Stable water isotopes are widely used to evaluate how well hydrological models reproduce observed catchment transit time and flow path behavior, but the available isotope record is often limited in temporal density. Here, we examined how different temporal resolutions of isotope time series affect modelled stream isotope simulations in the Weierbach catchment, Luxembourg. Weekly, fortnightly, and monthly subsets were derived from the available stream δ2H record, and the best matching simulation was identified for each subset from a previously saved ensemble. The fortnightly- and monthly-based simulations were then compared against the weekly isotope observations, which served as a common benchmark. The weekly-based simulation achieved an NSE of 0.47, the fortnightly-based simulation was 0.46, and the monthly-based simulation was lower with an NSE of 0.19. The time-series comparison showed that the monthly-based simulation departed more strongly from the observed dynamics, whereas the fortnightly-based simulation had less deviation from the weekly benchmark. These results indicate that reduced isotope data availability can affect which simulation in a Monte Carlo set is identified as most representative and how well it reproduces broader stream water behavior. Moderate reduction (fortnightly data) in isotope data availability may still support a useful model representation, but stronger reduction (monthly data) can noticeably weaken consistency with observed isotope dynamics. The results are relevant for water quality and catchment management applications where isotope time series are used to understand catchment residence time and guide hydrological model evaluation under limited data availability.