A quantitative understanding of the hydrochemical response of upland catchments is a fundamental goal of hydrologists and environmental chemists. Such knowledge is needed to ascertain the potential and realized effects, for example, of atmospheric deposition of acids and agricultural chemicals on the catchment (terrestrial and aquatic) ecosystems, of changes of climate on water quality, and of the role of hydrology in the transport of heavy metals and other trace contaminants. In particular, an improved understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, the overall carbon budget of lotic systems is of primary interest to aquatic ecologists seeking a fundamental scientific understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and therefore plays an important role in the transport of contaminants in streams. Heterogeneities in the amount of precipitation delivered to the surface of a catchment, in soil properties and vegetation coverage, and in incident solar radiation can pay an important role in the hydrological response of mountainous catchments. These heterogeneities may be particularly important in influencing how chemicals (e.g., DOC) are delivered from the terrestrial catchment to the stream. Complete characterization of heterogeneities over a catchment is essentially impossible, but is likely to be unnecessary for descriptions of hydrochemical dynamics at the scale of a whole catchment. A possibly fruitful hypothesis is that the heterogeneities that dominate the catchment response are relatively large scale and hence can be identified from readily available data--topography, aerial photography, and satellite imagery -- in conjunction with a modest data-collection program in the field. The proposed work will explore this idea. The effects of spatial patterns in topography, vegetation cover, snow accumulation and melt, and soil transmissivity on the hydrochemical response of the Snake River in Summit County, Colorado will be determined. Spatial data will be analyzed using techniques embodied within geographic information systems (GIS) and synthesized in the context of a hydrological model (TOPMODEL) that will be modified to take into account the identified landscape-scale heterogeneities. Field data on precipitation, flow, hydraulic conductivity, and dissolved constituents (sulfate and DOC, in particular) will be used to evaluate the working hypothesis of the links between large-scale heterogeneity, hydrology, and hydrochemical response.
