WET Hydrology & Hydrogeology

Population increases and climate change are expected to increase water stress in the semi-arid Okanagan Valley in the Interior of British Columbia. Groundwater discharge from the largest unconsolidated aquifer system in the Okanagan Valley to Okanagan Lake was directly measured between September 2011 and August 2013. Seepage meter measurements (331) and gradient calculations (73) were used to measure flow from the Kelowna aquifers in an effort to constrain groundwater values in a basin-scale water-balance model constructed to inform water use and planning decisions within the Okanagan Valley. The complexity of the subsurface environment in the Kelowna area led to the construction of a 2-D MODFLOW transect to provide a sensitivity analysis of the percentage of total flow captured within the study area using a reasonable range of hydraulic conductivity values for the known confining and confined layers. Forty-two MODFLOW scenarios estimated 26% - 100% of the total flow from the Kelowna aquifers to Okanagan Lake was captured within the study area. Long-term station seepage meter measurements showed a large range of annual variabil^ity with flux measurements which ranged from 10-¹¹ to 10-⁹/10-⁸ m³/m²/s. A substantial reduction in flows observed within the study area between study year one (4.1 x 10⁵ m³) and study year two (2.9 x 10⁵ m³) was potentially due to anthropogenic water extractions. The annual groundwater discharge estimate of 3.7 x 10⁵ m³ found flow from the Kelowna aquifers to be less than one percent of some previous estimates and likely less than seven percent of the volume being extracted from upgradient aquifers. Long-term provincial potentiometric monitoring indicated groundwater pumping rates have likely exceeded recharge in some Kelowna aquifers for the past 34 years. Other studies using modelling and geochemistry have suggested groundwater pumping is inducing recharge from adjacent fluvial water bodies in some areas. The low discharge from the Kelowna aquifers to Okanagan Lake suggests cautious groundwater extraction rates need to be established in the Kelowna area to ensure the groundwater system can continue to support both human and environmental water needs.

Quantifying recharge to the mountain block from headwater catchments in snowmelt dominated upland mountainous regions is an important aspect of hydrologic studies. This study contributes to understanding of the interaction between surface water, soil water and deep groundwater flow in headwater catchments. A novel approach was developed for estimating the bedrock hydraulic conductivity of a regional-scale fractured bedrock aquifer using discrete fracture network (DFN) modeling. The methodology was tested in the mountainous Okanagan Basin, British Columbia, Canada. Discrete fractures were mapped in outcrops, and larger-scale fracture zones (corresponding to lineaments) were mapped from orthophotos and LANDSAT imagery. Outcrop fracture data were used to generate DFN models for estimating hydraulic conductivity for the fractured matrix (Km). The mountain block hydraulic conductivity (Kmb) was estimated using larger-scale DFN models. Simulated Km and Kmb values range from 10⁻⁸ to 10⁻⁷ m/s, are consistent with estimates from regional modeling studies, and are greatest in a N-S direction, coinciding with the main strike direction of Okanagan Valley Fault Zone. Kmb values also decrease away from the fault, consistent with the decrease in lineament density. Simulated hydraulic conductivity values also compare well with those estimated from pumping tests. The estimates of Kmb were then used to represent the deep bedrock in a coupled surface water - groundwater model using MIKE SHE for the Upper Penticton Creek 241 headwater catchment in the Okanagan Basin. Although highly uncertain due to parameter uncertainty and calibration error, recharge to deep groundwater was ~4% of the annual water budget. An specified outward flux from the catchment boundary, representing ~6% of annual water budget, did not significantly impact streamflow calibration, indicating that such deep groundwater losses from the catchment can be accommodated in a model. This outflow may contribute to cross-catchment flow and, ultimately, to groundwater inflow to lower elevation catchments in the mountain block. The modeling exercise is one of the first in catchment hydrology modeling within steep mountainous terrain in which the lower boundary of the model is not treated as impermeable, and in which recharge to the deep bedrock and discharge to the surrounding mountain block were estimated.