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Abstract:Headwater catchments are sources of sediments, nutrients, and biota for larger streams, yet the hydrologic pathways that transport these materials remain unclear. Dynamics of storm¯ow generation related to landform attributes and antecedent rainfall were investigated in a steep forested headwater catchment at Hitachi Ohta Experimental Watershed, Japan. Such headwater catchments are deeply incised: the narrow riparian corridors have limited capacities to store and transmit water to streams. Storm runo was monitored at several nested scales within the catchment: (1) 2 . 48 ha ®rst-order drainage (FB); (2) incipient 0 . 84 ha ®rst-order drainage (FA) comprized of two zero-order basins; (3) 0 . 25 ha zero-order basin (ZB); and (4) 45 m 2 hillslope segment (HS), including subsurface matrix¯ow (MF) and preferential¯ow (PF). Results from applied tracer and staining tests as well as observations of piezometric, tensiometric, and subsurface temperature responses were also employed to elucidate hydrologic pathways during storms. During the driest conditions, water yield from FB was only 1%; runo occurred as saturated overland¯ow from the small riparian zone and direct channel interception. For slightly wetter conditions, subsurface¯ow from the soil matrix augmented storm¯ow. As wetness increased, two signi®cant non-linear hydrologic responses occurred: (1) threshold response in geomorphic hollows (zero-order basins) where runo initiated after an accumulation of shallow groundwater; and (2) selforganization and expansion of preferential¯ow pathways, which facilitate subsurface drainage. Storm¯ow increases observed during periods of increasing antecedent wetness depend upon temporal and spatial linkages and the unique hydrologic behavior of three components: (1) narrow riparian corridors; (2) linear hillslopes; and (3) geomorphic hollows. These linkages form the basis for an emerging hydrogeomorphic concept of storm¯ow generation for steep forested headwaters. Knowledge of storm¯ow response is critical to the assessment of management practices in these headwater areas as well as the routing of water and materials to larger stream systems.

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The Alto Loica experimental site was established to achieve some understanding of the hydrological characteristics and the effects of afforestation on soil and water conservation in semi-arid regions in Chile. Hydrological measurements were taken in the 16.25-ha catchment for seven years. Because of the high variability of annual rainfall, there was a drought year in which streamflow disappeared throughout the year. As the catchment contains intermittent streams, mean annual baseflow is only 5.9% of mean annual rainfall, while mean annual evapotranspiration is 62.6% of that. The seasonal fluctuations of soil moisture storage suggest that moisture conditions during the period of December through March may be the driest ones for the growth of vegetation. While Horton overland flow occurs over grassy and shrub covered hillside during high rainfall intensities, it contributes largely to sharp hydrograph peaks. During large rainstorms, the mean runoff coefficient was more than 50%. Hence, measures to reduce overland flow are required for afforestation as well as soil and water conservation.Discipline: Forestry and forest products Additional key words: Forest hydrology, water budget, overland flow, afforestation