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ARS Home » Pacific West Area » Boise, Idaho » Northwest Watershed Research Center » Research » Publications at this Location » Publication #178809
Title: The importance of canopy structure in controlling the interception loss of rainfall: examples from a young and an old-growth Douglas Fir forest

Author
item PYPKER, THOMAS - OSU
item BOND, BARBARA - OSU
item LINK, TIMOTHY - U OF I
item Marks, Daniel
item UNSWORTH, MICHAEL - OSU

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2005
Publication Date: 3/8/2005
Citation: Pypker, T.G., Bond, B.J., Link, T.E., Marks, Danny, and Usworth, M.H. The importance of canopy structure in controlling the interception loss of rainfall: Examples from a young and an old-growth Doughlas-fir forest, Elsevier, Agricultural and Forest Meteorology 130 (2005) 113-129.

Interpretive Summary: The canopy water storage capacity (S), direct throughfall fraction (p), the ratio of evaporation to rainfall intensity ( ) and interception loss (In), of a Douglas-fir forest are influenced by short (seasonal) and long-term (decades to centuries) changes in the forest canopy. Gross precipitation (PG) and net precipitation (Pn) were measured in a young (25-y-old) Douglas-fir forest and the results compared with measurements previously made in a nearby old-growth (>450-y-old) Douglas-fir forest (Link et al., 2004). Canopy rainfall variables were estimated using a regression-based method that estimates S, p and for individual storms using the relationship between PG and Pn. The individual storm estimates of S, p and for the young forest were applied to a rainfall interception model (Gash model (Gash, 1979)) to determine the effect of seasonal changes in canopy hydrologic variables have on estimates of In (young forest only). The Gash model was previously applied to the old-growth forest (Link et al., 2004). The young forest had significantly different S (1.40 mm '0.27) and p (0.12 '0.07) relative to the old-growth forest (S = 3.32 '0.35; p = 0.42 '0.07). Seasonal variation in canopy structure, such as deciduous leaf senescence and coniferous needle drop, were correlated with decreases in S. The differences in S and p between the two forests resulted in an In that was only slightly larger in the old-growth forest because the for the two forests were similar (young = 0.18 '0.06; old-growth = 0.17 '0.08). in the young and old-growth forests may have been similar because developmental changes associated with old-growth forest may alter the external resistance (ra) and the effective area for evaporation. The Gash model successfully predicted In for the young forest on a seasonal basis (3.29% error), but experienced larger errors (range = -91 to 36% error) for individual storms. The seasonal error and the error for individual storms improved when seasonal variations in canopy characteristics were incorporated in the model (seasonal error = 2.37%; individual storm error range = -12.0 to 21.7%). Therefore, short-term (seasonal) changes in phenology and long-term (decades to centuries) horizontal and vertical development of the forest canopy influence S, p, In and of Douglas-fir forests.

Technical Abstract: The canopy water storage capacity (S), direct throughfall fraction (p), the ratio of evaporation to rainfall intensity ( ) and interception loss (In), of a Douglas-fir forest are influenced by short (seasonal) and long-term (decades to centuries) changes in the forest canopy. Gross precipitation (PG) and net precipitation (Pn) were measured in a young (25-y-old) Douglas-fir forest and the results compared with measurements previously made in a nearby old-growth (>450-y-old) Douglas-fir forest (Link et al., 2004). Canopy rainfall variables were estimated using a regression-based method that estimates S, p and for individual storms using the relationship between PG and Pn. The individual storm estimates of S, p and for the young forest were applied to a rainfall interception model (Gash model (Gash, 1979)) to determine the effect of seasonal changes in canopy hydrologic variables have on estimates of In (young forest only). The Gash model was previously applied to the old-growth forest (Link et al., 2004). The young forest had significantly different S (1.40 mm '0.27) and p (0.12 '0.07) relative to the old-growth forest (S = 3.32 '0.35; p = 0.42 '0.07). Seasonal variation in canopy structure, such as deciduous leaf senescence and coniferous needle drop, were correlated with decreases in S. The differences in S and p between the two forests resulted in an In that was only slightly larger in the old-growth forest because the for the two forests were similar (young = 0.18 '0.06; old-growth = 0.17 '0.08). in the young and old-growth forests may have been similar because developmental changes associated with old-growth forest may alter the external resistance (ra) and the effective area for evaporation. The Gash model successfully predicted In for the young forest on a seasonal basis (3.29% error), but experienced larger errors (range = -91 to 36% error) for individual storms. The seasonal error and the error for individual storms improved when seasonal variations in canopy characteristics were incorporated in the model (seasonal error = 2.37%; individual storm error range = -12.0 to 21.7%). Therefore, short-term (seasonal) changes in phenology and long-term (decades to centuries) horizontal and vertical development of the forest canopy influence S, p, In and of Douglas-fir forests