Geophysical Based Site-Specific Mapping of Fragic Properties to Constrain Hillslope Hydrologic Controls On Variable Source Area Hydrology.

Fragipans impede the movement of water via generation of saturated conditions, which in turn increases runoff of agricultural nutrients.  The USDA Soil Survey provides scale-dependent information on the presence of a fragipan (and drainage class), but its resolution is not fine enough to delineate regions prone to saturation at the field-scale.  In this study, we used ground-penetrating radar (GPR) to generate a model of fragipan extent and depth in a portion (approximately a third) of the 39.5-ha-FD-36 watershed, within the Mahatango creek watershed, east-central Pennsylvania.  GPR transects (10-15 m apart) were run and fragic properties determined via soil pits.  Data from ground-penetrating radar, in conjunction with field verification of the presence of fragic properties, were used to build a model of the extent and depth of the pan. 

Model results of fragipan extent were compared to fragipan extent mapped in an order 1 soil survey of the watershed. On the south slope of the watershed, fragipan extent determined via GPR was comparable to that shown in the soil survey.  On the north slope, however, fragipan identification via GPR over-predicted fragipan extent relative to the soil survey.  It is presumed that bedrock was interpreted as a fragipan in the radar profile for regions of the north slope; the reason for this may have to do with the high soil moisture status during data collection.  In the future, more soil data will be used to confirm or improve the viability of the model.  Further, the role of soil moisture conditions (and a perched water table) on fragipan identification via GPR will be investigated.