HYDROLOGICAL CONTROLS ON DISSOLVED ORGANIC-CARBON DURING SNOWMELT IN THE SNAKE RIVER NEAR MONTEZUMA, COLORADO

A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientifi c concern for at least two reasons. First, quantifying the overall car bon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role i n the transport of contaminants. In the Snake River near Montezuma, Co lorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge i n the stream. The decline in DOC with time is approximately exponentia l, suggesting that a simple flushing mechanism can explain the respons e. We examined hydrological mechanisms to explain the observed variabi lity of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows throu gh a simple model that accounted for temporal changes in DOC. Conceptu ally the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrati ng meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for q uantitatively linking hydrological processes with carbon cycling throu gh upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics i n a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through th e various DOC source areas and the amount of DOC that can accumulate i n these areas.