The concept of a spatially distributed unit hydrograph is based on the fact that the unit hydrograph of a watershed can be derived from the time-area curve of the watershed by the S-curve method. The time-area diagram is a graph of cumulative drainage area contributing to discharge at the watershed outlet within a specified time of travel. Accurate determination of the time-area diagram is made possible by using a GIS. With GIS capability for parameter mapping, the assumption of a uniform spatial rainfall distribution is no longer necessary. Hence, the term spatially distributed unit hydrograph. Example of application for the Waiparous Creek in Alberta Foothills is given. IDRISI is used to develop a simple digital elevation model of the watershed, using 1 km x 1 km grid cells. A grid of flow directions is developed and used to create an equivalent channel network. Excess rainfall for each 1 km x 1 km cell is individually computed by the SCS runoff curve method and routed through the equivalent channel network to obtain the time-area curve. The derived unit hydrograph gave excellent results in simulating an observed flood hydrograph.

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