黄土丘陵沟壑区极端降雨事件及其对径流泥沙的影响

半干旱黄土丘陵沟壑区是我国水土流失的重灾区。在全球气候变化的大背景下,极端降雨事件时有发生,加重了区域水土流失防治的难度。因此,科学界定极端降雨事件、进而探讨其发生规律及其对径流侵蚀的影响尤为重要。通过整理定西市安家沟流域(35°35′N,104°39′E)17年的降水和径流侵蚀数据进行统计分析。以降雨量和最大30min雨强为指标,采用世界气象组织的标准划分了极端降雨事件。结果发现:(1)研究区内极端次降雨事件的雨量和雨强的临界值分别为40.11mm和0.55mm/min,次降雨量的多年平均值为18.87mm。17年间共发生12次极端事件,5月、7月、8月份的发生概率分别为16.67%、50%和33.33%。因此最佳防治时间段为7、8月份。(2)聚类分析表明极端降雨事件可分为三类:降雨量和雨强都大于临界值,占25%;降雨量大于临界值,而雨强小于临界值,占41.67%;雨强大于临界值,降雨量大于多年平均值而小于临界值,占33.33%。(3)在极端降雨事件作用下,径流系数和侵蚀模数要比对应的多年平均值高。总体而言,降雨量和雨强都很高的极端事件的破坏性最强,但高历时低雨强的极端事件所产生的破坏也不容低估。(4)沙棘林在生长演替的过程中显著增强了抵御土壤侵蚀的能力,对极端降雨事件有很好的防治作用。抵御极端降雨最弱的是坡耕地,主要是由于受到坡度大、植被覆盖率低以及人为干扰等因素的影响。

Extreme rainfall events and their effects on runoff and sediment yield in the loess hilly area

The loess hilly area is well-known for its severe soil erosion around China.Under the background of global climate change,extreme rainfall occurs frequently,and it aggravates the situation of regional soil erosion control.Therefore,it becomes more urgent how to establish the classification criterion of extreme rainfall events and discuss their effects on runoff and soil erosion.In this study,17 years of rainfall and runoff-erosion data in Anjiapo catchment in Dingxi were used for a statistical analysis.Using the indexes of rainfall amount and maximum 30-min intensity,the local extreme rainfall events were discussed according to the criterion of the World Meteorological Organization.The conclusions are as follows:firstly,the critical values of the extreme rainfall depth and maximum rainfall intensity are 40.11 mm and 0.55 mm/min,respectively.The mean rainfall amount is 18.87 mm.Generally,there are nine extreme rainfall events during this period,of which 16.67%,50% and 33.33% falls in May,July and August,respectively.Hence,July and August are the most pivotal period for controlling the soil erosion induced by the extreme rainfall.Secondly,the clustering analysis shows that the extreme rainfall events can be classified into three categories:the rainfall depth and intensity are above the threshold;the rainfall depth is above the threshold but the rainfall intensity is not;the rainfall intensity exceeds the threshold but the rainfall depth doesnot.These three parts hold 25%,41.67% and 33.33%,respectively.Thirdly,the runoff coefficient and erosion modulus under the extreme rainfall events are far higher than those of the mean values.In general,the rainfall which has the high intensity and depth are more prone to result in the severe sediment and runoff.Lastly,the vivacious plants such as sea buckthorn(H.rhamnoides L.)can gradually protect large amount of soil against extreme rainfall's destruction with the plant succession.The steep farmland,however,is the most sensitive land use to the extreme rainfall.The reason can be attributed to the steep slope,low vegetation coverage and severe human disturbance.