4.1 Model accuracy
Based on Hefei’s land use data in 2010, we simulated its land use in 2020 and compared the simulation results with the actual ones in 2020. In this study, the Kappa coefficient and FOM coefficient were both used to verify the accuracy of the simulation results. The results show that the Kappa coefficient is 0.93 (the closer to 1 its value, the higher the accuracy of simulation; a value greater than 0.80 indicates that the model accuracy is high statistically), and that the FOM value is 0.54 (the value greater than 0.2 indicates high accuracy)[33]. Therefore, the PLUS model is reliable, and suitable to simulate Hefei’s future land use change.
4.2 Characteristic analysis of predicted land use change
Using the PLUS model, we quantitatively predicted Hefei’s different land use in 2030, 2040, 2050, and 2060 in the scenario of economic development, and spatially visualised the prediction results using ArcGIS10.8 (Fig. 1). With the ongoing integration of the Yangtze River Delta and implementation of Anhui’s strong provincial capital strategy, Hefei is experiencing rapid economic growth and accelerated urbanisation. The rapid development of its urban economy has dramatically stimulated the demand for construction land, resulting in the continuous expansion of its construction land. Quantitatively, the dominant type of land use in the next 40 years will still be arable land, followed by construction land, water, forestland, grassland, and other types of land.
In Anhui Province, grain supply has long been tight, and economic development has increased the demand for construction land, resulting in a quantitative reduction in arable land. Therefore, Anhui is facing an obvious contradiction between economic development and grain security. Land is an important factor for guaranteeing economic development and grain supply. From the perspective of land system change, the above contradiction is essentially a spatial conflict between construction land and arable land. In 2020, Hefei’s land use was dominated by arable land overall but by construction land in the core economic development areas. Northern Hefei is mainly covered by arable land, construction land, and a small water area, whereas southern and eastern Hefei is mainly covered by water area, forestland, and grassland. The new construction land in 2030, 2040, 2050, and 2060 is formed due to the outward expansion of construction land present in 2020 and construction land in northern Hefei continues to increase. This pattern is in line with the main directions of Hefei’s ongoing urban expansion. The expansion of Hefei’s construction land mainly follows an extensive growth pattern. In terms of spatial structure, the overall pattern of Hefei’s urban expansion is still centred on the original urban areas. Driven by economic development factors, the ongoing urban expansion trend is continued; hence, non-construction land in peripheral areas is constantly and rapidly occupied by the extensive urban expansion.
4.3 State transition analysis of predicted land use change
To visualise the transformation of different land types in different cross-section years, the land use layers between adjacent cross-section years were superimposed to determine the transformation of different land types in each cross-section year, and then the results were visualised using ArcGIS 10.8 and R language. As shown in Fig. 3, the main land use change in Hefei from 2020 to 2060 is the transformation from non-construction land to construction land. Arable land transformed into construction land overwhelmingly accounts for the largest proportion. Specifically, the arable land area decreases from 759,668.04 ha in 2020 to 620,871.84 ha in 2060, a decrease of 18.27%; the construction land area increases from 194,392.71 ha in 2020 to 336,840.93 ha in 2060, an increase of 73.28%. Meanwhile, a certain quantity of construction land is transformed into non-construction land, mainly into types of land with certain ecological service value (e.g., water, forestland, and grassland). Evidently, to address the negative impacts (e.g., pollution) of economic development, the government of China has gradually adopted the principle of “ecological priority and green first” instead of “economic development first and then pollution harnessing”.
From 2020 to 2060, the new construction land is mainly contiguously distributed in Shushan District, Baohe District, Yaohai District, and Luyang District. The urban development of the former Chaohu City is restricted by the surrounding ecological land, so its construction land is expanded to surrounding areas. As the two wings of Hefei City, Feixi County and Feidong County undertake some ecological service functions; its construction land is distributed in the form of dotted grids and is rarely derived from forestland or grassland. Compared with the other areas of Hefei, Lujiang County has a higher DEM-derived elevation. The development of construction land is most significantly affected by DEM-derived elevation and soil type, and forestland, grassland, shrub land, and bare land are significantly affected by DEM-derived elevation and slope. Due to the combined effect and DEM-derived elevation and Ecological Protection Red Line of Anhui Province, Lujiang County’s grassland, shrub land, and forestland are not remarkably impacted. Therefore, some forestland and grassland in their edge areas are transformed into construction land in Lujiang County. According to the 14th Five-year Plan of Hefei City on Ecological Environmental Protection, Hefei will systematically restore and protect the natural environment of the Chaohu Lake basin during the 14th Five-year period (2021−2025), specifically achieving the following objectives by 2020: 1) the ecological authenticity and integrity of the Chaohu Lake basin are significantly restored; 2) construction land around Chaohu Lake shows dotted growth; 3) the water area of Chaohu Lake basically remains unchanged; 4) very little forestland and grassland are transformed into construction land. Evidently, human intervention can dramatically affect land use change. Hefei’s land use change from 2020 to 2060 shows that DEM-derived elevation and urban planning can influence the process of land system change. Land is among the essential natural elements for human survival, and the utilisable types of land are scarce and limited. Therefore, the determination of UDBs under China's territorial spatial planning can provide a spatial guarantee for smart growth in Hefei's economic development, thereby promoting the reasonable, scientific and high-quality development of Hefei’s urban economy.
4.4 Determination of UDBs
4.4.1 Methods and criteria for the determination of UDBs
In China, UDBs are mainly determined using three methods: the positive method, reverse method, and integrated method. Focusing on urban development, the positive method is to determine UDBs by using a growth model or based on constraints and current construction land status combined with the needs of urban development. This method is suitable for cities with fewer resource constraints and large development space. The reverse method emphasises the principle of “bottom line first and then blueprint”, namely, excluding the areas unsuitable for urban development and construction first and then using the remaining areas to determine UDBs. This method is mainly suitable for cities with obvious resource constraints and limited development space. As the combination of the above two methods, the integrated method is suitable for cities that have developed to a certain stage and attach equal importance to the development of incremental space and stock space. In this study, we must ensure the availability, scientificity, and systematicness of related data and consider the fact that northern Hefei has a large development space but other spaces are obviously constrained by resource factors (e.g., natural conditions). Therefore, we adopted the reverse method to superimpose Hefei’s predicted construction land with ecological service value, compared the results with its current urban planning, and dynamically adjusted Hefei’s UDBs. This provides a new approach for determining UDBs in territorial spatial planning.
Hefei’s predicted urban construction land reflects its future land demand for urban development, and the current resource constraint is mainly manifested as the ecological service value of idle land from the perspective of ecological environment. Using the method specified by Cao et al. [32], we evaluated the ecological service value of Hefei. The accuracy of the predicted land use data is 30 × 30 m. To ensure the consistency of data accuracy, we re-sampled the raster data of Hefei’s ecological service value, performed accuracy matching between the data, and used the natural discontinuous point method to set four levels for ecological service value (low, fairly low, medium, fairly high, and high levels). Additionally, the predicted construction land from 2020 to 2060 was superimposed with ecological service value. Figure 5 shows that Hefei’s urban construction land is basically distributed in areas with a low level of ecological service value. Evidently, Hefei’s current urban development pattern follows China's development concept of "ecological priority and green first", proving the correctness of its current urban development strategy. Therefore, determining Hefei’s UDBs based on the current research data is operable, and such UDBs have a binding force on Hefei’s future urban development, thereby preventing its unlimited urban sprawl, optimising its urban spatial structure, beautifying its spatial morphology, and improving its overall spatial efficiency.
4.4.2 UDB pattern
The UDB pattern must be determined based on the ongoing urban development scale, basic resource conditions, and urban development strategy. Based on the requirements for determining UDBs under the Guidelines for Preparing City-level Territorial Spatial Master Planning (Trial), we consider the development of future urban construction land, to provide spatial guarantee for urban development. To ensure the scientificity, operability and effectiveness of the determination of Hefei’s UDBs, we developed the following principles for the determination of Hefei’s UDBs:
1) Respect the reality and keep the baseline
Land use regulation is a key tool for macro regulation of urban development. China's urban planning system and land reserves are powerful tools for the government to regulate land, and reasonable government intervention and regulation is beneficial to the scale growth and transformational development of cities[34]. The land within the determined UDBs should include contiguous urban construction land, and approved and allocated urban construction land, and must be in line with the approved urban planning (including overall urban planning, overall land use planning, and detailed control planning). The reality must be respected. Hefei’s urban development has both advantages and disadvantages, so it is necessary to determine its UDBs by taking appropriate measures at an appropriate time, keeping the baseline for its urban spatial development.
2) Optimise the urban morphology and ensure reasonableness and scientificity
The determined UDBs should be as complete as possible. Additionally, ecological and landscape corridors between city groups should be adequately reserved to prevent disorderly urban expansion, and a certain amount of flexible space should be reserved for subsequent urban construction and development. Moreover, it is not advisable to overemphasise the beautifulness of the determined UDBs but to determine UDBs scientifically and reasonably. It is necessary to ensure the feasibility and systematicness of urban development and project implementation within the determined UDBs. This requires paying attention to the correlation between urban compactness and economic efficiency in territorial spatial planning, taking into account the economic development of multiple sectors, coordinating the subjective demands of various interests, and giving benefits to the production, living and recreational needs of residents[35].
3) Strategic guidance, and planning first
Hefei’s UDB pattern should be in line with the urban development strategy determined by its overall territorial spatial planning, manifest Hefei’s urban development directions (eastward advancement and southward expansion), and prioritise the construction land for provincial-level and city-level major strategic areas (e.g., the cradle and industrial clustering areas of "Quantum Centre", “USTC (University of Science and Technology of China) Silicon Valley” around the USTC campus, Binhu Financial Area, fund clustering area of Hefei New & Hi-tech Industrial Park, and Binhu International Science Communication Centre). Additionally, Hefei’s overall territorial spatial planning (2021 to 2035) stipulates its new urban development pattern. In this pattern, based on the principles of intensive and efficient production space, liveable living space, and beautiful ecological space, Hefei will develop an overall territorial spatial pattern characterised by “guidance by centres, parallel development at two wings, multi-polar support, splendid Linghu, and synergy of six belts”. Provincial-level and city-level major strategies can guide territorial spatial development. At the same time, in the preparation and implementation of territorial spatial planning, there is a need to improve the professionalism and technical ability to predict future land use changes by considering the urban compactness paradigm, incorporating planning concepts that reflect inclusive and contextualized participatory development processes. This improves the transparency and comprehensibility of the adaptation between project results and stakeholder interests, and facilitates the supplementation of public participation in planning to promote broad and inclusive territorial spatial planning[36–37]. Therefore, UDBs should be manifested in Hefei’s territorial spatial planning to provide support for Hefei’s strategic development.
4) Improve efficiency and ensure quality
Hefei’s UDBs should be concentrated along current urban construction land, deployed in areas with good locational and traffic conditions, and embrace the existing low-efficiency and idle land as a matter of priority. In 2010, Hefei incorporated Chaohu City into its administrative jurisdiction. Urban development will never be achieved without sufficient water supply. Hence, Hefei’s UDBs should be determined on the principle of "determining urban areas, locations, population, and production by water supply". In other words, the scale and layout of population, industries, and land use should be reasonably distributed based on water resource constraints (including the lower and upper limits of water supply). Under the constraints of the water environment, an overly compact urban form, while beneficial to physical health and social relationships and synergistic with the social well-being of urban residents, is to some extent detrimental to the well-being of urban residents[38]. Therefore, while strictly adhering to the bottom line, the delineation of urban development boundaries needs to grasp the appropriate development boundary form and provide a more relaxed environment and spatial constraints for the development of urban form. The determination of Hefei’s UDBs is intended to improve the efficiency of urban land use and development, develop a high-quality spatial growth pattern, and achieve the ultimate goal of smart growth.