large areas. Groundwater
flow runs through rocks and soil. Precipitation and meltwater percolate into the ground and reach a level, known as the water
table, at which all of the spaces in the rocks are filled with water. Groundwater flows from areas where the water table is higher
to areas where it is lower. The speed of flow averages less than 1 m (less than 3.3 ft) a day. When groundwater reaches streams, it
supplies a base flow that changes little from day to day and can persist for many days or weeks without rain or meltwater. During
periods of sustained drought, however, the water table can fall so low that streams and wells dry out.