Seasonal hydrologic response at various spatial scales in a small forested catchment, Hitachi Ohta, Japan

Headwater catchments are important and often overlooked hydrologic components of drainage basins. Hydrologic response at different scales was evaluated during a period of increasing antecedent wetness in a steep, humid catchment in coastal Japan. Outflows from a second-order basin (EB, 15.7 ha), two first-order basins (B, 2.48 ha and A, 0.84 ha), a zero-order basin (0.25 ha), and a hillslope segment (0.0045 ha) were measured for nine individual storms prior to and during the typhoon season of 1992. Antecedent rainfall greatly influenced the dynamic contributions from various watershed components. During drier conditions at the beginning of the typhoon season, storm runoff was small (< 2.5% of rainfall), largely generated by contributions from the narrow riparian zone. As wetness increased, subsurface flow from a hillslope segment contributed 2.0–3.4 times the runoff (on a unit contributing area basis) as the entire forest basin B. At least the lower 20% of the hillslope appeared to contribute actively to subsurface stormflow during late season events with very wet antecedent conditions. During major storms with high antecedent moisture, macropore flow comprised more than 25% of total subsurface flow during the peak and recession limbs of hydrographs. Almost no macropore flow occurred during storms with dry antecedent conditions. Zero-order basins with shallow soils appear to contribute to stormflow once a threshold of saturation is reached; after this point these geomorphic hollows contribute similar magnitudes of flow (per unit area) as subsurface hillslope drainage. Water yields during the two largest storms (with wet antecedent conditions) were higher in basin EB (16.6–25.6%) than in basin B (11.6–19.5%), partly owing to reduced canopy interception in the cutover portion of EB. During the two smallest storms (dry antecedent conditions) the opposite pattern occurred, likely because of the limited contributing area of stormflow near the stream channel. The dynamic hydrologic contributions of the near-channel zone, hillslope soils, and zero-order basins with respect to antecedent moisture and storm intensity offer new insights into the variable source area concept of streamflow generation and supplement more recent streamflow generation studies based on tracer tests.

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