The isotopic composition of meteoric water (that is, its ²H and ¹⁸O content) can be used as a tracer of the pathway it follows within a catchment, and the effect of evaporation during its flowing down the slopes and the valley.

2H and 18O in meteoric waters fall along a line reflecting the differences in their equilibrium fractionation factors (MWL, thick line). The diagram summarizes how hydrologic processes affect the isotopic composition of water causing deviations from the MWL (source: http://web.sahra.arizona.edu/programs/isotopes/oxygen.html#4)

Atmospheric precipitation carries and deposits nutrients onto the catchments. Among them, nitrogen (N) has become of great concern because the human emmissions have increased the atmospheric N load the ecosystems are receiving worldwide. The inorganic N compounds of human origin (majoritarily nitrate and ammonium) in atmospheric precipitation have a distinctive isotopic signature that can be used to trace their impact on the food web of the ecosystems.

Atmospheric precipitation also brings organic compounds. This can be a significant input of organic substrates to barren soils in high mountains, that can help to sustain a relatively high microbial activity. The isotopic composition of airborne organic C and N may serve to trace their fate and assess their importance to the soil microbiota.

Precipitation collector in Contraix station

In order to sample atmospheric precipitation, we have installed collectors at two different altitudes, in the vicinities of the automatic weather stations ol lakes Contraix (2,600 m asl) and Llebreta (1,600 m asl). Samples are collected monthly. Access to lake Contraix is difficult in mid winter, and strong winds prevent collectors from functioning properly. For this reason, we have moved the sampling location to a place less exposed to wind (and also to avalanches), at a lower altitude (2,300 m asl). In addition, we are sampling snow cover profiles integrating the whole winter precipitation at the highest altitude.