Effects of evaporative enrichment on the stable isotope hydrology of a central Florida (USA) river

The δ¹⁸O characteristics of water masses contributing to flow in the Econlockhatchee River basin, a subtropical low-gradient watershed in central Florida, were monitored to assess the potential for applying hydrograph separation models in the watershed. Daily sampling for a 2-month period in autumn 1992 revealed ranges in precipitation of -6.64 to -0.17‰, in surficial groundwater of -3.29 to -2.90‰, and in river water of -2.68 to -1.41‰. Results indicated that river δ¹⁸O was consistently more enriched than either groundwater or precipitation by about 1‰, prompting the hypothesis that evaporative enrichment significantly altered the δ¹⁸O of river water. Additional monitoring and mass-balance modelling of the entire basin (620 km²) and a subcatchment (70 km²) showed that evaporative effects could account for the observed enrichment. Although some enrichment occurred in headwater swamps and wetlands, the field data and modelling results supported the hypothesis that evaporation from the river channel significantly altered the δ¹⁸O of river water. Enrichment elsewhere in the hydrological cycle, such as during throughfall or temporary storage in wetlands or stormwater management ponds, may have contributed to the observed signal of evaporation, but could not be distinguished from headwater or river evaporation. It appears that a sufficient isotopic signal exists in central Florida precipitation to apply hydrograph separation models, but that evaporative isotopic enrichment should be included as a modelling element. Copyright (C) 2000 John Wiley and Sons, Ltd.