Glaciers are key components of the water towers of Asia and, as such, are relied upon by large downstream communities for domestic, agricultural and industrial uses. They store snow and ice during cold periods and release it as water during warm periods - often when it is needed the most. Across the Himalayan mountain range, accumulation and ablation occur simultaneously during northern-hemisphere spring and summer. The Parlung No.4 catchment, located southeast of the Tibetan Plateau, is heavily influenced by the southern westerlies and the Indian Summer Monsoon during spring and summer, when moist air is transported from the south through the Parlung Tsangpo valley. Parlung No.4 Glacier is a maritime type glacier with most accumulation occurring in spring (April-May). Prior examinations of the meteorology, energy budget and mass balance of Parlung No.4 glacier have provided key insights into the climatic sensitivity of the region’s glaciers, but their discharge seasonality and role in catchment hydrology remains largely unknown. Here, we conduct a multidecadal glacio-hydrological study using a fully distributed model for runoff simulation (TOPKAPI-ETH), forced with ERA5-Land and China Meteorological Forcing Dataset (CMFD) climate reanalysis products, downscaled using data from a local weather station to reconstruct meteorological time series at an hourly resolution from 2000 to 2018. TOPKAPI-ETH is calibrated exploiting the available field data, then validated against discharge measurements, geodetic mass balance, stake measurements and snow cover products from Landsat and MODIS. We show that the southern westerlies and the associated spring precipitation were having more influence on the catchment hydrology than the Indian Summer monsoon variability. Snow-melt was the largest contributor to runoff during the studied period (65%), followed by ice-melt (21%), their relative proportion being mostly controlled by spring precipitation amounts, mean summer temperature and monsoon onset timing. We expect that runoff quantity and its ice-melt contribution will increase in the future, but the timing and magnitude of the peak will largely depend on climate change impacts onto the accumulation and ablation seasons.