Stormflow-hydrograph separation based on isotopes: the thrill is gone--what's next?

Abstract

Beginning in the 1970s, the pr omise of a new method for separating stormflow hydrographs using 18 O, 2 H, and 3 Hp r o v e da ni r r e s i s t i b l e temptation, and was a vast improvement over graphical separation and solute tracer methods that wer e pr evalent at the time. Eventu- ally, hydrologists r ealized that this new m ethod entailed a plethora of assumptions about temporal and spatial homogeneity of isotopic composition (many of which wer e commonly violated). Nevertheless, hydr ologists forged ahead with dozens of isotope-based hydrograph- separation studies that wer e published in the 1970s and 1980s. Hortonian overland flow was pr esumed dead. By the late 1980s, the new isotope-based hydr ograph separation technique had moved into adolescence, accompanied by t ypical adolescent problems such as confusion and a search for identity. As experienced hydrologists continued to use the isotope technique to study stormflow hydr ol- ogy in for ested catchments in humid climates, their younger peers followed obligingly — again and again. W as Hortonian overland flow r e ally dead and forgotten, though? What about catchments in which people live and work? And what about catchments in dry climates and the tropics? How u seful were study r esults when several of the assumptions about the homogeneity of sour c e waters wer e commonly violated? What if two components could not explain the variation of isotopic composition measured i n t he stream during stormflow? And what about uncertainty? As with many new tools, once the initial shine wor e off, the limitations of the method became a concern — one of which was that isotope-based hydrograph separations alone could not r eveal much about the flow paths by which water arrives at a str eam channel during storms.