Sources of uncertainty in estimating stream solute export from headwater catchments at three sites

Uncertainty in the estimation of hydrologic export of solutes has never been fully evaluated at the scale of a small-watershed ecosystem. We used data from the Gomadansan Experimental Forest, Japan, Hubbard Brook Experimental Forest, USA, and Coweeta Hydrologic Laboratory, USA, to evaluate many sources of uncertainty, including the precision and accuracy of measurements, selection of models, and spatial and temporal variation. Uncertainty in the analysis of stream chemistry samples was generally small but could be large in relative terms for solutes near detection limits, as is common for ammonium and phosphate in forested catchments. Instantaneous flow deviated from the theoretical curve relating height to discharge by up to 10% at Hubbard Brook, but the resulting corrections to the theoretical curve generally amounted to \u3c0.5% of annual flows. Calibrations were limited to low flows; uncertainties at high flows were not evaluated because of the difficulties in performing calibrations during events. However, high flows likely contribute more uncertainty to annual flows because of the greater volume of water that is exported during these events. Uncertainty in catchment area was as much as 5%, based on a comparison of digital elevation maps with ground surveys. Three different interpolation methods are used at the three sites to combine periodic chemistry samples with streamflow to calculate fluxes. The three methods differed by \u3c5% in annual export calculations for calcium, but up to 12% for nitrate exports, when applied to a stream at Hubbard Brook for 1997–2008; nitrate has higher weekly variation at this site. Natural variation was larger than most other sources of uncertainty. Specifically, coefficients of variation across streams or across years, within site, for runoff and weighted annual concentrations of calcium, magnesium, potassium, sodium, sulphate, chloride, and silicate ranged from 5 to 50% and were even higher for nitrate. Uncertainty analysis can be used to guide efforts to improve confidence in estimated stream fluxes and also to optimize design of monitoring programmes

Long-term Trends from Ecosystem Research at the Hubbard Brook Experimental Forest

The Hubbard Brook Experimental Forest was established by the U.S. Forest Service in 1955 as a major center for hydrologic research in the Northeast. The Hubbard Brook Ecosystem Study originated 8 years later with the idea of using the small watershed approach to study element flux and cycling and the response of forest ecosystems to disturbance. Since that time, the research program at Hubbard Brook has expanded to include various physical, chemical and biological measurements collected by researchers from a number of cooperating institutions. Collaborative, long-term data are the keystone of the Hubbard Brook Ecosystem Study and have provided invaluable insight into how ecosystems respond to disturbances such as air pollution, climate change, forest disturbance, and forest management practices. This report highlights long- term ecological trends at Hubbard Brook, provides explanations for some of the trends, and lists references from the scientific literature for further reading

Dilution and the Elusive Baseline

Knowledge of baseline conditions is critical for evaluating quantitatively the effect of human activities on environmental conditions, such as the impact of acid deposition. Efforts to restore ecosystems to prior, “pristine” condition require restoration targets, often based on some presumed or unknown baseline condition. Here, we show that rapid and relentless dilution of surface water chemistry is occurring in the White Mountains of New Hampshire, following decades of acid deposition. Extrapolating measured linear trends using a unique data set of up to 47 years, suggest that both precipitation and streamwater chemistry (<i>r</i>² >0.84 since 1985) in the Hubbard Brook Experimental Forest (HBEF) will approximate demineralized water within one to three decades. Because such dilute chemistry is unrealistic for surface waters, theoretical baseline compositions have been calculated for precipitation and streamwater: electrical conductivity of 3 and 5 μS/cm, base cation concentrations of 7 and 39 μeq/liter, acid-neutralizing capacity values of <1 and 14 μeq/liter, respectively; and pH 5.5 for both. Significantly large and rapid dilution of surface waters to values even more dilute than proposed for Pre-Industrial Revolution (PIR) conditions has important ecological, biogeochemical and water resource management implications, such as for the success of early reproductive stages of aquatic organisms

Factors that Control the Range and Variability of Amorphous Silica in Soils in the Hubbard Brook Experimental Forest

In terrestrial ecosystems, the largest pool of amorphous silica (ASi) is stored in soils and is an important reservoir of biologically active Si for the global biogeochemical cycling of Si. Only limited data are available that quantify the size of this reservoir and often these estimates are made from the physical separation of silt-sized phytoliths, which can underestimate the ASi pool. Soil samples from five watersheds in a temperate-zone continental ecosystem at the Hubbard Brook Experimental Forest, New Hampshire, were analyzed for ASi using alkaline digestion. Soils from two of the watersheds were analyzed after experimental forest removal. In undisturbed watersheds, ASi was concentrated at the surface of the soil profile, similar to organic matter, and then progressively decreased with depth. This investigation supports our hypothesis that forest disturbance leads to redistribution of Si in the soil. In fact, although deforestation led to significant decreases in ASi in the upper soil horizons, total profile ASi (similar to 17,400 kg SiO2 ha(-1)) remained essentially unchanged, implying translocation downward. Significant increases in the transport of dissolved silica (DSi) by rivers have been observed With deforestation, however, in which the ASi pool in soils may play an important role. Additional studies should target the potential role of ASi as a buffer for DSi losses from deforested watersheds

Deforestation causes increased dissolved silicate losses in the Hubbard Brook Experimental Forest

Globally significant increases in the riverine delivery of nutrients and suspended particulate matter have occurred with deforestation. We report here significant increases in streamwater transport of dissolved silicate (DSi) following experimental forest harvesting at the Hubbard Brook Experimental Forest, NH, USA. The magnitude of the streamwater response varied with the type of disturbance with the highest DSi export fluxes occurring in the manipulations that left the most plant materials on the soil surface and disturbed the soil surface least. No measurable loss of amorphous silica (ASi) was detected from the soil profile; however, ASi was redistributed within the soil profile after forest disturbance. Mass-balance calculations demonstrate that some fraction of the DSi exported must come from dissolution of ASi and export as DSi. Land clearance and the development of agriculture may result in an enhanced flux of DSi coupled with enhanced erosion losses of ASi contained in phytoliths