CHAPTER III. RAINFALL, DRAINAGE AND EVAPORATION. The relations of evaporation and drainage to rainfall must now be studied to obtain some of the data required in estimating the expenditures of energy in growing crops. Experiments to determine the amount of drainage and evaporation from soils have repeatedly been made, but a small number of them, however, have been carried on for a sufficient length of time, especially in the United States, to be of assistance in settling general principles. The conditions that may have an influence on evaporation and drainage are so exceedingly complex, that a detailed examination of the available records which have been collated in the following tables, will be required to obtain results of practical value. As early as 1796 Dr. John Dalton, so well known to chemists as the author of the atomic theory, made a drain-gauge, consisting of a cylinder ten inches in diameter, and three feet deep, filled with soil, with arrangements for measuring the water passing through it. observations for three years (1796-98) showed that on the average twenty-five per cent. of the rainfall was removed from the soil by drainage, and seventy-five per cent. by evaporation. The last two years, grass was allowed to grow on the soil of the gauge, which must have had an influence on the results.* The average annual rainfall at Manchester, where the experiments were conducted, is about thirty-six inches. This form *Men. Lit. Phil. Soc. of Manchester, Vol. V, pt. 2, as quoted in J. R. Ag. Soc., 1871, p. 130. of drain-gauge, known as Dalton's gauge, was adopted by other experimenters, with some modifications of the apparatus, for collecting the drainage water. Mr. John Dickinson, of Abbots Hill, near King's Langley, Herts, England, made experiments with a Dalton's gauge, the results of which may be profitably studied in detail. His gauge was twelve inches in diameter, and three feet deep, filled with a sandy, gravelly loam, on which grass was growing.* The rainfall was measured with a common rain-gauge. The prominent facts recorded by Mr. Dickinson are given in tables 3, 4, 5 and 6, in convenient form to illustrate the observed variations in drainage and evaporation. TABLE 3. AVERAGE RESULTS FOR EACH MONTH FOR EIGHT YEARS WITH DICKINSON'S DRAIN-GAUGE. The heaviest rainfall, it will be seen, was from June to November, and the drainage in the summer half of the year, from April to September, was very small. The average annual rainfall of but 26.6 inches was considerably below the average of the locality for a longer series of years. The comparatively large actual, and percentage of evaporation in the summer months, will likewise be noticed, with the increased drainage in the *J. R. Ag. Soc., 1844, p. 146. winter months, notwithstanding the smaller amount of rainfall. These variations must be attributed, in the main, to the higher summer temperature, which would increase the evaporation from the soil, and lead to a more rapid exhalation of water by the grass in its more vigorous growth. In December, it will be seen, the average drainage exceeded the rainfall for the month, and the evaporation, which is estimated as the difference between drainage and rainfall, falls to zero. Evaporation from the soil undoubtedly occurred, and while the drainage records may be accepted as correct, the estimated evaporation needs an indefinite correction, which will again be noticed in comments on another table. In table 4 the yearly variations in rainfall, drainage and evaporation are given. TABLE 4. ANNUAL VARIATIONS IN RAINFALL, DRAINAGE AND EVAPORATION OBSERVED BY DICKINSON. The annual rainfall varied from 21.10 inches to 32.10 inches, a difference of 11 inches, and in several of the years there was evidently a severe drouth. The annual drainage varied from 6.95 to 17.65 inches, a difference of 10.70 inches. The difference between the rainfall and drainage is accounted for as evaporation, and, on this assumption, the moisture of the soil should be the same at the beginning and the close of the experiments, which may not be the case. This element of error will not, however, materially affect the general averages of the above series of years. The evaporation would, of course, be influenced by the mean temperature and humidity of the atmosphere, especially in the summer months, and the relative vigor of the growth of the grass on the soil of the gauge, besides other conditions which we need not notice here. The lowest evaporation recorded was 13.25 inches in 1840, with the very low rainfall of 21.44 inches, and 13.35 inches in 1836, with a rainfall of 31.00 inches. The highest amount of evaporation was 18.31 inches in 1843, with a rainfall of but 26.47 inches, which is less than the average of the eight years. If these extremes (which we are unable to explain, in the absence of a record of the peculiarities of these seasons, as to temperature, etc.) are omitted as exceptional, we find that in the remaining five years, with a rainfall ranging from 21.10 to 32.10 inches, the evaporation varied from 14.15 to 17.91 inches, a difference of only 3.76 inches, while the drainage varied from 6.95 to 14.91, a difference of nearly eight inches, from which it appears that the evaporation is less influenced by the rainfall than the drainage. The averages by months and years do not, however, bring out all of the facts that are required to explain the real relations of rainfall and drainage, and the record is presented in another form in table 5, which will clear up some of the apparently anomalous results which are noticed above. The remarkable variations in the relations of drainage and rainfall recorded in this table are suggestive. In 1841, the year of highest rainfall, 32.10 inches (or 5.48 inches above the average), there was drainage from the Dalton gauge in but four months of the year, namely, slightly more than half an inch in March, and an unusual amount in the last three months. In the first eight months of the year the rainfall was not quite 1.5 inches above the average for the eight years, and this is |