Impact of Reforestation on Local Climate and Environment in a Semi-arid Urban Valley, Northwestern China

Abstract:

Since 1999 Chinese government has invested more than 800 million Yuan to reforest the southern and the northern mountains surrounding urban Lanzhou - a typical semi-arid city located in a river valley, Northwestern China. Until 2009 obvious land use change occurred, with 69.2% of the reforested area been changed from grasslands, croplands, barren or sparsely vegetated land to closed shrublands and 20.6% been changed from closed shrublands, grasslands, and croplands to forests. This study assesses the impact of these changes on local climate and environment in winter using WRF (Weather Research & Forecasting) model incorporated with high-resolution remotely sensed land cover data for 1999 and 2009 and the FLEXible PARTicle (FLEXPART) dispersion model. Results indicate that the changes in albedo, surface exchange coefficient and surface soil heat conductivity related to the reforestation led to the changes in surface net radiation and the surface energy partitioning, which in turn affected the meteorology fields and enhanced the mountain-valley wind circulation. The amount of air exchanged between the valley and the outside increased after reforestation during the day, with the largest increase of 10 %, while it changed little during the night on winter sunny day with no snow cover. The sensitivity analysis using FLEXPART-WRF model indicates that the reforestation affected the spatial distribution of pollutants and slightly improved the urban air quality in winter. And the greening program of Lanzhou has special reference to other valley urbans.