Predictability of Effects of a Severe Local Storm in Pennsylvania

Abstract

On 24 May 1975, an isolated thunderstorm in east-central Pennsylvania produced over 4 inches of rain in 2 h over an Agricultural Research Service instrumented area. The return period for this storm appears to exceed 650 years. Simulation of the precipitation excess from all soils within the storm area using a modification of the Mein and Larson infiltration equation indicated surface runoff from a limited number of soil types?the same soils which exhibited severe erosion plus the alluvial soils of the stream channels. About 17% of the area affected by the storm accounted for nearly all of the measured surface runoff. Post-storm field investigation showed that the principal damage by erosion was restricted to corn fields on two soil groups, both silty clay loams over a fragipan. Soil losses due to sheet and rill erosion were estimated in the field as ranging from 15 to 110 tons acre?1. Soil losses resulting from gully erosion could have been as high as 300 tons acre?1 in some areas. Simulation of the event using modification of the universal soil loss equation shows that the location and amount of damage could have been predicted. Analysis of the impact of the storm illustrates that 1) the lack of a general wind field which would give the storm horizontal motion accounted for the extreme 2 h rainfall, 2) presently available techniques for the simulation of runoff and erosion can be successful under these most extreme conditions, and 3) the source-area concept of runoff and sediment production must be considered in modeling such events.