FIG. 3. Spatial distributions of the linear trends of (a)
Rmax, (b) R>10mm, (c)
R95p and (d) R<10mm over the
Loess Plateau during the presummer season from 1979 to 2018.
The FDR-adjusted Mann-Kendall test is applied to detect the significance
of the trend of the four presummer precipitation indices during
1979–2018 at each grid point over the LP. The black dots indicate the
grid points with the trend passing the significance test at 90%
confidence level. In general, the extreme precipitation increases in the
northwest and decreases in the southeast of the LP (Fig. 3), which is
basically consistent with the precvious studies (Zhao, Zhai et al. 2017,
Zhao, Li et al. 2017). However, significant differences are found among
the six UAs. Xi’an, Taiyuan and Luoyang are located in the Fenhe and
Weihe River Basin in the southeastern LP, with the fastest urbanization
development over the LP (Fig. 2). Under the climate background with
decreasing precipitation, the extreme precipitation decreases more
severely in Taiyuan UA and the rural areas. The rates of
Rmax, R>10mm and
R95p in Taiyuan UA are 0.2 mm·year −1,
0.4−0.6 mm·year−1 and 0.4−0.5 mm·year −1,
respectively, which are lower than those in the rural areas. The most
significant effect of urbanization is found in Taiyuan UA (Fig. 3c),
where the decrease rates of R95p in the urban and the
rural areas presents clearly different distributions. In obvious
contrast with Taiyuan, Luoyang is under a climate background with
increasing precipitation, and an obvious center with increasing extreme
precipitation is found in Luoyang UA. The increase rates of
Rmax, R>10mm and
R95p in Luoyang UA are 0.3 mm·year −1,
0.7 mm·year −1 and 0.9 mm·year −1,
respectively, which are higher than those in its rural region. Xi’an is
different from the two above UAs.
Xi’an UA is in a wet environment, but a dry core is formed in the UA
center due to the urbanization effect, which is obviously reflected in
the distribution of Rmax and R95p (Figs.
3a and 3c). The extreme precipitation rate decreases by 0.4−0.6 mm·year−1 in the center of Xi’an UA compared with that in the
rural areas. Hohhot and Xining are two UAs in the northern and western
parts of the LP, respectively, where the precipitation has increased
significantly in recent years. In the past two decades, the urbanization
of Xining has been relatively slow, but it is particularly sensitive to
the urbanization as a plateau city. The extreme precipitation in Xining
UA is more concentrated in the center, especially for
Rmax and R>10mm. Hohhot has
witnessed rapid urbanization in recent years, and its extreme
precipitation characteristics are similar to those of Xining. The trend
distribution of the number of light rain days is one of the most
sensitive indicators for the urbanization effect. Different from the
trend of extreme precipitation, the number of light rain days in the
five UAs consistently decreases except for Luoyang, which corresponds to
the change in the artificial impervious area (Figs. 2a and 4d).
The trends of extreme precipitation in Taiyuan, Luoyang, Hohhot and
Xining UAs almost pass the
significance test at 90% confidence level, while that in Xi’an does not
pass the significance test, with opposite trend in the urban and the
rural areas. The above analysis shows that the urbanization plays an
accelerator role in the urban climate evolution, and the urban
precipitation has become more intense and sensitive than that in the
rural areas.