## 4 Discussion  
### Comparison of Land Productivity Assessment  
1. Land productivity assessment based on Trends.Earth--the MODIS NDVI dataset (MOD13Q1) was chosen to estimate land productivity in the Han River Basin from 2001 to 2019, which can be implemented in the Trends.Earth plugin of QGIS 3.16 software. It was obtained by integrating three categories of trajectory, state, and performance according to the rules in Table 2. Figure 6 presents the spatial distribution of land productivity in the Han River Basin from 2001 to 2019 using this method. In this study, the state was computed using 2001-2014 as the baseline period and 2015-2019 as the comparison period to compare a region's current productivity with its past productivity. Specifically, three integrated degradation types, including stable, stable but under pressure, and early signs of decline, were combined as STABLE; the integrated degradation type of increasing was classified as IMPROVEMENT; the integrated degradation type of declining was labeled as DEGRADATION. 2. Land productivity assessment based on sustainability pattern--the sustainability analysis of land NPP presented in Figure 5 was taken for land productivity estimation. Specifically, three sustainability patterns of land NPP, including continuously and highly significant increase, continuously significant increase, and reverse continuously significant decrease, were combined as IMPROVEMENT; four sustainability patterns, covering continuously and highly significant decrease, continuously significant decrease, reverse continuously and highly significant increase, and reverse continuously significant increase, were merged as DEGRADATION; two sustainability patterns, involving continuously no significant change and reverse continuously no significant change, were grouped as STABLE. 3. Comparison of the results obtained using the above two methods--the DEGRADATION land estimated by the two methods was broadly consistent, but the assessments of IMPROVEMENT and STABLE presented some differences. (Figure 7). Specifically, the percentages of IMPROVEMENT, DEGRADATION, and STABLE in the Trends.Earth method were 87.05%, 1.92%, and 11.03%, respectively; the percentages of those in the sustainability pattern method were 58.90%, 1.32%, and 39.78%, respectively. Because certain land defined as IMPROVEMENT by Trends.Earth was classified as STABLE by the sustainability pattern method. Figure 5: Spatial distribution of sustainability pattern of land net primary productivity (NPP) in the Han River Basin of China from 2001 to 2019. In addition, this study chose a test area to investigate the above differences (Figure 8). The test region was defined as IMPROVEMENT in the Trends.Earth method but classified as STABLE in the sustainability pattern method. We discovered that the test region is mostly in a more stable state from the Google Earth images in 2001 and 2019. The Trends.Earth method appeared to overestimate IMPROVEMENT and DEGRADATION lands. As a result, the conclusions drawn from the sustainability pattern method were more reasonable. One explanation could be the data source for assessing land productivity. The Trends.Earth method was based on annual NDVI obtained from MOD13Q1, while the sustainability pattern method was based on annual NPP generated from MOD17A3. It is worth emphasizing that, according to the UNCCD-LDN scientific framework [67], the assessment of land improvement or land degradation must exactly correspond to the time period under consideration. In this study, the Trends.Earth compared the current productivity of an area with its past productivity using the period 2001-2014 as the baseline period and the period 2015-2019 as the comparison period. Thus, there may be cases where the assessment leads to an improvement, yet in reality it is still in the process of degradation. Therefore, it is essential to recognize that despite the positive results derived from the Trends.Earth method, land degradation in the Han River Basin could be severe.