In estimating the evaporation to be deducted from the rainfall for the purpose of determining the flow into a reservoir, it is important to bear in mind that the loss from a constant water surface is nearly one and a half times as great as from the intermittently saturated land surface.
With further experience it has become obvious that very few reservoirs are capable of equalizing the full flow of the three consecutive driest years, and each engineer, in estimating the yield of such reservoirs, has deducted from the quantity ascertained on the assumption that they do so, a certain quantity representing, according to his judgment, the overflow which in one or more of such years might be lost from the reservoir.
Having determined this evaporation for the second driest consecutive year and deducted it from the rainfall - which, as above stated, cannot be less than 87% of the mean of 50 years - we may, as shown on fig.
Wide, of the Path river, which should be deducted, as this stream is only the lower reach of the Tocantins.
When this is deducted from the gross profits of $5.60 prices found above, We have a net profit of $3.32 an acre, not an exorbitant one by any means.