| The changes of watershed vegetation cover, which can reflect the trend of ecological environment, but also had certain response and influence for precipitation and runoff. Kuye River is a major tributary of the Yellow River, located in storm prone areas, large floods happening more opportunities. The main land use within the watershed is grassland. With the help of the NDVI data of vegetation cover, combined with rainfall and runoff data in the basin, to explore the relationship between vegetation cover, rainfall and runoff of Kuye River watershed. Based on the NOAA-NDVI data sets and the Yellow River hydrological datasets, analyze the relationship between vegetation cover, precipitation and runoff changes, in multiple time scale, in order to reveal the vegetation cover response to precipitation, and its impacts on runoff, in Kuye River watershed. And thus for, proved a reference and a basis of the study of Precipitation-Vegetation-Runoff response mechanism of the Yellow River. The main research results and progress are as follows:1) 96 consecutive months of monthly maximum NDVI, had jumped significantly, and jumped to annual maximum in August or September in each year, while the maximum monthly precipitation and runoff in July or August reach to the top during the study period. Maximum monthly NDVI and monthly runoff were there for a significant response in precipitation, and NDVI response to precipitation with a lag. Monthly precipitation has a significant linear effect on runoff; it is the main factors affecting runoff. Vegetation cover has slowed effects on runoff; the role of relations between the two is more complex.2) During the study period, although the change of precipitation and runoff is rapid, but no statistically significant linear trend, interannual variation showed cyclical changes in about three years. In contrast, the maximum NDVI inter-annual variation is an increasing trend, and vegetation cover conditions improve overall. Precipitation has a significant positive effect on runoff. When the NDVI is over than 0.22, vegetation cover had obvious response to precipitation, and vegetation cover had no significant correlation with runoff directly; When the NDVI is less or equal to 0.22, the precipitation and runoff were negatively correlated with vegetation cover and the correlation was not significant.3) The study area of average monthly data reflects changes during year's conditions. Rainfall, runoff and vegetation cover reached the maximum value in August. Three similar trends was obvious linear relationship between them. Runoff and vegetation cover are very sensitive to precipitation. Meanwhile runoff and vegetation cover also shows directly significant positive correlation. The linear relationship of precipitation to runoff was significantly. But the linear relationship between NDVI of the runoff was not significant; there was a more complex relationship between the two. The average monthly runoff increase with the increase of precipitation and NDVI, whereas reduced.4) During the rainy season, the changes of precipitation and runoff condition is consistent with annual change, fluctuating period of about three years. Vegetation cover, during the rainy season, also shows ascendant trend. Precipitation is still the key factor on runoff during the rainy season. During the rainy season, when the NDVI is over than 0.22, vegetation cover is significantly affected by precipitation, vegetation cover and runoff positive correlation, not significant; when the NDVI is less or equal to 0.22, both rainfall and runoff had negative correlation to vegetation cover, with bad correlation. More precipitation in the rainy season, more focused case, the linear response of runoff to precipitation significantly. NDVI had a not significant linear relationship to runoff, with a negative effect. Non-rainy season precipitation could be found almost no rules, and runoff showed a certain cycle, the cycle is about 4 years. Rather than the vegetation cover during the rainy season there was no significant linear trend during the non-rainy season. Statistical analysis showed that when the NDVI is over than 0.22, vegetation cover during the rainy season is also strongly influenced by the non-rainy season precipitation; no significant correlation between rainfall and runoff during non-rainy season; non-rainy season rainfall was negatively correlated with vegetation cover of non-rainy season, with the poor correlation; non-rainy season runoff and vegetation cover are related, correlation not good. Relatively few precipitations during non-rainy season, the linear relationship between precipitation and runoff was not significant. Contrary, NDVI had a significant linear effect to runoff, runoff increasing with NDVI.5) Spring precipitation with no obvious rule, summer, autumn and winter precipitation showed about three to four cycles, in which summer precipitation> autumn precipitation> spring precipitation> winter precipitation. Spring and autumn are decreasing trend of runoff and summer runoff cycles of about 3 years and there are an increasing trend in winter runoff, runoff in summer> spring> autumn> winter. NDVI in spring and autumn had no significant linear, and with a slight decrease in winter. NDVI in summer there are increasing trend. Summer NDVI> autumn NDVI> Winter NDVI> spring NDVI. There is a poor correlation between rainfall and runoff in spring and winter, and a not significant correlation between autumn rainfall and runoff. But rainfall has a significant positive impact on runoff, which is only in the summer, rainfall doing the main factor to runoff. The seasonal average NDVI were positively correlated with counterpart's precipitation, but correlation is not good, the seasonal rainfall is not the main factors affecting to vegetation. The average NDVI and runoff in spring, the average NDVI and runoff in winter both had a significant negative correlation. Average NDVI and runoff in summer is related to each other. The average NDVI and runoff in autumn had a negative correlation, and correlation is not good. That, in spring and winter, there was a negative correlation between vegetation cover and runoff. That is to say that interaction between them in summer and autumn is not obvious. All in all, in spring and winter, due to relatively small rainfall, precipitation and runoff in the linear relationship was not significant, while the NDVI linear relationship with the runoff significant, runoff increasing with the NDVI; more precipitation and concentration in summer, precipitation and runoff were significantly linear correlation, the relationship between runoff and vegetation are more complex. The impacts, that vegetation cover on runoff, was existence, but it was not significant linear. The linear relationship between runoff and precipitation, NDVI was not significant in autumn.6) The process of precipitation changes had a dramatic effect on the runoff, between the two there was a significant positive correlation. The response of vegetation to precipitation was very sensitive. The runoff increased with precipitation increased; there was a significant linear relationship between the two. When the vegetation cover had increased, runoff reduced, a relationship is more complex between the two. |
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