Patterns and trends of cloud cover and photosynthetically active solar irradiation in southern England: implications for vegetation productivity

Abstract

The aim of the research described in this thesis was to investigate the spatial and temporal variability of photosynthetically active radiation (PAR) over the UK, in particular, to estimate both global PAR and the fraction of diffuse PAR at spatial and temporal scales appropriate for use with plant productivity models. In the UK, the spatial and temporal variation of PAR is primarily controlled by the fractional cloud cover and the solar geometry. Diffuse flux (skylight) penetrates further into the canopy than direct flux, so knowing the diffuse PAR fraction ( ) will improve the accuracy of plant productivity models, especially for canopies with significant 3D structure, such as forest.

The first part of the research investigated a novel sunshine sensor, the Delta-T Devices BF3, to test whether this simple low-cost instrument was an adequate substitute for the instruments normally used to measure the components of PAR. This was the first independent test of this instrument and it concluded that the BF3 was highly suitable for this purpose.

The main study developed and tested a method to map the amount of incident PAR ( ) and the diffuse fraction ( ), based on satellite sensor data. The main systems used were the Meteosat Visible and Infrared Imager (MVIRI) on board the Meteosat First Generation (MFG) satellite and the Spinning Enhance Visible and Infrared Imager (SEVIRI) on board the Meteosat Second Generation (MSG) satellite.

The resulting maps of global and diffuse PAR over the whole of the UK were validated using ground data from meteorological stations and instruments at eight FLUXNET sites around the country. It is estimated that global hourly PAR was accurate to < 50 (RMSE) and diffuse PAR fraction to <10 % (RMSE). This is the first time these variables have been mapped at moderate spatial resolution (1km) over the whole of the UK.

The Forest LIGHT (FLIGHT) model (North, 1996) was used to study the influence of and on forest canopy photosynthesis. The effect of diffuse PAR fraction on gross primary productivity was clear. With the same overall level of PAR, a forest canopy under ‘direct and diffuse’ illumination had an increase in GPP around 12 % compared to one under direct illumination only.

One of the major issues faced in this research was the lack of adequate ground data for validation. The research has shown that both the amount of PAR and the diffuse fraction are important factors in forest productivity, and that the Delta-T Devices BF3 instrument is well-suited as a source of validation data for a national network to monitor the gross primary productivity of forests in the UK.

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