Modeling the photosynthetically active radiation in South West Amazonia under all sky conditions

Abstract

Accurate estimates of photosynthetically active radiation (PAR) are critical for the development of realistic models of plant productivity. However, in many areas such as the vast Amazon region of South America, there have been few empirical studies of PAR. Here, we analyzed the relationship between PAR and broadband solar irradiance (R s ) and formulated models to estimate PAR in two experimental sites (pasture and forest) in the Brazilian Amazon. Three different models of increasing complexity were developed based on information from R s (model 1), R s and clearness index (k t ; model 2), and R s , k t , and water vapor pressure (model 3). Estimates of PAR were generated for each season and for the entire year. All models had very high determination coefficients and indices of agreement for both pasture and forest sites. This strongly supports the use of R s and k t to produce robust estimates of PAR. The results obtained by annual models were close than that found by seasonal models, demonstrating that a single annual model is able to estimate PAR, albeit with lower accuracy.

Photosynthetically active radiation and comparison of methods for its estimation in equatorial Singapore

“…3 Seasonal variation of daily PAR/H with daily rainfall than seasonal variations of PAR/H reported in higher latitudes, for instance in Hu and Wang (2014), but closer to the monthto-month variations in the tropics, for instance in Aguiar et al (2012) and Udo and Aro (1999). Our data also showed clear relationships between PAR and H with the proportion of diffuse radiation (H d ) to H (K d ).…”

“…3). That PAR/H is higher in wetter months than drier months is well documented in different geographical locations (Aguiar et al 2012;Jacovides et al 2007;Udo and Aro 1999). This is explained by the preferential absorption of longer wavelengths by water vapor, which reduces H more than PAR (Alados et al 1996(Alados et al , 2000.…”

“…The diurnal variation in PAR/H is variable between sites, e.g. with different degrees of midday depression (Alados et al 1996;Jacovides et al 2004;Udo and Aro 1999), midday increase (Ge et al 2011), slight increasing trend throughout the day (Aguiar et al 2012), as well as inconsistent diurnal changes from month to month (Bat-oyun et al 2012). The most commonly cited factor that explains the diurnal variation is water vapour.…”

“…The second approach used the empirical relationship between PAR and k t of the form described in Aguiar et al (2012), which parameterises PAR as a function of H and k t in a multi-variable equation. We find this to be a useful empirical function given its simplicity, the ability to estimate PAR even when directly measured H is not available and that there are clear relationships between PAR and k t (Aguiar et al 2012;Hu and Wang 2014;Jacovides et al 2007). The incorporation of k t may therefore improve the accuracy of prediction of PAR from H. The equation used, as described in Aguiar et al (2012), follows this form:…”

Relationship between photosynthetically active radiation with global solar radiation using empirical model over selected climatic zones in Nigeria

“…Another widely used method is to estimate PAR from the routinely measured global solar radiation (H) by considering the PAR fraction as a constant for a specific area [15][16]. The range of PAR fractions as reported in the literature suggest the desirability of local calibration to account for climatic and geographic differences such as cloudiness, day length and the diurnal pattern of solar radiation [17]. This problem necessitates PAR estimation by analyzing the characteristics of PAR with direct measured data, developing appropriate models for calculating PAR/H that can work well under various sky conditions in large areas.…”