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Heterogeneous Soil Water Dynamics around a Tree Growing on a Steep Hillslope

Laboratory of Erosion Control, Dep. of Forest Science, Graduate School of Agriculture, Kyoto Univ., Kyoto 606-8502, Japan

View larger version (53K): [in this window] [in a new window]   FIG. 1. (a) Topography of the observation area showing the locations of tree stems, tree canopy areas, soil water content measurement points, and throughfall measurement points; (b) side view of tree stands; (c) longitudinal section along the observation line for tree S1 with soil water content and pore water pressure measurement points P1 through P10.

View larger version (169K): [in this window] [in a new window]   FIG. 2. Collection system for stemflow along the upslope and downslope sides of the trunk of tree S2.

View larger version (31K): [in this window] [in a new window]   FIG. 3. Hyetograph and the change in soil water content () and pore water pressure head () in the upslope region at pore water pressure measurement points P3 and P5 and in the downslope region at points P6, P7, and P9. The blue-shaded periods correspond to the small and heavy storm periods shown in Fig. 4 and 5, respectively.

View larger version (27K): [in this window] [in a new window]   FIG. 4. Hyetograph (upper), spatial variation in soil water content change () (middle), and hydraulic head () distribution (lower) for pore water pressure measurement points P1 to P10 at (a) 170 min, (b) 220 min, (c) 230 min, and (d) 380 min for the small storm event of 5 July 2006, where is the difference between the current water content and the initial water content observed at the start of the storm event, and was computed as the sum of the observed pore water pressure () and the height for the measurement, which corresponds to the height of the soil–bedrock interface. The black dashed line in each panel in the lower row shows the initial distribution observed at the start of the storm event. The thick black line in each panel in the middle and lower rows represents the location of the tree stem.

View larger version (28K): [in this window] [in a new window]   FIG. 5. Hyetograph (upper), spatial variation in soil water content change () (middle), and hydraulic head () distribution (lower) for pore water pressure measurement points P1 to P10 at (a) 40 min, (b) 200 min, (c) 520 min, and (d) 2770 min for the heavy storm event of 17 to 19 July 2006, where is the difference between the current water content and the initial water content observed at the start of the storm event, and was computed as the sum of the observed pore water pressure () and the height for the measurement, which corresponds to the height of the soil–bedrock interface. The black dashed line in each panel in the lower row shows the initial distribution observed at the start of the storm event. The thick black line in each panel in the middle and lower rows represents the location of the tree stem.

View larger version (16K): [in this window] [in a new window]   FIG. 6. Accumulated heights of the open area rainfall (OR), throughfall (TF), and stemflow along upslope (SF-up) and downslope (SF-down) sides of the trunk for the heavy storm event on 17 to 19 July 2006. Stemflow height is shown in two ways: the observed total stemflow volume divided by the canopy projected area (Ac) or by the stemflow infiltration area (As).

View larger version (119K): [in this window] [in a new window]   FIG. 7. Stemflow on the downslope side of the tree trunk.

View larger version (18K): [in this window] [in a new window]   FIG. 8. Box plot showing the vertical distribution of water content increase rescaled by Eq. [3] (*) for pore water pressure measurement points P1 to P10 when the average increase for 50 observed points reached its maximum. Boundaries of the box indicate the 25th and 75th percentiles. The black and red lines within the box mark the median and mean, respectively, and error bars indicate the 10th and 90th percentiles.

View larger version (20K): [in this window] [in a new window]   FIG. 9. Relationship between the maximum increase in soil water storage from the surface through a depth of 55 cm (Smax) for each storm event and cumulative open area rainfall from the start of the event to the time when Smax was recorded for pore water pressure measurement points P1 to P10. In each panel, the initial soil moisture condition for each event was defined as wet or dry by comparing the initial water storage for the event with the average value for all events.

View larger version (29K): [in this window] [in a new window]   FIG. 10. Occurrences of earlier water content response in the deeper layer (ERDL) at pore water pressure measurement points P1 through P10 in relation to the event total rainfall (Ra) and the average water content at the start of the storm event (Wi). The green line shows the location of the tree.

View larger version (182K): [in this window] [in a new window]   FIG. 11. The root system of tree S3 in the downslope region from the stem.

View larger version (58K): [in this window] [in a new window]   FIG. 12. Maximum pore water pressure (max) at measurement points P1 to P10 in relation to the event total rainfall (Ra) and the average pore water pressure at the start of the storm event (i). The numbers in each column represent the order of the response time. The thick, vertical green line shows the location of the tree.