Incoming photosynthetically active radiation (PAR) in the Biebrza National Park, north-eastern Poland (Kopytkowo case study)

Photosynthetically active radiation (PAR) is one of the most important ecosystem steering factors. Here we present the results of 9-year (2013-2021) continuous measurements of incoming PAR made at the Kopytkowo site (53°35′30.8′′ N, 22°53′32.4′′ E, 109 m a.s.l.) in the Biebrza National Park in north-eastern Poland.  The area is a unique marsh ecosystem on a European scale. The PQS1 Quantum Sensor mounted at a height of 2.7 m a.g.l. was used to measure PAR with a time step of ten seconds. The data were then averaged into a 5-minute time step used in the study. The results were expressed in terms of photosynthetically active photon flux density (PPFD in µmol·m⁻²^-·s⁻¹^-). The results show that two seasons, corresponding to different PAR regimes in the Biebrza Basin can be distinguished. The first season (the warm part of the year) starts in the second half of March and lasts until the beginning of October. During this period, the development of convective cloudiness influences photosynthetically active radiation values. The winter season, which lasts for the remainder of the year, is characterised by a higher proportion of cloudy days, influencing the reduced values of surveyed radiation. In general, the annual and diurnal PAR course reflects the incoming radiation on the top of the atmosphere and its attenuation in the atmosphere.
For this reason, in addition to the standard climatological PAR characteristics, we have determined the clearness index in the PAR range. We found that this index can be expressed as a hyperbolic function of the Sun's height for the clear sky. Due to the limited availability of PAR measurement data, we have proposed simple parameterization of this element. Three types of models were taken into account: parameterizations based on the values of total solar radiation, parameterizations for a cloudless sky with the height of the Sun as an input parameter, and their development for parameterization of daily sums in any weather conditions using the daily air temperature amplitude as an input variable.

Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219 and the University of Lodz under project 4/IDUB/DOS/2021. The authors thank the authorities of the Biebrza National Park for allowing continuous measurements in the area of the Park.