Rainfall Redistributionand Chemical Characteristics Of Rainfall Components In A Savanna Ecosystem,Yuanjiang Dry-hot Valley,Southwest China

Forest plays an important role in redistribution of rainfall and nutrient cycling,which not only changes the spatial distribution pattern of rainfall input,but also influences the water balance and nutrient cycle in forest ecosystems.The seasonal drought and uneven precipitation distribution in the Yuanjiang Dry-hot Valley have resulted in a unique vegetation type of valley-type savanna,with prominent water and heat contradictions and severe ecological and environmental problems.Based on the mechanism of rainfall redistribution in forest canopy,the hydrochemical characteristics of rainfall redistribution and rainfall components were studied in Yuanjiang Dry-hot Valley in order to understand the chemical characteristics and variation rules of rainfall redistribution and rainfall components.?1?The results showed that the throughfall rate,stemflow rate and canopy interception rate of the shrubs in Yuanjiang dry-hot valley are 87.4%,1.7%and 10.8%,respectively.Compared with other forest types in China,the penetration rain rate of the shrubs in Yuanjiang dry-hot valley is relatively higher,while the canopy interception rate is relatively lower.As long as the rainfall in this area reaches 1.1 mm can produce throughfall,which has a low critical value among all vegetation types in China;the critical runoff rainfall of stemflow is only 1.4 mm,which is far lower than the other forest types in China.Rainfall and relative humidity are closely related to the components of rainfall redistribution,especially the rainfall and the components of rainfall redistribution have a very significant correlation?P<0.01?.The relative humidity and the components of rainfall have a significant correlation?P<0.05?,while the correlation with wind speed,water vapor pressure and components of rainfall redistributio was not signigicant?P>0.05?.?2?The pH value of atmospheric rainfall remain neutral all the year,with the ranges of pH=6.7-7.7.In the dry season,the pH value is higher than that in the rainy season.The pH values of penetrating rain and trunk runoff are 5.9-7.0 and6.1-6.6,respectively,which are slightly lower than that in atmospheric rainfall.This result indicated that the ion contentchanges after rainwater flows through forest canopy,resulting in slight acidification of throughfall and stemflow.In rainfall and rainfall components,the main ions are K⁺,Ca²⁺,Mg²⁺,SO₄^2-,NO₃^--N and PO₄^3--P.The content of each ion varies greatly in different periods.Except PO₄^3--P,the content of other ions in dry season is higher than that in rainy season,with the highest values in the middle of dry season.The content of PO₄^3--P is higher in rainy season and penetrating rain,and higher in stemflow in dry season.Rainfall has a strong elution and leaching effect on nitrogen elements on vegetation canopy.Total nitrogen content in rainfall?0.354-2.187 mg/L?,throughfall?1.148-6.133 mg/L?and stemflow?1.47-27.252 mg/L?was the largest in mid-dry season,and the smallest in mid-dry season or late rainy season.Total phosphorus content in rainfall ranged from 0.168 mg/L to0.062 mg/L.In throughfall the total phosphorus content?0.201-1.076 mg/L?was highest at the beginning of the dry season and lowest at the beginning of the dry season.In stemflow the total phosphorus content?0.101-1.077 mg/L?was highest at the middle of the dry season and lowest at the middle of the rainy season.And the change period of total phosphorus content in thoughfall was lagging behind that of stemflew.The annual net input of nutrient elements in rainfall and rainfall components are:K⁺,SO₄^2-,total nitrogen,Mg²⁺,NO₃^--N,total phosphorus,PO₄^3--P and Ca²⁺,and the values are 43.706,17.284,13.758,5.437,5.278,2.915,2.566 and-7.214 kg/hm²,respectively.Compared with other forest vegetation types in China,the annual net input of Ca²⁺in the savanna of Yuanjiang dry-hot valley was at a low level,while the annual net input of K⁺,SO₄^2-,PO₄^3--P were at the medium level,while the annual net input of Mg²⁺,total nitrogen and total phosphorus were at a higher level compared with other forest vegetation types in China.Through this study,in order to better understand the hydrological process of forests,understand the interaction mechanism between forests and water,reveal the hydrochemical changes between rainfall and vegetation after washing,filtering,adsorption and exchange,and provide theoretical basis for the analysis of nutrient cycling under this vegetation type.