Estimation of photosynthetically active radiation under an arid climate

Photosynthetically active radiation (PAR) plays a major role in the development of plant canopies but the PAR band of the solar radiation spectrum is seldom measured. Instead, PAR radiation is usually estimated from all-wave global solar radiation and this provides a reasonable alternative. For Qassim (26 degrees 12'N latitude, 43 degrees 42'E longitude and 751m altitude), Saudi Arabia, a model was developed to relate the PAR photon flux to measurement of all-wave solar radiation at the earth's surface. Based on two years (1993 and 1994) of data measurement for PAR photon flux and solar radiation, the conversion factor was found to be 1.86 mu mol/J with a correlation coefficient (r(2)) Of 0.98. (C) 1997 Silsoe Research Institute.