This research investigates the influence of seasonal hydroclimate variability on the triple oxygen and hydrogen isotope composition of small, shallow lake systems that show substantial intra-annual and interannual fluctuations in the water level. The study was conducted at Laguna Honda, a semi-permanent lake located in the semiarid Mediterranean environment of southern Spain. Over 1 year, the lake water level was monitored continuously and water samples from the northern and southern margin were taken monthly for major ion concentration and triple oxygen and hydrogen isotope analyses. Over the study period, the lake water level dropped from 1.4 to 0.6 m, while salinity increased from 23 to 130 g L-1 and the delta O-18, delta H-2, and O-17 excess of lake water varied from -2 parts per thousand to 15 parts per thousand, -26 parts per thousand to 51 parts per thousand, and -9 per meg to -87 per meg, respectively. Hydrological mass balance calculations indicate that precipitation, basin discharge, and evaporation control lake water level changes in Laguna Honda, and major inflow from other sources, such as groundwater, is absent. The lake water's isotope composition is mainly driven by changes in relative humidity (34 %-73 %), while precipitation and basin discharge can cause transitional mixing effects that, however, remain small in magnitude (<10 %). In the O-17 excess vs. delta ' 18O space, the lake water forms a loop evolving from low delta(18)Oand high O-17 excess in winter to higher delta(18)Oand lower O-17 excess in summer along a convex curvature, then back to low delta(18)Oand high O-17 excess with the beginning of the subsequent rainy season along a concave curvature. The triple oxygen isotope system allows the identification of non-steady-state conditions, which is challenging using delta(2)Hand delta(18)Oalone due to the linearity of trends in this isotope system. The large seasonal variability of triple oxygen isotopes should be considered when interpreting isotope data obtained from paleo-archives from lake sediments in semiarid and arid environments.