WATER REQUIREMENT OF CROPS UNDER FIELD CONDITIONS. 87 
relatively higher immediately above the soil surface than over the 
fallow land. The crop in its early stages also dries out the surface 
soil, so that the water content of the upper layers of the cropped plat 
is materially lower than the fallow land. This would tend to reduce 
the evaporation loss from the cropped plat. It would consequently 
appear that the correction term for the evaporation loss from the 
cropped plats is somewhat too large, which would tend to give too 
low a value for the water requirement under field conditions. It will 
be noted that the values obtained in the field experiments are, almost 
without exception, below the mean water requirement obtained from 
the pot experiments. (Table XL, p. 52.) Leather calls attention, 
however, to the fact that in the latter determinations small pots were 
used in a part of the experiments, which tend to give too high a value 
for the water requirement. He also suggests as a possible source of 
error the capillary rise of water from below the 9-foot level in the 
field experiments, but believes the error due to an unmeasured water 
supply from this source to be very small. The field experiments of 
the writers fully support him in this conclusion. 
BRIGGS AND SHANTz'S EXPERIMENTS. 
The writers' measurements of the water requirement of wheat under 
field conditions at Akron, Colo., were based upon the water removed 
by the crop as shown by the difference between the initial and final 
water content combined with the rainfall entering the soil, deter- 
mined by daily sampling. No correction for evaporation was 
attempted. The results are summarized in Table LXXV. The water 
requirement of wheat grown in the field was found by this procedure 
to be higher than that obtained in the pot measurements. However, 
when the rainfall during the growth of the crop was not considered, 
the water requirement of the wheat grown under field conditions 
agreed closely with that obtained in the pot cultures. The writers 
attribute this to the fact that the wheat was drawing its supply of 
moisture from the subsoil at the time of these rains, and failed to 
develop a surface root system in time to absorb the rainfall before it 
evaporated. 
Table LXXV. — Comparison of water requirement of Kubanka wheat crops grown under 
field conditions, 1910 and 1911, according to Briggs and Shantz (1913, p. 41)- 
Character of experiments. 
Water requirement. 
Including rainfall 
during growth. 
Based on 
dry matter. 
Based on 
grain. 
Excluding rainfall. 
Based on 
dry matter. 
Based on 
grain. 
Field experiments, 1910 . 
Field experiments, 1911. 
Pot experiments, 1911 .. . 
700 
862 
2,315 
2,380 
486 
466 
468 ±8 
1,780 
1,614 
1,196±15 
285 
