MEAN MONTHLY EVAPORATIONS. ----------------+-------------------------+------------------------- | | | Chestnut Hill Reservoir,| Mount Hope Reservoir, Month. | Boston, Mass. | Rochester, N. Y. | | ----------------+------------+------------+------------+------------ | | Per Cent | | Per Cent |Evaporation,| of Yearly |Evaporation,| of Yearly | Inches. |Evaporation.| Inches. |Evaporation. ----------------+------------+------------+------------+------------ January | 0.96 | 2.4 | 0.52 | 1.5 February | 1.05 | 2.7 | 0.54 | 1.6 March | 1.70 | 4.3 | 1.33 | 3.9 April | 2.97 | 7.6 | 2.62 | 7.6 May | 4.46 | 11.4 | 3.93 | 11.4 June | 5.54 | 14.2 | 4.94 | 14.3 July | 5.98 | 15.2 | 5.47 | 15.8 August | 5.50 | 14.0 | 5.30 | 15.4 September | 4.12 | 10.4 | 4.15 | 12.0 October | 3.16 | 8.1 | 3.16 | 9.1 November | 2.25 | 5.7 | 1.45 | 4.2 December | 1.51 | 3.9 | 1.13 | 3.2 +------------+------------+------------+------------ Total for year | 39.20 | | 34.54 | +------------+------------+------------+------------ Mean temperature| 48°.6 | 47°.8 ----------------+-------------------------+------------------------- A reference to data of the Weather Bureau will show that annual evaporation as high as 100 inches, or even more, may be expected on the plateaux of Arizona and New Mexico. Other portions of the arid country in the western part of the United States will indicate annual evaporations running anywhere from 50 to 90 inches per year, while on the north Pacific coast it will fall as low as 18 to 40 inches. =171. Evaporation from the Earth’s Surface.=—Data are lacking for anything like a reasonably accurate estimate of evaporation from the earth’s surface. It is well known that the loss of water from that source is considerable in soils like those of swamps, particularly when exposed to the warm sun, but no reliable estimate can be obtained for the exact amount. Nor is this necessary for the usual water-supply problems, since it is included in the difference between the total rainfall of any district and the observed run-off in the streams. Indeed evaporation from reservoirs is similarly included for reservoirs existing when the run-off observations are made. CHAPTER XVI.