## 5 Results
### Evaluation of the modeling results
#### 5.3.1 Qualitative comparison with previous studies  
Intensive efforts have been made to collect all credible published estimates of biomass, GPP and NEP from grassland biomes, particularly in the study region. The method used to predict NPP/GPP in previous studies range from biomass harvesting, simple above-ground NPP-NDVI correlations to complex ecophysiological simulation models, and cover a number of scales from point to regional and global. For qualitative comparison with our results, we compiled a number of published studies carried out in grasslands of Kazakhstan [28, 29, 30, 31, 32, 33, 34]. We also took some studies providing averages of vegetation productivity with regard to vegetation biome and climatic zone, which are calculated globally [60] or regionally [61]. Table 3 gives a summary of the comparison, which we based on the literature survey. The table also contains some studies carried out in grasslands in other regions (USA, Sahel, former Czechoslovakia). Qualitative agreement of productivity estimations in previous studies, particularly in the studies from Central Asia, with our LUE model is good. The compilations of the studies from Central Asia have defined the known range of NPP in the dry steppe biome, from 126 g C/m2/year to 326 g C/m2/year [28, 29, 30] and the semidesert short grassland from 114 gC/m2/year to 220 gC/m2/year [31, 32]. These values are highly consistent with values obtained in the present study, 145 gC/m2/year and 131 gC/m2/year for dry steppe and semidesert grassland, respectively. The NPP estimates for semidesert in Central Kazakhstan by [33] has in some extent higher values, from 90 to 310 gC/m2/year. This may be explained by more favourable precipitation conditions, which were observed in the years of their field investigations. The results of the present study go well with the scope of NPP values averaged for grassland biomes by [60] and [61]. Our results are also consistent with NPP and GPP estimations in other regions such as South of the USA [20] and Sahel in Niger [22]. Of particular interest is a quantitative comparison of NEP estimates. However, as NEP estimations need collection of much more field data than NPP/GPP estimations, they have been carried out relatively not often. Recent literature is characterized by a scarcity of NEP estimates from grasslands in Central Asia. A comparison with other regions revealed that the magnitude of NEP obtained in the present study was, in general, consistent with the NEP values in grasslands of Wyoming, USA, and former Czechoslovakia [20, 62, 63]. Theseresults boost confidence in the NPP, GPP and NEP values obtained in our LUE model with values from the previous studies in the study region and in other regions with similar vegetation classes. However, the comparison of the present results with the results of previous studies should be taken with a caution, because there is a lot of variability within the results of previous studies. Even though, that the mean values of NPP, GPP and NEE from this study are well within the range of previous grassland studies, the large variability within the previous studies puts the comparison into perspective.