Long-Term Dynamics of Aufeis and Their Association with Vegetation Phenology: A Case Study in the Chuluut River Valley, Mongolia

Multi-temporal satellite data were used to examine changes in aufeis area and spatial configuration in the Chuluut River valley, Mongolia, and to compare seasonal vegetation dynamics among sites with different return frequencies of aufeis. Annual aufeis masks were derived from Landsat imagery for 1986–2025, while vegetation phenology was assessed using Harmonized Landsat–Sentinel-2 (HLS) data for 2016–2025. Mean aufeis area was 11.98 km2 and declined significantly by approximately 0.084 km2 per year, accompanied by spatial reorganization. The strongest climatic association was found with April–June precipitation in the preceding year. Hydrologically, aufeis acts as a temporary seasonal water store, retaining part of winter discharge and releasing meltwater during spring. Its decline and redistribution may alter the timing and pathways of seasonal water release, reduce delayed moisture inputs to floodplain surfaces, and modify water availability for riparian and meadow ecosystems. In meadows, the start of season (SOS) occurred later under frequent than infrequent aufeis recurrence. Peak and mean summer normalized difference vegetation index (NDVI) values remained high after aufeis melt-out. The highest integrated seasonal NDVI values occurred under intermittent aufeis recurrence.