The relationships between the daytime CO2 flux density above a wheat crop and both solar irradiance and evapotranspiration are investigatedusing physical and biophysical measurements made in 1971 in Australia. Well-established flux‒gradient relations are used to estimate the hourly CO2flux density using the measured CO2 concentration at heights of 1 m and 2 m above the crop.Principal component regression identifies four key environmental variables (diffuse and beam shortwave irradiance, leaf-area index, humidity deficit) whose variations together explain almost 80% of the variations in the hourly CO2 flux. For the diffuse ratio Sd/Sg ≈ 0.9 (generally overcast skies), where Sd and Sg are the observed diffuse and global broadband shortwave irradiances (wavelength range 285 nm to 2800 nm), respectively, both light-use and water-use efficiencies are found to be around double those for Sd/Sg ≈ 0.1 (sunny skies). For a given value of Sg this doubling in the water-use efficiency is primarily due to the sensitivity of the CO2 flux to the diffuse ratio.Profile measurements of narrowband (350 to 950 nm) irradiance within the canopy confirm that the increase in light-use efficiency is primarily due to the greater interception of radiation in the canopy under an increased diffuse ratio, up to 15 percent in overcast conditions.