Quantifying Evaporation from Pervious Concrete Systems: Methodology and Hydrologic Perspective

Permeable pavements underlain by infiltration beds have been used as storm-water control measures (SCMs) for several decades. As a design practice, runoff volume reduction in those systems is attributed exclusively to subsurface infiltration. Neglecting evaporation in the hydrologic cycle of permeable pavement systems is based on the perceived insignificance of this factor rather than on scientific evidence. This paper presents research designed to fill the knowledge gap in the evaporation behavior of pervious concrete SCMs. A laboratory simulation was conducted to identify parameters affecting evaporation from pervious concrete systems and to obtain the evaporation rates typical for summer months in the Philadelphia area. Considerations used in the experiment design, methodology, the experimental program, and the results are presented here. The depth to water surface and the time since rainfall event were both found to be significant terms in predicting the evaporation rate. The concept of influence depth was established and characterized as approximately 250 mm. A predictive empirical equation describing 24-h evaporation rates as a function of the initial depth and the time since the last rainstorm event was developed. Generalizing the experimental results to field conditions, the percentage of the water budget accounted for by evaporation from a single rain event was found to be highly variable, ranging from negligible to moderate depending on the watershed configuration and the size of the rain event. The paper includes a discussion of evaporation behavior and optimization of the design parameters to promote evaporation from pervious pavement systems.