## 3 Results and Discussion
### Relationship between T/ET Ratio and Vegetation  
All the sites show different patterns of relationships between T/ET ratio and vegetation index (Figure 9). Arou and Daman represent the synchronous variation of T/ET ratio and NDVI as T/ET ratio grows and decline with increase and decrease of NDVI, respectively. The seasonal dynamic of T/ET ratio and NDVI are both single peak curve and spiked simultaneously. A similar synchronous variation in T/ET ratio and NDVI is shown by Sidaoqiao and Xiaiazi at the beginning and end of the growing season when compared with Arou and Daman; however, NDVI remains at a certain level while the T/ET ratio fluctuates in the middle of the growing season. In the case of the Xinier site, the relationship between the T/ET ratio and NDVI is quite different in that the variation of T/ET ratio seems to be irrelevant to NDVI. In general, four out of five sites show a positive and significant correlation between NDVI and T/ET ratio (Figure 9). Arou and Daman demonstrate an incredibly strong correlation between NDVI and T/ET ratio, with \(R^{2}\) values of 0.91 and 0.92, respectively, which indicates that vegetation has a very strong controlling impact on ET partitioning. Sidaoqiao also shows a positive correlation between T/ET ratio and NDVI with an \(R^{2}\) value of 0.56. At Arou site, the strong positive correlation between NDVI and T/ET ratio can be well fitted with a quadratic function, indicating a strong controlling impact of NDVI over T/ET ratio. When NDVI is low, the increase in NDVI leads to a slow increase in T/ET ratio; however, when NDVI reaches a certain value (about 0.4), the T/ET ratio increases rapidly with the increase of NDVI. Arou site is covered by alpine and mountain meadow and characterized by intermediate precipitation and low temperature. Grass grows slowly at the beginning of the growing season due to the low temperature and consequently there is low vegetation cover; most of the precipitation is allocated to soil evaporation and the T/ET ratio increases slowly at the beginning of growing season. However, grass develops rapidly as temperature increases, resulting in a rapid increase in the T/ET ratio. Figure 8: Frequency distribution of weekly T/ET ratio. In the case of Sidaoqiao, the T/ET ratio increases with the increase of NDVI but with a different pattern compared with Arou site. From Figure 10, it is seen that the T/ET ratio increases rapidly and slowly when NDVI is lower and higher than a certain value (around 0.4), respectively. Sidaoqiao is located in an extremely arid region where precipitation is the main factor driving plant growth. Once there is sufficient soil moisture accumulation for plant growth, the sparse desert shrubs on this site recover quickly, leading to a rapid increase in transpiration. However, as the amount of precipitation is low in this site, it cannot afford a high density of vegetation. Consequently, when NDVI reaches a certain value, shrub growth is constrained; thereby transpiration is limited and increases slowly or remains at a certain level. This is also consistent with the study of [PERSON] et al. (2019), which stated that in semi-arid areas the increase in woody plants often results in a decrease in vegetation cover, which leads to a decrease in T to ET. Figure 9: Seasonal dynamic of weekly T/ET ratio. When NDVI is low, the Xiaiazi site shows an increase in T to ET with an increase of NDVI; however, the T/ET ratio becomes insensitive when NDVI is beyond a certain range (Figure 10). This is shown in Figure 10 when the NDVI is about 0.95. The T/ET ratio shows a high level of variation, ranging from 0.2 to 0.8. This might be the reason for the low NDVI, due to which the potential canopy transpiration determined by environmental factors is much higher than the physiological factors, which also indicates that canopy transpiration is limited by physiological factors under low NDVI conditions. Therefore, the canopy transpiration increases linearly with the increase in NDVI, consequently leading to a rapid increase of T/ET ratio.