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Abstract:This study aims to evaluate the relative contribution of a zero-order basin to runo generation in a headwater catchment as well as to elucidate internal factors aecting hydrological response. Runo, piezometric heads, and soil temperatures were measured for a 0 . 25 ha zero-order basin (ZB) together with discharges from an adjacent 0 . 84 ha 1st-order basin (FA) and a larger 2 . 48 ha 1st-order basin (FB) which includes both ZB and FA. Data collected over a year showed ZB contributed to runo generation in FB at three dierent levels. While continuous runo was recorded from both FB and FA, no substantial runo was measured for ZB during dry conditions when discharge from FB 5 0 . 5 mm d À1 (level 0: no contribution). For wetter conditions above this threshold, the ZB augmented storm¯ow and the discharge ratios of ZB to FB (on a unit area basis) increased rapidly from zero up to unity with increasing wetness (level 1: non-linear contribution). During the wettest periods when discharge from FB 4 5 mm d À1 , all three basins generated runo of the same order per unit area (level 2: linear contribution). Piezometers installed above the soil-bedrock interface (0 . 5 to 1 . 2 m depth) along the longitudinal axis of ZB responded only in the lower locations when runo from ZB ( discharge from FB. Conversely, the major runo contribution from ZB to the discharge from FB generally coincided with a large piezometric rise near the head hollow. Soil temperatures in the head hollow¯uctuated even during some rainstorms, indicating that such a large piezometric rise was caused by a convergent subsurface¯ow from the further upslope. Thus, shallow groundwater, which developed above the trough of ZB, would not always extend from the base to the upslope but may appear simultaneously in the head hollow. This additional contribution due to upslope topography may create additional variability and non-linearity in runo response from ZB relative to planar hillslopes.

Stemflow was measured in a planted young stand of Japanese cypress for four years from ages 9 to 12. Canopy cover increased with growth from 55% to 94% during the measurement period. The ratio of stemflow SF to rainfall R, SF/R (the funneling ratio FR that represents the efficiency in collecting stemflow), was 5.9% (81.3) at age 9 on an annual basis; however, it abruptly fell to 2.8% (30.0) at age 10. Following the drop, SF/R recovered gradually with growth, reaching 3.8% at age 11 and 4.3% at 12, while FR remained almost constant at values of around 30. The relation between R and SF/R analyzed quarterly on a rain event basis revealed that changes in canopy structure and/or tree architecture caused the drop in SF/R in the April June period at age 10. Saplings of the species must compete for light and water until canopy closure because their growth rate is slower than that of competitors. As stemflow effectively supplies rainwater into the soil around the root system, it can be hypothesized that large SF/R and FR at age 9, and probably younger ages, are a strategy to acquire water for juvenile unclosed stands in dry summers.