Soil Moisture, Ground Water, and Streamflow Response to Rainfall In a Slope and Watershed Underlain by Paleozoic Sedimentary Rocks

Soil moisture and ground water level (GWL) were continuously measured in a concave slope (altitude 130 m, slope 32°) in the gauged Tatsunokuchi-yama experimental watershed (34°42′N, 133°58′E, 50-257 m, 22 ha) since late March 2007. The watershed is underlain by Paleozoic sedimentary rocks, and annual precipitation is usually less than 1200 mm with little snowfall. The experimental slope is forested, primarily Eurya japonica. An observation well was installed to a depth of 17.5 m at mid-elevation within the watershed. A strongly weathered sandy stone layer extends from about 0.3 m in the subsoil down to a depth of 6 m; thereafter, a fractured and weathered sandy mudstone layer exists down to 16 m. Below the 16 m depth, relatively unweathered sandy stone exists. GWL increased up to about 10 m below ground surface in the July rainy season and then continuously declined after early August because of less precipitation. During this period, the GWL recession rate converged to about 3×10^-7 m/s when GWL fell below 16 m, approximating the permeability obtained when boring. GWL decreased below 17.5 m in mid-October, then reappeared in late April 2008 about one month after 60 mm/d of rainfall. Soil water pF at the 40 cm depth exceeded 1.8 in late August 2007, then became < 1.8 in late April 2008. For pF<1.8, gravitational drainage is assumed to dominate and GWL increased within several hours even during small rainfall. But GWL decreased when pF>1.8 even if rainfall was 20 mm/d. Nevertheless, streamflow responded closely to rainfall during the entire observation period. Since ground water temperature remained about 14°C and Hortonian overland flow did not occur, rainfall appears to mainly discharge through fractured and weathered rock layers < 16 m deep.