Minjiang catchment is located in the subalpine area of east Tibet, the largest soil and water conservation zone of Yangtze River, where the natural and planted forests have been playing important roles in water conservation as well as carbon sequestration. However, the recent concern on trading water for carbon draws our attention to the large-scale trade-off between water and carbon in sub-alpine vegetation. In this paper, the spatial and temporal patterns including gross ecosystem productivity (GEP), net ecosystem productivity (NEP), evapotranspiration (ET), and runoff (Q) were simulated by a distributed water-carbon coupling model WaSSI-C (modified).
Results/Conclusions
Results showed upward trends in catchment ET, GEP and NEP with averages of 453 mm, 1081g C m-2 a-1 and 377 g C m-2 a-1 respectively, and meanwhile the downward trend in catchment runoff (Q) during 1982-2014. The increased vegetation water consumption is likely a mutual result of warming and large-scale vegetation recovery project. Water balance analysis showed that the conifer-dominated sub-catchments was much higher in water loss due to high evapotranspiration, compared with that of either sub-alpine meadow or mixed forest dominated catchments. The water loss was not offset by the high soil water infiltration in conifer forests during the growing season. During the whole study period, the average change rate in annual soil water storage (∆S) of sub-alpine meadow, mixed forest and coniferous forest were –44 mm, –18 mm and –5 mm respectively, both showed significant downward trends. Significant negative relationship was found between Q and NEP. The alpine meadow as the dominant vegetation type showed high water yield and low carbon sequestration, and the conifer-dominant and mixed forest vegetation showed low water yield and high carbon sequestration, moreover, the higher the forest coverage, the lower the water yield. Upward trends in net ecosystem productivity were found in the three vegetation types during the study period and the alpine meadow type was the most significant. A trade-off relationship between water yield and carbon sequestration was found in the whole sub-catchments. NEP/Q and forest cover showed strong negative relationship, indicates that the vegetation cover is the key to the spatial allocation of water and carbon benefits. In general, our results provide a comprehensive reference for future catchment management of waters and carbons.