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

Light use efficiency

Light or radiation use efficiency is a land cover specific variable that denotes how well the vegetation can convert the energy from the absorbed sunlight into biomass, and is expressed in kg dry matter produced per GJ of PAR absorbed.

There are at least three commonly used definitions of photosynthetic LUE based on (1) incident radiation, (2) total absorbed light, and (3) radiation absorbed by photosynthetically active vegetation (Gitelson et al, 2018). In WaPOR we use the last definition with light use efficiency expressed in kilograms (kg) dry matter (DM) produced per Gigajoule (GJ) of photosynthetically active radiation (PAR) absorbed: kgDM/GJPA. In scientific literature light use efficiency is often expressed in gram carbon per Megajoule (gC/MJ). Schlesinger (1991) noted that the carbon content of biomass is almost always found to be between 45 and 50% of dry matter biomass. In the current WaPOR analyses a carbon fraction of 0.45 in organic matter is assumed, which means the conversion is gDM = 0.45 gC, and together with 1kg/GJ = 1 g/MJ, we can convert:

One can distinct real and maximum light use efficiency, where LUE_real = LUE_max * stress (mainly soil moisture and temperature related stress). Since stress is already added to the NPP calculation of WaPOR the LUE used in WaPOR is the LUE_max. The approach does not take into consideration that maximum light use efficiency may not be constant over the cropping season as the physiological conditions of the plant varies with time (e.g. chlorophyl content).

Absorbed PAR (aPAR) is the amount of PAR actually absorbed by canopy layers, taking into account PAR reflected off the canopy back into the atmosphere and PAR reflected off the soil or background material back into the canopy. It is not the same as intercepted PAR (iPAR) which is the amount of PAR caught by various canopy layers as the PAR incident at the canopy top travels down through canopy layers to the ground. The difference between iPAR and aPAR depends on canopy closure, coverage over the background materials, canopy composition, density, and reflectance. If canopy has complete, dense coverage and consists of green leaves, iPAR could be a good proxy of aPAR. For open canopies, iPAR and aPAR may differ.

WaPOR application

Light use efficiency is used as an input to convert the (potential) maximum NPP value to actual NPP.

Methodology

The light use efficiency factors applied in WaPOR NPP calculations are based on the natural vegetation and cropland as classified in the ESA WorldCover Land Cover product. The LUE values have been derived from various sources including the Copernicus Global Land Component – Dry Matter Productivity algorithm theoretical basis document. There are also several scientific papers on light use efficiency values for ecosystems (e.g. Rong Gan et al, 2021, Mingzhu He et al, 2022) and cropland (e.g. Lobell et al, 2002, Gitelson et al, 2018)

The table below gives an overview of the optimized LUE values per World Cover land cover class.

Table 1: WaPOR land cover classes with parameterized Light Use Efficiency (LUE) values for all three levels (derived from the WaPOR NPP data manual v2 document)

Crop specific Light Use Efficiency values can be found in scientific literature. However, these values should be carefully applied, as LUE values do not have a common basis due to the use of different definitions, and can greatly vary (Gitelson and Gamon, 2015). Kooman and Rabbinge (1996) found that the difference in light use efficiency of potatoes between years was larger than between cultivars within a given year, and partly attributed that to the differences in rainfall. According to Monteith (1972) the light use efficiency varies, in the absence of water shortage, only between c3 and c4 crops. This implies that the exact crop type does not need to be known (Bastiaanssen and Ali, 2003).

When the LUE cannot be determined for unknown crops, perennial crops and fallow, a default LUE value 2.7 g/(MJ PAR) is used.

Once the L3 crop maps are available, the NPP can be updated using correction factors containing maximum LUE values specific for each type of natural vegetation (e.g. woodland, shrubland and grassland) and crop type (C3 and C4). The LUE correction factor is the ratio between the actual LUE and the LUE applied for the NPP calculation in WaPOR. The user can multiply the (preliminary) NPP or TBP data with the LUE correction factor.

The procedure to be followed is:

where:
NPP_crop = crop specific Net Primary Production (gC/m²)
NPP_WaPOR = Net Primary Production in WaPOR (gC/<m²)
LUE_max, WaPOR L2 = maximum Light Use Efficiency based on WaPOR L2 land cover (kgDM/GJPA)
LUE_max, crop = maximum Light Use Efficiency of the crop (kgDM/GJPA)
LUE_cf = Light Use Efficiency correction factor (-)

The NPP for pixels classified as cropland or fallow in the L2 land cover classification has been calculated assuming a standard C3 crop with a Light Use Efficiency factor of 2.7 kgDM/GJPA. For cropland pixels the light use efficiency correction factor can be calculated as follows: