基于扩展的Budyko模型定量评估平江流域森林恢复和气候变异对季节性径流的影响

流域季节性径流变化反映了年内水资源的动态特征。在以森林为主的流域中，森林变化和气候变异被普遍认为是影响流域水文过程的两大驱动因素。因此在全球气候变化背景下，研究流域森林恢复和气候变异对流域季节性径流的影响，可为协调区域碳-水关系和制订可持续的森林经营管理策略提供参考。选择鄱阳湖流域上游的平江流域为研究对象，根据流域历史森林覆盖率变化情况，将研究期划分为参考期（1961-1985）和森林恢复期（1986-2006），采用Mann-Kendall趋势分析研究流域长时期水文气象数据是否存在显著变化趋势。同时引入月干旱指数（潜在蒸散发和有效降雨的比率），将一年定义为能量限制季（1-6月）和水分限制季（7-12月），结合扩展的Budyko模型定量分析平江流域森林恢复和气候变异对季节性径流的相对贡献。在本研究流域整个研究期内（1961-2006），通过Mann-Kendall趋势分析发现，研究流域水分限制季径流呈现显著增加趋势，而能量限制季水文和气候变量变化趋势均不显著。其次，相较于参考期，流域森林恢复使能量限制季径流降低了11.71 mm/a （24.40%），使水分限制季径流增加了12.27 mm/a （17.23%）。同时，气候变异导致能量限制季径流减少了36.28 mm/a （75.60%），而使水分限制季径流增加了58.94 mm/a （82.77%）。上述研究结果表明，森林恢复对径流影响具有累积效应。森林恢复对季节性径流具有积极的调节作用，同时季节性径流对森林恢复的响应存在时间差，而且森林恢复对径流的影响在能量限制季和水分限制季具有相互抵消的作用，气候变异与森林恢复的影响效应类似。此外，本研究也证实，平江流域季节性径流变化主要是受气候变化主导，但森林恢复对季节性径流的贡献也不容忽视。

Quantitative assessment on the effects of reforestation and climate variability on the seasonal streamflow in Pingjiang watershed using the extended Budyko model

The seasonal changes of streamflow in the watershed reflect the dynamic characteristics of water resources within a year. In forested watersheds, it is generally accepted that forest cover change and climate variability are considered as two critical driving factors to affect hydrological processes. Thus, quantifying the effects of reforestation and climate variability on seasonal streamflow dynamics provides scientific basis for balancing regional carbon and water resources and developing sustainable forest management strategies in the context of global climate change. The Pingjiang watershed, which is located in the upper reach of Poyang Lake basin in subtropical China, was selected for this study. The entire study period consists of the reference period (1961-1985) and the reforestation period (1986-2006), based on the history of forest coverage dynamics. The method of Mann-Kendall trend analysis was used to detect whether trends in the hydroclimatic variables significantly varied over a long-term period. Meanwhile, the monthly aridity index (the ratio of potential evapotranspiration to effective precipitation) was applied to define two different seasons within a year as energy-limited season (January-June) and water-limited season (July-December), respectively. The extended Budyko model was used to quantify the relative contribution of reforestation and climate variability to the seasonal changes of streamflow in the studied watershed. First, based on Mann-Kendall method, streamflow in the water-limited season showed a significant increasing trend during the study period (1961-2006), but with no significant trend in hydrological and climatic variables for the energy-limited season. Second, reforestation decreased streamflow in the energy-limited season by 11.71 mm/a (24.40%), but increased that in the water-limited season by 12.27 mm/a (17.23%) compared with the reference period. By contrast, climate variability related streamflow was reduced by 36.28 mm/a (75.60%) in the energy-limited season but was increased by 58.94 mm/a (82.77%) in the water-limited season. These results demonstrated that effects of reforestation on streamflow can be cumulative over time and space. Reforestation had a positive role in regulating seasonal streamflow dynamics, while there could be a time-lag for the response of seasonal streamflow to reforestation. The effects of reforestation on streamflow was offset between the energy limited and water limited seasons, and the effects of climate variability on streamflow was similar with those of reforestation. Meanwhile, these results also demonstrated that the seasonal changes of streamflow in the Pingjiang watershed are primarily determined by climate change. However, reforestation may be another important driving force to affect seasonal streamflow variation.