Can the distribution of headwater stream chemistry be predicted from downstream observations?

Small streams with catchment areas <2 km2 make up the majority of all stream length and are of great ecological importance. Surveys of first and second order streams reveal great spatial and temporal variability in the water chemistry of these headwaters, but their assessment presents a serious challenge since systematic, representative data are usually only collected in larger streams and rivers. Using low flow synoptic survey data from seven mesoscale Swedish catchments, this study tests the hypothesis that downstream monitoring data can be used to predict key features of the distribution of chemistry in headwater streams [median and interquartile range (IQR)]. Three ecologically relevant analytes were tested: pH, acid neutralizing capacity (ANC) and total organic carbon (TOC). For all seven catchments, the outlets (36–127 km2) were considerably less acid with lower TOC than the median of the headwaters (<2 km2, N D 19–45). Among catchments, headwater median and IQR were positively correlated with the value at the outlet, for all three analytes. A univariate general linear model (GLM) was used to predict the headwater chemistry distribution for each catchment from its outlet chemistry, using the relationship established with the other six catchments. Headwater median pH and IQR of ANC were well predicted by a single downstream sample [median adj. R2 3⁄4 0Ð7, normalized root mean squared error (NRMSE) <0Ð7]. Other response variables were not as well predicted, with median adj. R2 ranging from 0Ð08 to 0Ð48, and NRMSE up to 1Ð1. A minority of models were significant at ̨ D 0Ð05, in part due to the limited availability of catchments with such extensive survey data. However, the clear trends observed suggest that with additional model development, downstream chemistry could ultimately provide a valuable tool for characterizing the range of chemistry in the contributing headwaters. Copyright  2010 John Wiley & Sons, Ltd.