RELATIONSHIPS BETWEEN STREAM DISCHARGE AND YIELD OF DISSOLVED SUBSTANCES FROM A COLORADO MOUNTAIN WATERSHED

Abstract

The dependence of concentration and yield of dissolved substances on water discharge was studied over a 150-week period in the watershed of Como Creek, Colorado, near the Continental Divide. Substances were divided into three groups on the basis of relationship between concentration and discharge: (1) substances whose concentrations decrease with increasing discharge (HCO3−, NO3−, Ca2+, Mg2+, Na+); (2) substances whose concentrations show no trend with discharge (NH4+, dissolved organic phosphorus and nitrogen, K+, SO42-); (3) substances whose concentrations increase with increasing discharge (dissolved organic carbon, H+,PO43-). The concentration-discharge relationships were compared between years and generally showed evidence of a common slope between years, even though discharge and weather varied considerably between years. Mean concentrations between years were compared statistically at a common discharge and showed no significant differences between years, suggesting common control mechanisms between years for a given substance. Substances were also compared with respect to yield (product of concentration and discharge). Three categories of yield response to increasing discharge were identified. All showed increasing yield with increasing discharge but with: (A) yield increasing slower than discharge (HCO3−, NO3−, Ca2+, Mg2+, Na+); (B) yield increasing at the same rate as discharge (NH4+, DON); (C) yield increasing faster than discharge (DOP, K+,SO42-, DOC, H+, PO43-). Simple groundwater-precipitation mixing models are not adequate to explain these differences in concentration and yield responses to discharge. A satisfying mechanism to explain the differences between responses of dissolved substances to discharge must take into account not only mixing but also the chemical reaction of incoming precipitation with soil and soil water and the intensity of biological demands for the substances. © Williams & Wilkins 1979. All Rights Reserved.