Water Lost by Surface Drainage 115 



ment the rainfall upon these types of land by irrigation as, even 

 with the coarse texture they have, to make them bear remuner- 

 ative crops of various kinds, as has been abundantly proved in 

 many places. 



Passing from the extreme type of "barrens" soil which we 

 have been discussing, there are extremely large areas of only the 

 less coarse loamy sands and sandy loams in all humid climates, 

 where supplementary irrigation, could it be practiced, would 

 greatly increase the average yields beyond the largest which are 

 possible with the best of tillage ; but the truth of this proposition 

 does not carry with it the corollary that it will pay to irrigate 

 them whenever there is an abundance of water to do so. 



Then, there are topographic conditions which greatly diminish 

 the effectiveness of the rain which may fall in a given locality. 

 When the fields are decidedly rolling, every one is familiar with 

 the fact that wherever heavy rains occur in short periods of time 

 very considerable percentages of such rains flow at once over the 

 surface to the lower lying lands, producing only damaging effects 

 upon the hillsides. Under such conditions, it is plain that the 

 measured rainfall of the growing season is not available for crop 

 production, even though the texture of the soil were such as to 

 retain the whole of it, could it rest upon the surface long enough 

 to be absorbed. Further than this, the brows of hills, where 

 they are exposed to the prevailing winds, lose a much higher 

 percentage of the absorbed soil moisture by surface evaporation than 

 is the case on the level plains or in the sheltered valleys, and 

 from this it follows that when the whole rainfall of the growing 

 season is only enough to make the soil produce at its full 

 capacity, the exposed hillsides must receive irrigation sufficient 

 to make good the losses by surface drainage and greater evapo- 

 ration, if equally large yields per acre are expected. 



Again, in rolling countries, where the higher lands are 

 porous, the rains which are there lost by deep percolation reap- 

 pear under the lower lands, to supplement the rain which falls 

 directly there, and often to such an extent as to make under- 

 draining a necessity. Where these conditions exist, and where 

 drainage is sufficient, so that crops may take advantage of the 





