Keito Nakajima, Ikuya Adachi, Tsutomu Yamanaka
The evaporation-to-inflow ( E / I ) ratio is an important index characterizing lake water balance and is often estimated using isotopic methods. However, the sources of uncertainty in such estimates remain poorly quantified. Here, we present results from four years of isotopic monitoring of Lake Kussharo, northern Japan, along with spatial observations of adjacent rivers at 37 sites. We compared isotopic estimates of annual and 4-yr mean E / I ratios with those from energy-balance-based numerical models of lake evaporation and catchment evapotranspiration. The 4-yr mean values from the isotopic approach (14.4 ± 0.1%) and the modeling approach (14.1 ± 1.2%) closely matched. Although inter-annual variations of the ratio were not negligible, they were smaller in the isotopic estimates, reflecting longer-term average conditions mediated by catchment storage. Sensitivity analyses revealed that the most critical factor for accurate estimation is the partial isotopic adjustment between lake evaporation and atmospheric water vapor. Using daily isotope data from a dynamic isoscape model further reduces errors in E / I ratio estimates. Finally, applying the isotopic approach with a sampling strategy optimized for the study lake allows E / I ratios to be estimated with errors of less than a few percentage points, providing insights into the spatial and temporal variability of lake water balance.