Conditions Modifying Duty of Water 207 



with all cultivated crops where the soil is not completely shaded 

 by the plants on the ground. 



6. The closeness of planting is another factor which affects 

 the duty of water when this is expressed in terms of land served, 

 rather than in terms of crop produced. This is particularly true 

 in climates where a rainy season contributes a considerable por- 

 tion of the moisture needed to produce a crop ; because if one is 

 contented with a small yield per acre, a comparatively thin stand 

 upon the ground, with thorough tillage, may often be brought to 

 full maturity with a relatively small amount of water applied 

 by irrigation, thus making the duty of water. to appear very high, 

 whereas if the plants were made to stand as closely as the sun- 

 shine would permit, much more water, when expressed simply 

 in acre -inches, would be required. The real duty, however, might 

 be even higher in the second case, when expressed in terms of 

 yield per acre. 



7. The fertility of the land is still another factor which 

 affects the duty of water, tending to make it appear less the 

 richer and more fertile the soil is, when the standard of com- 

 parison is the unit area rather than the yield of crop. This 

 apparent decrease in the duty results from the larger evaporation 

 of water which takes place from the more vigorous growth of 

 vegetation, and the closer stand which the larger amount of 

 available plant-food renders possible. In such cases as these, 

 however, the real duty of water is higher on the most fertile soil, 

 when this is based upon the actual yields per acre ; not so much 

 because the plant uses the water more economically, as that the 

 necessary loss from the soil itself is relatively less with the large 

 yield than it is with the small yield per acre. The loss from the 

 soil direct may even be actually larger with the smaller crop on 

 the ground, on account of a less complete shading and stronger 

 air movement close to the surface. 



8. The frequency of applying water also modifies the quantity 

 which will be used during a season. This may be true even 

 when the greatest skill is exercised in the application of the water. 

 In the first place, too frequent application of water in small 

 quantities at a time not only increases in a marked degree the 



