I hear the meterologists speak about how we have a approx. 10 inch rain defficite. Looking at the ponds and lakes around my house, they are down about 4 - 5 feet or more. How is the 10 inch addup to the 4-5 feet of low water levels in ponds?
Posted October 21, 2007
MIKE MOSS SAYS: Devin, Water levels in lakes or ponds depend on rain that falls over a large area, then runs off into streams that eventually concentrate in the ponds or lakes, or in some cases flows within the ground as groundwater to replenish the pond. This magnifies the effect of rainfall, especially if it is rather widespread, so that an inch of rain across the watershed that feeds the body of water may result in a water level rise considerably larger than one inch (as a simple thought experiment, imagine placing a level half inch of water in a 13x9 inch pan, and then pouring that into a deep bowl about 4 or 5 inches in diameter - the water would certainly rise to much higher than one half inch in the bowl - likewise, if you only filled the pan to one quarter inch and concentrated that into the bowl, the difference between a half inch in the pan and a quarter inch in the pan would lead to a difference much greater than one quarter inch in the bowl).
When there is a significant rainfall deficit over a large area, the situation is reversed and the lack of rain across the watershed may be magnified in terms of the resulting lowering of water level. There are of course many complexities to these relationships, including the effects of water usage by humans and the effects of evaporation losses, which may be greater than normal during periods of unusually low rainfall due to the tendency for those periods to frequently correspond with higher than normal temperatures that make water evaporate more rapidly than would otherwise be the case.