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. 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!

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, 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.