Vol. III. No. 47. 



THE AGEICULTURAL NEWS. 



39 



RELATIONSHIP OF WOODS TO DOMESTIC 

 WATER SUPPLIES. 



The fullowing paper, on the ' Relationship of woods 

 to domestic water supplies', taken from the Journal of 

 the Board of Agriculture, Uecember 190;i, gives 

 useful information upon this interesting subject : — 



This subject has, for more than twenty years, occupied 

 much of the attention of Forest Experimental Stations, 

 especially in Germany, France, Austria and Switzerland, and 

 in view of its importance the conclusions arrived at may be 

 usefully summarized. 



It has been asserted, and theoretically the contention is 

 doubtless correct, that masses of woodland increase the 

 rainfall. The causes of this result are sought for in the 

 reduction of temperature associated with forests, and in the 

 greater absolute and relative humidity of the air in woods. 

 But although it may be possible to obtain experimental 

 proof by means of elaborate and long-continued observations 

 in a region where extensive afforestation or deforestation is 

 taking place, it may at once be said that such tree-planting, 

 as is practically possible in Britain, can have ni;> appreciable 

 influence on the rainfall. Trees do, however, under certain 

 conditions of the atmosphere, condense dew on their leaves 

 and branches, and this effect may often be seen in the wet 

 state of the ground underneath trees on a foggy morning 

 when the surface elsewhere is comparatively dry. 



But the case is materially different where the fate of 

 the rain and snow that fall on a tract of woodland is 

 considered. The foliage, branches and stems of the trees 

 intercei>t much of the rain and snow, so that it never reaches 

 the ground at all, the amount so intercepted usually ranging 

 from 30 to 45 per cent, of the total, but much depends on 

 the character of the rainfall and on the species of tree. In 

 a district of heavy annual rainfall a smaller proiiortion 

 of the i)recipitation is caught by, and evaporated from, 

 the trees than where the rainfall is light. Similarly, 

 in the case of heavy and long-continued rain, as contrasted 

 with gentle showers ; in the latter case, in fact, but 

 little of the water reaches the ground through the leafy 

 canopy of a dense forest. Then again, much depends 

 on the kind of tree, evergreens intercepting more water 

 throughout a year than deciduous trees ; and a larger 

 proportion of the rainfall is evaporated from the leaves and 

 branches in summer than in winter. 



But although less rain-water reaches the soil of a wood 

 than finds its way to the ground in the oiien country, the 

 moisture in the soil is much better conserved in the former 

 than in the latter case. This is due partly to the exclusion 

 of the sun's rays by the foliage, jiartly to the absorbent and 

 retentive character of the decaying vegetable matter that 

 covers the ground of a dense and well-managed wood, and 

 partly to the air in a forest being more humid and thus 

 better fitted to discourage evaporation. The lace work of 

 tree roots, too, that occupy the soil of a forest, offers 

 mechanical resistance to the rapid surface-How and percola- 

 tion of water. It is also to be noted that roots penetrate to 

 great depths, and when they die, they leave holes through 

 which water readily penetrates from the surface. The 

 friable condition of the soil of a wood, too, permits ready 

 percolation of water, whereas in the open country the denser 

 character of the surface of the ground is less favourable to 

 the entrance of water. The consequence is that streams in a 

 wooded country are not so subject to rapid rises and falls, 

 the flow being maintained more equally throughout the year. 

 Where water-sujiply for domestic or industrial imrposes 



is concerned, the avoidance of violent freshets on the one 

 hand, and of scanty flow on the other, is alike desirable. Not 

 only may the water of sudden and heavy floods be lost 

 owing to the incapacity of the reservoir to contain it, but 

 such floods have also the disadvantage of carrying much mud 

 and similar jnaterial in suspension, and this gradually silts 

 u[> reservoirs, besides entailing increased expenditure in 

 filtering. 



It may be pointed out that the water of a reservoir 

 surrounded by well-stocked woodland is not subjected to the 

 same amount of violent agitation during gales as is the case 

 where such sheltering agency is absent. The mud and silt 

 deposited on the bottom, and especially along the margin, is, 

 consequently, left comparatively undisturbed, with corres- 

 ponding advantages in the matter of purity. 



When a catchment area is covered with trees, and with 

 the vegetable matter that accumulates on the surface of the 

 ground, the water that reaches the soil as rain is impeded in 

 its flow and its evaporation is hindered, so that the general 

 effect is equivalent to an increase in the size of the reservoir. 

 It is also important to note that snow melts more slowly 

 underneath trees than in the open country, so that at a time 

 of thaw the snow-water is yielded up more gradually. 



Forests not only affect the degree of moisture in soil, but 

 they also exert a considerable influence on the soil 

 temperature. Although this influence is greatest at the 

 surface of the ground, it is also perceptible to a depth of 

 several feet. On the average of a large number of 

 Continental Stations, it was found that woods of various 

 species and ages depressed the mean annual temiierature at 

 the surface of the ground by about i-Q" F., while even 

 at the depth of 4 feet the reduction of temperature was 2". 



This general cooling influence is due to a variety of 

 causes. The foliage of the trees excludes the sun's rays, the 

 decaying vegetable matter that covers the ground prevents 

 the free exchange of air between the soil and the atmosphere, 

 while the water in the soil absorbs much heat without its 

 temperature being much affected. 



While woods have a depressing influence on the mean 

 annual temperature, it is found that this effect is much 

 greater in summer than in winter. On the average of eleven 

 German Stations, the July temperature of the surface soil in 

 the forest was found to be 7" F. lower than that in the 

 open field, whereas in December the former was rather 

 warmer than the latter. Forests, therefore, tend to equalize 

 the temperature of water collected in them, the temperature 

 being slightly raised in winter and markedly reduced in 

 sununer. This result would appear to be of considerable 

 practical and hygienic importance, where a supply of water 

 for domestic purposes is concerned. 



To the credit of forests is also to be placed the fact that 

 they exercise a purifying influence both on the air and on 

 the soil, germs of all kinds being markedly scarcer in a well- 

 wooded district than in a similar extent of treeless country. 



Felling Trees by Electricity. Successful experi- 

 ments are reported from France with regard to the felling 

 of trees by electricity. According to a recent issue of Le 

 Jardin, in various forests the plan has been tried of using 

 a platinum wire heated to a white heat by an electric 

 current instead of a saw. By this means the tree is 

 severed more easily and rapidly than by the older methods ; 

 no sawdust is made, and the slight charring produced by the 

 burning wire preserves the wood. The new principle is said 

 to be eight times as si>eedy as when a saw is used. {Gardeners' 

 Chronicle, January 2.) 



