Drainage Runoff Separation of New and Old Water Based on Precipitation, Air, Water, and Soil Temperature Compared to Stable Isotopes ¹⁸O and ²H

In order to evaluate the proportion of old and new water in drainage runoff, we recorded air temperature, drainage discharge, drainage runoff temperature, soil temperature, precipitation totals, and temperature. The results of separation by temperature were compared with the results of chemical separation using the stable isotopes δ 18O and δ 2H measured in causal precipitation and monitored in drainage runoff. Separation was determined based on precipitation temperature in 18 rainfall–runoff events and on soil temperature in 20 rainfall–runoff events, with no significant difference between results of separation of drainage runoff based on temperature and that based on isotopes. Separation of runoff based on temperature is feasible only for simple isolated rainfall–runoff events as opposed to those involving multiple rainfalls. Time to peak discharge was identified as a viable factor to determine whether to employ separation based on soil temperature or on precipitation temperature. Time to peak discharge showed a strong correlation with the intensity of precipitation. The results suggest that, conditional on analysis of a larger dataset, isotope separation of drainage runoff and, possibly, runoff in watercourses may potentially be replaced with more economical and technically simple measurement of soil and precipitation/air temperature