Hydrology is a critical component of all ecosystems. Water moving through hill slopes and stream channels links terrestrial and aquatic ecosystems, drives nutrient cycling processes, and governs geomorphic and fluvial disturbance processes. Streamflow monitoring is a component of research at 11 of the 18 LTER research sites, and four sites have climate and streamflow records spanning more than 30 years. This article describes preliminary results -- supported by a 1995 LTER Network Office graduate student travel award -- from an ongoing comparative study of streamflow hydrology at four LTER sites: H.J. Andrews (AND), Coweeta (CWT), Hubbard Brook (HBR), and Luquillo (LUQ), as well as Caspar Creek (CC), a U.S. Forest Service Research Forest in California’s Coast Range (see table). These sites were selected because they have contrasting hydrologic characteristics and well-documented long-term streamflow and climate records. It is hoped that approaches developed in this study will be extended to examine other sites’ long-term streamflow records.
Streamflow patterns from undisturbed watersheds differ markedly among these five sites, reflecting differences in climate and vegetation (see table and figures). At CWT, HBR, and LUQ, precipitation is evenly distributed throughout the year, whereas precipitation at AND and CC occurs predominantly in winter (Figure A) (McKee and Bierlmaier 1987, Swift 1987, Ziemer and Albright 1987, Federer et al. 1990). This distinction produces relatively constant monthly streamflow at CWT and HBR, but much higher winter than summer streamflow at AND and CC (Figure B). AND and HBR also have a seasonal snowpack, whereas CWT only occasionally receives snow, and LUQ and CC lack snow. Melt of the seasonal snowpack contributes to prolonged high spring streamflow at AND and a rapid rise in spring streamflows at HBR compared to CWT and CC (Figure B). Forest canopies at CWT and HBR are dominated by deciduous broadleaf vegetation which transpires throughout the summer months, whereas forest canopies at AND and CC have evergreen coniferous vegetation which may transpire little during dry summer months. Potential evapotranspiration greatly exceeds precipitation in the summer at AND and CC (Bierlmaier and McKee 1989, Swift et al. 1975), whereas summer soil moisture deficits are smaller at CWT and HBR (Federer 1982).
The availability of these high-quality long-term streamflow data provide the opportunity to address a number of process-based hypotheses relating hydrology to ecology at long-term ecological research sites. For example, post-disturbance vegetation succession may differ among sites and produce contrasting post-disturbance streamflow patterns. Life history strategies of aquatic organisms and stream community structure may be related to streamflow variability at annual, seasonal, storm, or diurnal time scales. Nutrient fluxes may differ among sites according to the relative importance of rare, large precipitation and streamflow events. A two-year collaborative project is currently under way to further compare streamflow data among these five sites.
Climate and Vegetation Characteristics of Five Long-Term Streamflow Monitoring Site
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Site
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Location
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Climate
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Vegetation
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| H.J. Andrews | Oregon | winter rain/snow, summer drought | old-growth Douglas-fir forests |
| Coweeta | North Carolina | winter rain, summer rain | oak hickory forests |
| Hubbard Brook | New Hampshire | winter snow, summer rain | northern hardwood forests |
| Caspar Creek | California | winter rain, summer drought |
second-growth Douglas-fir, coastal redwood forests |
| Luquillo | Puerto Rico | winter rain, summer rain | sub-tropical and lower montane forest |