Isotopic fractionation of water in river pools. The effect of diel atmospheric variations and eventual depletion of heavy isotopes.

The hydrology of non-perennial (temporary) rivers under climate change (UPH 3) affects not only dry regions but could affect all climates, because most of the catchment headwaters already have (or will have) temporary regimes. Most temporary rivers undergo a pools phase when surface flow ceases but surface water remains in disconnected pools. These pools offer many ecosystem services, such as the provision of refuges for aquatic species. Nevertheless, the hydrology of this pools phase is poorly known because gauging stations or hydrological models do not inform about what happens after the cessation of flow.

We implemented a methodology to estimate the time since disconnection of pools from the river flow when they are sampled, based on the study of water stable isotopes. In pools disconnected from groundwater, the isotopic modification of the water allows us to estimate the relative volume of water evaporated (Gonfiantini, 1986). To test this methodology, within the Vallcebre Research Catchments (42º12’N and 1º49’E ) an artificial pool, covered with a transparent lid to prevent the input of rainfall, was installed. From July to November 2020, water volume of this pool was weekly measured and sampled for isotopic analysis. In parallel, meteorological variables were monitored and rainfall was also sampled for water stable isotopes.

The results obtained in the artificial pool were satisfactory but showed two aspects not reported in previous publications: i) the customary application of isotopic fractionation equations using weekly averaged atmospheric conditions strongly underestimated the observed changes, and ii) when about 60% of pool water had evaporated, its isotopy became so enriched with heavy isotopes that periods of heavy isotopes depletion instead of enrichment happened.

The analysis of the information available showed that the first problem could be attributed to the fact that time averages of weather conditions strongly overestimated air humidity during the periods of active evaporation. Therefore we decided to weight air humidity measures with proxies of evaporation flow like vapour pressure deficit or global radiation. When the fractionation equations were applied using flow-weighted air humidity, they adequately reproduced the observed pool water isotopy, including the late periods of heavy isotopes depletion.