楊松,張巖,阿尼克孜·肉孜,李宏鈞,劉彥
(北京林業(yè)大學(xué),水土保持與荒漠化防治教育部重點(diǎn)實(shí)驗(yàn)室,100083,北京)
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陜北黃土區(qū)植被恢復(fù)對(duì)切溝發(fā)育速率的影響
楊松,張巖?,阿尼克孜·肉孜,李宏鈞,劉彥
(北京林業(yè)大學(xué),水土保持與荒漠化防治教育部重點(diǎn)實(shí)驗(yàn)室,100083,北京)
切溝是黃土高原常見的地貌形態(tài),是流域產(chǎn)沙的主要來源,而植被常被看作抑制切溝發(fā)育的關(guān)鍵因子,為探究退耕還林(草)工程以來切溝發(fā)育速率的特征,本研究使用吳起研究區(qū)間隔6年的2期同時(shí)相QuickBird影像和分辨率為5 m的DEM,應(yīng)用GIS技術(shù)分析研究區(qū)內(nèi)的切溝發(fā)育速率及其影響因子。結(jié)果表明:2007—2013年研究區(qū)內(nèi)小流域的年均切溝面積變化比例均<0.5%,其中無淤地壩小流域的年均切溝面積變化比例為0.13%,淤地壩小流域的年均切溝面積變化比例為0.12%;相關(guān)分析表明:在該研究區(qū)內(nèi)溝間地的植被覆蓋度是影響切溝發(fā)育的最主要影響因子;通過對(duì)30個(gè)無淤地壩小流域的年均切溝面積變化比例和植被覆蓋度做非線性回歸(R2=0.713)可得,當(dāng)植被覆蓋度>0.6時(shí),其抑制切溝發(fā)育的效果最明顯;方差分析表明:其他影響因子值(植被覆蓋度和地貌因子)相近的條件下,有無淤地壩的6組小流域切溝發(fā)育速率無顯著差異??傊瑴祥g地植被覆蓋度是影響切溝發(fā)育的主要影響因子,而淤地壩對(duì)發(fā)育在溝緣線上的切溝無顯著影響,促進(jìn)植被恢復(fù)有利于抑制切溝發(fā)育。
黃土地區(qū); 切溝發(fā)育速率; 植被覆蓋度; 淤地壩; GIS; QuickBird影像
切溝是黃土高原常見的地貌形態(tài),以不能橫過妨礙耕作為主要特征[1],是流域產(chǎn)沙的主要來源[2]。切溝侵蝕被看作一種臨界現(xiàn)象[3-4],影響其發(fā)生、發(fā)展的因素包括氣候、植被、地形、地質(zhì)以及人類活動(dòng)等[1],而其中植被和土地利用常被看作比氣候更重要的影響因子[4-5]。Li Zhen等[6]則將植被因子引入地貌臨界模型中來預(yù)測(cè)切溝面積增長(zhǎng)率;在晉西黃土區(qū)李鎮(zhèn)等[7]通過回歸分析得到覆蓋度大于60%的植被在抑制切溝發(fā)育上效果顯著;Li Yong等[8]和Li Zhen等[6]在黃土高原地區(qū)的研究均表明,流域溝間地的土地利用類型中林草地面積比例增加能夠抑制切溝的發(fā)育;切溝的形成和發(fā)育與坡面侵蝕高度相關(guān),通過人為方式對(duì)坡面進(jìn)行綜合治理能夠有效地抑制切溝的發(fā)育[9],張巖等[10]基于陜北地區(qū)QuickBird影像,通過對(duì)比不同土地利用類型小流域的切溝面積變化,得出坡面水平條帶整地措施可以有效地抑制切溝的發(fā)育。
切溝侵蝕定量研究始于20世紀(jì)70年代[1],隨著科技的進(jìn)步,切溝監(jiān)測(cè)手段從傳統(tǒng)的卷尺、插釬等實(shí)地測(cè)量方法發(fā)展到使用GPS、高精度遙感影像等[11-17]。分辨率為0.61 m的QuickBird影像為在大中時(shí)空尺度中研究小型切溝提供了可能性[18],A.Vrieling等[19]用多光譜QuickBird影像和實(shí)地調(diào)查數(shù)據(jù)驗(yàn)證ASTER影像(分辨率15 m)提取切溝的可行性;秦偉等[20]在黃土丘陵溝壑區(qū)基于QucikBird影像和數(shù)字高程模型(DEM),成功提取坡面淺溝及其地形參數(shù);李鎮(zhèn)等[7]利用QuickBird影像估算出晉西黃土丘陵溝壑區(qū)切溝侵蝕速率。
吳起縣處于黃土丘陵溝壑區(qū)第四副區(qū),且為國(guó)家退耕還林(草)工程示范縣,自1999年退耕還林(草)工程以來,境內(nèi)植被覆蓋度顯著增加,生態(tài)環(huán)境明顯改善[21],然而對(duì)于切溝發(fā)育速率的研究卻鮮有報(bào)道。淤地壩通過攔泥淤地抬高侵蝕基準(zhǔn)面,可以防治溝道下切和溝岸坍塌,有效地減輕溝道侵蝕的作用[22]。有研究[22]表明淤地壩對(duì)淤積面以下部分的減蝕作用最明顯:溝道被泥沙淤埋,不再發(fā)生侵蝕;然而對(duì)于發(fā)育在溝緣線上切溝的影響卻不得而知。本文基于2期同時(shí)相QuickBird影像,應(yīng)用GIS技術(shù)分析吳起研究區(qū)內(nèi)植被恢復(fù)對(duì)切溝侵蝕速率的影響,并通過對(duì)比分析,探究淤地壩對(duì)發(fā)育在溝緣線上切溝的影響,以期為該研究區(qū)防治切溝侵蝕提供參考。
研究區(qū)位于延安市西北部的吳起縣境內(nèi),地理位置為E 108°8′7″~108°16′20″,N 36°48′6″~36°58′3″,總面積98.8 km2,海拔1 245~1 585 m。境內(nèi)山川梁峁多,屬于典型的黃土高原梁狀丘陵溝壑區(qū)。年均降雨量478.3 mm,且主要集中在7—9月,年均溫7.8 ℃,無霜期96~146 d,多年平均陸面蒸發(fā)量為400~450 mm,屬于典型半干旱地區(qū)[23]。2000—2009年,退耕還林(草)工程的實(shí)施,境內(nèi)林草覆蓋面積明顯增長(zhǎng),植被蓋度低于15%的面積比例從89.18%劇減到0.01%,植被類型以落葉闊葉和灌木草叢為主[21]。本研究在研究區(qū)內(nèi)隨機(jī)選取30個(gè)無淤地壩的小流域和6個(gè)有淤地壩的小流域(圖1),基本信息見表1。
圖1 小流域在研究區(qū)內(nèi)的分布Fig.1 Distribution of small catchments in the study area
流域面積Catchmentarea/hm2植被覆蓋度Vegetationcoverage地貌因子(AS2)Geomorphologicfactor灌木林地面積比例Proportionofshrubland/%草地面積比例Proportionofgrassland/%耕地面積比例Proportionoffarmland/%其他用地面積比例Proportionofotherland/%均值Mean10.300.630.9958.5734.104.422.91范圍Range3.6~26.10.52~0.760.12~3.4617.37~72.1727.83~82.637.12~45.362.57~14.82
注:AS2為切溝上坡來水的動(dòng)能,其中A為切溝的上坡匯水面積,S為切溝發(fā)育的臨界坡度。Note:AS2means the kinetic energy of flowing water from the inter-valley,while A means the area of inter-valley,S means the threshold slope of gully development.
2.1數(shù)據(jù)來源與處理
應(yīng)用2007年6月30號(hào)和2013年7月5號(hào)2期分辨率為0.61 m的QuickBird影像,計(jì)算研究區(qū)內(nèi)的植被覆蓋度,通過目視解譯劃分土地利用類型并提取溝緣線。根據(jù)研究區(qū)1∶10 000地形圖,建立分辨率為5 m的數(shù)字高程模型DEM(Digital Elevation Model),應(yīng)用Arcgis10.1獲取影像切溝發(fā)育的局部坡度以及輔助影像提取溝緣線,所有圖層數(shù)據(jù)均采用高斯克呂格投影和WGS_1984_UTM_Zone_49N坐標(biāo)系。
2.2切溝參數(shù)提取
2.2.1溝緣線及其局部坡度的提取溝緣線是流域內(nèi)溝間地和溝谷區(qū)的分界線,以上為溝間地,以下為溝谷區(qū)。李鎮(zhèn)等[18]用三維激光掃描全站儀實(shí)地測(cè)得的切溝DEM(分辨率0.15 m)驗(yàn)證基于QuickBird影像提取切溝參數(shù)的精確度,結(jié)果證明基于QuickBird影像提取切溝參數(shù)具有較高可信度。本研究首先對(duì)2期同時(shí)相影像進(jìn)行配準(zhǔn),采用目視解譯方法提取2007年溝緣線,再以2007年溝緣線為基礎(chǔ)提取2013年溝緣線,最后將小流域分割獲得溝間地和溝谷區(qū)?;诜直媛蕿? m的DEM,以10 m(2個(gè)像元)為半徑做緩沖區(qū),提取溝緣線以上10 m區(qū)域的坡度為局部坡度。
2.2.2切溝面積變化發(fā)育在溝緣線上的切溝面積的變化相當(dāng)于溝緣線所圍溝谷區(qū)面積的變化[7],已有研究表明切溝面積變化能夠說明切溝發(fā)育的嚴(yán)重程度[10]。將同一流域2007年和2013年的溝谷區(qū)面積做差,即為該流域內(nèi)的切溝面積變化。
2.2.3植被覆蓋度提取歸一化植被指數(shù)(Normalized Difference Vegetation Index,NDVI)是植被恢復(fù)的指示因子[24],其公式為
NDVI=(NIR-R)/(NIR+R)。
(1)
式中:NIR(Near Infrared)為近紅外波段(0.7~1.1 μm)的反射率;R(Red)為紅色波段(0.4~0.7 μm)的反射率。應(yīng)用Erdas軟件計(jì)算植被歸一化指數(shù),再根據(jù)所得NDVI值計(jì)算植被蓋度。植被覆蓋度與NDVI 的關(guān)系公式[25]為
fc=(NDVI-NDVIsoil)/( NDVIveg-NDVIsoil)。
(2)
式中:fc為植被覆蓋度;NDVI為像元NDVI值;NDVIsoil為裸露或無植被覆蓋區(qū)域的NDVI值;NDVIveg為純植被像元的NDVI值。為提高選取NDVIsoil和NDVIveg的準(zhǔn)確性,根據(jù)NDVI的累計(jì)統(tǒng)計(jì)數(shù)據(jù),以1%~99%為置信區(qū)間,取1%分位數(shù)對(duì)應(yīng)的NDVI值為NDVIsoil,99%分位數(shù)對(duì)應(yīng)的NDVI值為NDVIveg,計(jì)算2007年和2013年各小流域的植被覆蓋度。
2.2.4土地利用類型的提取為突出表現(xiàn)影像中植被特征,對(duì)QuickBird影像進(jìn)行RGB(4,3,2)標(biāo)準(zhǔn)假彩色合成,根據(jù)野外實(shí)地考察經(jīng)驗(yàn),通過影像上地物的大小、形態(tài)、色調(diào)以及紋理等推斷目標(biāo)地物,并參考第1次全國(guó)水利普查土地利用類型的劃分標(biāo)準(zhǔn)[26]提取研究區(qū)內(nèi)的土地利用類型。本研究所涉及到土地利用類型見表2。
表2 土地利用分類
3.1小流域切溝發(fā)育速率特征與影響因子分析
3.1.1切溝發(fā)育速率的統(tǒng)計(jì)特征對(duì)吳起研究區(qū)內(nèi)無淤地壩的30個(gè)小流域以及有淤地壩的6個(gè)小流域的年均切溝面積變化進(jìn)行頻率統(tǒng)計(jì)(圖2),可以看出,2007—2013年該研究區(qū)小流域的年均切溝面積變化比例均在0.5%以下,其中無淤地壩小流域中有將近87%的流域集中在0~0.2%之間,平均值為0.13%,而淤地壩小流域的年均切溝面積變化均在0.2%以內(nèi),平均值為0.12%。
3.1.2無淤地壩小流域內(nèi)切溝發(fā)育速率與其影響因子的相關(guān)性無淤地壩小流域內(nèi)的切溝發(fā)育與其影響因子的相關(guān)分析結(jié)果見表3,在該研究區(qū)內(nèi)切溝面積增加比例與流域溝間地的植被覆蓋度(2007年和2013年植被覆蓋度的平均值)的相關(guān)性最強(qiáng)(R=-0.619,P<0.01),即說明植被恢復(fù)能抑制切溝的發(fā)育;已有研究表明在黃土丘陵區(qū)灌木抑制土壤侵蝕的效果最為明顯[27],通過相關(guān)分析可以看出流域溝間灌木地林地面積的增加能夠有效地抑制切溝的發(fā)展,相關(guān)系數(shù)為-0.511,顯著水平0.01;地貌因子(AS2)與切溝發(fā)育的相關(guān)系數(shù)是0.482,顯著性水平0.01,地貌因子代表上坡來水的動(dòng)能,一直被作為影響切溝的重要因子,而在植被覆蓋度較高的情況下,其促進(jìn)切溝發(fā)育的作用在一定程度上減弱[11];流域的植被覆蓋度與灌木林地面積比例在0.01顯著性水平上呈現(xiàn)正相關(guān)(R=0.724),即該研究區(qū)內(nèi)的灌木林地是影響該地區(qū)植被覆蓋度的主要因子。
3.1.3植被覆蓋度的影響2007—2013年,研究區(qū)內(nèi)植被覆蓋度增長(zhǎng)28.7%,林地和草地面積達(dá)到94.5%。已有研究表明,在黃土高原地區(qū),植被覆蓋度大于60%能夠有效地減弱土壤侵蝕[6]。對(duì)30個(gè)無淤地壩小流域的年均切溝面積變化比例和植被覆蓋度做非線性回歸,其判定系數(shù)R2為0.713。由圖3可知,擬合方程為指數(shù)方程,年均切溝面積變化比例隨著植被覆蓋度的增大呈現(xiàn)出單調(diào)遞減的趨勢(shì),并在植被覆蓋度達(dá)到0.6之后趨于平緩。由此可以說明在該研究區(qū)內(nèi)植被覆蓋度達(dá)到0.6以上能夠有效地抑制切溝發(fā)育,與以往的研究結(jié)果[7]一致。
圖2 年均切溝面積變化比例頻率統(tǒng)計(jì)Fig.2 Frequency statistic for annual average change proportion of gully area
相關(guān)因素Impactfacter切溝面積增加比例Proportionofincreasingingullyarea植被覆蓋度Vegetationcoverage地貌因子Geomorphologicfactor(AS2)灌木林地面積比例Proportionofshrudland切溝面積增加比例Proportionofincreasingingullyarea1-0.619**0.482**-0.511**植被覆蓋度Vegetationcoverage1-0.3280.724**地貌因子Geomorphologicfactor(AS2)1-0.431*灌木林地面積比例Proportionofshrudland1
注:**表示顯著性水平為0.01,*表示顯著性水平為0.05。Note:**indicates significance level of 0.01,*indicates significance level of 0.05.
圖3 年均切溝面積變化比例與植被覆蓋度的非線性回歸Fig.3 Non-linear regression between annual average change proportion of gully area and vegetation coverage
3.2淤地壩對(duì)切溝發(fā)育的影響
為探究淤地壩對(duì)發(fā)育在流域溝緣線上切溝的影響,以6個(gè)淤地壩小流域?yàn)榛A(chǔ),從30個(gè)無淤地壩小流域中挑選切溝發(fā)育影響因子值(植被覆蓋度、地貌因子)相近的6個(gè)小流域,對(duì)其切溝面積變化進(jìn)行方差分析,結(jié)果表明2組數(shù)據(jù)在0.05的顯著性水平上無顯著差異。由表4可知,6組小流域內(nèi)影響切溝發(fā)育的影響因子值相對(duì)較為接近,各組年均切溝面積變化比例差別很小。主要原因可能是溝緣線上的切溝發(fā)育主要取決于上坡來水動(dòng)能,而在溝緣線以下溝谷區(qū)的淤地壩不能干擾上坡來水,因此無法對(duì)切溝的發(fā)育產(chǎn)生影響。
高精度遙感影像為小型切溝形態(tài)參數(shù)分析提供了可能性。本研究基于2期同時(shí)相QuickBird影像,利用GIS技術(shù)得到研究區(qū)內(nèi)無淤地壩流域和有淤地壩流域的年均切溝面積變化比例,并對(duì)影響切溝發(fā)育的因子進(jìn)行分析。研究發(fā)現(xiàn)該研究區(qū)內(nèi)影響切溝發(fā)育的主導(dǎo)因子是溝間地的植被覆蓋度,說明植被恢復(fù)能夠有效地抑制切溝的發(fā)育;通過對(duì)比有無淤地壩小流域可得淤地壩對(duì)發(fā)育在溝緣線上的切溝發(fā)育速率無顯著影響。筆者估算的切溝發(fā)育速率小于張巖等[10]在陜西綏德的結(jié)果,但是大于李鎮(zhèn)等[7]在晉西北估算結(jié)果,可見,黃土高原不同地區(qū)的切溝發(fā)育速率是有差異的。該研究區(qū)內(nèi)各個(gè)小流域之間的距離均在9.8 km以內(nèi),流域之間的地質(zhì)、氣候等因素的空間差異很??;因此忽略其對(duì)切溝發(fā)育的影響。在今后的研究中,應(yīng)根據(jù)不同黃土地區(qū)的降水、土壤性質(zhì)、植被覆蓋度、地貌特征以及土地利用類型等因素綜合分析不同黃土地區(qū)切溝發(fā)育的特征。筆者所選取的切溝是發(fā)育在溝緣線上的切溝,其發(fā)育主要取決于上坡來水的動(dòng)能,而處在溝谷區(qū)的淤地壩無法對(duì)上坡來水造成影響,淤地壩的減蝕作用通過淤積物進(jìn)入溝道抬高侵蝕基準(zhǔn)面,削弱重力侵蝕,淤地壩是否對(duì)溝谷區(qū)內(nèi)的切溝具有顯著影響還有待進(jìn)一步研究。
表4 2類流域比較
1)2007—2013年吳起研究區(qū)內(nèi)小流域的年均切溝面積變化比例均在0.5%以下,其中30個(gè)無淤地壩小流域中有將近87%的流域集中在0~0.2%之間,平均值為0.13%,而6個(gè)淤地壩小流域的年均切溝面積變化均在0.2%以內(nèi),平均值為0.12%。
2)相關(guān)分析顯示,在30個(gè)無淤地壩小流域中溝間地的植被覆蓋度是影響切溝發(fā)育的最主要因子(R=-0.619,P<0.01),說明該地區(qū)植被恢復(fù)是防治切溝發(fā)育的有效措施。在植被覆蓋度較高的情況下地貌因子對(duì)切溝發(fā)育的影響被一定程度的削弱,但二者之間還是具有較強(qiáng)的正相關(guān)性(R=0.482,P<0.01)。
3)對(duì)30個(gè)無淤地壩小流域的植被覆蓋度與年均切溝面積變化比例進(jìn)行回歸分析可得,當(dāng)流域溝間地植被覆蓋度>0.60時(shí)能夠有效地抑制切溝的發(fā)育。方差分析結(jié)果表明,有無淤地壩小流域的切溝面積變化無顯著差異,即淤地壩對(duì)發(fā)育在溝緣線上的切溝發(fā)育速率無顯著影響。
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Effects of vegetation restoration on gully development rates in Northern Shaanxi
Yang Song,Zhang Yan,Anikezi·Rouzi,Li Hongjun,Liu Yan
(Key Laboratory of Soil & Water Conservation and Desertification Combating,Ministry of Education, Beijing Forestry University,100083,Beijing,China)
[Background] Gully erosion is one of the major forms of soil erosion threatening land resources on the Loess Plateau.Gully development is a threshold-dependent process controlled by a wide range /of factors and vegetation is frequently considered as one of the key factors to control gully erosion.[Methods] In order to give great insight to gully development characteristics since the project of returning cropland to forest (grass) in 1999,30 small catchments without check dam and 6 catchments with check-dams were selected in Wuqi County of northern Shaanxi province,which is in the second sub-zone of loess hilly region with active gully erosion.Based on two same time phases QuickBird images of 0.61 m-resolution with a 6-year interval and digital elevation model (DEM) with pixel size of 5 m,valley shoulder lines in the studied small catchments were extracted by visual interpretation and the rates of gully area change were estimated with GIS technology.Average vegetation coverage,local slope,upslope drainage area,land uses,and the change rates of gully area from 2007 to 2013 were calculated to assess the factors influencing gully growth.[Results] 1) Annual average growth rate of gully area in all 36 studied catchments was less than 0.5%,with averagely 0.13% in the catchments without check dam and 0.12% in the catchments with check dams.2) Pearson’s correlation analysis among gully growth rates and the factors influencing gully erosion showed that vegetation coverage on inter-valley was of the most importance with r=0.619 at significance level of 0.01,followed by proportion of shrub land.The geomorphologic factor (AS2,where S is local slope gradient and A is drainage area),which was considered the predictor of gully initiation,was not related significantly to gully area growth rates owing to high vegetation coverage.3) The vegetation coverage increased by 28.7% and the proportion of woodland and grassland area increased to 94.5% from 2007 to 2013 in the studied area.Non-linear regression curve between growth rates of gully area and vegetation coverage in 30 catchments without check dam,with R2was 0.713,indicated that vegetation coverage larger than 60% in inter-valley significantly inhibited the gully erosion.4) Analysis of variation showed that there was no significant difference of the gully area growth rate between 6 pairs of catchments with and without check dams (with the similar vegetation coverage and geomorphology),indicating that check dam in the area of gully presented no significant effect on the gully development on valley shoulder line.[Conculsions] This study indicates that vegetation restoration owing to the project of returning cropland to forest or grassland reduced gully growth rate significantly,which could provide reference for understanding gully development mechanism and help to select the effective ecological measures to control gully erosion.
the Loess Plateau; gully erosion rate; vegetation coverage; check dam; GIS; QuickBird imagery
2015-09-28
2016-05-20
項(xiàng)目名稱:國(guó)家自然科學(xué)基金“黃土丘陵區(qū)退耕還林對(duì)切溝發(fā)育和侵蝕過程的影響機(jī)制”( 41271301)
楊松(1990—),男,碩士研究生。主要研究方向:自然資源監(jiān)測(cè)與管理。E-mail:564838726@qq.com
簡(jiǎn)介:張巖(1970—),女,副教授,碩士生導(dǎo)師。 主要研究方向:土壤侵蝕和水土保持。E-mail:zhangyan9@bjfu.edu.cn
S157.1
A
1672-3007(2016)04-0018-08
10.16843/j.sswc.2016.04.003