Quantifying Groundwater Contribution to Surface Flow in an Agricultural Watershed

There is increasing evidence that groundwater withdrawals are contributing to reductions in stream flow in the agricultural area of the Fraser Valley of British Columbia, Canada. Results of a two-year data collection effort within the Bertand Creek Watershed are presented which focus on creating a data set for development of a coupled surface water-groundwater flow model to investigate the effect of groundwater withdrawals on the magnitude of surface water flows. The field program was carried out during the summers of 2006 and 2007, the former among the driest rainfall summers on record while the latter received higher than normal rainfall. A multi- scale approach was adopted for the field program which employed streamflow gauging, seepage meter measurements, thermal gradient measurements, and eighty-five mini-piezometers to measure channel bed groundwater levels, hydraulic conductivities and to investigate near stream chemistry patterns. To overcome the challenge of measuring the low magnitude gain in streamflow between successive stream gauging locations, weirs were installed in segments of the creek in 2007. Preliminary results of the study indicate that groundwater contribution to surface flow within the watershed is dependant on overall dry season rainfall and on irrigation demand. Hydraulic conductivity testing and vertical hydraulic gradient record of the mini-piezometers at multiple depths within the creek bed were found to be instrumental in characterizing the magnitude of groundwater - surface water exchange. Comparison of successive streamflow measurements required continuous stream discharge record at gauging locations due to variations imposed on stream flow by water users.