Simulation of Ecosystem Carbon Flux and Impact of Drought in Liulin Watershed of Taihang Mountains

To accurately simulate the Gross Primary Productivity (GPP) and evaluate the impact of drought on GPP at a regional scale in Taihang Mountains, we calibrated three parameters of maximum light use efficiency, optimum temperature, and maximum temperature in the Vegetation Photosynthesis Model (VPM) by measured GPP data in 2021. The GPP was measured by the eddy flux station of the Liulin watershed in Taihang Mountains. Data from January to August 2022 were used to validate the VPM. Remote sensing data and meteorological reanalysis data were used to upscale GPP of Liulin watershed from 2000 to 2020. Measured precipitation data from precipitation stations in the watershed were used to calculate the standard precipitation index (SPI). The time lag effect and cumulative effect of SPI on GPP were further analyzed. Results show that VPM is applicable to the Liulin watershed, and its performance during the calibration period (R2=0.94, RMSE=0.47 g·C·m−2·d−1) and validation period (R2=0.91, RMSE=0.60 g·C·m−2·d−1) are good. The GPP of the Liulin watershed from 2000 to 2020 was between 446.4–828.1 g·C·m−2·y−1, with an average value of 545.6 g·C·m−2·y−1, and the spatial distribution was high west and low east. The GPP changed in an increasing trend from 2000 to 2020. A mutation of the GPP occurred around 2010, with an average annual GPP increase of 13.9 g·C·m−2·y−1 from 2000 to 2010 (p< 0.05), and the average annual GPP increase of 10.22 g·C·m−2·y−1 (p>0.05) from 2011—2020. SPI can well monitor the occurrence of drought in the Liulin watershed, and SPI showed an increasing trend but failed to pass the significance test of (P<0.05). The time lag and cumulative effects existed, with a cumulative effect of 9 months and a lag effect of 6–7 months. Thus, the VPM has high simulation accuracy in the Liulin watershed, the GPP showed a significant increasing trend, the time lag and cumulative effects indicate that drought has a long-term effect on GPP