OBSERVATIONS ON DETERMINATIONS OF DROUGHT-INTENSITY. 81 



also the inclusion of the terms just indicated. If we represent the 

 rate of the loss of moisture from all causes by dz 3 /dt, then the rate 

 of saturation d( /dt say, is 



d^ = dz 1 _dz }L , 1() . 



dt dt dt 

 and since this is affected by the circumstances of supply as well 

 as of loss, the one reacting complexly on the other, we must put 



M =^' (t.^B.O.sM.p.[&.T.i*Xo>] y ) (11) 



in which, however, it may be possible, as I have said, from a 

 purely practical point of view, to substitute for [cr.r /-t.S.w] the rate 

 of evaporation from water, de/dt say. 



20. Necessity for the study of elementary cases. — The degree of 

 saturation of the earth's surface to a sufficient depth to include all 

 sensible effects on vegetation, is probably one of the best measures 

 by which to determine the reciprocal of the intensity of drought. 

 It is however not easy practically to assign this limit of depth, 1 

 and this is one of the real difficulties that will occur in applying 

 the results of theoretical investigations and investigations made 

 with well-defined conditions. It must be quite clear from the 

 above discussion, and from the necessity of an expression like (11) 

 to symbolically represent the facts, that the analysis even of 

 elementary cases will be involved in no ordinary difficulty. By 

 studying the reaction of the involved phenomena upon limited 

 areas, whose surfaces, soils, and general circumstances are suscep- 

 tible of somewhat exact definition, something like an intelligent 

 general application of the facts learnt thereby may become possible, 

 provided the range of conditions in the analysed cases be 

 sufficiently wide. But short of such a study sound conclusions 

 seem hardly possible. And, even with the elementary solutions 

 to hand, it will be no easy task to apply them to the complexity 

 of natural conditions. For example the very necessity of limiting 

 our consideration to some particular depth, and the general 



1 See Bull. 121, Univ. Calif. Agric. Expt. Station — The Conservation 

 of Soil Moisture, etc , by Hilgard and Loughridge, 1898. 



F— Aug. 2, 1899. 



