We distinguish between hydrologic and agricultural droughts, although many people believe that they are the same. A geologic drought occurs when annual rainfall decreases substantially below normal annual rainfall, and when annual recharge is unable to restore river flow and groundwater tables to their normal winter highs. It often takes two or more years for these serious geologic droughts to occur. Agricultural droughts occur whenever seasonal rainfall from planting to harvest falls significantly below normal levels. Since our soils only hold a week or so's supply for the plant, yield falls below long term average yields during agricultural droughts. Rainfall is almost always below the evapotranspiration; so, yields typically are lower than their potential every year, but this is not considered a drought.

Note, the Hydrologic Year is often used when examining rainfall, flow in rivers or elevation of groundwater tables. For the Southeast, the hydrologic year begins at the normal end of the low rainfall period, which is also the time when trees and plants begin to decrease water use. Thereafter, water tends to accumulate in soil, groundwater tables rise, and river flow increases, as net rainfall exceeds net evaporation. In the latter half of the hydrologic year the opposite occurs.

Page created by James E. Hook on Dec 7, 1998. Last updated June 18, 2002.