## 4 Discussion  
Spatial Distribution Characteristics of Spatial and Temporal Variation of the WUE in the Loess Plateau Ecosystem  
The ecosystem WUE takes into account the relationship between the vegetation water consumption and dry material production, and it is an extensive physiological and ecological index used to assess the suitability of plant growth. In the water-scarce Loess Plateau region, it is important to clarify the distribution of the ecosystem WUE under different drought indices, as well as the changes and responses to the influencing factors. This paper investigates the seasonal distribution and changes of the WUE rates of different vegetation types on the Loess Plateau during 2001 to 2020. It was found that the WUE of the Loess Plateau showed obvious spatial heterogeneity in space, i.e., decreasing gradually from southeast to northwest [31; 32] (Figure 4). Seasonally, the WUE of the vegetation started to increase in spring when the temperature started to pick up, the precipitation and sunshine hours kept increasing, and the surface plants entered the growth period [33]. The average temperature in summer was higher than 22 \({}^{\circ}\)C, which was suitable for vegetation growth, with sufficient precipitation and the longest sunshine hours, meaning vegetation growth was vigorous [34], so the vegetation WUE was higher relative to other seasons. Spatially, it was high-value areas concentrated in the forest in the south and east of Loess Plateau. In autumn, the temperature started to drop, the sunshine hours gradually shortened, the vegetation gradually withered [35], and the vegetation WUE started to retract. The autumn WUE distribution was more consistent with the annual average WUE, showing low rates in the northwest and high rates in the southeast, and the high-value areas were mainly distributed in Shanxi Province and southern Shaanxi. The southeastern part of the Loess Plateau cools down slowly in autumn, which is more favorable for vegetation growth.  
Figure 5 shows that the highest WUE was found in the forests of the Loess Plateau, which was due to the fact that most of the forests comprise tree vegetation with larger leaves which have higher vegetation cover, stronger photosynthesis potential, and a well-developed root system, making them more capable of absorbing water from the soil compared to other vegetation types [36; 37]. In spring, when the temperature rises and the vegetation starts to grow, slight droughts do not have much effect on water utilization in forests, so the forest WUE changes are less influenced by the drought index. Crop is more affected by anthropogenic irrigation activities [38], but crop in arid areas can still meet the water consumption needs of crops through anthropogenic irrigation, so this land cover type  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Correlation** & **NDVI** & **Precipitation** & **Temperature** \\ \hline Annual & 0.71 & 0.28 & 0.36 \\ Spring & 0.23 & \(-\)0.43 & 0.43 \\ Summer & 0.46 & 0.32 & \(-\)0.16 \\ Autumn & 0.28 & \(-\)0.46 & 0.31 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Correlations between the vegetation WUE and meteorological factors in the Loess Plateau and the proportion of each grade.  
is least sensitive compared to other concentrated vegetation types. Summer is the season when crop fixes the most carbon, while evaporation is less than for other small vegetation types [39], so the WUE is higher in areas with crop distribution. The grasslands have a high adaptive capacity and are also the vegetation type comprising the largest area on the Loess Plateau, but due to the short root system of the grasslands [21], precipitation has become the main limiting factor, and the grasslands are subject to more severe water stress compared to other vegetation types. The hot temperatures in summer make the grassland WUE distribution more sensitive to drought indices.