Alberta Rainfall-Runoff Analysis September, 2002

Objectives of Study Accumulate rainfall input and runoff volume data for all significant storm responses recorded in Alberta. Analyze these results to assess geographical distribution of rainfall-runoff response throughout the province. Review consistency of this data with the runoff depth maps. Develop a crude hydrologic model for various areas in Alberta for use in predicting response to storm events for management of flood response activities. Investigate applicability of this hydrologic model for design purposes.

Methodology Use hydrograph re-creation tool to estimate direct runoff volumes for all significant runoffs recorded by the WSC gauge network in Alberta. Identify and build storm contours for all large rainfall events (>100mm) recorded by the Environment Canada meteorological network. Use storm on basin tool to estimate the rainfall input for all basins with gauges responding to the largest storms to populate a rainfall - runoff database. Use a simplified rainfall-runoff model to assess the geographic variation in results.

Data Analysis Over 2200 hydrographs identified and re-created based on WSC HyDat daily flow database. Over 140 storm contours developed based on Environment Canada rainfall data, dating back to Over 1100 rainfall - runoff data pairs assembled into a database for analysis.

Hydrograph Re-Creation Example

Storm On Basin Example - July 1986

Rainfall Runoff Model Most losses occur near the beginning of a storm, as interception and surface storage fills up and infiltration capacity approaches saturation. Once these losses have been accounted for, the amount of runoff will come close to the rainfall input. Therefore, the rainfall runoff plot will be asymptotic to a line with a slope of ~ 1. This line is shifted horizontally from the runoff = rainfall line by the total loss value (L). A parabolic transition curve is used for the model until all losses have been accounted for.

Rainfall - Runoff Model

Results

Analysis of Results Significant scatter in the results for each geographical area are evident. However, significant grouping of results by geographic region are also evident. Total loss appears to vary from about 50mm in the most productive areas to more than 200mm in the less productive areas. Reasons for the scatter include both data errors and physical factors.

Scatter Due To Data Errors Interpolation of point rainfall data to generate storm contours. Especially problematic for :  older storms with less point coverage  areas with lower density of point coverage  extreme storms with large gradients between the highest recorded values and surrounding stations Possible errors in the actual rainfall data and runoff data. This can include problems with equipment, as well as processing errors such as extrapolations of flow data using rating curves.

Scatter Due To Physical Factors Changes in antecedent moisture conditions between storms, affecting both infiltration and storage Temporal distribution of rainfall, with shorter duration storms providing more runoff than longer storms of the same magnitude Changes in characteristics of basin runoff and channel routing between storms Location of eye of storm relative to productive areas and storage devices Additional runoff during some storms from melting of snow on the basin

Conclusions Analysis of the rainfall - runoff relationship for basins in Alberta based on available rainfall and runoff data shows significant geographical variation consistent with the Runoff Depth map Although there is significant scatter in the results for all geographic areas, a simple rainfall - runoff model can be used to estimate runoff response to a storm, facilitating management of flood response activities The scatter in results, plus the need for definition of design storms for various areas in the province, would make application of this simple model for design purposes problematic.