Spatial variability in the stable isotopes of modern precipitation in the northwest of Iberia.

The stable isotopic (delta(18)O and delta(2)H) composition of modern precipitation in the northwest of Iberia was investigated. Samples collected from October 2004 to July 2006 at 24 stations together with isotopic data from other sources (12 stations) were used to construct a map of the spatial distribution of isotopes in precipitation in the study area. A regional meteoric water line was defined and deuterium excess values were determined, proving the origin of vapour masses from the Atlantic Ocean. Environmental conditions that account for spatial isotopic variability were identified. Altitude, continental, and latitude effects can explain the eastwards depletion in heavy isotopes. The altitude leads to a depletion of 0.17 per thousand of delta(18)O per 100 m rise. The decrease in delta(18)O values, as the distance from the coast increases, defines an inland isotopic gradient of about 0.1 per thousand of delta(18)O per 10 km. Latitudinal gradient of-0.55 per thousand of delta(18)O was estimated between 41 degrees and 44 degrees latitude north. Together, the altitude and continental effects account for 78 % of the delta(18)O variability in regional precipitation.