Watersheds are hydrologic units of the landscape which process water entering as precipitation, resulting in storage or loses through evapotranspiration, groundwater drainage, or stream runoff. In addition to water quantity, precipitation inputs and drainage losses transport nutrients to and from watersheds, and can be important components of nutrient dynamics of ecosystems. Small watersheds (12 to 76 ha) have been used as experimental units since the initiation of the Hubbard Brook Experimental Forest (HBEF) by the US Forest Service in the mid-1950s. The HBEF has a network of precipitation quantity and chemistry stations and nine first-order gauged watersheds, including several that are used for long-term experiments. Reference experimental watersheds are used to examine long-term patterns and trends in hydrology and water quality. Experiments conducted at the small watershed scale include evaluations of different forest harvestings (i.e., devegetation (w2), strip-cutting (w4), whole-tree harvest (w5)) and a chemical manipulation by calcium silicate addition (w1).
At the HBEF, experimental watersheds have relatively little deep seepage, so measurements of precipitation inputs (1400 mm) and stream outputs (900 mm) from gauging stations allow for estimates of annual evapotranspiration losses by difference (500 mm). The small watershed approach provides for relatively accurate water and material mass balances (budgets) to quantify ecosystem function and examine changes in response to changing inputs (e.g., atmospheric deposition, climate) or in response to experimental treatments. Small first-order watersheds that are relatively unimpacted by disturbance are a bell weather of ecosystem response to environmental/ecological change, such as climate change and air pollution. The small watershed approach is a relatively large scale for conducting experimental manipulations, which allows for the integration of ecosystem responses to experimental treatments through the monitoring of stream hydrology and water quality.