Impact of watershed delineation detail on hydrologic process modeling in low slope areas

Ascension Parish, located along the Mississippi River in southern Louisiana, is a low-lying, low slope landscape that primarily drains into the tidally-influenced Lake Maurepas. The predominant method of drainage within the parish is gravity drainage. Ascension Parish is currently one of the fastest growing areas in the United States. New developments have changed the area’s hydrology along with its landscape. Proper watershed delineation within the parish is critical for the management of and future improvements to the parish’s drainage infrastructure. Most of the recent drainage modeling in the parish has been performed using a light detection and ranging (LIDAR) digital elevation model (DEM). There are a number of software applications available which provide automated watershed delineation tools. Most automated watershed delineation tools only require a DEM as input; however, other data, such as a stream network shapefile, can be used to force the automated watershed delineation tool to consider certain known existing conditions. Stream network shapefiles can vary in the detail they provide. By running an automated watershed delineation tool using stream networks of varying detail, the effects of their detail on the watershed delineation process can be quantified. Results showed significant differences in watershed area and watershed orientation across the three different delineations completed using different stream network inputs. The detailed stream network breached inaccurate hydrologic barriers present in the DEM. These barriers were the main cause of differences between the three sets of watersheds. When delineating a watershed, the number of sub-basins that are created to represent the watershed is dependent on a user-defined stream threshold value. Watersheds may be represented in detail by many sub-basins or generally with only one sub-basin encompassing the entire delineated watershed area. Hydrologic models can be created using these different sub-basin delineations to represent the same physical area. The effects of different watershed sub-basin delineations on hydrologic process modeling can then be determined. Results showed significant differences in the model outputs across the three different sub-basin delineations used to represent the same watersheds; with an increase in sub-basin delineation detail, time to peak discharge decreased significantly while peak discharge rate increased significantly