of mass, we can write the following equation that must apply to all ecosystems:
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of mass, we can write the following equation that must apply to all ecosystems: Inputs Outputs Storage Where Inputs refers to the mass of some form of matter (eg, water or an element like nitrogen) loaded to the ecosystem, Outputs refers to the mass of the same compound lost from the ecosystem per unit time, and Storage refers to the change in the mass of that compound located in the ecosystem per unit time. Because this equation always holds, we can measure two of these fluxes and estimate the third by difference. For example, if we can measure all the inputs and the outputs, then the difference between these two fluxes is the term describing the storage of that compound in the ecosystem. Storage can be either positive (accumulating in the ecosystem) or negative (being lost from the ecosystem) This exercise provides some simple but illustrative examples of the application of this "Mass-balance" approach to studying ecosystems. Answer all questions listed below and hand in the last page by the end of class today. You will need to use a calculator for solving these mass-balance problems. Pay attention to the units in all problems! 1) Hydrologic budgets of watersheds Likens et al. (1985) studied the effects of forests on the water flow through watersheds in the Hubbard Brook Experimental Forest in 1979-1980. One of the key processes they were interested in estimating was the role of the trees and other plants in the hydrologic budgets of forested watersheds. They knew that plants move water out of soils and into the atmosphere through the process of evapotranspiration, but they did not know how important this process was at the scale of an entire watershed. They wanted to know how much water loss could be accounted for by evapotranspiration. Likens et al. (1985)1 estimated that their study watersheds received annual inputs of 115.6 cm of rainfall per year. Through intensive monitoring of the streams draining these watersheds, th estimated that 70.4 cm of rainfall was lost from the watershed via streams every year. They assumed that the forest was at equilibrium and that changes in water storage in soils were negligible (1-4 cm/year). Thus, they assumed that change in storage in the watershed was negligible. The only other major loss of water from the watershed was due to evapotranspiration. .Estimate (in cm/year) how much water is lost from the Hubbard Brook watersheds every year due to evapotranspiration. What percentage of the water inputs to these forests is lost to evapotranspiration? Write a one-sentence statement about how you would expect stream flows to respond if the . . watersheds were all clear-cut from logging. 2) Human effects on the global carbon cycle Likens, G.E.,.J.S. Eaton, N.M. Johnson, and R.S. Pierce. 1985. Flux and balance of water and chemicals. Pages 135-155 in: G.E. Likens (ed) An Ecosystem Approach to Aquatic Ecology. Springer-Verlag, NYExplanation / Answer
a) We are going to estimate the evapotranspiration by difference, given that we have the input of water, the loss by streams and changes in water storage in soils was negligible. All units are in cm/year.
115.6 cm/year rainfall - 70.4cm/year lost in streams = water lost in evapotranspiration (45,2 cm/ year aprox.)
b) Now they ask what percentage of the rainfall is lost due to evapotranspiration. Keep in mind, 115.6 cm/year is the 100%. what porcentage is 45.2 cm/year?
45.2 cm/year x 100 % = 39,45 %
115.6 cm/year
c) Clear-cut logging is a practice in which most or all trees in an area are uniformly cut down, when this happens, the soil cannot contain the water with the same efficiency and thus, the stream flows increases. It also increases the risk of channel destabilization and damage to the habitat surrounding producing floods.
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