Slide 5 of 75
Notes:
Water balance is the concept that the sum total of rainfall is equal to the amount of rain infiltrated, absorbed, and evapotranspired, and the volume of runoff generated from the surrounding watershed. In a pre-developed setting, much of the rainfall is absorbed by the surrounding vegetation, soil and ground cover. In a developed setting, the water balance changes and a disproportionate amount of rainfall becomes surface runoff. The changes in the water balance in urban streams are exemplified by changes in the volumetric runoff coefficient; "flashy" hydrographs; increased flood peaks, peak discharge, and frequency of bankfull flows; floodplain widening; and decreased dry weather flows.
This diagram shows how development and its corresponding increase in impervious cover disrupts the natural water balance. In the post-development setting, the amount of water running off the site is dramatically increased.