84 WATER REQUIREMENT OF PLANTS. 
the writers' measurements. Rye has a higher water requirement 
than wheat or barley in every case. The water requirement of oats 
is higher than barley in all cases except in Wollny's determinations. 
Peas gave a water requirement lower than barley in all cases except 
Leather's and the writers'. The water requirement of potatoes was 
lower than barley in all cases except in King's determination. 
DETERMINATION OF THE WATER REQUIREMENT OF CROPS 
UNDER FIELD CONDITIONS. 
CROPS GROWN WITH IRRIGATION. 
widtsoe's experiments. 
Widtsoe (1912) conducted an extensive field experiment for the 
purpose of determining the effect of different amounts of irrigation 
water upon the water requirement. A number of crops were em- 
ployed in the investigations. The results are given in Table LXXIII, 
the amount of irrigation water applied being given in the first column 
of the table. In addition to this irrigation water, there was available 
to each crop the rainfall of the season, which amounted to about 5 
inches, and also the water which was stored in the soil at the be- 
ginning of the season as the result of the winter rains. From earlier 
measurements Widtsoe has calculated the average amount of stored 
water removed from the soil by each of the crops tested. The sum 
of the rainfall and the stored moisture used bj each crop, according 
to Widtsoe's calculations, is given at the bottom of the table. This 
sum was added to each amount of irrigation water applied in calcu- 
lating the water requirement of the crop. This procedure assumes 
that the soil-moisture content was the same at the end of the season 
in all of the plats devoted to a given crop without reference to the 
amount of irrigation water which had been applied during the season 
and which varied from 4 to 70 inches. It is unfortunate that mois- 
ture determinations were not made in each plat at the beginning 
and at the end of the season in order to avoid the uncertainty arising 
from this assumption. 
285 
