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Fog Drip

Fog drip has mysteriously been omitted or de-emphasized in natural resource analyses and discussions. It is the water that collects on tree twigs and leaves and drops to the ground under forests. It is not measured by standard rain guages. As fog and clouds form or sweep across an area, much water collects on the trees. There is hardly a more beautiful sight than this condensation on trees in winter when, before it melts, is called hoarfrost. Before the "drip". Every branch, twig and needle is clothed in snowy crystals. It's pretty!

Harr 1981 found that net precipitation in Oregon's Bull Run watershed was 20% more in forested than in adjacent clearcut areas. This was attributed to fog drip.

The resource significance is great too. This drip in some area tallies 10 to 12 inches in areas where rainfall is 40 inches a year. When this much moisture is not measured (not even mentioned!), it cannot be a surprise that watershed "run-outs" never match well with the "rain-ins." In a 5000-acre watershed, a fog drip of 10 inches is about 1,260,400 cubic feet of water or 9,400,000 gallons of water. If precipitation layers go into GISs, so should an estimate of fog drip -- at least parametric estimates (high, low, and most likely) to study the picture, pattern, and consequences, if any, on decisions.

It is easy to see why it is difficult to estimate the amount of drip -- plenty of expensive gages are needed under the trees. As trees age, the collecting surface increases, then stabilizes. The surface is wonderfully dynamic and predictable. It represents, as well, a GIS land cover phenomenon. Landsat or other land cover images e.g., of hardwood and conifers are potential areas for mean fog-drip estimates. If tree cutting occurs, fog drip ceases (as does evapotranspiration of trees, etc.); no one measures the "wet" of the weedy field. The actual amount of drip may not add to the stream flow rate directly but it will surely wet the soil and change the percolation and infiltration rate of any precipitation that does occur. All sensitive water budget analyses include time since the last rain or soil wetting in the models. Soils experiencing fog drip are different than soils whose performance depends on the last rain event. Describing and simulating the dynamics of land use change, then vegetation aging, then harvest rates, then combining precipitation and fog drip is a wonderful challenge. The results can be mapped. The procedure does not have to be repeated by every GIS office, every worker.

New maps of this major changing ecological variable are likely to provide new insights into land management. Failure to include it in thoughts (if not analyses and actions) when dealing with precipitation and water budgets over large ecosystems is just silly. Including it may help unify concepts of trees, water, fish within water, fish that land mammals and birds eat, and people who like all of these wonderful, wild animals.

Dew and fog may have an effect equal or greater than precipitation. Billings and Drew (1938) said "Foggy nights or a foggy day keep bark more moist than a hard rain once a week." Such moisture affects stem flow and interception phenomena.

Compare fog drip and cumulatyive precipitation in an area. Correlations are hypothesized.

See WGEN (1985?) a model for generating daily weather variables, microfilm, PB 85 107100

Consider the possible presentation of monthly data as bar graph or continuous data as at the right.


Cahn, A.R. 1938. A climatographic analysis of the problem of introducing three exotic game birds into the Tennessee Valley and vicinity. Trans. 3rd N.A. Wildlife Conf. 3:807-817 (compares 2 areas, 2 years)

Durham, J. L. ed. 1984. Chemistry of particles, fogs, and rain . Butterworths, Boston 262p.

Harr, R.D. 1981. Fog drip in the Bull Mountain Municipal watershed, Oregon. abstract . Northwest Sci. Program , 54th Annual Meeting, Corvallis, Or. p. 49.

Harr, R.D. 1982. Fog drip in the Bull Run Municipal Watershed, Oregon. Water Resources Bul. 18(5):785-789.

Hicks, B.B. ed.1984. Deposition both wet and dry. Butterworths, Boston

Hori, T. Ed. 1953. Studies of fogs in relation to fog-preventing forests. Hokkaido

Imahori, K. 1953. On vanishing mechanism of advestion fog and the role of turbulence, in T. Hori (Ed.) studies of fog in relation to fog-preventing forest, Hokkaido, Japan

Tonna, G. 1973. A data processing method for determining the fog droplet size distribution by laser light scattering. Atmos. Environment. 7:1093-1102

White, E.J. and F. Turner. 1970. A method of estimating income of nutrients in a catch of airborne particles by a woodland canopy. J. Applied Ecol. 7: 441-461

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