Palustrine wetlands are an uncommon but ecologically important feature of the semi-arid landscape in the southern interior of British Columbia. The predominantly groundwater-fed wetlands occur in topographic depressions at a range of elevations, providing critical habitat for flora and fauna, and potential refugia during periods of drought and wildfire. The wetlands exhibit a range of salinities (EC ~200 to 20,000 uS/cm) that reflect differences in geology and evaporation to inflow (E/I) ratios, but which do not provide a reliable means to assess water balance. The stable isotope composition (2H and 18O values) of water was measured from May 2024 to November 2025 in precipitation and 36 wetlands and lakes located near Kelowna BC. A local meteoric water line (LMWL; 2H = 6.8 ± 0.19 * 18O – 17.9 ± 3.42; n = 106, R2 = 0.97) was established from the precipitation data. Surface water data defined a local evaporation line (LEL; 2H =4.4 ± 0.039 18O – 55.5 ± 0.39; n = 684, R2 = 0.99) that intercepts the LMWL at the annual amount-weighted isotopic composition of precipitation. Intersections between individual wetland surface water regression lines and the LMWL were used to estimate E/I ratios. Palustrine wetlands had E/I ratios ranging from 0.03 to 2.3. For sites having an E/I ratio >1, residual water fraction was calculated using the Craig-Gordon model. This approach provides a means to monitor shifts in water balance in wetlands that may be adversely impacted by interannual changes in precipitation, temperature or land use practices.