Evaluation of the hydrologic effects of stock ponds on a prairie watershed
Womack, Jennifer Marie
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The impact of stock ponds on the hydrology of the Northern Great Plains has largely been overlooked despite stock ponds' widespread presence. However, recent conservation efforts have placed a focus on restoring prairie stream systems, and water management decisions require more information regarding the effects of these ponds. This thesis addresses the unknown impacts of stock ponds by modeling Box Elder Watershed located just north of Fort Peck Reservoir in north central Montana. The hydrologic model helped quantify the peak flow rate and runoff volume changes caused by 28 stock ponds on what would have been the undisturbed watershed's flow regime. HEC-HMS was employed to model the watershed. Parameters for the model were abstracted from a GIS and measured in the field. The HEC-GeoHMS extension for ArcGIS 9.3.1 was used to construct necessary model files and create the model components. Measured runoff events from spring 2010 and spring 2011 were used to calibrate non-measurable parameters. To determine the ponds' impacts, frequency storms were run in the model for a watershed with all of the ponds included and a watershed with no ponds included. Results showed that the stock ponds are reducing peak flow rates by 12.66% to 24.08% and reducing runoff volumes by 10.1% to 16.84% depending on the return interval of the frequency storm modeled. In order to apply these results to other watersheds, an attempt was made to find a simplified relationship between pond parameters and an individual pond's influence on the system. This was unsuccessful. The results showed complex model interactions dictating runoff, and some degree of modeling may be required to determine a pond's importance to the hydrology, especially regarding peak flow rate. Finally, the ponds on Box Elder Watershed were ranked in a single list according to their influence on peak discharge and runoff volume. The list will help to prioritize the ponds on Box Elder Watershed for removal since cost and animal water requirements will prevent the removal of all ponds.