Stable isotopes of water have extensive applications for studying hydrological processes. Isotopic enrichment can be used to estimate evaporation from concrete-lined open water canals which is important for water resources management in arid regions. In this study, the isotopic composition (delta 18O and delta 2H) of water was sampled along two Canals in Phoenix, Arizona, USA (Arizona Canal; AC, and Crosscut canal; CC) over about one year (June 2023 to May 2024), prior to photovoltaic (PV) canal coverage. Measurements showed a trend of progressive enrichment of heavy isotopes along the length of both canals, indicative of evaporative enrichment. Using the Craig-Gordon (C-G) evaporation model, we estimated evaporation losses of 5.2 +/- 2.3% and 3.3 +/- 1.5% for AC and CC, respectively, with large seasonal differences and greater rates of evaporation in summer that reached 8.5%. The Canal Water Evaporation Line (CWEL) slope for the AC and CC was 5.0, with no significant differences between the two canals. Isotope-based canal evaporation losses were consistent with pan evaporation data, with Normalized Root Mean Square Errors (NRMSE) of 6 and 3% for AC and CC, respectively, against pan evaporation. This approach provides a means for monitoring evaporative losses from open water canals in arid areas which can aid water resources managers with closing the large delivery systems water budgets, and provides baseline data for assessing future solar panel canal installations in arid cities.