Detecting nonlinear threshold effects in water-carbon trade-offs driven by farmland expansion: An Ordered Logit Model approach 后印本

摘要: Meeting the escalating global food demand while maintaining sustainable ecosystem development poses a pressing global challenge. Since 1990, China's newly reclaimed farmland has been predominantly distributed in arid northwestern regions, where farmland expansion boosts food production but intensifies water consumption and threatens oasis ecological stability. However, conventional continuous models are incapable of identifying nonlinear threshold effects in water-carbon trade-offs, which are essential for targeted ecosystem management. To fill this research gap, this study developed a water-carbon trade-off degree (WCTD) indicator, discretized the WCTD indicator into ordered categorical variables, and adopted the Ordered Logit Model to quantify the impacts of farmland expansion on water-carbon trade-offs of the Heihe River Basin, China. Results showed that in the middle reaches of the Heihe River Basin, farmland expanded by 307.11 km² from 2001 to 2020, mainly due to conversion from grassland and unused land. The regional annual mean net primary productivity (NPP) and evapotranspiration (ET) both showed significant increasing trends. However, the relatively faster growth rate of ET resulted in a pronounced decrease in water use efficiency (WUE), with a linear trend slope of –0.0072 gC/(mm•m²•a) (P<0.050). The Ordered Logit Model results suggested that increased well density and temperature were both significantly positively associated with the deterioration of the water-carbon trade-off, and that grassland-to-farmland conversion imposed a stronger negative impact than unused land-to-farmland conversion. This is mainly because unused land has extremely low baseline WUE, and its reclamation improves soil and hydrological conditions, thereby greatly enhancing WUE. In comparison, grassland reclamation disrupts ET balance; water consumption increases more rapidly than carbon sequestration, which restricts WUE improvement. This study provides a scientific reference for rational farmland planning in the Heihe River Basin and constructs a discretized modeling analytical framework that can be applied to other arid regions to explore the nonlinear interaction mechanisms of human-driven water-carbon coupled systems.