Analysis for spatial-temporal matching pattern between water and land resources in Central Asia

Central Asia, the pioneering place of the ‘Belt and Road’, is under the threat of prominent water issues. Based on the Gini coefficient model and the matching index, the amount of the total renewable water resources and the cultivated land area were introduced to evaluate the matching pattern between the water and land resources in Central Asia. The water problem of Kazakhstan, being the most prominent, shows low water resources per unit area with the highest reclamation rate. The matching degree for the upstream countries of the Aral Sea (Kyrgyzstan, Tajikistan) was better than those of the downstream countries (Turkmenistan, Uzbekistan, Kazakhstan). The Gini coefficient in Central Asia was 0.32, smaller than that of the global average value (0.59). The overall water available for use and the matching cultivated land resources was reasonable. Large differences exist in the matching degree in water distribution and utilization among Central Asian countries. The matching index of water and land resources in Central Asia was 1.25, similar to the matching degree estimated from the Gini coefficient model. Moreover, rational measures are suggested to alleviate the issue of water and land resources matching in Central Asia.


INTRODUCTION
Central Asia, located in the hinterland of the Eurasian continent, lacks water resources (Gafurov et al. ). It is one of the regions threatened by serious water problems in the world (Unger-Shayesteh et al. ). The economic benefits of water use in Central Asia are lower than those in other areas of Asia (Lee & Jung ). Although the total amount of water resources is seemingly large in this region, in fact, only 24.4% is available for use by humans (Yang et al. ). On top of that, multiple transboundary rivers exist in Central Asia (Zhupankhan et al. ).
However, the mismatched spatial distributions of water and land resources, along with the intense human activities (e.g., overexploitation of water resources), has ultimately led to the serious water crisis in Central Asia's river basins. This is the main reason for ongoing water conflicts in the region's transboundary rivers and the ecological disaster of the Aral Sea (Chen et al. ).
Kyrgyzstan and Tajikistan, located in the upstream of the Aral Sea Basin, have the most abundant water resources and are called the 'water towers' of Central Asian countries. However, due to insufficient water conservancy engineering measures, water resources have flowed into downstream countries, so the amount of available water resources in upstream countries is limited. Meanwhile, the downstream countries of the Aral Sea Basin (Kazakhstan, Turkmenistan, and Uzbekistan) enjoy more available water resources, which flow from upstream countries, than their actual domestic water storage (Deng et al. ). Owing to the limited water storage, the downstream countries are also water-deficient, especially the oil-rich Turkmenistan, which has the saying that 'water is more expensive than oil'.
In the former Soviet Union era, the upstream countries focused on the construction of water conservancy facilities, providing downstream countries with water resources and power resources such as farmland irrigation water, while downstream countries provided more cultivated land and agricultural products for the upstream (Jalilov et al. ).
Since the independence of Central Asian countries, a host of international projects has been launched to achieve sustainable water management, but only a few have been implemented (Abdullaev et al. ). Uncompensated water use patterns for downstream countries have caused dissatisfaction in upstream countries. The issue of 'energywater-irrigation' has been plaguing relations between Central Asian countries (Bernauer & Siegfried ). Water and land resources, as the basic materials for production and life, are the basis for ensuring national food security (Zhang et al. a). For the five Central Asian countries dominated by irrigated agriculture, the shortage of water and land resources has delayed the process of agricultural modernization (Riquelme & Ramos ). Generally speaking, the mismatching between the formation area and consumption area of water resources will lead to water conflicts. The correct evaluation of the formation and consumption of water resources and its spatial matching with land resources is critical to alleviating water problems (Yang et al. ). Most of the current state-of-the-art studies have focused on the water problems in Central Asia from the aspects of the state of water resources, the water cycle processes, the transboundary river management, and so on (Yang et al. ). Although previous studies exist for quantitative analysis, they mainly focus on the impact of climate change on the water cycle and the water environment of the basin (Zhu et al. ). However, there is a lack of study on the matching patterns between water and land resources. Therefore, a quantitative study on spatial distribution difference and the matching pattern change of water and land resources is the basis of a spatial optimal allocation of regional water resources, offering important guidance for efficient utilization of water and land resources in Central Asia.
Spatial-temporal dynamics of water and land resources matching have attracted many researchers' attention. Most Due to the complexity of transboundary water resources in Central Asia, this paper takes the five countries of Central Asia as the basic unit, then uses total renewable water resource and total water withdrawal as the water resource parameters, and the cultivated area as the cultivated land resource parameter to study the spatial-temporal matching relationship of water and land resources in five Central Asian countries. The total renewable water resources in this area are 2.28 × 10 11 m 3 and the cultivated area is 3.84 × 10 7 hm 2 . The rivers in Central Asia are mostly inland rivers and lakes, mainly including the Amu Darya, the Syr River, the Chu River, the Aral Sea, and so on ( Figure 1). Its terrain is high in the east and low in the west, dominated by plains and hills. The total area of drylands is 5.17 × 10 6 km 2 (above 80% of the global total temperate desert area) in Central Asia, belonging to the temperate desert climate and grassland continental climate (Li et al. ). The vegetation is mainly grassland and desert. In most areas, the climate is dry with strong evaporation (Yao et al. ). The annual precipitation is below 300 mm, and the annual precipitation in the desert near the Aral Sea and Turkmenistan is only 75-100 mm. The territory area of the five Central Asian countries varies greatly, with Kazakhstan's territory being vast, twice the total area of the other four countries. However, although Uzbekistan has a total area of only one-sixth of Kazakhstan, its population is nearly 40% greater than that of Kazakhstan.

Data sources
Since the data of water and land resources in Central Asia are not continuous, this paper uses the data of 1992, 1997, 2002, 2007, 2012, and 2016

Theory of water and land resource matching
Matching index model of water and land resources 1. Basic concept: The matching index of water and land resources is expressed by the total water resources volume per hectare. It is a quantitative relationship that reflects the spatial and temporal distribution of water resources and cultivated land resources available for agricultural production in a specific region. It focuses on regional water resources and cultivated land, and coordination and suitability of regional water resources and  agricultural water used to the whole water consumption (agricultural water, industrial water, and municipal water) in Central Asia (Liu et al. ). The calculation formula is as follows: where R wl i is the matching index of water and land resources of the ith country in Central Asia; W i is the total renewable water resource of the ith country, 10 8 m 3 ; L i is the area of the cultivated area of the ith country, 10 4 hm 2 ; α i is the proportion of agricultural water in the ith country to the total amount of social and economic water.
The matching index of water and land resources in Central Asia reflects the mean value of the matching index of water and land resources among countries in Central Asia. The calculation formula is as follows: where R p j is the matching index of water and land resources in area j; R wl i is the matching index of water and land resources of the ith country in area j; and n is the number of i countries in area j.

Gini coefficient model
The Gini coefficient was proposed by Italian economist Corrado Gini in 1921 when studying income inequality. It has been recognized and applied in many fields. The Gini coefficient (G), also known as the Lorentz coefficient, is determined by calculating the area of the Lorentz curve graph using MATLAB software. First, the Lorentz curve needs to be fitted. Assuming that the area between the actual distribution curve of income and the absolute equal distribution curve is A, and the area between the actual distribution curve of income and the coordinate axis is B, then the Gini coefficient is calculated as follows: Smaller G value indicates relatively even distribution, while larger G value indicates relatively uneven distribution.
Because the spatial distribution of water resources is diversified, the Gini coefficient is also applicable in the study of the patterns of water and land resource matching.
According to the spatial distribution characteristics of water and land resources, the Gini curve construction  (Table 1).

RESULTS
The structure and spatial distribution of water and land resources in Central Asia

Composition and utilization of water resources
The total freshwater in the five Central Asian countries is above 1 trillion m 3 , but the real available water resources are only 206 billion m 3 , of which, surface water is 187 billion m 3 and the non-repetitive groundwater is 19 billion m 3 (

Composition and utilization of land resources
The total area of Central Asia is nearly 4.00 × 10 8 hm 2 .
Regarding land types, grassland is the largest proportion

The production potential of land resources in Central
Asia is huge, but it is constrained by water resources.  Meanwhile, a series of ecological and environmental problems in the process of land use seriously restrict the sustainable use of land resources in Central Asia. Effective approaches to these problems are the key to maintaining sustainable development in Central Asia.

Spatial distribution of water and land resources
The water resources per unit area can reflect the richness and shortage of regional water resources, and the reclamation rate can reflect the degree of reclamation of regional cultivated land resources. Five countries in Central Asia are taken as the basic regional units, then the natural breakpoint method of GIS is used to divide the Central Asian region into five grades according to the water resources per unit area and the reclamation rate, producing the spatial distribution pattern of water and land resources ( Figure 4).

The spatial distribution of water resources in Central
Asia is not uniform and is characterized by a larger amount of water resources in Kazakhstan and a lower amount of water resources in Kyrgyzstan (Figure 4(a)). The distribution of cultivated land in Central Asia is uneven ( Figure 4(b)). Moreover, the spatial distribution of water resources and cultivated land resources in Central Asia is inconsistent (Figure 4). Kazakhstan is the most prominent country, whose water resources per unit area is the lowest, but the reclamation rate is the highest in the region. This spatial dislocation of water and land resources in Central Asia severely limits the increase of regional grain production and sustainable use of resources, which in return, exacerbates the contradiction of water problems in Central Asia.

Matching pattern of water and land resources in Central
Asia

Matching degree of water and land resources based on the Gini coefficient
The total renewable water resource is taken as the water resource parameter, and the cultivated land area is used as the land resource parameter. The Gini coefficient of Central Asia in 1992Asia in , 1997Asia in , 2002Asia in , 2007Asia in , 2012, and 2016 is shown in Figure 5.
The Gini coefficient showed little variation in 1992, 1997, 2002, 2007, 2012, and 2016, and the matching The average Gini coefficient was 0.60 ( Figure 6). The total water withdrawal indicates that the matching situation of water and land resources in Central Asia is extremely poor.

Spatial matching pattern of water and land resources based on the matching index model
Since potential errors may exist in the unit division of the Gini coefficient, the matching index model can be used to study the matching situation of water and land resources.
Based on the data of total renewable water resource, cultivated area, and proportion of agricultural water in five Central Asian countries in 1992, 1997, 2002, 2007, 2012, and 2016, the water and land resources matching model is used to obtain the matching index of water and land resources in Central Asian countries, as shown in Table 3.
To Combined with the multi-year average of Table 3, the matching index of Kazakhstan is the smallest and the matching index of Tajikistan is the largest, the latter being six times that of the former. By taking the five countries as a whole and calculating the multi-year average matching index of water and land resources from 1992 to 2016, the matching    index of water and land resources in Central Asia is 1.25 (Table 3)   show the renewable water resources is not a key factor affecting water issues in Central Asia (Table 4).

Suggestions on water resource management
The Central Asian states share many common problems and unaddressed tasks in the management of water resources (Zhupankhan et al. ). Based on the current situation of water and land resources matching in Central Asia, the following suggestions are put forward.
1. Improve the water resources management and allocation system in Central Asia and formulate a scientific plan for the relative matching of water and land resources. The matching of water and land resources in Central Asian  in Central Asia is not the total renewable water resources, but the total water withdrawal used by various countries.
The upstream countries are rich in water resources, but the matching degree of water and land resources lags behind the downstream countries. Therefore, the five Central Asian countries should consider the relevant compensation factors when formulating the transboundary water resources allocation plan.
3. Strengthen cooperation with the international community, which requires cooperation among the countries of Central Asia that goes far beyond the current situation.
Major reforms are necessary and external pressures from neighboring Russia and China are likely required to make this happen (Howard & Howard ). Therefore, Central Asia needs to actively carry out cooperation with the international community, introduce advanced water resources management programs, draw on national policies with outstanding effects on water control, and ultimately achieve social-economic-ecological sustainable development in Central Asia.

Limitations of this study
1. The total amount of water and land resources will change due to industrialization, urbanization, water conservancy construction, and so on. Therefore, future studies should consider the dynamic changes in the matching of water and land resources in the border areas of Central Asia.
At the same time, the matching of water and land resources should be refined to evaluate the micro-states of crop water demand and land use in each state, which also needs further research.
2. This paper analyzes the matching of water and land resources in Central Asia on the time and space scales.
However, due to the fact that appropriate data were not available, it is impossible to estimate historical data and make scientific predictions for future data. Further research can improve this to make a more comprehensive analysis.

CONCLUSIONS
This paper takes the five Central Asian countries as the research unit, and uses the total renewable water resource and total water withdrawal as the water resources parameters, and constructs the model of the Gini coefficient and the matching index between water and land resources.
Then, reasonable suggestions are put forward according to the current situation of water problems in Central Asia. The matching situation of water and land resources in Central Asia is generally better. The average Gini coefficient is 0.32, and the water and land resources matching index is 1.25. The main contradiction of the Central Asian water problem is the distribution and utilization among countries.
Therefore, relevant departments should improve the water resources management and distribution system in Central Asia, formulate reasonable water distribution, water supply, and compensation water agreements to achieve social-economic-ecological sustainable development in Central Asia.