INFLUENCE OF FORESTS UPON PRECIPITATION. 125 



influence of sun and wind, must have an influence, however small, on 

 the local water conditions, which may or may not become appreciable 

 in human economy. 



The question of forest influence on water supplies may be considered 

 under three heads, namely: (1) Influence upon precipitation or upon 

 the distribution of atmospheric water; (2) influence upon the disposal 

 and conservation of available water supplies; (3) influence upon the 

 "run-off" or the distribution of terrestrial waters. 



INFLUENCE OF FORESTS UPON PRECIPITATION. 



The question whether forest cover has or has not an influence upon 

 the quantity of rain or snow that will fall over a given area is discussed 

 at length on pp. 111-117 of this bulletin and also upon additional 

 data in my annual report for 1888. It may suffice, therefore, to state 

 only the conclusions which may fairly be drawn from our present 

 knowledge and experience : 



(1) Finding the air strata above forest stations moister and cooler, 

 although only slightly so, than over field stations, we would infer that 

 the tendency to condensation over wooded areas might be greater than 

 over open fields. Experience and measurements seem to sustain this 

 reasoning. 



(2) These cooler and relatively as well as absolutely moister air strata 

 carried away by air currents must modify conditions near the forest 

 and possibly increase in its neighborhood also the tendency to forma- 

 tion of dew and to other precipitation under certain conditions. 



(3) While the forest may not everywhere increase precipitation over 

 its own area and near it, yet the presumption is that large systems of 

 forest growth over exteusive areas alternating with open fields may 

 establish sufficient differences in temperature and moisture conditions 

 and in air currents to modify, if not in quantity yet in timely and 

 local distribntion, the fall. 



(4) It must not be overlooked that the extent, density, height, and 

 composition, and relative position of the forest in making it a climatic 

 factor are important conditions, and furthermore that there are certain 

 rain conditions prevailing in climatic zones (rainy or rain-poor locali- 

 ties, with periodical, seasonal, or irregular rains) which are due to cosmic 

 influences and can not be altered, yet maybe locally modified by forest 

 cover. Hence experience in one climatic zone or under one set of con- 

 ditions can not be utilized for deductions in another. 



(5) Altogether the question of appreciable forest influence upon pre- 

 cipitation must be considered as still unsolved, with some indications, 

 however, of its existence under certain climatic and topographical con- 

 ditions in the temperate zone, especially toward the end of winter and 

 beginning of spring. 



As one of the most striking examples of an increase of precipitation, 

 seemingly due to forest planting, the experience at Lintzel, on the Liine- 



