Precipitation-induced isotopic variations in streamflow

The overall stream response to intense precipitation is a result of complex pathways and mechanisms through which precipitation runoff, reservoir outflow, displaced soil moisture, shallow subsurface flow, and other factors act upon the stream. As a result, the relative contribution of all the hydrologic components is significant to the formulation of accurate stream response simulation models. Using stable isotope tracers provides new insights into the controlling factors of stream response, which otherwise are masked in traditional stream hydrograph studies.

In particular, variation in the naturally occurring stable isotope ratios of oxygen and hydrogen can be effectively used to trace the pathways of water movement in the catchment area. Evaporation leads to distinctive shifts in the isotopic composition of water, making it possible to differentiate input to the stream from precipitation versus from sources such as the stock pond and stored soil water.

As part of the Department of Energy's Water Cycle Pilot Study, measurements of the stable isotope ratios of oxygen and hydrogen in precipitation, stream water (Fig. 1), and soil water have been used to study the response of a drainage system to two closely spaced storm events in the Walnut River watershed in the southern Great Plains. The stream catchment area is mostly composed of grasslands with some agricultural lands and a stock pond located near the headwaters region that outflows into the creek (Fig. 2).

The isotope data showed that during the initial hours following the storms, flow upstream was dominated by the outflow from the pond, while precipitation runoff was the major factor influencing the downstream flow. The gradual replacement of evaporated pond water by precipitation is reflected in the isotopic ratios of the upstream samples. The downstream samples exhibited an increasing input from displaced soil moisture, as the influence of precipitation runoff diminished.

Future work will focus on incorporating isotopic data into a numerical hydrologic model designed to help forecast flood assessment and to analyze effects of land usage on stream response.-MADHAV V. MACHAVARAM, KATHY E. BASH FORD, MARK E. CONRAD, AND NORMAN L. MILLER. "Precipitation-Induced Isotopic Variations in Streamflow, 17th Conference on Hydrology.