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Oxygen isotope distributions from lacustrine carbonates provide insights into climate and hydrological change, but it is difficult to isolate the influences of catchment precipitation δ 18 O, water temperature, and evaporation on lacustrine carbonate δ 18 O values. Recent work shows the potential for using a combination of clumped (Δ 47) and triple oxygen isotope (Δ ′ 17 O) measurements to identify the roles of temperature and evaporation on carbonate δ 18 O values in lakes, allowing precipitation δ 18 O values to be inferred and facilitating paleoclimate reconstructions. However, modern calibration of this approach has been mostly limited to arid regions with a high ratio of evaporative losses over inputs (X E) and low relative humidity (h < 0.7). Developing this tool for paleoclimate and paleoelevation reconstructions requires expanding the modern calibrations to a greater range of climatic and hydrologic conditions. We sampled four lakes in different hydrologic states under a single, high humidity climate regime (h = 0.7 – 0.9) in the Lake Junín region of central Peru. Clumped isotope temperatures from lake carbonates reflect water temperatures during carbonate formation. Lake hydrology is the main control on the Δ ′ 17 O values of carbonates and waters: Δ ′ 17 O values are lowest in the larger lakes with higher X E when compared to smaller, headwater lakes where evaporation is minimal and Δ ′ 17 O is indistinguishable from that of precipitation. Reconstructed unevaporated catchment precipitation δ ′ 18 O (δ ′ 18 O rucp) values from lake waters rely on accurate characterization of λ lake , the triple oxygen isotope evaporation slope. We explore the influence of humidity on λ lake using both new observations and modeled data. Accounting for local humidity improves λ lake estimates, which allows for more accurate reconstructions of δ ′ 18 O rucp. We generate a δ ′ 18 O rucp value of − 15.2 ± 2.1 ‰ from modern carbonates and waters (n = 15) in the Lake Junín region, which is similar to amount weighted mean annual precipitation −14.1 (± 2.2 ‰). This study illustrates that (1) Δ ′ 17 O can be used to differentiate between lakes with differing X E in humid climates, (2) lake carbonate Δ ′ 17 O and δ 18 O values can be used to evaluate the influence of evaporation on lake water δ 18 O values in a range of climates, and (3) modeling λ lake under appropriate humidity conditions improves δ ′ 18 O rucp estimates from lake carbonate Δ ′ 17 O. • Expansion of lake water and carbonate Δ ′ 17 O observations to humid systems. • Carbonate Δ 47 and Δ ′ 17 O track lake water temperature and isotopic composition. • Δ ′ 17 O can track the evaporative state of lakes (X E) in both humid and arid regions. • Refined Δ ′ 17 O- λ lake relationship improves δ 18 O rucp estimates in humid systems. • Even in the absence of humidity constraints, Δ ′ 17 O data improve δ 18 O rucp estimates. [ABSTRACT FROM AUTHOR]