Components of the Radiation Balance of Irrigated Plots in a Dry Monsoonal Environment

Abstract

Over irrigated crops of cotton grown in a dry monsoonal environment, net radiation was measured daily over a period of one year together with total radiation, duration of sunshine, and wet and dry bulb screen temperature. Daily estimates of total, effective terrestrial, and net radiation were obtained from several empirical relationships, and these were compared with measured values. The reliability of alternative modes of estimating these components of the radiation balance was assessed. When estimating total radiation from relative duration of sunshine, a hyperbolic relation was found more effective in accommodating low values than the commonly used linear relationship. When estimating effective terrestrial radiation from a relationship used by Penman, it was necessary to derive constants appropriate to this environment using data an clear days. Effective terrestrial radiation was also estimated using a relationship based upon a proposal by Swinbank which does not include a vapor pressure term. Although wet-season estimates were found to be in significantly better agreement with observation when vapor pressure was included, satisfactory estimates can nevertheless be obtained without vapor pressure data. Significantly better estimates were obtained when total radiation was used rather than relative duration of sunshine in accounting for the effect of cloudiness on the downward long-wave radiation flux. Errors in the estimated effective terrestrial radiation were found to be the major contributors to inaccuracy in the estimation of net radiation.