Changes of vegetation and hydrological dynamics in warming climate in a mountainous watershed

The dynamic global vegetation model LPJ-WHyMe is improved and used, after calibration, to study vegetation and water cycle in a mountainous watershed in the Qilian Mountains in western China and their responses to the warming climate in recent decades. Major results show uphill expansion of all vegetations following the accelerated warming and moistening of the CO₂-enriched climate since 1979. Associated with these habitat shifts are the changes of the water use efficiency (WUE) of these plants. Herbaceous plants have shown improved WUE with a peak at elevations 3500-4000 m asl, marked by greater increase of net primary production (NPP) than water use in the elevated warming. However, boreal needleleaf evergreen forest (BNE) show a slight decrease of WUE, though minor in higher elevations. These WUE changes of the vegetation along with increased warming and moistening in the high elevations (>3500 m asl) have redefined the water resources availability of the mountainous watershed. The increased WUE by herbaceous plants below 4000 m asl leaves more snowmelt water and precipitation for runoff, R. This addition of runoff is offset however by the increased surface evaporation and plant transpiration in higher elevations attributed to increased coverage of plants and warmer temperature. This near balance between the opposite effects on R from changes of herbaceous plants is brought to a net reduction of R by the decreased WUE of BNE plants and their expansion in altitude. These changes explain the reduction of total R yield in the study basin observed in the recent decades.