The Sinwulu catchment has a tropical monsoon climate, with a
mean annual precipitation of 2300 mm. Due to the huge amount of
rainfall triggered by typhoons, on average, 78% of annual precipitation
is concentrated in summer and autumn. Thus, the precipitation in the
dry season from October to the following April is just 22% of annual
precipitation. Besides, the mountain climate features wide temperature
variations; in the catchment between the piedmont alluvial area
and the mountainous area temperatures sometimes will abruptly
drop 10 °C. The average temperature in the Sinwulu catchment is
15 °C where the altitude exceeds 1000 m and 24 °C where it is below
1000 m.
Major lithologies in the Sinwulu catchment include slates in the
Pilushan Formation, and Black Schist, Mixed black/green Schist and
Marble in the Tananao Complex (Fig. 1C). The Pilushan Formation is
395 km2 in area, occupying 62% of the catchment area; the Black
Schist in the Tananao Complex is 53 km2 in area, 8% of the catchment
area; the Mixed Schist is 158 km2 in area, 25% of the catchment area;
and the Marble is 10 km2 in area, just 4% of the catchment area. The
formation's prevailing attitude is a strike of N20°–60°E with
southward dips of 35°–70°, and with various discontinuous planes
distributed within.