| The worldwide frequency and intensity of drought had increasedin the world since nearly half a century. With the aggravation of global change, research of interaction between climate change and terrestrial ecosystem has become a hot issue. Net primary productivity of vegetation was an important indicator of terrestrial ecosystems. And changes of NPP can reveal directly the impact of climate and environmental changes on terrestrialecosystems. Drought can directly affect the process of vegetation photosynthesis. Drought also interfered with the process of photosynthesis through other forms, and then indirectly had impact on the change of vegetation productivity. In recent years, the frequency and intensity of drought had increased dramatically in Wei River Basin, and it had important impact on net primary productivity of the ecosystem. Therefore, this paper can reveal the impact of the drought on the carbon cycle by the study of the impact of the drought on the net primary productivity in Wei River Basin. This paper can provide theoretical basis for the ability of terrestrial ecosystem to resist disastersand maintaining the balance of ecosystem, and had great significance to water resources to keep sustainable development of regional economy and rational utilization of water resources.According to meteorological data and remote sensing Image data of the Wei River Basin during 1961-2014, based on the standardized precipitation evapotranspiration index (SPED, CASA model and Zhou Guang-sheng and Zhang Xin-shi model, this paper analysisspatial and temporal distribution and variation characteristics of drought and NPP by using the trend analysis method and M-K test at annual and seasonal scale in Wei River Basin, and the effects of drought intensity, drought occurrence frequency and drought area on net primary productivity (NPP) were investigated. The main conclusions are as follows:(1)By comparison, this paper showed that SPEI can accurately reveal the drought features in Weihe basin. SPEI generally showed a significant downward trend, and the degree of drought deepenedin Wei River Basin during 1961-2014.The arid area coverage rate increased, drought was more serious nearly 20 years. Spatial variation characteristics of drought frequency are obvious at annual and seasonal scales. The frequency of drought increased from east to west in general at annual scales. Frequency of drought was in the range of 30%-35%, and the spatial distribution was different atseasonal scales. The drought intensity was in the range of 18.17%-24.85%. Drought intensity was weaker in northeast and southwest of Wei River Basin, and drought intensity of the central basin was bigger.(2) The applicability of CASA model, the Zhou Guang-sheng and Zhang Xin-shi model was good in the Wei River Basin by comparing the model and the data. It can make up for the lack of spatial or temporal analysis in the use of a single model by combining with the two models, so as to revealaccurately the spatial and temporal distribution characteristics of NPP in the Wei River Basin. Based on the CASA model, average annual NPP increased in the Wei River Basin during 2000-2014. The characteristics of seasonal NPP showed that the maximum value is in summer, then spring, autumn, winter. The annual average NPP showed that the maximum value is in forest region, then Bushveld region, grassland region, farmland region, other vegetation coverage area. Based on the Zhou Guang-sheng and Zhang Xin-shi model, the annual average NPP showed a downward trend in Wei River Basin during 1961-2014. Spatial distribution of NPP showed that the maximum value is in southeast of Wei River Basin, and the minimum value is in northwest of Wei River Basin. In addition to winter, the annual average NPP showed a decreasing trend in other seasons.(3) During the study period, the change trend of the SPEI and NPP was synchronous in the Wei River Basin. The effect of drought on NPP was different in various vegetation coverage areas. The change trend of the SPEI and NPP was synchronous in farmland and grassland regions. Correlation coefficient of SPEI and NPP was smaller, and its relevance did not pass the significant test in forest and Bushveld regions. In drought weakened area, downward trend of the annual average NPP weakened. The change trend of SPEI and NPP was synchronous in various vegetation coverage areas, and this characteristic is especially obvious in Bushveld region.(4) This characteristic showed that the greater the frequency of drought, the lower the net primary productivity of the vegetation. This characteristic is more obvious at the annual scale, summer and winter. The frequency of droughtand NPP showed a negative correlation in all kinds of vegetation coverage areas. And the correlation coefficient of drought frequency and NPP showed that the maximum value is in building region, then grassland region, Bushveld region, farmland region, grassland region.This characteristic showed that the more frequent occurrence of drought, the smaller the net primary productivity of the vegetation.(5) This characteristic showed that the greater the drought intensity, the lower the net primary productivity of the vegetation. The correlation between drought intensity and NPP was significant negative. This characteristic showed the net primary productivity of vegetation could decrease, when drought intensity increased. In slight drought region, moderate drought region and severe drought region, decreasing amplitude of net primary productivity increased by 11.33%,19.46% and 35.18%. When drought intensity was more serious, area ratio of abnormal low NPP was greater and net primary productivity was lower. The annual average NPP of farmland region greatly decreased when there was a slight drought or moderate drought. The annual average NPP of forest and Bushveld region greatly decreased when there was a severe drought.(6) When area of drought was larger, the net primary productivity of vegetation was lower. There was a significant positive correlation between the drought occurrence area and the abnormal area of NPP. The correlation coefficient was the largest in spring, and it was the lowest in winter. |
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