3.2. Trends variation of presummer precipitation in UAs
In order to further investigate the effect and contribution of climate
change and urbanization on the presummer precipitation in different
periods, we divide the five UAs into urban and rural areas separately
according to the map of artificial impervious area cover. The UA area is
covered by the artificial impermeable area, and the rural areas are
mapped by using a buffer method. A buffer zone of 20 kilometers around
the urban area is defined as the rural buffers (Fig. 1). The
precipitation differences between the urban and the rural areas are
calculated and compared in this section. The 30-m resolution land cover
map is regridded into a 0.1° GAIA map according to the spatial
resolution of the CMFD dataset. For each grid of the new
GAIA map, if the area of the
original urban grids accounts for over 50% of its total area, it is
defined as an urban grid; otherwise, it is defined as a rural grid. The
precipitation indices are then spatially averaged over the urban and the
rural grids separately, and the trend of each index is calculated based
on the spatial mean during 1979–2018.
The analysis of section 3.1 reveals that the urbanization plays a role
in amplifying the local climate change. Although the climate change is
the main contributor to the urban precipitation trend, the effect of
urbanization has been gradually identified in recent years. We apply the
Student’s t-test to detect the change point of the precipitation over
the LP and divide the study periods into two stages. Most of the extreme
precipitation indices in the five
UAs demonstrate a sudden change
point in the middle or late 1990s (Table 1). This is in accord with the
previous studies indicating an obvious inter-decadal variation of the
precipitation over the LP, with the significant turning point in the
middle and late 1990s. During this period, the UAs had been growing more
rapidly. Therefore, it is inferred that the climate change plays a
primary role in the first stage of the precipitation series, and the
urbanization effect gradually emerges in the second stage.
Figures 4 to 7 and Table 1 display
the evolution of
Rmax, R10mm,
R95p and the light
rain days in the five UAs and their rural areas. In the first stage, the
extreme precipitation exhibits consistent trends in the urban and the
rural areas of the five UAs (Fig. 4 and Table 1). However, the
discrepancies between the urban and the rural widen in the second stage.
The Rmax,
R10mm and R95p keep growing slowly in
both the urban and the rural areas of Xi’an before 2001, however, the
trends of extreme precipitation become discrepant between the urban and
the rural areas after 2001, with increase in the urban areas and
decrease in the rural areas. The Rmax,
R10mm and R95p in the urban and the
rural areas of Taiyuan maintain a decreasing trend, which becomes
steeper in the urban areas than in the rural areas in the second stage.
For example, the trends of R95p in the urban and the
rural areas of Taiyuan after 2001 are −1.09 mm·year −1and −0.06 mm·year −1, respectively (Table 2). The
Rmax, R10mm and R95p in
Xining, Hohhot and Luoyang show a consistent increase trend in both the
urban and the rural areas. In the second stage, the increase rates in
the urban areas get greater than that in the rural areas, all of which
are significant at a 0.10 significance level (Table 2). The linear
trends of the number of light rain days in the five
UAs show similar characteristics
in the two stages, but the variation is significant. In the second
stage, the number of light rain days in the rural areas of Hohhot,
Taiyuan and Xining increases more significantly than that in the urban
areas, on the contrary, that in the urban areas of Xi’an decreases much
more than that in the rural areas. As a result, the number of light rain
days increases less in the urban areas than in the rural areas. The
above analyses illustrate that the climate change is the dominant factor
for the urban precipitation in the first stage, but the urbanization
effect is further revealed in the second stage.