Net radiation — soil heat flux relations as influenced by soil water content variations

Net radiation, soil heat flux, incoming and reflected solar radiation, and soil water content were measured during several clear day periods following approximate 10-cm applications of water to loam soils at Phoenix, Arizona, and at Sidney, Montana. The regression of soil heat flux on net radiation changed significantly as the soil dried, with the difference between them being a linear function of the volumetric soil water content of the uppermost 2 to 4 cm of soil. The net radiation-soil heat flux difference for soil in an air-dry state was only about one-half of what it was on the day after irrigation. Techniques discussed allow evaluation of what the net radiation-soil heat flux difference would be under conditions of no surface saturation deficit at any time of year from measurements of net solar radiation, soil water content, and air temperature, thereby improving the utility of many evaporation models. The data also indicate that water content measurements may be replaced by more easily measured soil albedo.