Estimation of Bedrock Infiltration on a Weathered Granitic Mountain Covered by Japanese Cypress Forest using Water-Budget and Eddy Covariance Methods

At a watershed underlain by weathered granitic bedrock (5.99 ha) and covered by Japanese cypress forest, we estimated apparent bedrock infiltration (QB) by extracting evapotranspiration measured by eddy covariance (EEC) from the residual of the water budget (EWB = precipitation – runoff) over an 8-year period (2001–2008). The degree of interannual variation in EEC was relatively small throughout this period. Conversely, EWB showed larger annual variations and positive dependence on annual precipitation. The difference between EEC and EWB was greater in the years with more precipitation. Therefore, estimated values of QB also showed positive dependence on annual precipitation. Observed average annual precipitation, runoff, EEC, and estimated QB were 1549, 722, 742, and 86 mm year -1 , respectively. Consequently, QB is believed to have amounted to 0–10% (average 5%) of total precipitation. The findings suggest that episodes of rainfall with medium and high intensity induce lower and higher QB values, respectively, with similar amounts of precipitation.