The Hydrologic Cycle

W ater, our most precious resource. Over the millennia, great civilizations have both flourished and perished due to the availability of water. Today, industrialized societies are still, and possibly more so, dependent on reliable water supplies. In Missouri , each person uses about 75 gallons of water per day for household and other domestic uses. Even with more efficient use and other conservation efforts, as the population increases, so does the need for water.

A Renewable Resource

 The amount of water on Earth has essentially never changed. It is continuously being recycled, moving from one storage place to another, including lakes, streams and oceans, underground, in glaciers and ice caps, plants, and in the atmosphere. This recycling movement is called the water or hydrologic cycle. The hydrologic cycle explains the exchange of water between the atmosphere, ground and surface of the Earth. The hydrologic cycle is perhaps the most important natural phenomenon on Earth; it is the driving force behind most other natural processes. The movement of water through the cycle annually replenishes our water supplies, thus making water a renewable resource. This replenishment takes place throughout the year, but is more pronounced during the spring.

 The hydrologic cycle is an ever fluctuating, dynamic system. Small changes often occur in the quantities of water located in the various segments of the cycle. Many of these fluctuations relate to seasonal changes. For example, the amount of rainfall, the effect of temperature on evaporation, and the uptake of water by plants during the growing season all affect how much water will be available in any segment of the water cycle. Thus, the movement of water in the cycle is always changing. Even small changes in the cycle at a regional or local scale may look like large changes to us as in the form of droughts or floods.

A water budget is used to understand its effect on local water resources and to predict or estimate quantities of available water from surface or ground water sources. The water budget for Missouri is illustrated above. The Bonne Femme Watershed averages about 39 inches of precipitation a year. Of this, 9 inches run off the land surface to streams and rivers, and the remaining 30 inches is returned to the atmosphere through a combination of evaporation and transpiration (the process where water that passes through a plant's pores back to the atmosphere).

 The hydrologic cycle assures a reliable, although fluctuating, water supply by annually replenishing, or recharging, both surface and ground water sources. When water removal (evaporation, transpiration, water supply, etc.) exceeds replenishment (precipitation), water levels fall as usually observed during the summer and fall months; conversely, water levels rise when replenishment exceeds removal, usually during the winter and spring months.