FOREST INFLUENCES. 307 



It can not be too strongly impressed upon those who disclaim any influence of forest cover on 

 climate, because the cosmic causes by which this is produced are immeasurably greater, that there 

 are two classes of climate always to be considered separately, namely, the general climate and the 

 local climate. The latter is of most importance to us, and alone can be modified by small causes. 

 We modify it by building a house around us, thus altering the temperature and moisture conditions 

 of the atmosphere so inclosed; but the question is, whether we can alter these conditions on a 

 larger scale by such means as alternating forest areas and fields or by large bodies of forest. We 

 are not so much concerned as to whether the total rainfall over the continent is increased, but 

 whether the distribution of precipitation in time and quantity over and near a forest area is 

 influenced by its existence; whether we or our crops feel its absence or presence in our immediate 

 neighborhood; whether the protection it seems to afford and the changes it seems to produce in 

 the meteorological phenomena are or are not real and of sufficient magnitude to influence our 

 forest policy. 



We can understand readily that if any influence exists it must be due, in the first place, to 

 the mechanical obstruction which the forest cover presents to the passage of air currents and 

 to the action of the sun's rays upon the soil — it must result from a difference in insolation, and 

 consequent differences in temperature and evaporation over forest and field. It is also readily 

 understood that the influence can become appreciable only when large enough areas exhibiting 

 such differences are opposed to each other, capable of producing local cunents of air which may 

 intercommunicate the characteristics of the one area to the other. The size and character of the 

 forest growth, its density, height, situation, and composition are, therefore, much more important 

 in determining its influence than has been hitherto supposed. It is not trees, but masses of foliage 

 which may be effective. A large sheet covering an extended area from the influence of the sun 

 would produce almost the same differences in meterological conditions that a forest cover is 

 expected to produce. 



While, then, we may admit a priori that extent or area and condition of the forest cover are 

 important, we have as yet no data from which to calculate any proper size or proportion, and the 

 attempts to fix a certain percentage of forest cover needed for favorable climatic conditions of a 

 country are devoid of all rational basis. 



Leaving the question of forest influences upon climate as still awaiting final solution, we may 

 speak with much more confidence of the effect which forest cover exerts upon the disx>osal of 

 water supplies. This effect can be much more readily studied and shows itself much more 

 conspicuously. It is perhaps also much more important to human economy, for it is becoming 

 more and more apparent that our agricultural production is dependent not so much upon the 

 amount of rainfall as upon the proper disposal of the waters that fall. 



Eecognizing this truth, the American Association for the Advancement of Science in 1891 

 sent the following resolution to the Secretary of Agriculture: 



The American Association for the Advancement of Science respectfully submits for the consideration of the 

 Secretary of Agriculture that the future of successful and more productive agriculture depends very largely upon a 

 lational water management; meaning thereby not 011I3 the use of water for irrigation in the arid and subarid 

 regions, but the rational distribution and use in the humid regions of available water supplies by meanb of 

 horizontal ditches and irrigation systems, combined with 2>roper mechanical preparation of the soil, and with 

 diainage systems, with the object of fully utilizing the water for plant production and providing for the safe and 

 harmless removal of the surplus. 



The present policy of forest production and of allowing our waters to run to waste not only entails the loss of 

 their beneficial influence upon plant production, but permits them to injure crops, to wash the fertile mold from the 

 soil, and even to erase and carry away the soil itself. 



Tt is upon these considerations that the association respectfully suggests to the honorable Secretary the 

 desirability oi utilmDg the Weather Bureau, the various agricultural experiment stations, and other forces, in 

 forming a systematic service of water statistics, and m making a careful survey of the condition of water supplies, 

 which may serve as a basis for the application of rational principles of water management. 



How poorly we understand the use of these supplies is evidenced yearly by destructive 

 freshets and floods, with the accompanying washing of soil, followed by droughts, low water, and 

 deterioration of agricultural lands. 



It may be thought heterodox, but it is nevertheless true, that the manner in which most of the 

 water of the atmosphere becomes available for human use (namely, in the form of rain) is by no 



