When the Water Account Runs Low

When the Water Account Runs Low

1A dry streambed can make it seem as though the water cycle has stopped. It has not. Water keeps moving among the atmosphere, land, and bodies of water, but it does not arrive everywhere in equal amounts or on a regular schedule. A drought develops when a place receives unusually little precipitation over an extended period. Precipitation includes rain, snow, and other water that falls from clouds.

2The sun supplies energy for evaporation, the change of liquid water into water vapor, a gas. Plants also release water vapor through their leaves in a process called transpiration. As water vapor cools, it can condense into tiny liquid droplets that form clouds. However, the presence of clouds does not guarantee rain for the land below.

3Think of underground water as a savings account. During wetter periods, some water enters the ground and replenishes that account, replacing water that has been removed. This movement from the surface into soil is called infiltration. Water that travels across the land instead is called runoff. Underground water can move slowly through spaces in soil and rock.

4During drought, the account can shrink. Wells and plant roots continue to draw on underground supplies even when little new water enters the ground. Hot conditions can also increase water loss through evaporation. If water leaves a region faster than it returns, a deficit develops: the supply falls short of what is needed. A place can therefore have water underground while still facing a serious shortage.

5One brief shower may be welcome without breaking a stubborn drought. Here, stubborn suggests a drought that is difficult to end, not weather that makes decisions. Parched soil may need repeated periods of rain before deeper stores recover. The word parched emphasizes how extremely dry the soil has become. A storm might wet the surface while doing little to restore supplies farther below.

6Drought can loosen its grip gradually. Scientists track precipitation, stream levels, soil moisture, and underground supplies to judge recovery. These clues do not always improve together. A stream may begin flowing before underground stores have recovered. Following several clues gives a fuller picture than looking out the window after a single rainy afternoon.