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Comparison of Methods for Estimating Sulfate Export from a Forested Watershed¹

ABSTRACT

Seven methods of estimating sulfate export from a forested watershed in central Pennsylvania were compared. One method involved simple linear regression of discharge and concentration data, two methods were based on regression models in which the concentration data were stratified by flow frequency or duration, two methods weighted the mean concentration by discharge, one method used a simple unweighted mean concentration, and one weighted export according to the length of the sampling period interval. The regression models produced the highest export estimates but were considered statistically invalid because the data set did not adhere to the statistical requirements of independence, normality, and linearity. Weighted and unweighted mean concentration methods gave the lowest estimates of export because they gave less emphasis to high flows during which the majority of sulfate was exported from the watershed. The period-weighted method gave intermediate to high export estimates because it took into consideration the large export of sulfate during high flows. This method appeared to offer the best estimate of sulfate export from the watershed studied within the conditions and limitations of the data set. Although limited to sulfate export from a forested watershed, these results would appear to be applicable to any ion, particularly if the solute concentration of the ion is flow related.

Key Words: ionic export • nutrient flux • nutrient losses

¹ Contribution from The Pennsylvania State Univ. Agric. Exp. Stn. Journal Series no. 7141, University Park, PA 16802.

² Former Graduate Assistant and Associate Professor, School of Forest Resources, The Pennsylvania State Univ., University Park, PA 16802 and Principal Hydrologist, USDA Forest Service, Northeastern Forest Exp. Stn., University Park, PA 16802.

Received for publication February 14, 1985.

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