National Research Program


Watersheds

Water and Solute Mass Balance

Scientists use watershed mass balance calculations to estimate the changes that occur when water interacts with a watershed's vegetation, soils, and bedrock. A comparison of the mass of water and accompanying dissolved and suspended materials that is entering a watershed with what exits that watershed enables them to better understand weathering and other abiotic and biotic reactions and provides them with a means to better understand human influence on water quality.

Sites of the Water, Energy, and Biogechemical ProgramTaking this approach a step further, USGS scientists used mass balance calculations to compare the differences and similarities of five small, relatively undisturbed watersheds that have greatly differing climate, geology, physiography, and ecology. Data were collected over a six-year period (October 1991 through September 1997) in a study of the five watersheds of the USGS Water, Energy, and Biogeochemical (WEBB) program. In evaluating the monthly and annual watershed mass balance of water and major inorganic solutes in the five watersheds (located in Colorado, Wisconsin, Vermont, Georgia, and Puerto Rico), the variety of conditions (alpine/subalpine, boreal, temperate, subtropical, tropical) provided a gradient in which to study and contrast processes within and across watersheds, and to determine the primary regional controls on yields of the major dissolved inorganic solutes.

Results reported in a 2006 journal article included the following:

  • For solutes derived primarily from rock weathering (Ca, Mg, Na, K, and H4SiO4), net fluxes (outputs in streamflow minus inputs in deposition) varied by two orders of magnitude, which is attributed to a large gradient in rock weathering rates controlled by climate and geologic parent material;
  • The net flux of solutes that are largely atmospherically derived (NH4, NO3, SO4, and Cl) was similar among sites; SO4 was the most variable and NH4 and NO3 were generally retained (except for NO3 in Colorado);
  • The timing and amount of water and solutes exported from each watershed are linked to regional climate. The Puerto Rico watershed had the highest annual precipitation and it, and to a lesser extent the Georgia watershed, was sometimes greatly affected by hurricanes and tropical storms. Snowmelt dominated the runoff patterns in Vermont and Colorado, where snowmelt periods are relatively brief (one to two months). The Wisconsin watershed had the lowest annual precipitation and the least variability in monthly runoff; snowmelt influence was relatively minor due to lower snowfall amounts and high infiltration capacity.
Additional information on the results and information on data uncertainties (e.g., lack of dry-deposition in input fluxes) are available in the journal article:
Peters N.E., Shanley, J.B., Aulenbach, B.T., Webb, R.M., Campbell, D.H., Hunt, R., Larsen, M.C., Stallard, R.F., Troester, J., and Walker, J.F., 2006, Water and solute mass balance of five small, relatively undisturbed watersheds in the U.S.: Science of the Total Environment, v. 358, p. 221- 242.

Also see the Water, Energy, and Biogeochemical Budgets (WEBB) program home page for related research or contact Earl Greene, eagreene@usgs.gov.

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