SAMPLING-RATE EFFECTS ON THE PROPERTIES OF DYE BREAKTHROUGH CURVES FROM GLACIERS

Dye-tracer techniques are widely used to infer the character of subgla cial drainage systems. Quantitative analysis of dye breakthrough curve s focuses on the determination of tie water throughflow velocity (u), the dispersion coefficient (D) and the dispersivity parameter (d = D/u ). Together, these parameters describe the rate of passage of tracer t hrough the drainage system and the extent to which the dye cloud becom es spread out during passage. They have been used to infer the nature of now conditions within a drainage system and temporal changes in sys tem morphology. Estimates of all three parameters, however, are depend ent upon the sampling interval at which measurements of dye concentrat ion to define breakthrough curves are made. For a given breakthrough c urve, dispersion coefficient increases with the sampling interval, whi le the throughflow velocity shows no systematic variation. As a result , the dispersivity also lends to increase with the sampling interval. Investigations of the sensitivity of parameter estimates to the sampli ng interval reveal that reliable estimates call be obtained only if th e sampling interval is less than 1/16 of the time from dye injection t o peak dye concentration. As a general guide, we suggest that, ideally , quantitative analyses of dye breakthrough curves should therefore be conducted only when this criterion can be met.