PEECOLATIOIf, AND EVAPORATION. 



45 



Tlic six wettest summers, winters, and years give the following 



Thus, with six summers' rainfall averaging 45 Y>er cent, above 

 the forty-two years' mean, the percolation averaged 233 per cent, 

 above it, and the evaporation only 35 per cent, above it. In the 

 winters and years the difference was much less, but even in 

 the six wettest years, with a rainfall averaging one-third above 

 the mean, the percolation increased more than three times as 

 much as did the evaporation. 



The following averages are given by the six driest summers, 

 winters, and years : — 



Thus, with six summers' rainfall averaging 36 per cent, below 

 the forty-two years' mean, the percolation averaged 90 per cent, 

 below it, and the evaporation only 33 per cent, below it. In the 

 winters the reduction in the rainfall made very little difference 

 to the amount of evaporation, for with a rainfall 37 per cent, 

 below the mean, the evaporation was only 13 per cent, below it, 

 ■while the percolation was as much as 68 per cent, below it, the 

 reduction in evaporation being scarcely one-fifth that in percolation. 

 The average winter percolation being nearly eight times as much as 

 the average summer percolation, we see what a very great difference 

 a small rainfall in the winter will make to the annual percolation. 

 "With about three-quarters the average annual rainfall, the perco- 

 lation is reduced -to nearly half the average, while there is only 

 a reduction of one-fifth in -the evaporation. 



These results may now be applied to the rainfall in the catchment- 

 basins of the rivers Colne and Lea. With a total divergence of 

 17-05 ins. in the rainfall, there is a divergence of 8-20 ins. in the 

 percolation, or rather less than half. Assuming that the difference 



