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This study demonstrates the potential of triple O-isotopes to quantify evaporation with recharge on a salt lake from the Atacama Desert, Chile. An evaporative gradient was found in shallow ponds along a subsurface flow-path from a groundwater source. Total dissolved solids (TDS) increased by 177 g/l along with an increase in delta O-18 by 16.2 parts per thousand and in delta D by 65 parts per thousand. O-17-excess decreased by 79 per meg, d-excess by 55 parts per thousand. Relative humidity (h), evaporation over inflow (E/I), the isotopic composition of vapor (*R-V) and of inflowing water (*R-WI) determine the isotope distribution in O-17-excess over delta O-18 along a well-defined evaporation curve as the classic Craig-Gordon model predicts. A complementary on-site simple (pan) evaporation experiment over a change in TDS, delta O-18, and O-17-excess by 392 g/l, 25.0 parts per thousand, and -130 per meg, respectively, was used to determine the effects of sluggish brine evaporation and of wind turbulence. These effects translate to uncertainty in E/I rather than h. The local composition of *R-V relative to *R-WI pre-determines the general ability to resolve changes in h. The triple O-isotope system is useful for quantitative hydrological balancing of lakes and for paleo-humidity reconstruction, particularly if complemented by D/H analysis.