Determining water needs for crops from climatic data

The best measure of water use by crops is found by soil sampling to obtain the changes in soil moisture. Reproducible results are obtained by this method, if there is a long, rainless period during the growing season, and if the irrigations are far enough apart so that at least three samplings may be obtained between waterings. The difference in evaporation between black and white atmometers has shown a high correlation with the use of water by crops and also with the solar radiation as measured by an Eppley pyrheliometer. Loss of water from a given area is associated with the energy received from the sun, and the ground coverage of the plants as long as the soil moisture is above the permanent wilting percentage. Rates of use of water by crops may vary from year to year, but if the ground coverage is equal, the use is the same in any given year in spite of the size of the individual plants. Large differences in loss of water by crops may occur if the coverage of the ground by the plants differs. The correlation between evaporation from a white atmometer and from a standard United States Weather Bureau pan suggests that the former could be substituted for the latter for measurement of evaporation.