droughts
Mood:  silly
Meteorological Drought: This type of drought is the simplest to identify and it is probably what almost everybody would think of when the word drought is mentioned. A meteorological drought occurs when there is a lack of precipitation over a certain period of time. Notice that I have not given any specific numbers for precipitation deficits or for the length of time. The reason for not being specific is that these numbers will vary from site to site across the state and especially from state to state and region to region across the country. For example, the amount of precipitation that falls is not the only factor that determines whether or not an area is in a meteorological drought. What is important is how effective the precipitation is in watering the land. If temperatures are cool during a dry period, then there will be less evaporation of water and less transpiration of water by plants. As a result, the precipitation that does fall is more effective in watering the landscape. Should hot conditions exist during a period with a lack of precipitation, there will be more evaporation and transpiration and the precipitation that falls is less effective in watering the landscape.

Hydrological Drought: This type of drought reflects the conditions of the water systems on the landscape including both surface and ground water systems. There is no method to identify positively when an area is under hydrological drought conditions, although very low stream flow and very low ground water level conditions are part of a hydrological drought. Hydrological drought conditions may not exist during the early parts of a meteorological drought, because water levels do not respond immediately to a decrease in precipitation. Surface waters respond more quickly than do ground water levels. However, once peak conditions in a meteorological drought are reached, it is likely that the area is also under hydrological drought conditions. This is what was happening from the end of last summer through the end of 2001. On the other hand, a hydrological drought often will extend beyond the meteorological drought. Precipitation may start to fall again at rates that are effective in increasing water levels, but it may take a period of time for water levels to reach conditions that are closer to average. Again, surface water systems will respond more quickly to increased precipitation than ground water systems. Another way to look at hydrological drought is to think of this type of condition as reflecting long-term conditions of the water system. It will take a lot of precipitation to overcome the record low precipitation of 2001 and the lower than average precipitation of late 2000!

Agricultural drought: This type of drought is the opposite to the hydrological drought as far as the time period of interest. This drought reflects short-term conditions in the hydrological system; particularly how much moisture is consumed by transpiration of plants and how much water is in the soil zone. Essentially, it is a way of identifying whether or not there is enough moisture available for plants to use. An area can fall into an agricultural drought while not necessarily being in either a meteorological or hydrological drought. If precipitation amounts are low in a given month or for just a few weeks and it is a very warm period, then plants will find that there is very little moisture available for their immediate use. On the other hand, an agricultural drought may exist before and then during the early stages of a meteorological drought. However, a brief period of rain during a meteorological drought can alleviate the agricultural drought at least temporarily.