Characteristics of oasis change in the Hexi Corridor
Oasis area variation at the river basin and county scales
The distribution of stable oasis in three river basins and seventeen administration regions is shown in Fig. 1a. The total oasis area in the Hexi Corridor has increased from 10,707.7 km2 in 1986 to 14,950.1 km2 in 2015 (Fig. 1b), with an increase factor of 1.4 from the start to the end years and an average annual increase of 140 km2. At the river basin scale, the HHRB has the largest oasis area with 47% of the total oasis area, followed by SYRB with 40%. The SLRB charactered by drier environments has the least oasis area of 13%. The oasis change types in the Hexi Corridor over the last 30 years are mainly “expansion”, which is supplemented by “retreating” (Fig. 1b). The oasis area variation of administration regions during the past thirty years is shown in Fig. 1c. It is observed that the variation tendency of the oasis area at administration regions scale was the same as that on the river basin scale. The oasis areas in Liangzhou District, Ganzhou District, Minqin County, Yongchang County, Suzhou District, and Shandan Country were more than 1000 km2 in most time. Conversely, the oasis area in Jiayuguan District and Sunan County was less than 200 km2.
Variation of oases area in three river basins of the Hexi Corridor during the past thirty years. (a) was generated using ArcGIS 10.3, www.esri.com.
The stable oasis and maximum oasis distribution
The stable oasis was extracted from the area where the oasis exists in all seven periods, and the maximum oasis area was depicted from the area where the oasis existed once in the past thirty years. It can be seen that the stable oasis area is 9062 km2, while the maximum oasis area reaches 16,374 km2, which is almost two times larger than that of the stable oasis.
The stable oases distribute in alluvial and pluvial fans, the river plains in middle reaches, and the catchment area in the lower reaches (Fig. 2). The maximum oases extended from the stable oases, which mainly located at the edges of the alluvial–proluvial fans, low-lying areas next to rivers and ditches, and the oases-deserts ecotone.
The distribution of stable oases and maximum oases in the Hexi Corridor. The map was generated using ArcGIS 10.3, www.esri.com.
The constraints of oasis development
Geomorphological characteristics of oasis distribution
The geomorphological conditions, formed in the geological history period, is critical for the process of oases development. To investigate the possible relationship between limiting factors and oasis distribution, the distribution frequency, which is the ratio of number in specific condition among all oasis raster number, was introduced and the scatter plots and normal distribution fitting curves were plotted. Figure 3a shows the altitude of the oasis is mainly between 1000 m that is near the lowest value in the study area to 2500 m. The elevation of stable (maximum) oasis peaks in 1500 (1450) m, and accounted for 3.5% (4.5%), which suggests that when oases expand, they tend to occupy the lower elevation. The oases are mainly located in the plains along rivers or irrigation canal systems where slopes flatter than 5° (Fig. 3b), most of them are located in the level ground with a slope flatter than 3°. The area of the stable and the maximum oases located in flat place (slope = 0) account 64% and 76%, respectively, which indicated that the oasis expansion mainly occurs on flat ground. The analysis of the oasis on eight slopes shows that the majority of the slope oasis is concentrated in the north slope and the northeast slope, accounting for about 60%, while the east slope and the northwest slope also have a part, accounting for 30% (Fig. 3c). The aspect of slope oasis expansion mainly takes place in sunny slope (Northwest, West, Southwest, south, southeast), which due to that almost all of the shady slope has been covered by oasis. On the contrary, there are many deserts in sunny slope, as long as the necessary moisture conditions will be occupied by the oasis. The different aspects result in varying amounts of solar radiation, which affects evapotranspiration and consequently water balance in the soil. More specifically shady aspects have more moisture for vegetation growth due to less evapotranspiration, on the other hand, sunny aspects experience potentially higher rates of evapotranspiration, supporting less moisture for vegetation growth26. The fitted normal distribution formula of DEM frequency for stable and maximum oases are given in Fig. 3a, the fits reached a significant level (p < 0.001).
Topographic characteristics of oasis distribution: (a) the elevation, (b) the slope, and (c) the aspect distribution frequency of stable and maximum oases. (the black dot represents the distribution frequency of stable oasis, the red dot represents the distribution frequency of maximum oasis, the black dotted line is the fitted curve of the distribution frequency of stable oasis, and its formula is the black one, the red dotted line is the fitted curve of the distribution frequency of maximum oasis, its formula is the red one).
Hydrology restriction of oasis distribution
The distribution frequency of precipitation in stable oasis and the maximum oasis was obtained by superposition analysis of gridded precipitation and oasis distribution. The most stable oasis distributes in the area where the precipitation is between 50 and 300 mm (Fig. 4a), which is limited by the lack of precipitation in the arid region. In fact, such low rainfall is not enough to sustain vegetation27. Plant growth and production are closely tied to shallow hydrologic systems, which is almost all of this from upstream runoff in an arid land. Therefore, the water depth, the sum of precipitation and the available runoff on grid-scale, was induced to examine the impact of hydrology on oasis development. Both the stable and maximum oasis located in the area of the water depth is larger than 400 mm (Fig. 4b). Moreover, the fit curve of maximum oasis is flatter than that of the stable oasis, which suggests that when oases expand, they tend to occupy lower water depth where the land cover was desert due to water shortage. The fitted normal distribution formulas of precipitation and available water depth for stable and maximum oases were shown in Fig. 4, all the fits passed the test (p = 0.001). We suggest that the oasis developed in the area with water depth more than 400 mm, which has important guiding significance to the site selection of oasis development.
The distribution frequency of (a) precipitation and (b) water depth in the stale and maximum oasis (the legend is the same as Fig. 3).
Temperature constraints of oasis distribution
Precipitation and air temperature are two primary climate parameters, which affect vegetation growth by changing soil moisture and heat energy, especially in arid-cold regions28. Using the mean, maximum, and minimum air temperature data, we examined how they are related to the distribution of oases. The results show that the oases distribution relate to mean air temperature ranges between 6 and 10 °C, with a maximum of around 7–9 °C (Fig. 5a), to minimum air temperature ranges between − 14 and − 7 °C, with a maximum of around − 10 to − 8 °C (Fig. 5b), to maximum air temperature ranges between 18–26, with a maximum of around 22–24 °C (Fig. 5c). Figure 5 also shows that oasis expansion occurs where the mean temperature, lowest temperature, and highest temperature are high. When the mean temperature is lower than 6 °C or higher than 10 °C, vegetation growth becomes restricted. The fitted normal distribution formulas of mean, maximum, and minimum temperature for stable and maximum oases were shown in Fig. 5, all the fits passed the test (p = 0.001).
The distribution frequency of (a) mean temperature, (b) minimum temperature, and (c) maximum temperature in stable and Maximum oases (the legend is the same as Fig. 4).
Driving force of oasis variation
The driving forces of oases variation were analyzed quantificationally based on the grey incidence model. The grey relative degrees (GRD) between oasis expansion and population, AWD, and GDP are pervasively high, and the general order is: rural laborer > total population > AWD > Primary industry > GDP > tertiary industry > secondary industry (Table 1).
The GRD of Population, especially for the rural laborer, is the highest. The increase of the nonagricultural population directly stimulated urban residential, commercial, industrial, transportation, and other related industry development. Consequently, urban land expanded in this area. The population growth was a major factor in oasis variation29. During the past 30 years, the population increased from 1.06 to 5.07 million (378% increase), while the oasis area increased from 10,707 to 14,950 km2 (39.6% increase) in the Hexi Corridor. The rise in population will unavoidably lead to an increase in arable land for survival.
Secondly, the GRD between oasis expansion and AWD is pervasively high with the value around 0.9. The water resource including the precipitation and runoff play an important role in the spatial expansion of oasis. The Hexi Corridor located in a typical arid region, where the most vital limiting factor for both vegetation growth and economic development is the limited water resource. Concerning the shortage of water resources, it is difficult to irrigate many newly reclaimed agricultural oases in the Hexi Corridor. That is to say, water resources cannot afford continuous growth. Thus, AWD of oases is significantly positively correlated with the area of oases.
Thirdly, the GRD between economic factors, including GDP, Primary industry, Secondary industry and Tertiary industry, is about 0.6, which is relatively low comparing to that of population, water resource. The GRD of primary industry is highest with a value of 0.7 among the economic factors. Separately, for the type of agriculture oases contains most of the administration regions in the study area, the GRD of the primary industry was considerably higher than that of secondary and tertiary industry, the agriculture was their first driving force. For the resource-based cities and towns, like Jinchuan District, Jiayuguan City, the GRD of secondary and tertiary industry is essentially equal to that of primary industry, the secondary industry and tertiary industry played a vital role in oasis development.
Source: Ecology - nature.com
