Extended growing season reduced river runoff in Luanhe River basin

Global warming has substantially altered vegetation growth and phenology In temperate biomes worldwide, whose impacts on river runoff have not been fully understood, especially at the watershed scale. Investigating the relationships among phenological shift, changes in vegetation growth, and river runoff is essential to improve our understanding of the ecosystem response to ongoing climate change. Using in-situ runoff records, phenological dates that were extracted from the normalized difference vegetation index (NDVI) over the period 1982-2015, and grey relational analysis, this study investigated the relationships between changes in river runoff and phenological variables in Luanhe River basin, China. It was found that the growing season in the Luanhe basin was significantly extended by 0.60 +/- 0.08 d per year associated with climate warming over the period 1982-2015, which is mainly related to the earlier start of growing season (SOS, 0.40 +/- 0.05 d per year), rather than the delayed end of growing season (EOS, 0.11 +/- 0.01 d per year). The vegetation growth, defined as the average growing season NDVI (GS_NDVI), significantly increased in more than 88.4% of the study area, on average the GS_NDVI increased by 1.30 +/- 0.03 x 10(-3) d per year. On the other hand, runoff significantly reduced by 1.42 nun per year over the study period, but large differences were observed between upper and middle-lower reaches region of the Luanhe basin. The grey relational analysis confirmed the primary regulation of river runoff by precipitation, but the length of growing season and vegetation growth also played a key role in the changes of runoff. These results provide new insights into the interaction between vegetation dynamics and water balance at the watershed scale, and highlight to couple the phenological processes into eco-hydrological models to improve the modelling accuracy.