Relation of precipitation to moisture storage and crop yield

Because of the low rainfall in the Great Basin, with 75% falling during the fall, winter, and early spring months, a cropping arrangement of alternate wheat and fallow has come to be regularly practised on dry-farms. From late October to the middle of April, 69.2% of the precipitation for the first winter and 45.1% for the second winter was conserved as soil moisture. Use of moisture by the growing crop, movement of water to depths beyond 6 feet, increased evaporation, and greater tendency for runoff have all contributed to differences between the first and second winters of a fallow-crop cycle. In addition to the variation between the first and second winters, a loss of approximately 30% occurred, due mainly to evaporation an occasionally runoff. During the summer-fallow period rains seldom make any significant addition to the stored soil moisture. Over a whole fallow-crop cycle approximately 30% of the precipitation was conserved. High yields of dry-farm winter wheat in the Great Basin are directly associated with fall emergence. When plants fail to emerge before winter or early spring, the maximum yield is usually not more than 20 bushels. Late crops suffer greater hazards than earlier maturing stands. The deficiency of late emergence usually cannot be remedied by high spring rainfall. The water cost of dry matter and also the acre inches of water used in producing a bushel of wheat seemed to decrease with yield within the limits of the range of moisture involved. Taking an average of both divisions of the study, 756 pounds of water were used to produce a pound of dry matter, 0.454 acre inch of water was used to produce a bushel of wheat; 2.45 bushels of wheat were produced by each inch of soil water plus spring rainfall; and 0.87 bushel was grown by each inch of total water. When acre yield of wheat was correlated with precipitation, it was found that the highest and most significant was with total water available followed by total rainfall. While the correlation of spring rainfall to yield was not high enough to be significant, it is recognized that without spring rain late-maturing stands in certain seasons may result in partial failure.