132 



THE INFLUENCE OF AGRICULTURE ON CLIMATE. 



[1854 



Twenty-five years after M. de Humboldt, I explored in my 

 turn the valley d'Aragua, having fixed my residence in the little 

 towu of Maracaibo. The inhabitants had now remarked, that for 

 several years, not only had the lake ceased to diminish, but that 

 it had even riseu very perceptibly. Some fields that were for- 

 merly covered with cotton plantations were now submerged. 

 The Isles de las Nuevas Aparacidas, which had risen from the 

 waters in 1796, had again become shoals dangerous to naviga- 

 tion ; the tongue of earth de la Cabrera, on the north side of the 

 valley, had become so narrow that the slightest rise in the water 

 of the lake covered it completely; a continuous N. E. wind was 

 sufficient to flood the road which led from Maracaibo to New 

 Valencia ; in short, the fears which the inhabitants of the lake 

 had entertained for so long a period had entirely changed their 

 nature; they were now no longer afraid of the lake drying up; 

 they saw with dismay that, if the water continued to rise as it 

 had done lately, it would, in no long space oi time, have drowned 

 some of the most valuable estates, &c. Those who had explained 

 the diminution of the lake by supposing subterraneous canals, 

 now hastened to close them up in order to find a cause for the 

 rise in the level of the water. 



In the course of the last twenty-two years important political 

 events had transpired, Venezuela no longer belonged to Spain ; 

 the peaceful valley d'Aragua had been the theatre of many a 

 bloody contest; war to the knife had desolated this beautiful 

 country and decimated its inhabitants. On the first cry of inde- 

 pendence raised, a great number of slaves found freedom by 

 enlisting under the banners of the new republic; agricultural 

 operations of any extent were abandoned, and the forest, which 

 makes such rapid progress in the tropics, had soon regained pos- 

 session of the surface which man had won from it by something 

 like a century of sustained and painful toil. With the increasing 

 prosperity of the valley, many of the principal tributaries to tho 

 lake bad been turned aside to serve as a means of irrigation, so 

 that the beds of some of the riveis were absolutely dry for more 

 than six months in the year. At the period which I now refer 

 to, the water was no longel- used in this way, and the beds of the 

 riveis were full. Thus with the growth of agricultural industry 

 in the Valley d'Aragua, when the extent of cleared surface was 

 continually on the increase, and when great farming establish- 

 ments were multiplied, the level of the water sunk; but by and 

 by, during a period of disasters, happily passing in their nature, 

 the process of clearing is arrested, the lands formerly won from 

 the forest are in part restored to it, and then the waters first cease 

 to fall in their level, and by and by show an unequivocal disposi- 

 tion to rise. 



In crossing the steppe of Baraba, in his way from Tobolsk to 

 Baraoul, M. de Humboldt perceived everywhere that the drying 

 up of waters increases rapidly under the influence of the cultiva- 

 tion of the soil. 



Europe also possesses its lakes; and we have still to examine 

 them from the particular point of view which engages us. M. 

 de Saussure, in his first inquiries in regard to the temperature of 

 the lakes of Switzerland, examined those which are situated at 

 the foot of the first line of the Jura. The Lake of Neufchatel is 

 eight leagues in length, and its greatest breadth does not exceed 

 two leagues. On visiting it, Saussure was struck with the extent 

 which this lake must formerly have possessed; for, as he says, 

 {he extensive level and marshy meadows which" terminate it on 

 the south-west, had unquestionably been covered with water at a 

 former period. 



The effi-ct of forests, considered in this point of view, would 

 therefore, be to keep up the amount of the waters which are 

 jlesfcined for mills and canals; anil, next, to prevent tins rain 



water from collecting, and flowing away with; too great rapidity. 

 That a soil covered with trees is further less favourable to 

 evaporatiuu than ground that has been cleared, is a truth that 

 all will probably admit without discussion. To be aware that it 

 is so, it is enough to have travelled, a short lime after the rainy 

 season, upon a road which traverses in succession a country that 

 is free from forests, and one that is thickly wooded. Those 

 parts of the road that pass through the unencumbered country 

 are found hard and dry, while those that traverse the wooded 

 districts are wet, muddy, and often scarcely passable. In South 

 America, more, perhaps, than anywhere else, does the obstacle to 

 evaporation from a soil thickly shaded with forests, strike tho 

 traveller. In the forests the humidity is constant, — it exists long 

 after the rainy season has passed ; and the roads that are opened 

 through them, remain through the whole year deeply covered 

 with mire; — the only means known of keeping forests-ways dry, 

 is to give them a width of from 260 to 330 feet, that is to say 

 to clear the country in their course. ' 



If once the fact is admitted, that running streams are diminished 

 in size by the effect of telling the forests and the extension of 

 agriculture, it imports us to examine whether this diminution 

 proceeds from a less quantity of rain, or from a greater amount 

 of evaporation, or whether perchance it may be owing to the 

 practice of irrigation. 



I set out with tho priuciple that it must be next to impossibl 9 

 to specify the precise share which each of these different cause* 

 has in the general result. I shall, nevertheless, endeavour to 

 appreciate them in a summary way. The discussion will have 

 gained something, if it be proved that there may be diminution 

 of running streams in consequence of clearing off the forests 

 alone, without the whole of the causes being presumed to act 

 simultaneously. 



With regard to irrigation, it is necessary to distinguish between 

 that case in which an extensive farm has been substituted for an 

 impenetrable forest, and that in which an arid soil, which never 

 supported wood, has been rendered productive by the industry 

 of man. In the first case, it is very probable that irrigation will 

 have contributed but little to the diminution in the mass of 

 running water; it may readily be imagined that the quantity of 

 water used up by a dense forest, will equal, at all events, if not 

 exceed, that which will be required by any of the vegetables 

 which human industry substitutes for it. In the second case, 

 that is to say, where a great extent of waste country has been 

 brought under cultivation, there will evidently be consumption of 

 water by the vegetation which has been fostered upon the surface ; 

 agricultural industry will thus tend to diminish the quantity of 

 water which irrigates a country. It is extremely probable that 

 it is to a circumstance of this kind that we must ascribe tire 

 diminution of the lakes which receive so large a proportion of tho 

 running streams in the north of jtsia. It is almost unnecessary 

 to add, that in circumstances of this kind the effect which is duo 

 to the simple evaporation of rain water is not increased ; the loss 

 by this means must be rather less, because from a surface covered 

 with plants evaporation takes place more slowly than from one 

 that is devoid of vegetation. 



In tho considerations which I have presented upon the lakes 

 of Venezuela, of New Granada, and of Switzerland, the diminution 

 may be directly ascribed to a less mean annual quantity of rain ; 

 but it may, with equal reason, be maintained to be a simple 

 consequence of more rapid evaporation. 



There are, in fact, a variety of circumstances, under the influence 

 of which the diminution of running streams can be shown to be 

 connected with more active evaporation. I shall confine myself 

 to the mention of two particular instances, one noticed by 



