## 4 Discussion
### Improvements of MTL-LUE, MTG, and BTL in Simulating GPP over Mountainous Areas
#### 4.1.3 Improvement of BTL over BEPS  
BEPS is a two-leaf process-based model using biological, climatic, and soil variables to characterize the vegetation photosynthesis process [29; 34], which has been adopted as an effective tool to simulate the carbon and water fluxes over the globe [66; 67]. In the BEPS model, each pixel is assigned the same value of radiation and temperature. BTL  
Figure 8: Relationships between elevation and slope and SVF over Wanglang Reserve. The solid line and ribbon represent the mean and standard deviation values in each elevation range (calculated at the interval of 100 m), which was obtained from the DEM, slope, and SVF maps at the 30 m resolution.  
is an eco-hydroological model that integrates BEPS with a spatially distributed hydrological model, which was designed to obtain accurate estimations of the carbon and water fluxes over mountainous areas [18]. BTL improves the GPP estimation from BEPS over mountainous areas mainly through adding: (1) the effect of elevation on temperature, i.e., lower temperatures in pixels at higher elevation in the watershed than those at lower elevations [52; 53], (2) spatial heterogeneity of incoming solar radiation, i.e., shaded terrain always receives less radiation than the sunlit terrain [38; 39; 40; 41; 86], and (3) the movement of soil water, by assuming that each pixel is connected to its surrounding 8 pixels, i.e., soil water of pixels with a higher elevation in the watershed could move to pixels with a lower elevation (as shown in Figure 7c) [18; 20; 87].  
Results showed that the GPP estimates from BTL had a higher annual value than those from the BEPS model for the forest, shrub, and grass pixels, with the mean value increasing by 264 gC m\({}^{-2}\)yr\({}^{-1}\). Over mountainous areas, the difference between GPP estimates from BTL and BEPS at the watershed scale depended on the combined spatial effect of various environmental indicators on the vegetation photosynthesis process. The capacity of vegetation photosynthesis was not directly associated with incoming solar radiation, temperature, or soil water. For example, when considering the single effect of incoming solar radiation on GPP, if the incoming solar radiation itself is high, a further increase in radiation would only result in a subtle GPP increase due to light saturation [29]. However, higher radiation would increase evapotranspiration and decrease soil water, and thus the GPP would decrease. Therefore, at higher radiation, GPP variability depends on the gap between the GPP increase (resulting from higher radiation) and the GPP decrease (resulting from lower soil water). The ratio between annual GPP estimates from BTL and BEPS was found to increase as SVF and elevation increased, and as slope decreased, possibly due to a higher SVF leading to higher incoming solar radiation, and a lower slope leading to increased soil water [20]. The lower ratio between annual GPP estimates from BTL and BEPS was observed in the aspect range of 90\({}^{\circ}\)-110\({}^{\circ}\), with the possible reason being that the radiation in this aspect range is most suitable for vegetation photosynthesis (i.e., GPP increased a lot by high radiation and decreased little by lower soil water).