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Role of heat storage in evaporation studied; observed data show that when Winnemucca Lake approached dryness and adjacent Pyramid Lake still exceeded depth of 200 ft, annual evaporation from 2 lakes was same; however, monthly quantities differ as result of heat storage in deeper lake.

Direct penetration of rainfall to the ground-water has long been recognized as an important source of many ground-water supplies. This subject, with the results of experiments begun in 1833, was discussed in the Proceedings of the Institute of Civil Engineers in 1850 [see 1 of references at end of paper]. The portion of the rainfall which penetrates to the ground-water is the residual item after all other uses of rainfall have been met. It is the part of the rainfall which is absorbed by the soil and not used as plant-transpiration or soil-moisture evaporation. Such deeply penetrating rainfall is equivalent to the permanently absorbed water as defined by Lee [2]. Direct accretions to ground-water from rainfall, as the term is used herein, is limited to such accretions from the area on which the rain falls. Even with small amounts of rainfall there is always some concentration of runoff in low areas from which some deep penetration may occur. This may be an important item in some areas; it is small on flat areas, for small rains or on coarse soils.

Much work has been done on measurements of evaporation from tanks and the coefficients which should be used to reduce such tank-observations to the evaporation from large water-surfaces. This work has made available fairly dependable methods and records from which the probable evaporation from large water-surfaces in nearly all areas can be estimated as closely as may be essential in most water-supply studies. However, all such methods involve factors of adjustment and are necessarily less satisfactory than direct records from such large water-surfaces.