USE OF WATER BY SPRIXG WHEAT OX GREAT PLAINS. 31 
inches for wheat, depending upon the season and the locality. Yields 
have a direct relation to the quantity of water consumed above this 
minimum. In general, the quantity required as a minimum to any 
production as well as the quantity above this minimum required for 
each unit increment of yield increases with change of location from 
north to south. 
The study now made shows the incessant and heavy demand of the 
crop for water from the time it commences gi^owth until it matures. 
The peld depends upon this demand being met uninterruptedly. 
When the water to meet this demand is not available in the soil, the 
rate of use is necessarily decreased. Such decrease always compro- 
mises the yield. Fortunately for the success of agriculture, death 
of the plant from a shortage of water is a very indefinite and long- 
delayed occurrence. When field crops suffer distress, they sacrifice 
vegetative growth to seed production and only cease their efforts 
after they have exhausted all available water and produced more or 
less seed. Under conditions where the water supply is normally 
insufficient or very seldom more than sufficient to meet the possible 
maximum demands of the crop under full possible development, it 
is consequently the normal condition for the available soil water to 
be exhausted when the crop matures. If death of the crop were as 
sudden a phenomenon as the exhaustion of soil water, the percentage 
of crop failures would be much higher than it is. 
Abbreviated column headings used in the tables and repeated in 
the text have distinguished between water used from the soil and 
watter supplied by precipitation. Actually, of course, the entire 
use is from the soil, and the division is between the quantity already 
in the soil at the beginning of the period studied and the precipita- 
tion by which this quantity is replenished from time to time. The 
available water in the soil is the reservoir from which the crop 
draws. Its quantity may fluctuate from time to time as the precipi- 
tation is greater or less than the current needs of the crop. So long 
as it is not depleted to the point of exhaustion the crop appears able 
to maintain its normal rate of use indicated by an uninterrupted 
curve. When depletion reaches the critical point marking the divi- 
sion between available and nonavailable water, the rate of use is 
interrupted and the crop suffers. Precipitation replenishes the 
supply, and the use of water is resumed. A sufficiently detailed study 
of this condition would show intermittently low and high rates of 
use. In the present study these are averaged together for consider- 
able periods of time. The result of this is to show a low rate of use 
for the period. Wlien such a condition exists early in the season the 
vegetative growth is either reduced or not developed, and the rate 
of use when water is available is thus reduced. This is the condition 
reflected in the curves that show a low rate of use and a small quan- 
tity of water used. This is the condition referred to by such state- 
ments in the text as '* The crop suffered from drought throughout the 
season." 
Maximum possibilities can be realized only when the water sup- 
l^ly is at all times adequate to pennit use at an uninterrupted rate. 
The present determinations of this rate show that at the northern 
stations the wheat crop in the field requires an inch of water every 
five (^r six days. At the southern stations the demand rises to an 
inch of water every four or even every three days. 
