## 4 Discussion  
In this study, the improved water-carbon coupled DTVGM ecohydrological model was used to simulate the ET and GPP in the six catchments of the water-source region of the Xiong'an New Area. Overall, the model was able to capture the ET and GPP variations quite well in the simulation periods, with relatively error of mostly below 10.0% and a high correlation coefficient of up to around 0.9. Compared to the MODIS data, the DTVGM simulation results also show some underestimations in the dry season for ET, while GPP is underestimated in the wet season and overestimated in the dry season, which is relevant to the vegetation, runoff and soil parameterization. The simulated ET is also consistent with the GLEAM ET Product [42] with a relative error of about \(-8.7\%\), which further proved the reliability of the DTVGM results. Some researches show that the large seasonal amplitudes MODIS GPP data is overestimated in the wet season and low values are underestimated. The reason may be due to the artificially high values of MODIS FPAR and the MODIS light-use efficiency approach, which does not account for the saturation of canopy photosynthesis under clear sky conditions [43; 44; 45]. Based on the DTVGM simulation, ET decreases from northeast to southwest and ranges from around 400 mm/year to 500 mm/year, with an average value of 446.67 mm/year. GPP ranges from around 600 gCm\({}^{2}\)/year to 800 gCm\({}^{2}\)/year from northeast to southwest, with a mean value of 702.57 gCm\({}^{2}\)/year. These results also agreed with previous studies that included ET [18] and GPP simulations in this region [28; 46]. Overall, we suppose that the DTVGM ecohydrological model could provide reasonable simulations for ET and GPP in the experimental scenario analysis to identify the dominant impact factors. Figure 13: Attributions of monthly GPP changes in the six catchments. The attributions of ET and GPP changes were evaluated based on ten experimental scenarios. At the annual scale, ET increased by about 2.4 mm/a in the region. The increasing precipitation dominated the ET uptrend absolutely, probably on account of the water limit in the semi-arid region [47]. Additionally, similar results were also found in other semi-arid regions [48]. Meanwhile, rising air temperature and decreasing humidity also contribute to the ET uptrend by increasing the vapor-pressure deficit and then accelerating ET [49]. The elevated atmospheric CO\({}_{2}\) concentration (eCO\({}_{2}\)) and solar dimming are the main causes for the decrease in ET, which is similar to the findings in [19]. It has been widely recognized that eCO\({}_{2}\) tends to induce the stomatal closure, resulting in a decrease in transpiration [50]. Additionally, solar dimming reduces the solar radiation on the land surface and limits the available energy allocation to the latent heat. The decreasing windspeed also contribute to a small decrease in ET. The GPP increases to about 6.0 gC/m\({}^{2}\)/a in the region. The eCO\({}_{2}\), increasing precipitation and rising temperature are responsible for the GPP increases, while solar dimming, decreasing humidity and windspeed contribute to decreases in GPP. These results are also supported in previous studies [21; 51]. The eCO\({}_{2}\) and rising temperature may directly lead to increasing carboxylation rates and GPP. Increasing precipitation promotes the available water resources for plant growth and also contributes to a GPP increase with a sigmoid relationship [52]. Meanwhile, decreasing humidity leads to vapor-pressure deficit increase, which would result in stomatal conductance declines and a slowing of photosynthesis [53]. Besides the negative effects of solar dimming on ET, it would also reduce the photosynthetically active radiation for carbon fix [54]. The results also show that the effects of LUCC on both ET and GPP are slightly negative, at about \(-\)0.2% and \(-\)1.0%, respectively. This may be related to the slight land-use change, less urbanization and human activities in the mountainous regions.