Abstract:The fill-spill of surface depressions (wetlands) results in intermittent surface water connectivity between wetlands in the prairie wetland region of North America. Dynamic connectivity between wetlands results in dynamic contributing areas for runoff. However, the effect of fill-spill and the resultant variable or dynamic basin contributing area has largely been disregarded in the hydrological community.Long-term field observations recorded at the St. Denis National Wildlife Area, Saskatchewan, allow fill-spill in the basin to be identified and quantified. Along with historical water-level observations dating back to 1968, recent data collected for the basin include snow surveys, surface water survey and production of a light detection and ranging-derived digital elevation model. Data collection for the basin includes both wet and dry antecedent basin conditions during spring runoff events.A surface water survey at St. Denis in 2006 reveals a disconnected channel network during the spring freshet runoff event. Rather than 100% of the basin contributing runoff to the outlet, which most hydrological models assume, only approximately 39% of the basin contributes to the outlet. Anthropogenic features, such as culverts and roads, were found to influence the extent and spatial distribution of contributing areas in the basin. Historical pond depth records illustrate the effect of antecedent basin conditions on fill-spill and basin contributing area. A large pond at the outlet of the St. Denis basin, which only receives local runoff during dry years when upstream surface storage has not been satisfied, has pond runoff volumes that increase by a factor of 20 or more during wet years when upstream antecedent basin surface storage is satisfied and basin-wide runoff contributes to the pond.
Abstract:Methods developed to process raster digital elevation models (DEM) automatically in order to delineate and measure the properties of drainage networks and drainage basins are being recognized as potentially valuable tools for the topographic parameterization of hydrological models. All of these methods ultimately rely on some form of overland¯ow simulation to de®ne drainage courses and catchment areas and, therefore, have diculty dealing with closed depressions and¯at areas on digital land surface models. Some fundamental assumptions about the nature of these problem topographic features in DEM are implicit in the various techniques developed to deal with them in automated drainage analysis. The principal assumptions are: (1) that closed depressions and¯at areas are spurious features that arise from data errors and limitations of DEM resolution; (2) that¯ow directions across¯at areas are determined solely by adjacent cells of lower elevation; and (3) that closed depressions are caused exclusively by the underestimation of DEM elevations. It is argued that while the ®rst of these assumptions is reasonable, given the quality of DEMs generally available for hydrological analysis, the others are not. Rather it seems more likely that depressions are caused by both underand overestimation errors and that¯ow directions across¯at areas are determined by the distribution of both higher and lower elevations surrounding¯at areas. Two new algorithms are introduced that are based on more reasonable assumptions about the nature of¯at areas and depressions, and produce more realistic results in application. These algorithms allow breaching of depression outlets and consider the distribution of both higher and lower elevations in assigning¯ow directions on¯at areas. The results of applying these algorithms to some real and hypothetical landscapes are presented. #
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