862 EXPERIMENT STATION RECORD. 



ical structure of the soil, character of its surface, distribution of the 

 precipitation, and the season. The greater the evaporation and the 

 water capacity and the less the permeability of the soil, the smaller is 

 the quantity of percolating water, other things being equal. For these 

 reasons percolation diminishes in proportion to the increase of fine par- 

 ticles (clay and humus) in the soil or to the decrease of large noncapil- 

 lary spaces, and vice versa. The downward flow of water increases 

 with the thickness of the soil layer up to a certain limit (about 60 cm.), 

 above which an increase in thickness is without influence on percola- 

 tion. In the case of light-colored, loose, and even surfaces the perco- 

 lation is greater than in dark-colored, compact, and uneven surfaces. 

 The covering of a soil with lifeless material (dead plants, stones, coarse- 

 grained soils) causes a considerable increase, while a covering of liv- 

 ing plants causes a remarkable decrease in the amount of percolating 

 water. The influence of the distribution of precipitation on the abso- 

 lute quantity of water carried off from beneath the surface is shown in 

 the fact that the largest quantities of percolating water in bare soils 

 occur in seasons of greatest precipitation. Those districts where the soil 

 freezes and becomes impermeable in winter form an exception to this 

 rule. In such cases drainage is generally susi^euded until the spring. 

 With regard to the ratio of the water percolating through the soil to 

 the quantity of precipitation, it is generally true that relatively the 

 larger proportion of the water supplied is carried away by drainage 

 when the season is coldest. In soils covered with living plants the 

 quantities of percolating water do not follow the course of precipitation, 

 but are greatly reduced by transpiration by plants. For these reasons 

 the greatest percolation occurs in all cultivated regions irrespective 

 of the distribution of moisture in the cold season of no growth, in winter 

 or in spring, according to the predominance of warmth.^ 



THE HEIGHT OF THE GROUND WATER. 



Variations in the height of the ground water depend partly upon the 

 physical structure of the soil,partlyupon the quantity and distribution 

 of precij)itation. In horizontal strata the height of the ground water 

 depends upon the depth of the impermeable subsoil, and approaches 

 nearer to the surface of the soil in proportion to the coarseness of the 



ij. Dalton, Mem. Lit. Phil. Soc. Manchester, 5, part II. J. Dickenson, Jour. Roy. 

 Agr. Soc, 5. Maurice, Bibl. univ. Geneve, Sci. et Arts, 1. Gasparin, Cours d' Agri- 

 culture, 2, p. 116. G. von MoUendorfF, Die Regenverhiiltnisse Deutschlands, 1862. 

 E. Risler, Arch, des Sci. de la Bibl. univ., 1809. F. Pfaff, Sitzungsber. konig. bayer. 

 Akad. Wissensch., 1 (1868), p. 311; 2 (1869), p. 125. J. N. Woldrich, Ztschr. osterr. 

 Ges. fiir Meteorol., 6, p. 97. J. H. Gilbert, Proo. Inst. Civil Engineers, 45, part III; 

 105, part III. E. Ebermayer, Die physikalischen Einwirkungen des Waldes auf Luft 

 und Bodeu, Berlin, 1873, p. 215; Forsch. Geb. agr. Phys., 12, p. 147; 13, p. 1. E. 

 Wollny, Der Eiufluss der Pflanzendecke und der Beschattung auf die physikalischen 

 Eigenschaften und die Fruchtbarkeit des Bodens, Berlin, 1877, p. 137; Forsch. Geb. 

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