Simulated seasonal water flux trends

A summary of the simulated annual site water balance is shown in Table 11.2. Drainage accounts for the majority of the incoming precipitation, comprising 84 and 74% of the annual water balance in 1999 and 2000, respectively. ET accounted for a smaller proportion of the water balance, approximately 18 and 23% of incoming precipitation for the respective years. Evaporation of water from the canopy, litter, and shallow soils were slightly less than half (~43%) of the annual ET. Overland flow at the site was negligible since the soils rarely froze and exhibited high infiltration capacities typical of forested systems. Storage changes accounted for a minor portion of the water balance, and were mainly affected by the timing of the seasonal shift from dry to wet conditions.

Simulated water balance trends for the 1999 and 2000 water years are shown in Figures 11.8 and 11.9. All trends

Table 11.2 Simulated water-balance summary

Component Year

1999 2000

Table 11.2 Simulated water-balance summary

Component Year

1999 2000

Precipitation

2596

2482

Drainage

2184

1833

Evaporation

200

250

Transpiration

278

321

Storage change

-48

88

Note: All units are in mm.

Note: All units are in mm.

1999

2000

Figure 11.7 Measured and simulated (0-0.30 m) soil water contents. Reproduced with permission from Link et al., Simulation of water and energy fluxes in an old growth seasonal temperate rainforest using the Simultaneous Heat and Water (SHAW) model; published by American Meteorological Society, 2003

2000

Figure 11.7 Measured and simulated (0-0.30 m) soil water contents. Reproduced with permission from Link et al., Simulation of water and energy fluxes in an old growth seasonal temperate rainforest using the Simultaneous Heat and Water (SHAW) model; published by American Meteorological Society, 2003

S 15

g 10

r 15

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep wy 1999

Figure 11.8 Simulated WRCCRF water balance for the 1999 water year

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piration

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Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

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if\

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Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep wy2000

Figure 11.9 Simulated WRCCRF water balance for the 2000 water year

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep wy2000

Figure 11.9 Simulated WRCCRF water balance for the 2000 water year were smoothed with a 15-day moving average to clearly depict seasonal variations in water-balance components. The storage term is the total storage in the system, dominated by the soil and snowpack reservoirs. Positive storage indicates that the system was gaining water. The lower plot in Figures 11.8 and 11.9 shows an expanded view of the evaporation and transpiration components.

The beginning of the water years was characterized by dry conditions, which transitioned to wet conditions in the mid-fall months. Replenishment of the soil reservoir is indicated by several weeks of positive storage, before drainage started to occur. The timing of the simulated drainage closely corresponded to the observed initiation of streamflow in the intermittent channel that drains the site (see Figure 11.4). Drainage during the 1999 winter was closely associated with precipitation input, given the lack of snowfall during this season. Drainage during the 2000 winter was also associated with precipitation but exhibited a distinct snowmelt pulse in March. The transition from a water-surplus to a water-deficit condition where ET exceeded precipitation occurred in mid-May of 1999 and mid-June of2000. Shortly after this transition, drainage ceased and the soil water reservoir was progressively depleted by transpiration and small rates of litter and soil evaporation.

Simulated ET was very low in the winter, remained relatively high through the late spring and summer, and gradually decreased starting in early August. Simulated evaporation was very low during the winter, and increased throughout spring due to frequent rainfall and warm temperatures. Evaporation decreased rapidly during the summer with the onset dry conditions, but remained slightly above zero due to isolated precipitation events and litter drying. Transpiration began almost immediately after soils warmed following snowpack ablation, peaked in the early summer, and gradually declined through late summer. During the spring and fall, ET patterns tended to mirror each other, when wet-canopy conditions increased evaporation and reduced transpiration.

The simulated ET pattern at WRCCRF contrasted with measured ET trends at a nearby dry ponderosa pine forest stand that experiences a similar Mediterranean climate (Metolius FLUXNET site). Unlike the WRCCRF canopy, the Metolius site exhibited lower ET rates in summer relative to the spring months (Anthoni et al. 2002; Anthoni et al. 1999). The higher ET rates at WRCCRF were also reflected in the ratios of sensible to latent heat fluxes, or Bowen ratios (^BR) from the ecosystem. During the warm season (day 165 to 235), estimated ^BR values at WRCCRF were lower (1.22-1.32) than were observed at the Metolius site (1.51-1.70) (Wilson et al. 2002), indicating higher ET rates.

The weekly average turbulent heat flux partitioning during the dry season from July through September is shown in Figure 11.10. The plot format is similar to the one used by Wilson et al. (2002) to compare the warm-season jiBR between different ecosystems. Dotted lines on the plots are constant values of jiBR and solid diagonal lines are constant values of net turbulent fluxes that are within 1% of net radiation during this period when computed on a weekly basis. Numbers on the

1999

2000

1999

-

\

1 = 2

= 1.' _

\

. X

43 31

\

-

6

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b = 0.5

x /

/9i0^

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= 0.33

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2000

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119 /

12

\

\

: X

\

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-

1

1

Daily latent heat flux (MJ m-2 day-1) Daily latent heat flux (MJ m-2 day-1)

Figure 11.10 Simulated evolution of average weekly turbulent fluxes during the dry season at WRCCRF. Numbered points are the average weekly flux from the first week of July (week 1) through the fourth week of Sept. (week 12). Dotted lines denote constant Bowen ratios (^BR) and solid lines denote constant net turbulent fluxes

plots represent sequential weekly periods where 1 is the first week of July and 12 is the last week of September. The arrows on the plots show the general trend of the turbulent flux partition. The climate at the site becomes more continental throughout the summer as vapor pressure deficits increase and net radiation declines. During this period, latent heat fluxes declined less rapidly than sensible heat fluxes, producing the observed ^BR decrease from values >1 in July to values <1 in September. This trend is opposite to the trend observed at the Metolius FLUXNET site in the Pacific Northwest under similar climatic conditions (Anthoni etal. 2002). The decreasing ^BR trend indicates that this canopy continued to transpire water during late summer dry conditions, whereas the other canopy, located in a drier area, exhibited reduced transpiration rates. This trend may result from higher water availability (a combination of shallow groundwater at the WRCCRF and ash-derived soils with a high plant-available water content) coupled with the presence of an effectively conducting root and stem system for supporting the transpiration flow. Consequently, the canopy can maintain relatively high transpiration fluxes through the dry season.

The simulation results for the WRCCRF canopy raise questions about how ET fluxes vary spatially in mountainous areas. For example, relatively high late-season transpiration fluxes were simulated for the WRCCRF canopy, which consists of old-growth vegetation in a relatively wet, valley-bottom location with deep loamy soils. Other younger forests (with less root development) and forests on steep, rocky slopes, more typical of montane environments may not be able to access as much water late in the dry season. Conversely, mature stands in drier sites may compensate with greater rooting depths to maintain transpiration and productivity. Detailed water and energy-balance measurement sites are not commonly located on slopes due to logistic difficulties of conducting measurements in these locations. We suggest that future forest water flux studies should focus on mountain slope areas, as has been suggested for carbon flux studies (Schimel et al. 2002). It is important for studies to include a focus on below-ground components, such as soil and rooting depth, to constrain parameters that are frequently estimated when parameterizing process models.

11.4 CONCLUSIONS

Results from the SHAW simulations exhibited good agreement with measured throughfall and soil moisture profiles and with estimates of transpiration and ET fluxes from the crane plot. We therefore feel that the model can be used to provide good estimates of the waterbalance dynamics of the WRCCRF site. Model results indicated that for the two study years, ET on average accounted for approximately 9 and 12% of the annual water balance, respectively. Evaporation of intercepted water during the rainfall season accounted for almost

25% of the water balance, probably due to relatively high canopy saturation storage resulting from the abundant canopy epiphyte populations. Transpiration rates peaked in early summer, followed by a decline resulting from decreasing net radiation and soil moisture depletion. Decreasing Bowen ratios during the summer dry period indicated that the system was able to maintain relatively high transpiration rates relative to drier forests, probably because of a shallow water table, high plant-available water content of the site soils, and the effective root system of the old-growth vegetation.

11.5 ACKNOWLEDGMENTS

Support for this research was provided by the Western Regional Center (WESTGEC) of the National Institute for Global Environmental Change (NIGEC), the U.S. Forest Service, and the Agricultural Research Service, Northwest Watershed Research Center. Sap flux data were provided by Dr. Nathan Phillips (Boston University) and Dr. Barbara Bond (Oregon State University). Eddy-covariance data were provided by Matthias Falk and Dr. Kyaw Thaw Paw U (University of California, Davis). Site photo was provided by the Forest Science Data Bank, a partnership between the Department of Forest Science, Oregon State University, and the U.S. Forest Service Pacific Northwest Research Station, Corvallis, Oregon. Significant funding for the data bank was provided by the National Science Foundation Long-Term Ecological Research program (NSF Grant numbers BSR-90-11663 and DEB-96-32921). The authors also thank two anonymous reviewers whose comments improved this manuscript.

REFERENCES

Anthoni, P., B. E. Law, and M. H. Unsworth. 1999. Carbon and water vapor exchange of an open-canopied ponderosa pine ecosystem. Agricultural and Forest Meteorology 95: 151-168.

Anthoni, P., M. H. Unsworth, B. E. Law, J. Irvine, D. D. Baldocchi, S. V. Tuyl, and D.Moore. 2002. Seasonal differences in carbon and water vapor exchange in young and old-growth ponderosa pine ecosystems. Agricultural and Forest Meteorology 111(3): 203-222. Baldocchi, D. 1997. Flux footprints within and over forest canopies. Boundary Layer Meteorology 85: 273-292. Bowling, L. C., P. Storck, and D. Lettenmaier. 2000. Hydro-logic effects of logging in western Washington, United States. Water Resources Research 36(11): 3223-3240. Dingman, S. L. 2002. Physical Hydrology. 2nd ed. New York:

Macmillan College Publishing. Dyrness, C. T. 2003. Soil Descriptions and Data for Soil Profiles in the Andrews Experimental Forest, Selected

Reference Stands, RNA's, and National Parks: Long-Term Ecological Research. Corvallis, OR: Forest Science Data Bank: SP001 [Database] 2001 [cited May 8 2003]. Available from http://www.fsl.orst.edu/lter/data/abstract.cfm?dbcode= SP001.

Flerchinger, G. N. 2003. The Simultaneous Heat and Water (SHAW) Model: User's Manual [electronic publication]. Technical Report NWRC 2000-10 2000, Northwest Watershed Research Center, USDA Agricultural Research Service, [cited March 15 2003]. Available from ftp://ftp.nwrc. ars.usda.gov/download/shaw/SHAWUsersManual.pdf.

Flerchinger, G. N., J. M. Baker, and E. J. A. Spaans. 1996a. A test of the radiative energy balance of the SHAW model for snowcover. Hydrological Processes 10: 1359-1367.

Flerchinger, G. N., K. R. Cooley, and Y. Deng. 1994. Impacts of spatially and temporally varying snowmelt on subsurface flow in a mountainous watershed: 1. Snowmelt simulation. Hydrological Sciences Journal 39(5): 507-520.

Flerchinger, G. N., C. L. Hanson, and J. R. Wight. 1996b. Modeling evapotranspiration and surface energy budgets across a watershed. Water Resources Research 32(8): 2539-2548.

Flerchinger, G. N., W. P. Kustas, and M. A. Weltz. 1998. Simulating surface energy fluxes and radiometric surface temperatures for two arid vegetation communities using the SHAW model. Journal of Applied Meteorology 37: 449-460.

Flerchinger, G. N., and F. B. Pierson. 1997. Modelling plant canopy effects on variability of soil temperature and water: Model calibration and validation. Journal of Arid Environments 35: 641-653.

Flerchinger, G. N., and K. E. Saxton. 1989. Simultaneous heat and water model of a freezing snow-residue-soil system I. Theory and development. Transactions of the American Society of Agricultural Engineers 32(2): 565-571.

Franklin, J. F., and T. A. Spies. 1991. Composition, function, and structure of old-growth Douglas-fir forests. In Wildlife and Vegetation of Unmanaged Douglas-Fir Forests, edited by L. F. Ruggerio, K. B. Aubry, A. B. Carey, and M. H. Huff. Portland, OR: US Department of Agriculture, Forest Service, 71 -80.

Granier, A. 1985. Une nouvelle methode pour la mesure du flux de seve brute dans le tronc des arbres. Ann. Sci. For. 42(2): 193-200.

Harr, R. D., W. C. Harper, J. T. Krygier, and F. S. Hsieh. 1975. Changes in storm hydrographs after road-building and clear-cutting in the Oregon coast range. Water Resources Research 11(3): 436-444.

Harr, R. D., A. Levno, and R. Mersereau. 1982. Streamflow changes after logging 130-year-old Douglas fir in two small watersheds. Water Resources Research 18: 637-644.

Harr, R. D., and F. M. McCorison. 1979. Initial effects of clearcut logging on size and timing of peak flows in a small watershed in western Oregon. Water Resources Research 15(1): 90-94.

Hicks, B. J., R. L. Beschta, and R. D. Harr. 1991. Long-term changes in streamflow following logging in western Oregon and associated fisheries implications. Water Resources Bulletin 27: 217-226.

Ishii, H., J. H. Reynolds, E. D. Ford, and D. C. Shaw. 2000. Height growth and vertical development of an old-growth Pseudotsuga-Tsuga forest in southwestern Washington State, U.S.A. Canadian Journal of Forest Research 30: 17-24.

Jones, H. G. 1992. Plants and Microclimate. 2nd ed. New York: Cambridge University Press.

Jones, J. A.,andG. E. Grant. 1996. Peak flow responses to clear-cutting and roads in small and large basins, western Cascades, Oregon. Water Resources Research 32(4): 959-974.

Jones, J. A., and G. G. Grant. 2001. Comment on ''Peak flow responses to clear-cutting and roads in small and large basins, western Cascades, Oregon: A second opinion'' by R. B. Thomas and W. F. Megahan. Water Resources Research 37(1): 175-178.

Keppeler, E. T., and R. R. Ziemer. 1990. Logging effects on streamflow: Water yield and summer low flows at Caspar Creek in northwestern California. Water Resources Research 26: 1669-1679.

Lee, X. 1998. On micrometeorological observations of surface-air exchange over tall vegetation. Agricultural and Forest Meteorology 91: 39-49.

Link, T. E. 2001. The water and energy dynamics of an old growth-seasonal temperate rainforest. Ph.D. dissertation, Environmental Sciences Graduate Program, Oregon State University, Corvallis, 169.

Link, T. E., G. N. Flerchinger, M. Unsworth, and D. Marks. 2004a. Simulation of water and energy fluxes in an old growth seasonal temperate rainforest using the Simultaneous Heat and Water (SHAW) model. Journal of Hydrometeorology 5(3): 443-457.

Link, T. E., M. Unsworth, and D. Marks. 2004b. The dynamics of rainfall interception by a seasonal temperate rainforest. Agricultural and Forest Meteorology 124(3-4): 171-191.

Maidment, D. R. 1993. Handbook of Hydrology. New York: McGraw Hill.

Nash, T. H. III, and V. Wirth, eds. 1988. Lichens, Bryophytes and Air Quality. Stuttgart: Gebr. Borntraeger Verlagsbuchhandlung, Science Publishers.

Paw U, K. T., T. H. Suchanek, S. L. Ustin, J. Chen, W. Winner, S. Thomas, T. Hsiao, R. Shaw, T. King, M. Falk, D. Pyles, and D. Matista. 2004. Carbon dioxide exchange between an old growth forest and the atmosphere. Ecosystems 7(5): 513-524.

Phillips, N., B. J. Bond, N. G. McDowell, and M. G. Ryan. 2002. Canopy and hydraulic conductance in young, mature and old Douglas-fir trees. Tree Physiology 22: 205-211.

Proctor, M. C. F. 1982. Physiological ecology: water relations, light, and temperature responses, carbon balance. In Bryophyte Ecology, edited by A. J. E. Smith. London: Chapman & Hall.

Schimel, D., T. G. F. Kittel, S. Running, R. Monson, A. Turnipseed, and D. Anderson. 2002. Carbon sequestration studied in western U. S. mountains. Eos, Transactions American Geophysical Union 83(40):445, 449.

Sellers, P. J., D. A. Randall, G. J. Collatz, J. A. Berry, C. B. Field, D. A. Dazlich, C.Zhang, G. D. Collelo, and L. Bounoua. 1996. A revised land surface parameterization (SiB2) for atmospheric GCMs. Part I: Model formulation. Journal of Climate 9(4): 676-705.

Shaw, D. C., J. F. Franklin, K. Bible, J. Klopatek, E. Freeman, S. Greene, and G. G. Parker. 2004. Ecological setting of the Wind River old-growth forest. Ecosystems 7: 427-439.

Stednick, J. D. 1996. Monitoring the effects of timber harvest on annual water yield. Journal of Hydrology 176: 79-95.

Thomas, R. B., and W. F. Megahan. 1998. Peak flow responses to clear-cutting and roads in small and large basins, western Cascades, Oregon: A second opinion. Water Resources Research 34(12): 3393-3403.

Thomas, B. R., and F. W. Megahan. 2001. Reply. Water Resources Research 37(1): 181-183.

Thomas, S. C., and W. E. Winner. 2000. Leaf area index of an old-growth Douglas-fir forest estimated from direct structural measurements in the canopy. Canadian Journal of Forest Research 30: 1922-1930.

Unsworth, M. H., N. Phillips, T. Link, B. Bond, M. Falk, M. Harmon, T. Hinckley, D. Marks, and K. T. Paw U. 2004. Components and controls of water flux in an old growth Douglas fir/western hemlock ecosystem. Ecosystems 7: 468-481.

Waring, R. H., and S. W. Running. 1998. Forest Ecosystems: Analysis at Multiple Scales. 2nd ed. New York: Academic Press.

Wigmosta, M. S., L. W. Vail, and D. P. Lettenmaier. 1994. A distributed hydrology-vegetation model for complex terrain. Water Resources Research 30(6): 1665-1679.

Williams, M., B. J. Bond, and M. G. Ryan. 2001. Evaluation of different soil and plant hydraulic constraints on tree function using a model and sap flow data from ponderosa pine. Plant Cell and Environment 24: 679-690.

Wilson, K. B., D. D. Baldocchi, M. Aubinet, P. Berbigier, C. Bernhofer, H. Dolman, E. Falge, C. Field, A. Goldstein,

A. Granier, A. Grelle, T. Halldor, D. Hollinger, G. Katul,

B. E. Law, A. Lindroth, T. Meyers, J. Moncrieff, R. Monson, W. Oechel, J. Tenhunen, R. Valentini, S. Verma, T. Vesala, and S. Wofsy. 2002. Energy partitioning between latent and sensible heat flux during the warm season at FLUXNET sites. Water Resources Research 38(12): 30.

Wright, K., K. H. Sendek, R. H. Rice, and R. B. Thomas. 1990. Logging effects on streamflow: Storm runoff at Caspar Creek in northwestern California. Water Resources Research 26: 1657-1667.

Ziemer, R. R. 1981. Storm flow response to road building and partial cutting in small streams of northern California. Water Resources Research 17: 907-917.

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