[AHW33-01] Effect of spatial scales on runoff / sediment transport in mountain catchments
Keywords:spatial scale dependence, spatial heterogeneity, water discharge, sediment discharge, numerical model, spatially distributed information
Examining the patterns of spatial variability of runoff and sediment transport should be the key to understand hydrologic and geomorphic processes in watersheds and improve the process-based representation of numerical models. Therefore (1) we reviewed the relations between drainage areas and various properties of runoff and sediment transport comprehensively, particularly addressing spatial-scale dependence and spatial heterogeneity. (2) We summarized and elucidated the processes controlling scale dependence and spatial heterogeneity based on the observed results from selected intensively studied sites. (3) We reviewed and analyzed the treatment of spatial scales in numerical models of runoff and sediment transport, to elucidate current problems and to identify avenues for future research.
(1) Some properties on water and sediment transport showed increase and/or decrease pattern relative to the catchment area, although other properties showed no distinctive relation to catchment areas, partly because of a lack of measured data. Most earlier studies specifically examined either spatial-scale dependence or spatial heterogeneity. Properties such as peak specific discharge showing clear spatial-scale dependence should be controlled by mechanisms that change with the spatial scale. Properties such as base flow specific discharge showed spatial heterogeneity, which suggested that mechanisms controlling base flow did not change much with the spatial scale for the measured ranges of catchment areas but heterogeneous properties of landscapes exerted strong effects.
(2) We selected well studied Fudoji and Kiryu (Shiga), Tanzawa (Kanagawa) and Saru watersheds (Hokkaido) for analysis. For baseflow generation, various ratio of contribution from weathered bedrock layer to soil layer derive spatial heterogeneity in small catchments and this variation tapered with increased catchment area due to the mixing of several flow components. Sediment transport also varied spatially depending on the catchment characteristics. Variation tapered with increased catchment area. For some catchment, sediment transport decreased with catchment area due to the increased storage components and decreased ratios of sediment source areas to the total catchment area for larger drainage areas. While for other catchment, sediment yields increased in larger catchment, due to landscape legacies of sediment supply, storage, and transport.
(3)We examined following questions. (a) Which spatial scale is used in existing modeling and how was it decided? (b) What spatial information was used? (c) Do the governing equations differ with spatial scale? Results show that (a) the smallest spatial scale in distributed models was determined by the spatial resolution of spatially distributed information used in modeling, although lumped models were applicable at any scale. (b) Physical parameters used in modeling were not based on field-based information, except elevation, land use and vegetation cover. (c) Results demonstrated that the governing equations differ depending on locations within the watershed (i.e., hillslopes and channels) that present different processes for runoff and sediment transport. Nevertheless, no report of the relevant literature described the changes in governing equations with the changes in spatial scales. We also reviewed modeling studies that focused on how we should treat the spatial scale. For runoff models, they had reported minimum and the maximum spatial scales for lumping runoff mechanisms and the relative importance of spatial data used as input parameters. While we were unable to find similar studies for sediment transport models.
The effect of spatial scales on runoff / sediment transport and processes behind that pattern have been demonstrated to some extent. However, many of the numerical models have not been sufficiently considered these information, yet.
(1) Some properties on water and sediment transport showed increase and/or decrease pattern relative to the catchment area, although other properties showed no distinctive relation to catchment areas, partly because of a lack of measured data. Most earlier studies specifically examined either spatial-scale dependence or spatial heterogeneity. Properties such as peak specific discharge showing clear spatial-scale dependence should be controlled by mechanisms that change with the spatial scale. Properties such as base flow specific discharge showed spatial heterogeneity, which suggested that mechanisms controlling base flow did not change much with the spatial scale for the measured ranges of catchment areas but heterogeneous properties of landscapes exerted strong effects.
(2) We selected well studied Fudoji and Kiryu (Shiga), Tanzawa (Kanagawa) and Saru watersheds (Hokkaido) for analysis. For baseflow generation, various ratio of contribution from weathered bedrock layer to soil layer derive spatial heterogeneity in small catchments and this variation tapered with increased catchment area due to the mixing of several flow components. Sediment transport also varied spatially depending on the catchment characteristics. Variation tapered with increased catchment area. For some catchment, sediment transport decreased with catchment area due to the increased storage components and decreased ratios of sediment source areas to the total catchment area for larger drainage areas. While for other catchment, sediment yields increased in larger catchment, due to landscape legacies of sediment supply, storage, and transport.
(3)We examined following questions. (a) Which spatial scale is used in existing modeling and how was it decided? (b) What spatial information was used? (c) Do the governing equations differ with spatial scale? Results show that (a) the smallest spatial scale in distributed models was determined by the spatial resolution of spatially distributed information used in modeling, although lumped models were applicable at any scale. (b) Physical parameters used in modeling were not based on field-based information, except elevation, land use and vegetation cover. (c) Results demonstrated that the governing equations differ depending on locations within the watershed (i.e., hillslopes and channels) that present different processes for runoff and sediment transport. Nevertheless, no report of the relevant literature described the changes in governing equations with the changes in spatial scales. We also reviewed modeling studies that focused on how we should treat the spatial scale. For runoff models, they had reported minimum and the maximum spatial scales for lumping runoff mechanisms and the relative importance of spatial data used as input parameters. While we were unable to find similar studies for sediment transport models.
The effect of spatial scales on runoff / sediment transport and processes behind that pattern have been demonstrated to some extent. However, many of the numerical models have not been sufficiently considered these information, yet.