The Colombian Andean Mountains host the headwaters of the main basins of the country. However, the interactions between high-mountain ecosystems and the isotopic composition of water in this region has been poorly studied. Here we present and analyze the first set of stable isotopes data collected in the Central Andes of Colombia. Stable isotopic composition of stream water and precipitation was determined for a period between 2017 and 2018 in the Upper Claro River Basin. The driving factors influencing the spatial and temporal variability of δ 2H, δ 18O and d-excess were identified and compared to daily air temperature and precipitation at seven meteorological stations. A Local Meteoric Water Line was defined as δ²H = 8.13 δ 18O + 12.5, R 2=0.98. δ 2H, δ 18O and d-excess values of precipitation were more negative during the rainy season and changes were more related to precipitation events and amounts than to temperature. An altitude effect of -0.11 ‰ / 100 m and -0.18 ‰ / 100 m was estimated for stream water and precipitation, respectively, where the latter showed a non-linear behavior. The data set was compared to stations of the Global Network of Isotopes in Precipitation (GNIP) database in Colombia and a back-trajectory analysis of air masses was conducted to compare with d-excess. δ 18O weighted means changed with respect to the position of the Central Andes and the altitudinal range 2,100 to 3,100 m a.s.l.. High d-excess can be attributed to moisture recycling enhanced by the local ecosystems and the travel of precipitable water from the Amazon basin across the northern Andes. The results showed a high range of variation due to the differences in elevation, seasonality and atmospheric circulation patterns across the year. The present study contributes to fill the gap of spatial and temporal isotopic composition data in the northern Andes as well as to the implementation of the first “National Network for Isotopes” in Colombia.

This study updates information on the evolution of glacier shrinkage in Cocuy-Güican mountains since the maximum glacier extent of the Little Ice Age (LIA), and presents the first mass balance data of Ritacuba glacier since 2009, that is compared to the available mass balance for the Conejeras Glacier (Los Nevados National Park). This study also discusses the hydrological significance of Colombian glaciers which is still largely unknown because of the very limited information available. Glaciers in Cocuy-Güican covered 13.2 km2 in 2019 that compared to the 127.8 km2 during the maximum LIA represents a shrinkage of 89.7%. Glacier cover observations in 1955, 1994, 2010 and 2019, reveal that the rate of ice loss was the largest from 1994 to 2010 (0.59 km2 yr-1) and was then more than halved from 2010 to 2019 (0.34 km2 yr-1). This slowdown in glacier retreat is in line with a moderate negative mass balance measured for 2009-2019, with an accumulated loss of 1,766 mm w.e. The progressive confinement of glaciers to higher elevation and optimal topographic context together with a lack of recent marked climatic anomalies, could explain that Cocuy-Güican glaciers have temporally reached near equilibrium state condition. This is in stark contrast with Conejeras glacier where 47,000 mm w.e. has been lost in the same period. The available data on runoff and isotopic traces of streamflows and precipitation suggest a primary control of precipitation on the hydrological variability of the high elevated sites, compared to glacier melt water.