Landscape Evolution and Characteristics of the Dyke-Pond Polder under Land Consolidation in Huzhou, Zhejiang Province

Objective The unique polder landscape formed through a series of artificial interventions of “water conservancy-agriculture-settlement” in low-lying water network areas is a living heritage that continues to evolve through the interaction between humans and the land. Since 1970, climate change and socio-economic development have driven the implementation of land consolidation in China, such as linking polders and land levelling, which have significantly changed the scale, structure and organization of polders, resulting in a spatial transformation of the dyke-pond polders in Huzhou from small polders to standardised large polders. The landscape transformation triggered by land consolidation far exceeds urbanization, to a certain extent, causing the polder landscape to lose its diversity, territoriality and continuity.

Methods Taking the dyke-pond polder system in the Yangtze River Delta as an example, this study adopts a multi-scale framework combining parameterisation and mapping analysis to reveal the spatial and temporal evolution patterns and mechanisms of the polder landscapes in the period of 1970−2020. Firstly, based on historical documents and field surveys, the spatial interpretation of land consolidation measures and processes is carried out, and a multi-temporal polder spatial database covering the three key stages of land consolidation is constructed through the GIS platform. Secondly, a landscape evolution evaluation model is constructed at the regional scale to analyse and quantify the land consolidation measures and landscape changes in hydrological, agricultural and settlement systems at the regional scale. Then, at the unit scale, Mapping was used to extract typological characteristics and reorganisation patterns of dyke-pond polder (DPP) landscape elements. Finally, the mechanism of polder landscape evolution under the influence of land consolidation is revealed, and adaptive protection and development strategies for polder cultural landscape are proposed.

Results The study found that land consolidation and organisation measures encompass multiple dimensions of water, agricultural and settlements. Depending on the extent of their transformation of the landscape, they can be divided into three phases: 1949−1978, 1979−2000 and 2001−2020. In the time-series dimension, the linking polders project simplified the structure of the water network outside the polder units, resulting in a 44% reduction in water network area. The number of polders decreased from 2,872 to 171, with the average area increasing 18-fold (from 0.42 km² to 7.5 km²); arable land shrinking accelerated due to the expansion of fishponds and settlement agglomeration. The spatial dimension shows a gradient of response with dramatic transformation in the western low-lying areas and progressive evolution in the southeastern plains. From 1970 to 2000, farmland and ponds tended to be centralised, fishponds gradually expanded, and farmland was transformed into a multilevel grid. From 2000 to 2020, with the popularity of hardened pond embankments and standardised fishponds, the ecological cycle of the dyke-pond polder weakened, and the pond-to-base ratio dropped to 1︰9. In addition, mulberry forests continued to decline, and the expansion of the colony was dramatic, with the farmland patterns becoming increasingly fragmented. The original three types of dyke-pond polder system: Mulberry-dyke fishponds, field-pond polder and mulberry-dyke polder are gradually transformed to standardised fishponds, networked mulberry-dyke polder, and fragmented polder. The mechanism analysis shows that the policy “disaster prevention-production-development” goal progressively drives the transformation of the polder landscape from naturalisation to homogenisation to fragmentation. The evolution of polder landscape is the result of the dynamic game between rigid policy intervention and flexible natural adaptation, with obvious spatial differentiation characteristics, such as the differentiation of polder area, the differentiation of settlement expansion, etc., which can be generally summarised as the dramatic transformation of the low-lying areas, and the gradual response of the plains. The historical linear elements such as water networks and dykes, are the genes of polder fields, which constrain the division of land parcels through the layered accumulation effect. However, production-oriented intensification gradually disintegrates the polder gene, leading to the decline of the long-evolved 'water-field-village' pattern and the disruption of the synergy between nature and culture.

Conclusion Based on this, the study proposes an adaptive conservation strategy with opportunity constraints and drivers. Protecting water resources and other elements that are coherently and sustainable in the spatial transformation of polder fields, and reintegrates historical water facilities into the daily lives of the people of the region by giving them new functions. Focusing on natural constraints and policy leadership, designating protected areas and coordinated development zones according to regional characteristics. Develop ecological agriculture, cultural tourism and other landscape industrialization paths to achieve a balance between industrial development and agricultural landscape protection. In the face of cultural fragmentation, the agricultural landscape formed through long-term historical evolution should be used as the foundation for building a systematic polder cultural landscape network. This study aims to provide scientific and reliable decision support for the inheritance of the characteristics and adaptive development of the polder landscape under climatic and socio-economic pressures, and to provide a transferable framework for the spatial evolution research and conservation development of other cultural landscapes.