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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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The unique topography of the pothole region of the North American prairies creates challenges for properly determining basin contributing area. Numerous depressions or potholes within the landscape impound runoff. However, potholes can ‘fill‐spill’ resulting in surface water connections between the potholes. Surface water connectivity between potholes ultimately influences basin contributing area. Currently, automated methods, such as landscape analysis tools, treat depressions in the landscape as artifacts and simply fill the depressions to delineate a drainage basin. Using this method to calculate contributing area assumes that all surface storage has been satisfied (threshold) and the drainage basin will contribute 100% of its area for all runoff events. However, most runoff events in the prairie pothole region are pre‐threshold events that contribute only a portion of surface runoff to the outlet. These pre‐threshold events have surface storage that varies because of antecedent water levels and have a variable or dynamic potential to store further runoff in the basin. Government agencies have developed methodologies for determining pre‐threshold contributing areas, but these methodologies do not incorporate current technologies and, as a result, have limitations. We propose an automated method for determining contributing area that incorporates the fill‐spill of prairie potholes. The algorithm, which uses the D‐8 drainage direction method, automates a methodology for identifying and quantifying runoff contributing area. Any algorithm that determines pre‐threshold contributing area, must allow the DEM to be filled in an incremental manner. This will simulate increasing pond levels, and the resulting decrease in available storage in the basin, in response to runoff events. The SPILL algorithm is an iterative solution that increases the magnitude of input runoff events and records the decreasing change in available surface storage and the increase in contributing area until the storage threshold is reached and the contributing area reaches 100%. Through application of the algorithm on prairie pothole region basins, we test proposed conceptual curves that describe a hypothesized non‐linear relationship between decreasing potential storage in the landscape and contributing area. Results indicate that the proposed conceptual curves represent the relationship between potential surface storage and contributing area in the test basins very well. Copyright © 2012 John Wiley & Sons, Ltd.

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