Drought: Reading the Cycle, Hearing the Change

Water in Motion, Water in Storage

1A stream can shrink beneath a cloudless sky even while water continues moving through the water cycle. The cycle does not promise that every place receives water whenever it needs it. Understanding drought means looking not only at moving water, but also at water stored in soil, snow, lakes, and underground.

2Energy from the sun helps liquid water evaporate into water vapor. Plants also release water vapor through transpiration. As moist air cools, water vapor can condense into droplets or ice crystals that form clouds. Water returns to the surface as precipitation, including rain and snow. Winds can carry moisture far from the place where it entered the atmosphere.

3After precipitation reaches land, several paths are possible. Some water flows over the surface toward streams. Some soaks into soil, where plant roots can take it up. Some travels deeper and replenishes groundwater. Snow can store water until warmer conditions cause melting. These stores release water at different rates, so a stream's flow may reflect precipitation from much earlier.

4Drought develops when a place has an unusually long period with too little precipitation for its usual conditions and needs. Warm conditions can increase water loss through evaporation and plant use, adding to the shortage. Meanwhile, people may continue drawing water from rivers, reservoirs, or wells. Water can leave local stores faster than those stores are replenished, even though the global cycle continues.

5A heavy shower does not necessarily end a drought. Rain may dampen the surface while deeper soil remains dry. Some water may run off instead of soaking in, depending on the ground and the storm. Groundwater and reservoirs can take longer to recover than a sidewalk takes to dry. Recovery depends on where water goes, not simply on whether rain falls.

6For that reason, people monitoring drought examine several clues: precipitation, soil moisture, streamflow, and water levels in storage. One clue may improve before another does. A puddle shows that rain reached one surface; it cannot reveal the condition of every local water store. Reading several clues together gives a fuller picture than watching the sky alone.