Albedo, fAPAR, Land Surface Temperature, Light Use Efficiency (LUE), NDVI , Precipitation, Solar radiation, Statics and Weather data

Solar Radiation

The availability of solar energy is the main driver for evapotranspiration and biomass production. Unless water availability is limited, places that receive more solar radiation (through latitudinal location, sun angle and/or number of sunny days) are likely to have higher water consumption and higher crop yields. Atmospheric conditions determine how much of the solar radiation that reaches the top of the earth’s atmosphere reaches the land surface.

WaPOR

This intermediate data component calculates the amount of solar radiation (expressed in Wm-2d-1) that reaches the land surface of a specific location on a specific day, based on the combined effect of location, date, local topography and atmospheric conditions. It is delivered on a daily basis for all three levels. Solar radiation values typically range from around 50 (when transmissivity is very low) to around 300 Wm-2d-1. In addition to the daily solar radiation, another data component, the instantaneous solar radiation, is calculated separately. This data component calculates the amount of solar radiation (in Wm-2) at time of satellite overpass and is used as input to compute the instantaneous soil moisture.

Solar radiation is derived from:
- the surface incident shortwave flux (swgdn)GEOS-5 (level 1 and level 2 areas outside the MSG coverage, e.g. Colombia); or
- the shortwave radiation product of MSG (level 2 and level 3 areas in Africa, Asia and Europe); and
- the slope and aspect derived from Copernicus DEM digital elevation data.

Transmissivity is derived from MSG shortwave radiation (or GEOS-5) by also computing the shortwave radiation at the top of the atmosphere. However, because of the low spatial resolution of MSG data we use this coarse resolution transmissivity together with the solar radiation calculations which are done at the resolution of the different levels. The most important inputs for the solar radiation calculations are the slope and aspect which are provided at the different level resolutions (300m, 100m and 20m).

The transmissivity is used as a proxy to distinguish between direct and diffuse radiation. At high transmissivity values most of the solar radiation is attributed to direct radiation and the effects of the aspect and slope of the terrain are much more visible. At lower transmissivity values a larger percentage of the solar radiation is considered diffuse. Diffuse solar radiation is estimated by ignoring the aspect and slope of the underlying terrain.

Table 12: Overview of Solar Radiation data component

Methodology

The amount of solar radiation that reaches the land surface is determined by a combination of factors. Latitudinal position, day of the year and local topography all determine the incidence angle of the sun at a specific location. Topographical features such as slope and aspect can be extracted from a digital elevation model (DEM) and are used to calculate the solar zenith angle to the surface. All these factors are combined to calculate the potential solar radiation for any location on the land surface at a given day.

However, not all the potential solar radiation reaches the land surface. To determine the actual solar radiation reaching the earth’s surface, the potential solar radiation is adjusted for atmospheric transmissivity, a measure of the amount of solar radiation that is propagated through the atmosphere. The transmissivity is derived from surface downwelling solar (sds) radiation measurement which are regularly made during the day by geostationary meteorological satellites. Atmospheric transmissivity can be calculated by comparing the calculated solar radiation at the top of atmosphere with the measured sds radiation.

The atmosphere causes the scatter of a part of the incoming solar radiation. This effect increases as the transmissivity decreases. Under clear atmospheric conditions most of the solar radiation reaches the surface directly, as can be seen by the sharp shade of sunlit objects. Under hazy or cloudy conditions, shades are less sharply delineated as the scattering of solar radiation cause the radiation to come in from different directions. This effect has to be taken into account: the total available solar radiation that reaches the land surface is the sum of the direct and indirect (diffuse) solar radiation. Both are calculated with the transmissivity determining the ratio between them. A diffusion index is calculated which is provided as a function of the transmissivity. The diffusion index is 1 when transmissivity is low, indicating that no direct solar radiation is available, the diffusion index is 0 when transmissivity is high, indicating that no diffuse solar radiation is available. The next step involves the calculation of the solar radiation during different moments of the day. This requires complicated geometry mathematics, particularly for slopes. More detail on this part of the methodology can be found in Allen et al. (2006b).

Solar radiation is calculated separately for all three levels as the inputs are resampled for each level.

The method to produce the instantaneous solar radiation (used as input in the soil moisture processing chain) is also applied at all three levels but differs from the one of the daily solar radiation described above. It is based on the implementation of the Solar Radiation Model r.sun whose detailed equations can be found in Suri and Hofierka (2004).