## 1 Introduction  
Drylands cover more than 30% of the Earth's surface and comprise a variety of ecosystems, such as shrublands and grasslands, which are large reservoirs of carbon as well as potential carbon sinks and sources to the atmosphere [1]. In Central Asia, where drylands cover about 85% of the total territory, shrubland and grassland carbon sinks represent the major pool for carbon absorption and are believed to offset a significant proportion of carbon emissions associated with fossil fuel combustion [2]. Thus, the ability to monitor carbon sequestration in drylands is of great interest in relation to understanding the current status of the global carbon cycle and to meeting requirements of the Kyoto Protocol. The terrestrial carbon cycle is a highly dynamic system that includes several storage pools and different components of the flux. Components of the terrestrial carbon flux are gross primary production (GPP), net primary production (NPP) and net ecosystem production (NEP). The amount of carbon that is fixed from the atmosphere, i.e., converted from CO\({}_{2}\) to carbohydrates during photosynthesis, is called GPP, which is carbon assimilation by photosynthesis ignoring photorespiration. Net ecosystem production, NEP, is the difference between gross primary production (GPP) and total ecosystem respiration (\(R_{e}\)), which determines the amount of carbon lost or gained by the ecosystem without disturbances, such as harvests and fire [3]:  
\[\textrm{NEP}=\textrm{GPP}\textrm{ - }R_{e} \tag{1}\]  
where \(R_{e}\) is the sum of all autotrophic (\(R_{a}\)) and heterotrophic (\(R_{h}\)) respiration over some time period from instantaneous fluxes to annual total and is defined by the following equation:  
\[R_{e}=R_{a}+R_{h} \tag{2}\]  
Net Primary Production (NPP) is an important component of the carbon cycle and key information of ecosystem performance. NPP represents the net new carbon stored as biomass in stems, leaves or roots of plants and defines a balance between GPP and autotrophic respiration \(R_{a}\)[4, 5]. The classic model for the net primary production of ecosystem is given by:  
\[\textrm{\emph{GPP}}\textrm{ (g C/m^{2}/yr)}=NPP\textrm{ (g C/m^{2}/yr)}+R_{a}\textrm{ (g C/m^{2}/yr)} \tag{3}\]  
Analysis of biomass inventory data obtained by field survey provides a capability for wide-area monitoring of biomass production and carbon sequestration using the regression models between remote sensing data and measured biomass data [6, 7]. Another concept is used for the physical-based models, where the key factors for plant development are extracted from remote sensing data to calculate the production of plant biomass [4, 5, 8]. These and other related studies used the approach first described by [9] to calculate biomass by linking the incoming radiation to vegetation production through an empirical biophysical conversion factor - the light use efficiency (LUE). According to [9], gross primary productivity can be calculated as a product of LUE and the amount of photosynthetically active radiation absorbed by plants (APAR):  
\[\textrm{\emph{GPP}}=\textrm{\emph{LUE}}\textrm{ * \emph{APAR}} \tag{4}\]This equation includes both above-ground and below-ground compartments of biomass. In the equation above, GPP can be estimated using remotely sensed data. The current methodology for estimating GPP and NPP from satellites combines the amount of photosynthetically active radiation absorbed by the canopy (APAR) with LUE, most commonly as a biome specific constant, modified taking into account the constraints imposed by climatic limitations through using a climatic stress factor, SI, as part of LUE [3,10-12,13]. APAR depends upon the geographic and seasonal variability of day length and potential incident radiation, as modified by cloud cover and aerosols, and the geometry of displayed leaf material. For calculation of APAR from the photosynthetically active radiation (PAR), the fraction of photosynthetically active radiation absorbed by vegetation (fPAR) is needed.