1. Introduction
The mountain–oasis–desert unit is the main topographic feature of inland river watersheds in China [
1]. For watersheds in this arid region, water from melted snow and glacier in the mountain flows through an oasis similar to a water tower, thereby providing fresh water to people and nature before disappearing into the desert. Through this morphotectonic pattern, as an important landscape, oases support human activities and economic development in arid regions [
2] but are generally water deficient [
3]. Its stability is directly related to the sustainable development of regional economy and ecological security [
2].
In recent decades, the expansion of the irrigation district, population growth, and economic development in the middle-stream oasis of the inland river basin has considerably modified hydrological cycles and reduced surface runoffs to downstream reaches. Consequently, this situation has led to dried-up rivers, reduced outflows to terminal lakes, and the deterioration of the ecosystem in downstream areas [
4,
5,
6,
7]. These effects have manifested in dried-up inland lakes, such as the Aral Sea in Central Asia [
8,
9], Lake Urmia in Iran [
10,
11], and the Shiyang River in Northwestern China [
12]. Thus, assessing whether an oasis is stable under the present development pattern is important in the pursuit of sustainable development. Moreover, evaluating a suitable oasis scale is crucial to the government development plan.
Several studies on stability evaluation and suitable oasis scales have focused on the Endorheic Basin in Northwest of China, specifically, the Keriya River Basin [
13], the Manas River Basin [
2], the Tarim River Basin [
14], the Weigan River [
15], and the Heihe River Basin [
16]. Research methods in stability evaluation and suitable scales have continuously progressed owing to the rapid development of ecological hydrology. In the past, suitable oasis cropland areas were calculated by establishing a regression equation using the total amount of runoff water resources and the cropland area of the oasis [
17]. Wang et al. [
18] applied contrastive analysis to different oasis landscapes and their homologous water–energy balance relationship and proposed the concept of the “green degree” to assess stability evaluation and suitable oasis scales. In addition, a suitable farmland scale model had been established from the perspective of crop water footprint and water resources available in an oasis city [
19]. Recently, Hao et al. [
16] developed an approach for calculating oasis and cultivated land scales by combining water–energy balance and wind–sand dynamic theories with ecological health assessments in the Heihe River Basin.
Agricultural irrigation consumes the largest proportion of the water supply of an oasis, and the suitability of a cropland scale can directly affect the stability of an oasis. Previous research on stability evaluation and suitable oasis scales has generally neglected the impact of changes in planting structure and agricultural water consumption. Additionally, no relevant studies have been conducted on the Shule River Basin, which is adjacent to the Heihe River Basin. The Dunhuang Oasis, which is the study area of this research, is located west of the Shule River Basin and an important region along the Silk Road.
Similar to the case of the middle reaches of the Heihe River Basin, the Dunhuang Oasis also has a sharp contradiction of the water resource issue between the economy and the ecosystem. In the past decade, planting structures constantly changed with the expansion in oasis irrigation areas. Moreover, water exploitation and utilization rates reached nearly 100% in the oasis, in which agricultural water consumption accounted for nearly 90% of total available resources [
20]. Hence, natural oasis ecosystems are unable to receive necessary water resources. Dunhuang City has proposed a water resource plan, namely, the “Comprehensive Planning of the Rational Use of Water Resource and Protection of Ecosystem Services in the Dunhuang Basin” [
21] to improve the ecological environment of its natural oasis. The main purpose of this plan is to allocate large amounts of water resources to the natural oasis in the lower reaches of the Dunhuang Basin and leave only a limited amount of water for the irrigation district of the Dunhuang Oasis.
The objective of this study is to propose a novel research idea to evaluate the stability of the oasis and calculate the suitable oasis scale by combining water–energy balance with altered planting structures. First, this study evaluates stability during the long-term expansion process of the Dunhuang Oasis from 1987 to 2015. Next, it calculates a suitable oasis scale to a certain stability degree. Finally, it discusses to what extent the current water plan can improve the stability of the oasis and provides a scientific reference for the sustainable development of the oasis.
4. Discussion
The planting structure exhibited two marked changes. The first planting structure change happens between crop food and cotton. Except for cotton’s high value and cultivation suitability, the rapidly increasing cotton lands, which were mainly attributed to farmers, had autonomy in terms of land use activities since the early 1980s under China’s economic reform policy. Against this background, farmers envisioned market economy ideas, and production activities were closely associated with market demands. In addition, the trial planting of grapes in the sandy soil region began in 1996. The second planting structure change happens between cotton and grape; this is because of successful grape trial experiments in the sandy soil region, and from then on, a large number of farmers began to install grape trellises in fields previously planted with cotton or wheat. Hence, the planting structure of the oasis changed once again.
Although the total cropland area increased by 18.01 km2 from 2007 to 2015, agricultural water consumption decreased by 0.611 × 108 m3. This result may be beneficial to the transformation of crop patterns and water-saving irrigation measures. Traditional field canal irrigation was the primary irrigation pattern used in the past, but, from the beginning of 2010, advanced water-saving irrigation models, such as micro, pipe, and greenhouse micro irrigation, were implemented in the entire oasis.
Table 5 shows that stability was in a stable level in 1987 and 1990 owing to low agricultural, domestic, and industrial water consumption. During this period, though each industry was gradually developing, stability was not extremely stable, which may have been due to limited water resources. Farmers’ enthusiasm for production and cropland areas increased under reform and opening-up policies, but irrigation measures were difficult. At the same time, tourism in the Dunhuang Oasis began to flourish. When the national economy improved, the stability of the oasis fell to a metastable level from 1996 to 2000 and reached a dangerously unstable level from 2007 to 2010. Water resource exploitation and utilization rates nearly reached 100% in the oasis, in which agricultural water consumption accounted for nearly 90% of overall available resources [
27]. Serious ecological problems, such as accelerated desertification and salinization, shrunken terminal lake and declining groundwater level, accompany rapid economic development [
27]. Water has become the primary restricting and bottleneck factor in the socioeconomic development of the Dunhuang Oasis.
In this case, a series of water resource plans was implemented to address this issue, in which the most important program “Comprehensive Planning of the Rational Use of Water Resource and Protection of Ecosystem Services in the Dunhuang Basin” was proposed. This program aims to reduce the croplands of state farms, implement agricultural water-saving measures, and carry out a water diversion project from the Sugan Lank Basin to the Danghe River. Approximately 0.835 × 10
8 m
3 of water allocated from the water diversion project is intended for the improvement of the ecological environment, which plays a crucial role in alleviating the water crisis by increasing groundwater levels and recovering the vegetation in marginal areas of the oasis [
20]. The system dynamic model simulated the agricultural water consumption under different scenarios in the Dunhuang Oasis and shows that the proportion of agricultural water consumption in overall water consumption can be reduced from 92.50% in 2010 to 86.30% in 2025 [
20], but, if reduced by 168 km
2, to attain a stable level, the agricultural water consumption should be decreased to at least half of what it is now. In this study, the suitable oasis area is far less than the actual area, which is a common issue in the Endorheic watershed oasis in Northwest China [
2,
16,
28]. Reducing cropland in the oasis is the most direct and effective means, which is difficult to achieve. Specifically, individual croplands in the Dunhuang Oasis should be reduced to preserve the ecological environment. However, several reasons highlight the difficulty of this solution. (1) The reduction of croplands will harm the economic interests of farmers and subsequently decrease their quality of life. Thus, the possibility of criminal problems due to poverty should be considered. (2) The reduction of croplands is not in line with the Chinese policy of farmland protection. Hence, only on the basis of maintaining water-saving irrigation, reducing domestic water consumption, improving industrial water consumption efficiency, forbidding sprawl inside and outside the oasis, and increasing the amount of water allocated to the oasis from the water-transfer project, can the stability of the oasis be improved and the sustainable development of the regional economy and ecology be maintained.