The variation of rainfall runoff after vegetation restoration in upper reaches of the Yellow River by the remote sensing technology

The runoff changes are controlled by combined effects of multiple regional environmental factors in the alpine hilly region of Northwest China

The imbalance between the water supply and demand in arid and semiarid regions is becoming increasingly serious due to global warming and human activities. It is of great significance to reveal the variation characteristics of runoff and its main controlling factors for the sustainable management of regional water resources. However, few previous studies have considered the integrated effects of multiple control factors on runoff variation at different periodic scales. We collected meteorological and hydrological data from 1960 to 2019 in the Huangshui watershed and explored the correlation degree between runoff and regional environment factors such as precipitation (P), potential evaporation (ET₀), mean temperature (T), normalized difference vegetation index (NDVI). The wavelet coherence indicates that there was a high degree of positive phase consistency between runoff changes and P, ET₀, T and NDVI at an approximately 12-month period scale, with lag times of approximately 1, 2, 1 and 0 months, respectively. The P was the single factor most closely related to runoff, and its combined with ET₀ dominated the runoff change during the whole study period. The Budyko frame combined with elastic coefficient analysis showed that the climate change were the main reasons for the increase in annual runoff in change period I (1981-1990), and changes in the underlying surface due to human activities and vegetation variation was the main reason for the decrease in runoff in change period II (1991-2019). The wetter climate brought more rainfall input but this did not make runoff appear an obvious upward trend. Therefore, for alpine regions with sensitive and fragile ecological environment, the balance between human water consumption, vegetation ecological water demand, and precipitation should be weighed. The combination of wavelet coherence analysis and Budyko framework is helpful to better determine the potential driving factors of regional runoff change.

Quantitative Effects of Climate Change on Vegetation Dynamics in Alpine Grassland of Qinghai-Tibet Plateau in a County

Alpine grassland in the Qinghai-Tibet Plateau is known to be sensitive to climate change. To quantify the impacts of climate change on alpine ecosystems at small scale, Wulan County in Qinghai Province was taken as the research object, and the relationships between vegetation dynamics and climate changes and the direct and indirect effects of climate factors on vegetation dynamics were analyzed using the methods of ordinary least squares, Pearson correlation analysis and path analysis, on the basis of MOD13A3 data and meteorological data from 2001 to 2020. The results showed that the Normalized Difference Vegetation Index (NDVI) in the growing season of the county and 5 vegetation types showed similar fluctuation processes and relationships with climate factors during the study period. The growing season NDVI (GSN) of shrubland and desert steppe respectively were the highest and lowest. The yearly mean values of GSN over the county ranged from 0.151 to 0.264, and increased extremely significantly with years at a rate of 0.0035 yr−1. Spatially, GSN gradually decreased from northeast to southwest, and 97.2% of the county area showed an increasing trend in GSN. With years, growing season evaporation (GSE) decreased extremely significantly at a rate of 29.6 mm yr−1, while growing season average relative humidity (GSARH) showed a significant increasing trend at a rate of 0.16% yr−1. The correlations and effects of GSE, GSARH, and growing season precipitation (GSP) on vegetation dynamics were weakened in turn. GSE was the main direct effect factor, and the latter two were the indirect effect factors through GSE. The total contribution rates of GSE, GSARH and GSP to vegetation dynamics was about 78.0%. However, growing season average temperature (GSAT) had little effect on vegetation dynamics. This study provides information for understanding the characteristics of vegetation dynamics of alpine grassland in the Qinghai-Tibet Plateau at a small scale.