A WATER BALANCE APPROACH FOR QUANTIFYING MOUNTAIN FRONT RECHARGE (MFR)

Many valley bottom communities in mountainous regions rely on replenishment of groundwater resources from Mountain Front Recharge (MFR); the main components of which are Mountain Block Recharge (MBR) and stream infiltration from valley side catchments. Regional water balance calculations to quantify MFR may be possible, but uncertainty exists due to data availability and representativeness concerns. The water balance approach was applied for MFR quantification for the Okanagan Basin mountainous watershed in southern British Columbia (BC) using data that are typically available throughout the Province. Preliminary results of a pilot project indicate regionally-averaged partitioning of mountain block precipitation on an annual basis to 68% actual evapotranspiration (AET), 19% surface water run-off (RO), and 13% MBR. Subsequent downgradient contributions to MFR from stream infiltration across the mountain front were estimated to reduce the surface run-off component to approximately 13%. Potential uncertainty in MFR was primarily the result of uncertainty of AET, and additional uncertainty was associated with determination of the RO and stream loss components of the water budget. Despite limitations, the water balance approach can be useful in providing preliminary estimates of MBR/MFR at the regional scale with uncertainties less than 1 order of magnitude. The uncertainty associated with this approach is typically less than that of Darcy calculations that rely on hydraulic gradients and poorly constrained estimates of hydraulic conductivity. Future research, data collection, and field instrumentation, focussed on reducing uncertainty of the water balance components in mountainous areas would assist in water resources assessment for valley bottom communities.