Quantifying the wetland water balance: A new isotope-based approach that includes precipitation and infiltration

Stable isotopes have been used to quantify lake and wetland pond water loss to evaporation by applying the modified Craig and Gordon (1965) model. This model and its derivatives employ simplifying assumptions that ignore the additions of precipitation and infiltration outputs and assume evaporation is the only loss term over the prediction period. Here we develop a coupled water and isotope mass-balance model to account for precipitation additions. Our model uses physical and isotopic observations to quantify pond evaporation and infiltration losses over the ice-free period. We tested and applied the model to four wetland ponds at the St Denis National Wildlife Research Area, Saskatchewan Canada, where we have long-term datasets. Modeled infiltration rates from the ponds ranged between 0.99 and 9.2 mm/d and open water evaporation rates ranged between 0.88 and 2.8 mm/d. Both were consistent with independent estimates. Infiltration amounts were highest in the ephemeral ponds (that dry out within days or weeks of the spring melt period). In these ponds, infiltration exceeded evaporation. In permanent ponds, that is ponds that do not dry out; evaporation exceeded infiltration. Evaporation amounts were most substantial for permanent ponds that were not sheltered by topography or riparian vegetation. Overall, our coupled water and isotope mass-balance model combined with physical and isotope observations was able to quantify the spatially and temporally variable evaporation and infiltration fluxes within and between ponds.