A hydrologic investigation of soils developed on a fractured crystalline rock area of Jamul, California, was conducted for the purpose of quantifying soil moisture recharge by in situ methods. Changes in soil water tension and electrical resistance were monitored in the upper five feet of the soil profile during the 1980 hydrologic year. Soil water tension was later converted to soil moisture content and estimates of recharge were obtained by plotting changes in soil moisture content versus depth, and measuring the area between the curves. Soil moisture recharge estimates ranged from 0 to 1.50 inches for a below-average precipitation year. The average recharge in soils where precipitation infiltrated below the zone of evapotranspiration was 0.64 inch. Infiltration in soils formed as a weathering product of basic igneous rock was limited to a maximum depth of 25 inches. Firm weathered material at shallow soil depths severely restricted infiltration, and no recharge was observed in shallow soils underlain by firm weathered material. Some evidence exists that soil moisture infiltrating down to the soil-firm weathered material interface may tend to accumulate and form a perched zone before flowing horizontally along the interface. Based on this investigation, ground water recharge in fractured crystalline rock terrain may be a result of infiltration of precipitation into near surface fracture zones with a high hydraulic conductivity rather than percolation through overlying soils.
