Net and solar radiation relations over irrigated field crops

Abstract

Many of the meteorological methods used to estimate evapotranspiration from crop surfaces require net radiation information. Since net radiation data are not generally available and solar radiation is measured at several locations throughout the world, it would be desirable to estimate net radiation from solar radiation. Consequently, a relation was sought between solar radiation and net radiation measured over irrigated field crops of alfalfa, barley, wheat, oats, cotton, and sorghum. Data collected under field conditions were analyzed by linear regression techniques. Standard deviation from regression was 0.02 ly/min for individual days and for individual crops. The regression equations changed from day to day; therefore, were of little value for estimation purposes. Seasonal data for individual crops were pooled and analyzed by linear regression. The resulting standard errors were about twice as large as those for individual days, ranging from 0.02 to 0.05 ly/min. When the data for all crops and all days were pooled and analyzed by linear regression, the resulting standard error was 0.06 ly/min. Thus, estimating net radiation for any of the crops from solar radiation would result in a standard error of 38 ly, or approximately 10%, for a 12-hour day having 649 ly incoming radiation. The regression equation may be solved using either hourly or daily solar radiation data. In the absence of net radiation data either the pooled regression or the individual crop regressions may provide sufficiently accurate estimates for some applications—for example, estimating evapotranspiration to be used in the design of irrigation projects. However, the error appears to be too large where daily evapotranspiration results are required. Therefore, measurements of net radiation are still desirable. Since reflected solar radiation is one of the components of net radiation, a better relation might be expected between net radiation and net solar radiation incoming minus reflected solar radiation). The inclusion of the reflected solar radiation data does not reduce the standard errors. Calculation of albedos of the various surfaces from daily totals of incoming and reflected solar radiation indicated a range of 0.14 over wet soil to 0.24 over dry soil, and up to 0.27 over crop surfaces. The average albedo of crop surfaces was 0.24. Row crops tended to have lower albedos until the maximum canopy was developed; then the albedos were similar to those of continuous crops such as alfalfa. Broadleaf plants tended to have larger reflections than grasses. The crops studied rank in order of increasing albedos as sorghum, wheat, barley, oats, cotton, and alfalfa. Increasing the surface albedo may result in water conservation by reducing the amount of energy absorbed which could be used in evaporation. It appears that the greatest effect could be achieved by increasing the albedo of wet bare soil.