Effects of Precipitation, Air Temperature, and Land Use on Organic Carbon Discharges from Rhode River Watersheds

We studied discharges of organic carbon from eight contiguous small watersheds on the Atlantic Coastal Plain in Maryland for up to 24 yr. Six of these watersheds were second or third order with mixed-land-use, while two were first order (one completely forested and one highly dominated by cropland). These watersheds have perched aquifers, so all groundwater discharges as well as surface runoff were measured at V-notch weirs and flumes, which included volume-integrating flow-proportional samplers. Interannual variations in annual and seasonal precipitation during this study spanned approximately the range of 160 yr weather records in the region. Annual total organic carbon (TOC) area yields from the overall Rhode River watershed varied 8-fold, correlations with precipitation were highly significant, and a power function regression explained 54% of the variance in annual TOC fluxes. TOC fluxes were higher from upland forest than mixed land use, and highest from the cropland-dominated watershed. The fluxes from first order watersheds were more variable with precipitation. In the spring, TOC fluxes were highest and most correlated with precipitation, compared to other seasons. Precipitation volume also explained much of the variance in annual and spring TOC concentrations from upland forest and cropland, with concentrations three to five times higher in very wet years than in very dry years. Variation in winter and summer air temperature was correlated with TOC concentrations from forested watersheds, and linear regressions explained 19 to 42% of the variance in TOC. A regression model was used to construct graphical and tabular summaries. Particulate organic carbon and dissolved organic carbon (DOC) concentrations and the ratio of DOC to TOC were highly correlated with water discharge for a second order, mixed land use watershed, and power function regressions explained 21 to 43% of the variance. For the first order, single-land-use watersheds the ratio of DOC to TOC was also highly correlated with discharge.