Certified Erosion, Sediment, & StormWater Inspectors (CESSWI) Practice Test

Isohyetal maps are tools used to represent the distribution of rainfall across a specific area by connecting points of equal rainfall amount. They provide valuable information for predicting surface runoff volumes because runoff is significantly influenced by the amount and intensity of precipitation that an area receives. When rainfall amount is accurately depicted through isohyetal maps, it allows for a better understanding of how much water will potentially run off into streams, rivers, or other bodies of water, thereby helping to forecast flooding or manage stormwater.

The effectiveness of isohyetal maps in predicting surface runoff lies in their ability to illustrate variations in rainfall, which is a crucial factor in determining how much water infiltrates the soil and how much will become surface runoff. Other factors like soil characteristics, land use, and vegetation cover also play roles in determining runoff, but the fundamental driver is the precipitation amount.

In contrast, while soil texture analysis can inform how much water the soil can absorb before runoff begins, peak flow calculations are typically a consequence of rainfall amounts rather than a predictor in their own right. Vegetation density can also affect runoff by influencing infiltration rates, but without the crucial data provided by rainfall distribution depicted on isohyetal maps, these factors alone cannot effectively predict surface runoff volumes.