•Vol. XII. No. 2-87. 



THE AGRICULTURAL NEWS: 



I3:J 



SUBSOIL WATER. 



Tbo series of dry seasnns experienced recently in 

 the majority of West Indian colonies naturally cause 

 attention to be drawn to questions of ai>ricultural 

 Avater- supply. In the following article by -Mr. H. A. 

 Terapany, 1! Sc, Superintendent of Agriculture for 

 the Leeward Islands, the priiic;iples which govern 

 agricultural water-supply are described. The fact 

 that, even during times of dronght. a large' reser- 

 voir of water exists below the soil is not always 

 recognized, nor are the laws which govern the movement 

 of the supply towards the roots of vegetation growing 

 above. I'hese, among other points are dealt with in 

 the following account of the subject of subsoil water. 



All supplies of fresh water are in the first instance 

 derived from the atmospheric contleiisatioii of rain (including 

 snow) and dew, which are precipitated on the land surface 

 and serve directly to minister to the needs of plant life. Of 

 the supplies received in this way, a portion soaks into the 

 land; the amount which is capable of being thus absorbed, 

 •will depend on the texture of the soil and the underlying 

 subsoil. When this is coarse and open the capacity of the soil 

 for absorbing water is relatively great. On the other hand, in 

 the case of close-textured soils, in which the finer particles of 

 the clay order of magnitude predominate, the rate of 

 absorption is naturally .slower. In any case, when the rate 

 of precipitation exceeds the absorptive capacity of any soil, 

 the excess of water passes off along recognized channels in the 

 form of surface drainage or .storm water, and unless special 

 facilities exist for conservation, becomes lost to the area on 

 ■vi'hich it falls. 



The water which is absorbed by the soil, however, tends 

 to accumulate and form an underground reservoir. This 

 accumulated supply may conveniently be termed subsoil 

 ■water, and serves as a source from which water is drawn 

 by capillarity from below through the layers of soil to 

 supply the needs of growing plants, supplementary to the 

 amount available from actual precipitation. 



In an article in the Yearbook of the United States 

 l^epartment of .Vgriculture for 1911*, the question of the 

 subsoil water of the central regions of the United States is 

 dealt with, and the considerations presented in the conclud- 

 ing part of the present article are partly derived therefrom. 



In general, subsoil water permeates the soil, subsoil and 

 underlying rocks. The quantity present usually increases 

 from the surface downwards to a point varying in depth 

 according to the prevailing conditions, and after that, 

 diniini"hing as the pressure of the overlying rocks and soil 

 increase.s. Under the conditions dealt with in the article in 

 question, it is estimated that the amount of water contained 

 in the first hundred feet from the surface is equal to one- 

 quarter of the total volume of ordinary porous soil, subsoil, 

 rock. In other words, this is an equivalent to a reservoir of 

 water 2-5 feet in depth, or a supply equal to the total rain- 

 fall over average years under the cocditions in question. 



It will be seen that the aggregate quantity of water thus 

 available is considerable, and it is useful to conceive it as an 

 -ictual reservoir susceptible of increase and diminution, and 

 differing from an open pond chiefly in unevenness of its 

 dipper level, this latter effect being due to obstruction of free 

 movement by the earth within which it lodges. 



* 'Subsoil Wiiter of Central Uiiiied States', by W. ,1. 

 -ilcGoa, I'nited States Uepartmunt of Agriculture Yu.-nbook, 

 1!)11, p. 479. 



Water contained in subterranean reservoirs in this way 

 moves under gravity, tending to flow from a higher to a 

 lower levels at rates which depend on the permeability of the 

 material through which it passes. Moreover, it is subject to 

 capillary movement, and thus unlike free water, the upper 

 level of the subsoil water will tend to conform to irregular- 

 ities of the ground level. 



Under the influence of these two forces the upper level 

 of the subsoil water is indefinite; the moisture content of 

 soils increasing gradually downwards until a point is reached 

 at which the surrounding subsoil or rock is saturated. 



Wells derive their water-supply from subsoil water, a.s 

 also do by far the greater part of normal brooks and river.s 

 (apart from storm water), and the varying levels of these 

 mark variations in the level of the subsoil water by which 

 they are supplied. 



The level of saturation is known as the water-table; it is 

 convenient to distinguish between the level of the saturation 

 which is effective in supplying capillarity, from that (gener- 

 ally somewhat lower) level at which water is delivered freely 

 into wells. The former is termed the 'agricultural water- 

 table', the latter the 'well-water table'. 



The quantity of subsoil water permeating the earth 

 varies to some extent with the' texture and structure of the 

 materials, and these factors also influence its movements to 

 a very considerable degree. Through gravels and sands it 

 flows with a freedom approaching that of open streams, 

 whilst through clays and close-textured rocks it may move 

 only at an imperceptible rate. When the underlying rooks 

 consist of permeable limestone however, large cavities and 

 fissures may in course of time become created, as the result 

 of the solvent action of water containing dissolved carbon 

 dioxide, and thus form those subterranean reservoirs anil 

 channels commonly termed underground rivers and lakes; 

 which receive drainage of the surrounding rocks, and facili^ 

 tate generally the movements of subsoil water. 



The limit of vertical movement of soil water is, of 

 course, the distance between the surface of the soil and 

 the agricultural water-table, but when the latter is 

 removed to considerable depths below the surface, the water 

 available for plant growth in this way becomes increasingly 

 restricted. It may be broadly stated that, under average 

 conditions, capillarity acts freely to a depth of 4 or o feet; 

 fairly, to a depth of 10 feet; and slowly, to 30 feet or more. 



In certain cases, especially those of very heavy low- 

 lying lands, the water-table may approach very near to, or 

 even rise above, the soil surface. Under these conditions, lands 

 become water-logged, and it is then necessary to lower the 

 level of the water table by drainage, to permit of the lands- 

 being utilized. 



In districts liable to suffer from drought it is obviously 

 a matter of importance agriculturally to know whether the 

 reserve supply of subsoil water, available for eking out th'i 

 rainfall, approaches the surface to within 10 feet, or comes 

 within 30 feet, or lies so much deeper as to be beyond the 

 reach of capillarity. 



Moreover, a proper appreciation of the conditions 

 governing supplies of subsoil water is of importance ia 

 relation to attempts to utilize them as industrial and domestic 

 sources of water. 



In the foregoing article the main principles relating to 

 this question have been outlined. It is proposed subsequently 

 to deal with certain aspects of the case having special refer- 

 ence to conditions obtaining in West Indian islands. 



