## 4 Discussion  
The period 1982-2015 was divided into two shorter sub-periods in this paper to highlight the variation features of various factors within each sub-period. For example, Figure 2 suggests that the major contribution to the vegetation growth in period S comes from period S2 and that implementation of the \"returning farmland to the forest\" policy effectively promoted vegetation growth and ecological construction in the Loess Plateau. In addition, reductions in vegetation in area I of Loess Plateau are mainly associated with some cities located in the northern portion of Loess Plateau (Figure 1), where urbanization is accelerated, and the construction land has increased. Therefore, changes in land use caused an obvious decrease in vegetation in this region over the past 34 years. In the southern portion of Loess Plateau in area I, a perennial open-pit coal mining has caused serious damage to the surface vegetation. [PERSON] et al. (2017) showed that the average growth rate of vegetation in the whole Loess Plateau during the period of 1982-2013 is 0.025/10a. In this paper, the vegetation change rate of the Loess Plateau from 1982 to 2015 is 0.022/10a, which is similar to [PERSON] et al. (2017). In addition, the vegetation growth of the Loess Plateau in S2 period is far greater than that of S1 and the vegetation growth of the whole region of China is mainly related to human activities, such as the implementation of the policy of \"returning farmland to forest\". Figure 17: Spatial distribution of time delay impacts on vegetation growth associated with PWV, RHU, and SSD, where (**a**)–(**d**) refer to 0 month, 1 month before, 2 monthe before and 3 month before, respectively in the thee subfigures. Several studies have been carried out to analyze extreme climate indices during the period of 1982-2013 [10; 19]. Figure 7 shows that extreme temperature indices (TXx) increased rapidly during S and S1, and the values are larger in S1 period than that in S period, but TXx revealed a downward trend during S2. In addition, the order of vegetation increased rate in the Loess Plateau in three periods are S2\(>\)S\(>\)S1. Such a phenomenon may suggests that the increase in TXx is may hindered by improved vegetation conditions. Comparison of the correlations between extreme climatic indices and vegetation index during S, S1, and S2 showed that the decrease rate of extreme temperature cold indices and increase rate of extreme temperature warm indices slowed due to the continuous increase in vegetation cover in the Loess Plateau. We found that the slopes of TX10P, FD0, and CSDI are downward during S2 period but with a slowing rate; these values were lower than those from S and S1. By comparing the Figure 2c,f,j, we found that the vegetation cover in the Loess Plateau increased significantly during S2, and the decrease rate of cold index in the Loess Plateau during S2 period may be associated with the increase of vegetation. These findings also revealed that the upward slower trends of the warm indices of extreme temperature (such as TX90P, SU25, GSL, TMAXmean, and TMINmean) and the increase in vegetation coverage during S2 over the Loess Plateau. This finding may suggest that vegetation exerts a negative impact on extreme event occurrences, which is rarely reported. Additionally, spatial-temporal analysis of PWV is presented in this paper, which has not been previously investigated, in contrast to precipitation, which has been widely analyzed in the Loess Plateau [32; 41; 42; 43]. PWV revealed good consistency with the elevation distribution of Loess Plateau, agreeing with the results of [PERSON] et al. [28] and [PERSON] et al. [44]. Our study suggests that the increase in temperature in the Loess Plateau over the past 34 years has caused the climate to become drier, as indicated by the reduced RHU observed. [PERSON] et al.