Spatio-temporal evolution and influencing factors of water resource carrying capacity in Shiyang River Basin: based on the geographical detector method

In this article, the comprehensive evaluation of water resource carrying capacity (for short WRCC) in Shiyang River Basin is the objective. Based on the comprehensive evaluation model of regional WRCC, the spatial and temporal change characteristics of the WRCC in Shiyang River Basin in 2007, 2011 and 2016 have been analyzed. Moreover, the influence elements of water resource carrying capacity in Shiyang River Basin are detected by the geographical detector. The results show that: (1) In terms of the spatial dimension, water resources in Shiyang River Basin are not in accordance with the distribution of social and economic development and population distribution, presenting a prominent contradiction between supply and demand of water resources; (2) in terms of the temporal dimension, the pressure on water resources in Shiyang River Basin has gradually increased, and the area of overload, light overload and serious overload has been augmented; (3) the geographical detector indicates that the influences of the water resources system and coordinating system factors are declining, and the influences of the social economy system and ecological factor are obviously increasing. The influences of the interaction with any two factors are stronger than that of a single factor, and the synergistic influences of two factors are aggravated. doi: 10.2166/ws.2020.057 s://iwaponline.com/ws/article-pdf/20/4/1409/705385/ws020041409.pdf Liang-jie Yang (corresponding author) Xiao-rong Yang Wei Wei Jing-hu Pan College of Geography and Environmental Science, Northwest Normal University, Lanzhou 730070, China E-mail: yangljmnx@163.com


INTRODUCTION
Water resources are not only basic natural resources, but also strategic economic and social resources (Fang et  The internal elements of a water resources system are interconnected and interact with each other to form a large complex system. It is necessary to construct an index system to reflect the carrying capacity of water resources (Guo et al. ). WRCC in different regions has been evaluated by constructing index systems and multiple quantification methods in academic circles. There are two principal methods to construct the index system. One is the system analysis method. Aimed at sustainable development theory, the system is divided into subsystems composed of different levels and elements from the perspective of system theory (Su et al. ; Zhou et al. ), taking into overall consideration the complexity and systematization of water resources. The disadvantage of this method is the complex calculation and limited application (Wu et al. ). The other one is the comprehensive evaluation method, relying on the comprehensive evaluation model of WRCC by borrowing the comprehensive evaluation model of ecological carrying capacity, and setting an evaluation standard to give a comprehensive evaluation for regional WRCC.
The main models include: pressure state response (PSR) model (Yan et al. ), driving state response (DSR) model (Wang & Liu ), driving pressure state impact response (DPSIR) model (Chen et al. ), and driving pressure state impact response management (DPSIRM) model (Guo et al. ). However, the systematicity of water resources is ignored in the study, and it is difficult to unify the index selection and evaluation criteria (Wu et al. ). The quantitative evaluation methods mainly include the comprehensive evaluation method (Liu et al. ; Zhang et al. a), principal component analysis method (Xu et al. ; Li & Zhang ), multi-objective analysis method (Fu et al. ), system dynamics method (Dürr et al. ), etc.
The regional water resources composite system is a composite system which is composed of the coupling of society, economy, ecological environment and water resources (Wu et al. ). Taking two cities and five counties in Shiyang River Basin as the research objects, combining the comprehensive evaluation method with the system analysis method, a comprehensive evaluation model is constructed of regional water resource carrying capacity which takes water resources, social economy, ecological environment and coordination system as its subsystems. The spatiotemporal pattern of water resource carrying capacity in Shiyang River Basin was analyzed by using the spatial analysis and visualization function of ArcGIS, and the factors affecting water resource carrying capacity were analyzed by geographic detector. The following problems will be solved: (1) carrying capacity and state between supply and demand of water resources in Shiyang River Basin; (2) the economic and population pressure on water resources in Shiyang River Basin; (3) the coordinated development degree of the water resources composite system; (4) factors influencing the carrying capacity of water resources.
Through the above research, it is expected that a scientific basis will be provided for the sustainable utilization of water resources in Shiyang River Basin and regional coordinated development.

Study site
The study site is located in the east of Hexi corridor of Gansu Province, west of Wushaoling Mountain and at the north foot of Qilian Mountain. It is one of the three major inland river basins in Hexi corridor, between longitudes 101 41 0 -104 16 0 and latitudes 36 29 0 -39 27 0 . In terms of administrative divisions, it involves two cities and five counties (districts), including Liangzhou district, Gulang county, Minqin county and the north of Tianzhu Zangzu Autonomous County, Yongchang county and Jinchuan district and Yugur Autonomous County of Sunan (for short Sunan county), and the total area of the basin is about 40,578 square kilometres (Yang et al. ; Guo et al. ). In regards to the climate, the region is characterized by little precipitation but strong evaporation, with the average annual precipitation reaching 213 mm (Deng et al. ; Zhang et al. ). The river basin is composed of eight major rivers from east to west, including Dajing River, Gulang River, Huangyang River, Zamu River, Jinta River, Xiying River, Dongda River and Xida River. River water supply is an important source of mountain precipitation supply and snow melt water supply from the mountain. The basin consists of eight large and mediumsized reservoirs. The average annual water resources in the basin are 1.776 billion cubic metres, the average annual surface water resources are 1.5037 billion cubic metres, the average groundwater resources are 2,613.1 million cubic metres, and the average per capita water resources are 734 cubic metres (Shiyang River Basin management ). An overview of the study area is shown in Figure 1.

Data sources
In this study, 19 indicators were selected from the aspects of water resources, society-economy and ecological

METHODS
Comprehensive evaluation model of water resource carrying capacity

Index system design and weight calculation
The water resources system is a composite system of water resources, society, economy and ecology. In the selection of indicators, the population, resources, social, economic and ecological effects on the carrying capacity of water resources should be taken into account to truly reflect the coexistence of the components of the carrying capacity of water resources. In this study, considering principles such as the representative, comprehensive and scientific indicators and data availability, and combining the issues of Shiyang River Basin in arid and semi-arid climate features and the water resources supply-and-demand situation, 19 specific indicators from four subsystems, covering water resources, social economy, and ecology are selected (Table 1), and a water resource carrying capacity comprehensive evaluation index system in Shiyang River Basin is constructed.
Due to the different calculation units and orders of magnitude of each index, it is impossible to directly compare the indices. For the above reasons, using the extreme method for non-dimensional indicators of the original data makes the data comparable between the indicators, and finally the purpose of scientific calculation is achieved (Li et al. ). The entropy value method is used to determine the weight of each indicator after water indicators of the resources composite system are standardized (Table 2 and Figure 2).

Model construction
In order to quantitatively analyze the carrying capacity of water resources, a comprehensive evaluation model of regional water resource carrying capacity will be constructed from four aspects: economic pressure, population pressure, carrying pressure and coordination level. The calculation formula is (Liu et al. ): In this formula, CI stands for the comprehensive evaluation index of regional water resource carrying capacity; E p I and P p I for economic pressure index and population pressure index respectively; α, β for the  Table 3.
This study refers to Liu et al. (), the comprehensive evaluation study on the carrying capacity of water resources in China which was made by Jiajun Liu. The classification standard of the comprehensive evaluation index is determined according to the actual situation of water resources, society, economy and ecology in Shiyang River Basin (Table 4).

Geographic detector
Factor detection is used to quantitatively detect whether a certain geographical factor affects the reason for the difference in the spatial distribution of index values and its weight (Lv et al. ):  σ 2 for the variance of water resource carrying capacity in The water demand of the ecosystem Negative Forest coverage rate (%) (0.0622) The state of regional water conservation Positive COD (t) (0.0269) The chemical oxygen demand Negative The rate of wastewater treatment (%) (0.0448) The level of water conservation Positive  (1) The index value is forward-processed, and the proportion of the evaluation index is transformed (2) Calculate information entropy of index e j ¼ Àk P m i¼1 (P ij × InP ij ) e j stands for the information entropy of each indicator; k ¼ 1/ Inm, and m for the sample region where the index value is located (Wu et al. ) (3) Determine the weight of each indicator W j stands for the weight of each index; n for the number of indices (Wu et al. ) each research region of the city; σ 2 u D,i for the variance of the sub-regional water carrying capacity (Li et  Interactive detection was used to identify the combined actions of different driving factors on the explanatory power of the analysis variables (Wang & Xu ). In the following rules (Table 5) the symbol '∩' means that the X i and X j are taken together (or, the conjunction of X i and X j ). The q(X i ) and q(X j ) denote the effect of the explanatory variables X i and X j , respectively, on soil heavy metal accumulation, and the q(X i ∩ X j ) denotes the q-statistic of the joint (interactive) effect of X i and X j . Specifically, the characterization 'nonlinear-weaken' indicates a small interactive effect of X i and X j compared with their effects considered separately; the term 'uni-weaken' suggests a mild interactive effect that lies between the separate effects of X i and X j ; the term 'bi-enhance' indicates that the interactive effect of X i and X j is bigger than each one of the separate effects of X i and X j ; the characterization 'independent' means that the interactive effect is equal to the sum of the separate effects of X i and X j ; and the term 'nonlinear-enhance' indicates that the interactive effect of X i and X j is stronger than the sum of their separate effects (Yang et al. ).

RESULTS AND DISCUSSION
Comprehensive evaluation and analysis of water resource carrying capacity

Economic pressure index analysis of water resources
The economic pressure on water resources carried in the Shiyang River Basin is relatively large and increasing, with significant spatial differences ( Figure 3 and Table 6). In terms of temporal dimension, the annual average value of the economic pressure index of water resources in Shiyang pressure index of these two places is much lower than the average of the river basin (0.691), which shows that the water demand is far lower than the water supply, so it is a relatively abundant water area. The economic pressure of water resources in Wuwei city is the largest, where the multi-year average of economic stress is 1.21, 63 times more than that of Sunan. The second is Jinchang city   Min(q(X1), q(X2))<q(X1ՈX2)< Max(q(X1), q(X2)) Single factor nonlinear weakening q(X1ՈX2)>Max(q(X1), q(X2)) Two-factor enhancement Nonlinear enhancement (0.93), which indicates that Wuwei city and Jinchang city have great economic pressure on water resources, and the water demand far exceeds the water supply of the region.
The economic pressures of water resources in Yongchang county, Minqin county and Gulang county have always been bearable, and water resources can basically support economic development.

The analysis of population pressure index
The population pressure on water resources in Shiyang River Basin is relatively less, and the trend is downward with obvious spatial differentiation ( Figure 4 and Table 6).
In terms of temporal dimension, the water resource popu- are 2.2, 3.1 and 3.1, which shows that the water resource demand of water is greater than supply, and water shortage is serious, and carrying pressure increases significantly. The The carrying pressure index of Gulang county and Tianzhu county has always been at the level of 1-3, and the average C p I is 1.68, 2.2 respectively. It indicates that water resources, social economy, and ecology have reached a state of mutual restraint and stability, the pressure of water resources is greater than the supporting force, and demand is greater than supply. For Sunan county and Wuwei city, the C p I is less than 1, and the multi-year average respectively is 0.5968, 0.3011, which indicates that the water resource pressure in Sunan county and Wuwei city is less than the supporting force, and water resource supply is greater than demand.

The analysis of coordination index
In 2007

The analysis of comprehensive evaluation index
Basing on the comprehensive index CI of water resource carrying capacity (Table 2)   Yongchang county changed from overloaded to lightly overloaded, and water resources utilization is not coordinated; Wuwei city changed from bearable to overloaded, and the water resources are in the state of slight uncoordination; Sunan county changed from bearable to surplus; Gulang county has changed from slight to overloaded; Tianzhu county has changed from an overloaded area to a bearable area.
Analysis of influencing factors of water resource carrying capacity

Factor detection results of geographical detector
There is a close correlation between the carrying capacity of water resources and the indices of the water resources composite system. Therefore, geographical detector is applied to detect 19 indicators of the water resources composite system and get the explanatory power of each indicator factor of water resource carrying capacity (Zhang et al. b). The factors with significant verification and high explanatory power (>0.5) are selected as the main influencing factors of water resource carrying capacity in three periods (Table 7).  Interaction of influencing factors on water resource carrying capacity The regional water resources composite system is a complex, giant system that is composed of society, economy, ecological environment and water resources. The intensity of the subsystem factors will have a profound impact on the WRCC.
Taking 2016 as an example year, 36 factor pairs are obtained through interactive detection of the nine factors (the influence q-value of these factors is greater than 0.5).
The result shows that the influence of the pairwise interaction of each factor is more than 0.5, indicating that the two factors of enhancing pairs have the stronger synergistic effect. It is

CONCLUSIONS
In this research, the data of water resources and socio-economics from the years 2007, 2011 and 2016 is used to analyze the spatial and temporal evolution characteristics of water resource carrying capacity in Shiyang River Basin, a comprehensive evaluation model of regional water resource carrying capacity is constructed, and the geographical probe method is applied to detect the factors that affect the carrying capacity of water resources. The results show the following: (1) In the spatial dimension, the distribution of water Wuwei city, is on the verge of overload and a slightly overloaded city in which the economy develops rapidly with large population, but with less water resource.
Minqin county belongs to the serious overload, which is far away from water sources and near desert with extreme shortage of water resources.
(2) In terms of temporal dimension, the water pressure in Shiyang River Basin gradually increases, and the areas on the verge of overload, and of slight overload and severe overload increase. The shortage in Minqin county, Yongchang county and Wuwei city worsens.
Minqin county changes from a light overload area to a serious overload area, Yongchang county changes from an overload area to a light overload area, and Wuwei changes from a bearable area to an overload area. The carrying pressure of water resources in Sunan, Gulang and Tianzhu counties decreases slightly, among which Sunan county changes from a bearable area to a surplus area, and Gulang county changes from a light overload area to an overload area. Tianzhu county, which used to be on the verge of overload, has become a bearable area.
(3) The results of geographical detector indicate that the influencing factors of water resource carrying capacity in Shiyang River Basin are different in time, and the root factor of water resources is the sustained supporting force, that the factors of the water resources system and coordination system have weaker influence while the socio-economic and ecological factors are significantly enhanced, and the influence of human activities is enhanced. The interactive results further reveal that the interaction of any two factors is stronger than that of a single factor, and two-factor enhanced pairs have the strongest synergistic effect.
The research area has its particularities including desert, oasis plain and mountain range, which is the ecological barrier area in northwest China. Shiyang River is a typical arid inland river, in which water resource shortage and overload is becoming more and more serious. Strengthening the efficient utilization of water resources, the implementing of the inter-basin water transfer project and storage project will be beneficial for alleviating these problems in Shiyang River Basin. The utilization coefficient of the canal system should be improved and the traditional irrigation method should be ameliorated in the oasis agriculture area. The water resources in upstream water areas (Sunan county and Tianzhu county) are in surplus. Due to the complex topographic and geomorphic conditions, the protection of water resources and the ecological environment would be strengthened to realize the sustainable development of water resources and the social economy in Shiyang River Basin.
(I) The study applied geographical detector to detect the factors that affect the carrying capacity of water resources.
Furthermore, the WRCC in the study area was analyzed more precisely, but the influence factors were not analyzed in other studies microscopically. (II) This paper analyzed the WRCC from the perspectives of socio-economy, coordination, water resources and ecology. In particular, ecology was first studied as an independent indicator. This study has a certain reference value for the WRCC in the Shiyang River Basin and its sustainable utilization in the future.
Although the study analyzes the WRCC in the Shiyang River Basin from a macroscopic perspective, the balance between supply and demand of water resources is not analyzed from a microscopic perspective (such as water footprint). Hence, further exploration is needed in the future.