Estimating photosynthetically active radiation (PAR) using air temperature and sunshine durations

Photosynthetically active radiation (PAR) is a necessary input in applications dealing with plant top and underground dry matter production, plant physiology and natural lighting in greenhouses. Unfortunately, a worldwide routine network for the measurement of PAR is not yet established and it is often calculated as a constant ratio of the earth received solar radiation (Rs). Generation of simple models for independently and accurately estimating PAR from global solar radiation or other meteorological data is an important solution to the mentioned problem. In this paper, the ratio of PAR to Rs and their relations were analyzed. Then three global solar radiation estimation models were improved and calibrated based on the measured daily temperature and sunshine duration data for estimating PAR and PAR/Rs in an intermountain region of southern Iran. The average, maximum and minimum values of PAR to Rs ratio were 0.5857, 0.8841 and 0.4224, respectively during study years. Modification and calibration of Angstrom model for estimation of PAR values showed that the value of "a" and "b" coefficients are 0.188 and 0.338, respectively. In addition, the value of Hargreaves model coefficient calibrated as 0.0998 . The values of daily PAR were predicted based on global solar radiation by a simple linear equation. This simple equation constant was determined 0.584 that is very close to summation of Angstrom model coefficients, 0.526, in clear sky condition. The values of MBE, RMSE and NSE confirmed that three investigated models were appropriate for predicting photosynthetically active radiation.