Calculation of canopy resistance using air temperature difference between canopy top and bottom

Abstract: In order to investigate the calculation method of maize canopy resistance under alternative furrow irrigation with water deficit, field experiments were carried out at the experiment field of crop water requirement of Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences in 2009 and 2010 seasons. Leaf stomatal conductivity, solar radiation, air temperature, canopy temperature and humidity, and wind speed were measured simultaneously on sunny day at maize growing stage of seeding, jointing, heading, and filling. Three models of canopy resistance, including the model of Jarvis (1976), Ortega-Farias et al (2004), and Brisson et al (1998), were introduced. The temperature difference between canopy top and bottom (?Tc) was determined as an environmental factor representing the response of leaf resistance to soil moisture with the relationship between maize leaf resistance and the temperature difference between canopy top and bottom (?Tc) with different soil moisture at different growing stages. Therefore, with improving the multi-factors model of Jarvis (1976), the canopy resistance of maize under alternative furrow irrigation with water deficit was developed by introducing a variable of ?Tc, i.e. the canopy stomatal model based on ?Tc. There were four response functions of canopy resistance to environmental factors, i.e. solar radiation, vapor press deficit, air temperature, and temperature difference between canopy top and bottom. These response functions was determined with the relationship between the stomatal conductivity of maize leaf and four environmental factors. Parameters of the environmental stress function of the canopy resistance model developed in this study were optimized using the iterative calculation of the least square method. Parameters of b1, b2, c1, d1, g1, and g2 in the model were 1.12, 380, -0.387, 0.0048, -0.236, and 1.087, respectively. The canopy resistance model was validated with comparison with the model of Jarvis (1976), Ortega-Farias (2004), and Brisson et al (1998), respectively. Results indicated that daily mean canopy resistance calculated using Jarvis model (1976), Ortega-Farias model (2004), the multi-factors model based on ?Tc, and Brisson model (1998) was 355.93, 318.75, 300.61, and 253.42 s/m, respectively. The simulated values of Ortega-Farias model (2004) and Jarvis model (1976) were greater than the measured values by 9.70% and 22.50%, respectively. The calculated values of Brisson model (1998) were lower than the measured values by 12.78%. The calculated values of the canopy stomatal model based on ?Tc were higher than the measured values by 3.36%. Mean absolute error (MAE), standard deviation (SD), and degree of fitting (d1) between the measured values and calculated values of the model based ?Tc were 2.42, 6.77, and 0.89, respectively; and the coefficient of determination (R2) was 0.86, indicating the model proposed had better precision than the model of Jarvis(1976), Ortega-Farias(2004)和Brisson et al(1998). Taking crop, soil and meteorological factors into account, the application scope, data acquisition, and precision of the canopy resistance model based on ?Tc were better than other methods. Therefore, the improved Jarvis model-based on the temperature difference between canopy top and bottom could be employed to calculate maize canopy resistance under alternative furrow irrigation with water deficit.