Hydrological processes in permafrost regions are still hard to represent in models because of a lack of sufficient process understanding based on field observations in different permafrost settings. This work delivers an important contribution to the knowledge of hydrological processes, specifically the water balance of a thermokarst lake in ice -rich permafrost. The goal of this research is to quantify the water balance based on field measurements of hydraulic components as well as modeling. The study site is located on Kurungnakh, an island in the central part of the Lena river delta in northern Siberia underlain by continuous ice- rich permafrost. The investigated lake, Lucky Lake, covers an area of about 3 km² and has a volume of approximately 20 * 10^6 m³. Field measurements of the water balance components were conducted in a period from July 15th to August 26th. Precipitation was recorded by an automatic rain gauge, at a nearby site on Kurungnakh island. Open channel inflow from a neighboring thermokarst lake into Lucky Lake was identified. It was possible to estimate the amount of water input by using a measurement device based on electro magnetism (Flo-Mate). The outflow of Lucky Lake was determined every ten minutes by using the stage -discharge relationship of an installed weir and a radar sensor. The evaporation of the thermokarst lake was calculated using climate data from weather stations on Kurungnakh island and the neighboring island Samoylov. The lake water storage was measured using an automated water level sensor in 30 minutes intervals. Water levels in the shallow active layer were recorded at seven measurements sites within the catchment of Lucky Lake. Changes of the active layer, hydraulic conductivity in the thawed layer and the groundwater level on the permafrost table were measured at different locations during the mentioned period. This contribution shows the measured and estimated water balance components for the thermokarst lake. During the period of measurement the discharge decreased significantly, due to unusual small amount of precipitation in this summer. The evaporation measured during this time was as usual high compared to the rest of the year. These two main drivers for the water balance explain the development of water storage within the lake, as it also decreased. In consequence evaporation dominated the water balance of Lucky Lake during the measurement period by far