_Evaporation_ resulting from surface flooding or use in shallow furrows may, again, cause the loss of from 30 to 50% of the water that actually reaches the land; so that in the latter case, between seepage and evaporation the irrigator may lose the effect of three-fourths of the water he pays for. _Loss by Percolation._—Finally, the water may be wasted on the land itself in leachy soils by over-use, _i. e._, it may percolate to a large extent beyond the reach of the roots when the flow is continued too long; as will always be the case when the head (supply) ditches are laid too far apart, so that the water may be wasting into the country drainage just below the upper ditch long before the water in the furrow reaches the lower one; as illustrated in the upper one of the subjoined diagrams. That this will not happen when the head ditches are nearer together, is shown in the lower diagram. [Illustration: FIGS. 46, 47.—Diagram showing loss by percolation when head ditches are too far apart.] The means of avoiding the mechanical losses have already been discussed, and may be summarized thus: tightening of leaky ditches; use of water in deep furrows; and ascertaining the rapidity of percolation (see p. 242) so as to obtain a proper gauge for the time during which water should run, and for the distances at which head ditches or furrows should be placed. The importance of thus diminishing the losses of water is obvious when it is considered that if the duty of water can be reduced to twenty instead of forty or fifty acre-inches, twice the area can be irrigated with the same amount of water, or the cost of water correspondingly reduced. It should be noted that when the land is leachy it may be pure waste to continue the flow beyond a few hours; but the irrigation must then be more frequently repeated. EVAPORATION. Alongside of and supplementary to the best possible utilization of the rainfall and irrigation water, the prevention of unnecessary evaporation has to be considered. Evaporation from the soil’s surface implies not only unnecessary loss of water that should have remained for the use of the crop, but also the depression of temperature which, as a rule, is unfavorable to the best development of vegetation. It is only in case of extreme stress from hot, drying wind that such evaporation and the consequent depression of the temperature of the surface soil can be of advantage to the farmer.