Objective Watershed is a comprehensive spatial unit that organizes natural processes and human activities with water system as a network. In hydrological geography, the spatial relationships of upstream-downstream, mainstream-tributary and left-right bank constitute the spatial differentiation basis of the natural background of the basin, and profoundly shape the distribution and form of human settlement environment elements. Therefore, as the coupling carrier of the “natural-social” dual system, the hydrological spatial structure of the basin has a profound impact on the formation, evolution and differentiation of the living environment. However, when discussing the human settlement environment in the basin, the existing studies mostly focus on the macro-watershed integrity or micro-settlement cases. The discussion on the correlation between the spatial differentiation characteristics within the basin and the human settlement environment is still insufficient and there is a certain degree of research fragmentation. There is a lack of systematic analysis framework based on the internal hydrological spatial structure of the basin to reveal its impact mechanism on the differentiation pattern of human settlement environment. Therefore, the purpose of this study is to construct a three-dimensional analysis framework of hydrological space, and to systematically explore the differentiation pattern of human settlement environment by taking the spatial structure characteristics of river network as the core explanatory variable, so as to provide a new analytical tool and theoretical perspective for understanding the man-land relationship in complex geographical environment.

Methods In this study, GIS spatial analysis and field investigation methods were comprehensively used. Taking the Longitudinal Range-Gorge Region as the research scope, the middle section of Lancang River Basin and the south bank of Yuanjiang River Basin were selected as typical case areas. By integrating the three dimensions of vertical (upstream-downstream), horizontal (main stream-tributary) and orientation (left bank-right bank), a three-dimensional analysis framework of hydrological space based on the interactive differentiation logic of natural hydrological process and human settlement behavior based on the spatial reference system of water system network was constructed to systematically reveal the spatial differentiation mechanism of human settlement environment in the basin.

Results The spatial pattern of human settlements in the Longitudinal Range-Gorge Region shows significant differentiation characteristics: the spatial differentiation of the longitudinal dimension originates from the significant elevation gradient, and the differentiation of human settlements is driven by two paths of hydrothermal conditions and water system structure. The hydrothermal condition path is analyzed by taking Xiaochangdu Village in Mangkang County, Tibet and Changle Village in Shaxi Town, Jianchuan County as a comparative case. The water system structure path is based on the V-shaped canyon in the Liutong River section of the Lancang River Basin and the U-shaped valley in the Yezhi Section as a comparative case; the horizontal dimension focuses on the spatial heterogeneity between the mainstream and tributaries, and the core lies in the difference of water network relationship between the mainstream and tributaries and the response of human settlements. The corridor effect of the main stream scale promotes the coexistence of contact, communication and adaptability of different cultural groups. The north-south distribution of the main line of the Yunnan-Tibet Tea-Horse Ancient Road along the main stream of the Lancang River is a typical example. The tributary basin is limited by the topological structure, and the internal connectivity is strong, and the connectivity with the main stream and other sub-basins is weak, forming a “barrier effect”, which shapes the settlement unit with single ethnic composition and convergence of cultural characteristics on the micro scale. The formation of different settlement patterns in the Tongle River Basin and the Laifeng River Basin verifies this point; the orientation dimension reveals the spatial differentiation between the left bank and the right bank of the river. The core is derived from the significant geological features of the two sides and the differences in the governance systems coupled with them in history. The three-dimensional differentiation phenomenon of vertical, horizontal and azimuth is not isolated, but through the spatial coupling of natural elements and human processes, forming a multi-dimensional superposition of complex influence mechanism, reflecting the coupling process of “hydrological characteristics-spatial differentiation-habitat behavior-comprehensive effect” of human settlements in the basin.

Conclusion The three-dimensional analysis model of hydrological space constructed in this study regards the basin as an organic whole organized by the network structure of water system by integrating the vertical, horizontal and azimuthal dimensions, and establishes the systematic correlation between the natural geographical characteristics of the basin and the spatial pattern of human settlements. The abstract “human-water relationship” is implemented in the specific spatial relationship, which makes it possible to compare and analyze cross-scale and cross-case, and provides a new analytical framework for understanding the complex human-land relationship. It has promoted the deepening of the study of watershed human settlements from “factor description” to “structure-mechanism” analysis.