南方红壤区坡面次降雨产流产沙特征

黄俊,亢庆,金平伟,姜学兵,李乐,韦聪谋,刘斌,寇馨月,徐舟

(1.珠江水利委员会珠江水利科学研究院,510610,广州;2.珠江水利委员会珠江流域水土保持监测中心站,510610,广州; 3.水利部黄土高原水土流失过程与控制重点实验室,450003,郑州)

南方红壤区坡面次降雨产流产沙特征

黄俊1,2,3,亢庆1,2,金平伟1,2,姜学兵1,2,李乐1,2,韦聪谋1,2,刘斌1,2,寇馨月1,2,徐舟1,2

(1.珠江水利委员会珠江水利科学研究院,510610,广州;2.珠江水利委员会珠江流域水土保持监测中心站,510610,广州; 3.水利部黄土高原水土流失过程与控制重点实验室,450003,郑州)

研究坡面次降雨土壤入渗及产流产沙特征,对于坡地水土流失防治及水土资源管理利用具有重要意义。基于广东省五华县水土保持试验推广站野外径流场2014年的观测数据,研究不同下垫面条件下坡面入渗及产流产沙特征。次降雨入渗量与地表植被盖度间存在显著相关关系(P＜0.05,R=0.95)。入渗量随降雨强度增加,呈先增后减的变化趋势,存在入渗量达最大值的临界降雨强度;人工林及灌草地临界降雨强度为19.9～27.8mm/h,均高于自然撂荒地。次降雨径流深和土壤流失量与地表植被盖度间,存在显著负相关关系(P＜0.05),相关系数超过0.9。土壤侵蚀率随径流系数呈幂函数增加趋势,径流中泥沙浓度随径流系数、降雨侵蚀力呈对数函数增加趋势,土壤流失量随降雨侵蚀力及次降雨径流深均呈线性递增趋势。次降雨径流深20mm为土壤流失量变化拐点,径流深超过20mm后,土壤流失量随径流深递增速率较快。

临界降雨强度;土壤入渗;植被覆盖;径流场

南方红壤区地形地貌复杂、降水丰沛,加之经济社会快速发展,导致水土流失问题十分严重[1]。南方红壤区水土流失面积约占其总面积的30%[2],加强对其水土流失规律研究,可为南方红壤区水土保持及水土资源管理利用提供科学参考。

坡面作为土壤侵蚀的重要组成单元,其降雨产流产沙特征研究具有重要意义[3]。次降雨产流是坡面土壤侵蚀源动力[4],是影响坡面土壤侵蚀的关键因素[5]。降雨特征及植被覆盖是影响坡面产流产沙的关键因素[6]。鲁克新等[7]进一步研究发现,降雨强度是影响径流量的关键因子,而植被覆盖及坡度是影响侵蚀量的关键因子。顾礼彬等[8]在黔西高原研究发现,坡面产流产沙量与年内降雨分布关系密切,产沙量与降雨量间存在显著的指数关系。李君兰等[9]基于模拟试验条件研究发现,含沙量的变化主要受降雨强度及坡度交互作用影响,而坡长对含沙量未有显著影响。吕玉娟等[10]研究发现,红壤区不同土地利用方式对产流产沙量影响较大,农地径流产沙量远大于经果林,且后者呈逐渐降低趋势;汪邦稳等[11]基于对比试验也得出类似结论。耿晓东等[12]基于室内模拟降雨试验发现,红壤坡面入渗率随降雨强度增大,呈先增加后降低变化趋势,产流速率主要受到降雨强度的影响;代数等[13]基于人工模拟降雨试验研究,认为平均降雨强度是影响黄壤旱地坡面产流产沙的关键因素,径流泥沙量与将雨强度间均存在幂函数关系。李新平等[14]采用通用水土流失方程,计算不同下垫面小流域尺度土壤侵蚀量,发现常绿阔叶林土壤侵蚀量较灌木牧草具有更好的保持水土效果。孙佳佳等[15]研究发现,马尾松纯林地保持水土效益一般,但采用“马尾松+牧草”复合土地利用方式,可减少土壤侵蚀模数90%以上。尽管坡面产流产沙特征相关研究成果丰硕,但有关红壤区不同土地利用方式、不同降雨强度及地表植被盖度条件下坡面产流产沙特征研究仍需进一步深入。本文基于南方红壤区野外定位观测径流小区,研究不同土地利用方式下、不同降雨强度及地表植被盖度对次降雨坡面入渗、产流产沙特征影响,以期为南方红壤区坡地水土流失防治及水土资源管理提供参考。

1 研究区概况

试验在广东省五华县水土保持试验推广站野外径流场(E 115.62°,N 24.09)开展。该径流场属珠江流域的广东韩江上游典型的红壤丘陵区,亚热带季风气候,年降雨量为1 300～1 900 mm。该区以山地丘陵为主,水土流失类型齐全,面蚀、沟蚀和崩岗侵蚀等各类型均有大量分布。地表植物主要有马尾松(Pinusmassoniana Lamb.)、桉树(Eucalyptus grandis Hill)、柚树(Citrusmaxima(Burm.)Merr.)、木荷(Schima kwangtungensis Hung T.Chang)、绢毛蔷薇(Rosa sericea Lindl.)、芒萁(Dicranopteris dichotoma(Thunb.)Bernh.)、蔗鸪草(Eriachne pallescens R.Br.)等。土壤田间持水量及饱和含水量分别为26.7%～36.7%和42.5%～44.4%,有机质质量分数为0.78%～1.69%。该试验站于2008年共修建8个人工径流小区(20m×5 m),各小区具体参数如表1所示。为保证本研究结果的可比性,各径流小区采用相关的管理方式,灌木保持在0～50 cm、牧草保持在0～20 cm,小区内枯枝落叶进行不定期清理,以保证各小区下垫面条件基本一致。

2 材料与方法

依托于水利部发布的《全国水土流失动态监测与公告项目管理办法》(试行)(办水保[2014]257号),基于五华县水土保持试验推广站8个人工径流小区开展2014年自然降雨观测试验。

降雨量(日降雨量、次降雨量和降雨过程)由雨量筒和自记雨量计连续监测,并使用RainRecordⓒ1.06软件包计算得到次降雨侵蚀力数据。小区土壤含水量采用传统烘干法测定。地表植被盖度采用照相法和目估法相结合确定,具体参照《径流小区和小流域水土保持监测手册》执行。土壤含水量及地表植被盖度均为每15～20 d测定一次。小区径流量通过集流池和分流池水位计算得到,泥沙质量浓度通过人工取样获得,并计算次降雨产沙量数据。

表1　径流小区详细参数Tab.1 Detailed information of the runoff plot

观测数据均来源于自然降雨事件。由于自然降雨事件降雨强度相对于模拟降雨试验偏小,且次降雨历时均相对较短;所以,次降雨过程中植被截留、土壤蒸发及填凹水量也相对较小。经估算次降雨事件植被截留、土壤增发及填凹水量均不超过次降雨量的4%,因此,忽略次降雨事件中植被截留、土壤蒸发与填凹水量。那么坡面次降雨入渗量为

式中:Inf为次降雨入渗量,mm;Rain为次降雨量,mm; Run为次降雨径流深,mm。

为对比分析各处理侵蚀产沙能力,计算各处理次降雨坡面土壤侵蚀率,该指标反映降雨过程中单位时间、单位面积上的土壤侵蚀量。其计算公式如下:

式中:ER为侵蚀率,t/(hm2·h);SL为土壤流失量,t/ hm2;T为次降雨产流历时,h。

经过初步筛选,使用2014年该径流场产生的共计280组(35组×8个径流小区)有效观测数据。使用Microsoft Excel 2013软件包进行数据处理,使用Origin Pro 8.5软件包绘图。

3 结果与分析

3.1次降雨入渗量

各处理次降雨入渗量变化范围为6.2～90.0 mm,其中,处理2和1入渗量平均值分别为最大和最小,各处理平均入渗率统计结果如图1所示。各处理间,入渗量存在极显著差异(P＜0.01)。处理2入渗量显著大于处理1、3、7和8(P＜0.05),处理1和7均显著低于处理2、5和6(P＜0.05)。次降雨平均入渗量与地表植被盖度间,存在显著的正相关关系(P＜0.05,R=0.948 1),人工林及灌草地次降雨入渗量,均高于自然撂荒地。

图2为各处理次降雨入渗量与降雨强度散点图。可以看出,入渗量随降雨强度增加,呈先增加后降低变化规律,这与耿晓东等[12]和黄俊等[16]研究结果一致。水分入渗主要依靠土壤中非毛管孔隙和部分毛管孔隙,当降雨强度增大后,坡面水深增加,静水压力变大,导致入渗率增大,入渗水量增加[17];此外,雨滴打击作用,使得部分静止毛管水变成流动下渗水[18],使得土壤入渗量得到一定程度提升。当降雨强度增加到一定值,雨滴打击动能过大,而破坏表层土壤结构或形成土壤结皮[19],导致土壤入渗率急剧降低,入渗水量呈下降趋势。

图1 各处理次降雨入渗量及地表植被覆盖度统计分析Fig.1 Statistical results of the infiltration amount and land surface vegetation cover under different treatments

图2　各处理次降雨入渗量与降雨强度散点图Fig.2 Scatter diagram between the infiltration amount per individual rainfall and individual rainfall intensity under different treatments

各处理次降雨入渗量(Inf)与降雨强度(Rain)间的关系可采用方程式Inf=aR2ain+bRain+c定量描述,其拟合结果如表2所示。由方程式确定的系数可以看出,各处理Inf变异中有40%～60%是由Rain引起的,表明Rain是影响土壤入渗的关键因素。对拟合方程求一阶导数,可以得到Inf最大时对应的临界降雨强度。临界降雨强度与地表植被盖度间存在显著相关关系,相关系数为0.960 3(P＜0.05)。人工林地及灌草地临界降雨强度在20.0～27.8 mm/h之间波动,均高于自然撂荒地处理。自然撂荒地表植被盖度相对较低,一旦降雨强度增加,雨滴打击地表作用,使得表土结构遭到破坏,土壤渗透性能大幅降低;因而该处理临界降雨强度相对较低。

表2　各处理次降雨入渗量与降雨强度拟合方程Tab.2 Fitted equations between the infiltration amount and rainfall intensity for different treatments

3.2次降雨径流侵蚀量

各处理次降雨径流深和土壤流失量统计结果见表3。各处理间径流深及土壤流失量均无显著性差异(P＞0.05),但自然撂荒地处理径流深及土壤流失量较其他处理均偏大。受试验条件限制,本文并未得出径流深及土壤流失量与坡度间规律性关系;但总体而言,坡度越大,径流深及土壤流失量均越大。王全九等[20]和陈正维等[21]分别在黄土和紫色土野外径流小区试验中,在一定坡度范围内,也得出类似研究结论。径流深及土壤流失量与地表植被盖度间存在显著负相关关系(P＜0.05),相关系数分别为-0.953 6和-0.926 3;地表植被盖度越大,次降雨入渗量越大,径流量(深)越小,其侵蚀产沙及输沙量也越小,土壤流失量亦越小。

次降雨径流深及土壤流失量随次降雨量呈线性递增变化规律,可采用线性函数定量描述二者变化关系,各处理拟合方程均达到显著或极显著水平(P＜0.05),且方程确定系数R2均＞0.9;这表明径流深及土壤流失量变异中,有90%以上是由次降雨量引起的,说明次降雨量是影响坡面径流深及土壤流失量的关键影响因素[6-7,22]。但也有研究表明,幂函数和二次抛物线也能较为准确地描述次降雨量与径流量及土壤流失量间的动态变化关系[23],这可能与次降雨特征有密切关系。

表3　各处理次降雨平均径流深和土壤流失量Tab.3 Mean runoff depth and soil loss under different treatments under different treatments

图3 各试验处理径流系数与侵蚀率及泥沙质量浓度和降雨侵蚀力与泥沙质量浓度及土壤流失量散点图Fig.3 Scatter plots between runoff coefficient and erosion rate as well as sediment concentration,rainfall erosivity and sediment concentration as well as erosion loss under different treatments

图3为各试验处理次降雨径流系数与侵蚀率及泥沙质量浓度、降雨侵蚀力与泥沙质量浓度及土壤流失量间散点图。土壤侵蚀率、泥沙质量浓度与径流系数和泥沙质量浓度、土壤流失量与降雨侵蚀力间均存在正相关变化关系,这与A.Cerdà等[24]的研究结果一致。径流系数越大,降雨后径流量越大,其侵蚀及携沙输沙能力均越大,因而导致土壤侵蚀率及泥沙质量浓度均越大。降雨侵蚀力越大,雨滴打击,导致土壤颗粒分散形成泥沙,既提升径流中泥沙质量浓度,又增加土壤侵蚀量。

土壤侵蚀率与径流系数间存在显著正相关幂函数关系(ER=a RbC,P＜0.05);ER为土壤侵蚀率,t/ (hm2·h);RC为径流系数;a和b为方程拟合参数。各处理ER变异中,有47%～67%是由RC引起的。RC越大,单位降雨产流量越大,水流侵蚀及携沙输沙能力均越大;且随着RC增加,水流这种侵蚀携输沙能力迅速变大,由于ER与RC间非线性关系,因而可以采用正相关幂函数描述ER随RC的动态变化关系。泥沙质量浓度与径流系数、降雨侵蚀力间,存在均显著的正相关对数函数关系(P＜0.05)。若仅考虑单个因素的影响作用,可以看出各处理SL变异中,有46%～78%是由RC引起的,有41%～51%是由RE力引起的。径流中SC变化,主要受到径流侵蚀产沙能力的影响,降雨初期RC及RE增加,均导致表层土壤颗粒持续被剥离,而增加了SC;但随着降雨持续进行,坡面流道趋于稳定,可侵蚀泥沙颗粒量逐渐减少,考虑到降雨初期SC迅速增加,而后逐渐放缓,趋于稳定的这一非线性变化关系,因而可以采用对数函数定量描述SC随RC及RE变化趋势。土壤流失量与降雨侵蚀力间存在显著的正相关线性关系(SL=aRE+b;式中SL为土壤流失量,t/hm2; a、b为方程拟合参数。P＜0.05),各处理SL变异中,有64%～72%是由RE引起的。SL是降雨过程中的1个累计量,RE越大,水流剥离表土颗粒而产生的侵蚀量越大,SL随RE表现为线性递增的变化规律。

图4为各处理次降雨土壤流失量与径流深散点图,可以看出随径流深变化,土壤流失量呈线性递增变化规律,可采用SL=aRun+b的方程,定量描述二者关系,这与A.J.Parsons等[25]和A.M.Raya等[26]研究结果一致。各处理拟合方程均达到极显著水平(P＜0.01),且方程确定系数R2＞95%,表明采用线性函数,描述SL与Run间动态变化关系是合理可行的。M.C.Zheng等[2930]认为幂函数也能较为准确地反映降雨径流量与产沙量间动态变化关系,这与降雨特征、研究尺度和下垫面条件等均有密切关系[31-32]。此外,当次降雨径流深＜20 mm时,SL随Run增加速率较快,当Run＞20mm时,SL增加速率放缓。这可能因为Run=20mm左右时,坡面易侵蚀剥离土壤颗粒已经被分散,并随水流脱离坡面表层土壤,而此后尽管水流侵蚀及携沙输沙能力均相对增加,但坡面流道及床面均趋于稳定,土壤流失量增加放缓。以Run=20mm为界,将8个处理分为2组进行线性拟合(图4中①和②)拟合方程分别为SL= 0.139 7Run-0.360 8(R2=0.883 2,P＜0.05,Run＜20mm)和SL=0.108 7Run-0.388 3(R2=0.782 6, P＜0.05,Run＞20mm)。

图4　各处理次降雨土壤流失量与径流深散点图Fig.4 Scatter plot between runoff depth and soil loss under different treatments

4 结论

1)人工林地及灌草地次降雨入渗量均高于自然撂荒地。次降雨入渗量与地表植被盖度间存在显著正相关关系(P＜0.05,R=0.948 1)。次降雨入渗量随降雨强度变化,呈先增后减的变化趋势,存在使入渗量达最大值的临界降雨强度。人工林地及灌草地临界降雨强度在19.9～27.8 mm/h间波动,均高于自然撂荒地。临界降雨强度与地表植被盖度间存在显著正相关关系(P＜0.05,R=0.960 3)。

(2)次降雨径流深、土壤流失量与植被覆盖度间相关系数分别为-0.953 6和-0.926 3(P＜0.05);径流深及土壤流失量随次降雨量增加呈线性递增变化趋势。土壤侵蚀率随径流系数增加呈正相关幂函数递增变化规律,径流中泥沙质量浓度随径流系数和降雨侵蚀率增加,均呈正相关对数函数递增变化趋势;而土壤流失量随降雨侵蚀力、径流深增加,均呈线性递增变化规律。次降雨径流深20 mm为土壤流失量变化拐点,20 mm之上,土壤流失量随径流深递增速率较大。

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Characteristics of slope runoff and sediment yield under individual rainfall events in southern red soil region

Huang Jun1,2,3,Kang Qing1,2,Jin Pingwei1,2,Jiang Xuebing1,2,Li Le1,2,Wei Congmou1,2, Liu Bin1,2,Kou Xinyue1,2,Xu Zhou1,2

(1.Pearl River Hydraulic Research Institute,Pearl River Water Resources Commission,Ministry ofWater Resources,510611,Guangzhou,China; 2.Soil and Water Conservation Monitoring Center of Pearl River Basin,Pearl RiverWater Resources Commission,Ministry ofWater Resources, 510611,Guangzhou,China;3.Key Laboratory of Soil and Water Loss Process and Control on the Loess Plateau,Ministry ofWater Resources, 450003,Zhengzhou,China)

[Background]It is significant for slope soil loss control,soil and water resources utilization and management to study slope soil infiltration,runoff and sediment yield characteristics at slope scales. [M ethods]Based on the field artificial runoff plots in the Promotion Test Station of Wuhua County in Guangdong Province,the characteristics of soil infiltration and runoff and sediment yield were investigated and analyzed with the natural rainfall events of 2014.The experimental treatment includes:the natural abandoned land(NAL),artificial forest(AF),as well as shrubs and grass land(SGL).[Results] There was a significant correlation(P＜0.05,R=0.95)between infiltration amount(Inf)and the surface vegetation cover(Lsvc).Infincreased firstly and then decreased with the rainfall intensity(Rai) increasing.There was a critical Raiwhich induced the largest infiltration amount.The critical Raifor theAF and SGL ranged from 19.9 to 27.8 mm/h,both were larger than that under the NAL.There are significant negative correlations(P＜0.05)between runoff depth,soil loss and Lsvcwith all the correlation coefficients over 0.9.Erosion rate was positively related to runoff coefficient by a power function,the sediment concentration increased with runoff coefficient and rainfall erosivity increasing by logarithmic functions,soil loss increased with both rainfall erosivity and runoff depth increasing by linear functions.[Conclusions]The runoff depth of 20 mm was an inflection point for soil loss,the soil loss rate was high when the runoff depth was over 20 mm.Compared with the NAL,the AF and SGL obviously improved soil infiltration and reduced runoff and sediment yield.

critical rainfall intensity;soil infiltration;land surface vegetation cover;runoff plot

S157.1

A

1672-3007(2016)02-0023-08

10.168431/j.sswc.2016.02.004

2015-10-28

2015-11-03

项目名称:水利部黄土高原水土流失过程与控制重点实验室开放课题基金资助项目“红壤区小流域水沙输移规律研究”(2016006);国家自然科学基金青年项目“天然林演替为人工林对林地水量平衡及土壤水影响作用机制”(41501019)

黄俊(1983—),男,博士,工程师。主要研究方向:水土保持与水土资源管理。E-mail:jie1002520@sina.cn