Change Feature of Net Primary Productivity of Natural Vegetation and Its Impact Factor in the Source Region of Yellow River in Recent 50 Years

Climatic observation data of monthly average temperature,maximum temperature,minimum temperature,relative humidity,precipitation,wind speed and sunshine duration in the source region of Yellow River during 1959-2008,the net primary productivity(NPP) was computed by revised Thornthwaite Memorial Model.The annual and interannual changes of NPP were analyzed and the impact climatic factors of NPP are discussed.The main results are as follows: In 1959-2008,the annual precipitation in this region has a increasing trend and the linear fitting ratio of annual precipitation change was 5.316~11.318 mm·(10 a)-1.The precipitation obviously increased during spring and summer in this areas.The annual temperature shows a ascending trend too.The linear fitting ratio of annual temperature change is 0.273~0.322 ℃·(10 a)-1 and the ratio significantly increased in winter.The annual maximum evapotranspiration(ET0) displayed a ascending trend too,with a linear fitting ratio from 5.286 to 8.026 mm·(10 a)-1 and the rate of increasing obviously appeared in summer.The surface humid index displayed a ascending trend as well as precipitation,but the linear fitting ratio is 0.011~0.026(10 a)-1 and the surface mositure index quickly increased in winter.There are five periods of dry and wet alternations in recent 50 years.Before 1990s,it was a light wet stage,but the light dry stages appeared about high frequency from 1998 to 2004.The annual NPP in this region had a increasing trend.The linear fitting ratio of annual NPP is 95.502~190.72 kg·hm-2·(10 a)-1.The major climatic impact factors of the NPP are the precipitation,maximum evapotranspiration and average minimum temperature in the regions.