張 飛,金章東,肖 軍,高 強(qiáng),石岳威
(1. 中國(guó)科學(xué)院地球環(huán)境研究所 黃土與第四紀(jì)地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室,西安 710061;2. 青海省水文水資源勘測(cè)局,西寧 810001)
青海湖流域理物理侵蝕速率對(duì)氣候變化的響應(yīng)
張 飛1,金章東1,肖 軍1,高 強(qiáng)2,石岳威2
(1. 中國(guó)科學(xué)院地球環(huán)境研究所 黃土與第四紀(jì)地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室,西安 710061;2. 青海省水文水資源勘測(cè)局,西寧 810001)
氣候要素是否影響地表物理侵蝕速率目前還存在諸多爭(zhēng)議。以往對(duì)此方面的研究均集中在高降雨區(qū)域,而對(duì)于低降雨條件下氣候因子的控制機(jī)制研究目前還很有限。青海湖流域?qū)夂蜃兓置舾?。本文通過(guò)對(duì)青海湖流域最大兩條河流布哈河和沙柳河每日的溫度、降雨、徑流和河流懸浮物濃度進(jìn)行為期一年的持續(xù)監(jiān)測(cè),探討干旱/半干旱區(qū)氣候要素對(duì)地表物理侵蝕速率的影響。研究結(jié)果顯示:在氣候要素中,降雨以及其產(chǎn)生的徑流是青海湖流域物理侵蝕速率最主要的控制因素,而溫度的影響較弱。最重要的是,降雨強(qiáng)度決定著最終的侵蝕速率。布哈河和沙柳河流域一次瞬時(shí)高強(qiáng)度降雨能產(chǎn)生全年侵蝕通量的30%以上。此外,相同流量條件下,季風(fēng)前期的侵蝕速率要高于季風(fēng)中、后期,反映了季風(fēng)前期低溫條件下的冰凍作用,以及春季降塵產(chǎn)生的大量細(xì)顆粒物質(zhì),增加了雨季來(lái)臨時(shí)侵蝕物質(zhì)的輸出。這些認(rèn)識(shí)對(duì)于理解長(zhǎng)時(shí)間尺度上青海湖沉積物鉆孔中沉積速率和古氣候要素的關(guān)系具有重要意義。
物理侵蝕速率;降雨;流量;氣溫;青海湖流域
厘清到底什么因素控制著地表的侵蝕速率,對(duì)于認(rèn)識(shí)大范圍的構(gòu)造和地形問(wèn)題至關(guān)重要,例如對(duì)侵蝕過(guò)程控制機(jī)制的認(rèn)識(shí)可用于探究構(gòu)造隆升和氣候之間潛在的相互反饋;確定極端和頻繁事件對(duì)地形塑造所起的作用;以及分辨沉積盆地沉積物的構(gòu)造和氣候信號(hào)等(DiBiase and Whipple,2011)。已有的研究顯示,地表侵蝕速率強(qiáng)烈依賴于巖性、地形坡度和氣候(Gabet et al,2008;Pelletier and Rasmussen,2009;DiBiase and Whipple,2011)。在這些因素中,目前對(duì)氣候因素的控制作用還頗有爭(zhēng)議。Thiede et al(2004)測(cè)試了喜馬拉雅山的剝蝕速率,指出高的侵蝕速率發(fā)生在最高的降雨區(qū)域,強(qiáng)調(diào)了侵蝕速率隨降雨量的增加而增加。其他學(xué)者的研究也發(fā)現(xiàn)降雨和徑流與侵蝕速率之間呈現(xiàn)良好的相關(guān)性(Dadson et al,2003;Moon et al,2011)。而另外一部分學(xué)者的研究卻并未發(fā)現(xiàn)侵蝕速率和降雨或徑流之間的相關(guān)性,或者認(rèn)為氣候?qū)η治g速率僅有微弱的影響(Riebe et al,2001;Aalto et al,2006)。此外,當(dāng)前對(duì)地表侵蝕的研究多集中在構(gòu)造活動(dòng)強(qiáng)烈的高降雨區(qū),例如中國(guó)臺(tái)灣,阿爾卑斯山,安第斯山,喜馬拉雅山等(Burbank et al,2003;Dadson et al,2003;Gabet et al,2008;Willett,2010;Carretier et al,2013)。但是對(duì)干旱-半干旱低降雨氣候條件下侵蝕過(guò)程的機(jī)理研究還十分匱乏。
位于青藏高原東北隅的青海湖是我國(guó)最大的內(nèi)陸封閉性微咸水湖,地處東亞季風(fēng)、印度季風(fēng)和西風(fēng)急流的匯聚帶,對(duì)氣候和全球環(huán)境變化十分敏感,是開(kāi)展我國(guó)西部環(huán)境變化、青藏高原隆升過(guò)程、環(huán)境效應(yīng)及它們與全球聯(lián)系等研究的熱點(diǎn)區(qū)域。前人通過(guò)沉積物鉆孔開(kāi)展了大量青海湖古、現(xiàn)代環(huán)境演變的研究(黃麒,1988;Lister et al,1991;王蘇民和施雅風(fēng),1992;王云飛,1993;張彭熹等,1994;沈吉等,2001;張恩樓等,2002;劉興起等,2003;Xu et al,2006;Liu et al,2009;An et al,2012;Li et al,2012;安芷生等,2015;Jin et al,2015),在這些研究中一項(xiàng)重要的指標(biāo)就是沉積速率,這是與當(dāng)?shù)厍治g速率直接關(guān)聯(lián)的。不同降雨頻率和降雨量條件可能會(huì)產(chǎn)生侵蝕通量的差異,最終產(chǎn)生湖泊沉積物中不同的沉積速率。這個(gè)侵蝕速率到底是如何受氣候因素調(diào)控的,目前還不清楚。因此,開(kāi)展青海湖流域現(xiàn)代侵蝕過(guò)程的研究對(duì)不同時(shí)間尺度氣候反演具有重要的現(xiàn)實(shí)意義。
青海湖(36°32′—37°15′N,99°36′—100°47′E)位于青藏高原東北隅,是我國(guó)最大的微咸水湖(圖1)。2007年湖面海拔3194 m,湖區(qū)面積4260 km2,流域面積29660 km2(Jin et al,2009)。湖區(qū)地勢(shì)為西北高而東南低,形成東為日月山、南為青海南山、西為天峻山、北為大通山脈所環(huán)繞的內(nèi)陸湖盆地,海拔多在4500 m以上,最高可達(dá)5200 m。湖區(qū)山地約占流域面積的68.6%,其余為河谷和湖積平原。湖區(qū)北部大通山走向北西西,山脈基巖裸露。剛察縣以西主要為三疊紀(jì)砂、頁(yè)巖,東段則為晚寒武紀(jì)變質(zhì)巖,主要巖性為片巖及片麻巖,并有花崗巖侵入體;湖區(qū)西部布哈河谷北側(cè)出露早古生代砂巖、片巖、千枚巖、混合片麻巖和火山巖,并夾結(jié)晶灰?guī)r(中國(guó)科學(xué)院蘭州分院和中國(guó)科學(xué)院西部資源環(huán)境研究中心,1994)。
圖1 布哈河和沙柳河水文氣象數(shù)據(jù)監(jiān)測(cè)點(diǎn)位Fig.1 Monitoring sites of hydrological and meteorological data in the Buha and Shaliu River
青海湖流域?yàn)楦咴箨懶詺夂颍庹粘渥?,日照?qiáng)烈;冬寒夏涼,暖季短暫,冷季漫長(zhǎng),春季多大風(fēng)和沙暴;雨量偏少,雨熱同季,干濕季分明(中國(guó)科學(xué)院蘭州分院和中國(guó)科學(xué)院西部資源環(huán)境研究中心,1994;陳桂琛,2008)。根據(jù)環(huán)湖氣象站的記錄,青海湖年平均降雨量(1951—2005)為336.6 mm,蒸發(fā)量(1959—2000)約為~925 mm(Jin et al,2009)。青海湖流域約90%的降水集中在5—9月(1961—2001)。每年平均溫度變化為0.9—2.7℃,記錄的最高溫度28℃出現(xiàn)在7月,而月平均溫度為12.4℃;最冷的月份為1月,其月平均溫度為-12.7℃,最低溫度-31℃。布哈河和沙柳河流域多年平均潛在蒸發(fā)量為1650 mm和1463 mm。這兩個(gè)流域的月平均氣溫最高出現(xiàn)在7月而最低值出現(xiàn)在1月。從10月中旬開(kāi)始到次年的4月,氣溫一般處于0℃(Zhang et al,2013)。位于湖區(qū)西部的布哈河是青海湖流域最大的河流,貢獻(xiàn)了入湖徑流量的40%以上。布哈河長(zhǎng)286 km,下游最寬處達(dá)22 m,其流域面積為14337 km2,幾乎占了總流域面積的一半。布哈河起源于平均海拔高于4600 m并帶有冰川覆蓋的疏勒南山,其2007年的總流量為10.05×108m3(Zhang et al,2013)。沙柳河起源于最高海拔在5200 m以上的大通山脈,長(zhǎng)109.5 km,流域面積1442 km2(中國(guó)科學(xué)院蘭州分院和中國(guó)科學(xué)院西部資源環(huán)境研究中心,1994)。
本文選取青海湖流域最大的兩條河流布哈河和沙柳河進(jìn)行了為期一年的定點(diǎn)水文氣象數(shù)據(jù)的每日持續(xù)監(jiān)測(cè),布哈河和沙柳河流域的監(jiān)測(cè)時(shí)間分別是2007年和2009年,監(jiān)測(cè)地點(diǎn)在布哈河水文站和沙柳河的剛察水文站(圖1)。監(jiān)測(cè)內(nèi)容包括每日河流懸浮物濃度(SPM)、河水流量、降雨量和氣溫。需要說(shuō)明的是,在這兩個(gè)河流流域,僅監(jiān)測(cè)了5—10月每天的SPM濃度,這是因?yàn)樵谄溆嘣路荩ǜ杉荆?,SPM的濃度非常低。
布哈河和沙柳河流域的物理風(fēng)化速率(PER)通過(guò)懸浮顆粒物濃度(SPM)和河水流量的(Discharge)數(shù)據(jù)估算。計(jì)算公式為:PER = SPM×Discharge。
3.1 日降雨、氣溫、流量、懸浮物濃度變化特征
根據(jù)監(jiān)測(cè)的布哈河和沙柳河水文氣象數(shù)據(jù)(圖2):布哈河2007年的降雨量為389.8 mm,當(dāng)年的日平均溫度變化-20.5—16.5℃(圖2a)。沙柳河流域2009年的降雨量為431.3 mm,當(dāng)年的日平均溫度變化-25.5—12.0℃(圖2d)。兩條河流5—9月的降雨分別形成了布哈河和沙柳河75%和85%的年徑流量,而從12月到次年2月幾乎沒(méi)有降雨產(chǎn)生。在春季,青海湖流域內(nèi)的降雨主要以雪和冰雹的形式發(fā)生,此時(shí)逐漸增加的氣溫導(dǎo)致了冰雪的融化,冰融水開(kāi)始補(bǔ)給河水。
布哈河2007年的總流量為10.05×108m3,每日平均流量從2月的2 m3· s-1增加到8月的220 m3· s-1(圖2b)。沙柳河2009年的總流量為4.23× 108m3,日平均流量變化從2月的0.1 m3· s-1到7月的107 m3· s-1(圖2e)。
每日SPM濃度監(jiān)測(cè)的數(shù)據(jù)顯示:布哈河流域SPM濃度的變化從干季的<0.001 kg · m-3到季風(fēng)期的2.280 kg · m-3,沙柳河流域的SPM濃度從干季的<0.003 kg · m-3到季風(fēng)期的2.350 kg · m-3。
所有這些水文氣象數(shù)據(jù)均顯示了一個(gè)共性,即它們的高值出現(xiàn)于雨季,低值出現(xiàn)于干季。其中兩個(gè)河流流域氣溫的變化從1月到12月呈現(xiàn)良好的正態(tài)分布,徑流的變化在7月出現(xiàn)第一次峰值,隨后到8月份逐漸降低,到9月份出現(xiàn)第二次峰值。懸浮物濃度的變化基本與徑流變化相似,一個(gè)明顯的區(qū)別是布哈河流域的懸浮物濃度全年最高值出現(xiàn)在季風(fēng)來(lái)臨的初期(圖2)。
3.2 物理侵蝕速率與氣候要素之間的關(guān)系
3.2.1 物理侵蝕速率與溫度之間的關(guān)系
圖3a和圖3b展示了沙柳河和布哈河流域的日物理侵蝕速率與氣溫之間的關(guān)系。整體上而言,物理侵蝕速率在雨季達(dá)到全年的最高值,與溫度變化相似,例如兩個(gè)河流的最高值均出現(xiàn)在7月,此時(shí)的溫度也基本上處于全年的最高值。但是物理侵蝕速率的詳細(xì)變化卻未能與溫度形成良好的對(duì)應(yīng),表現(xiàn)在物理侵蝕速率的變化并不是一個(gè)正態(tài)分布狀態(tài)。圖4a和圖4b顯示兩個(gè)流域的物理侵蝕速率與溫度之間呈現(xiàn)指數(shù)函數(shù)的關(guān)系,相關(guān)系數(shù)為0.22和0.17,處于弱正相關(guān)關(guān)系。這個(gè)結(jié)果表明,此區(qū)域的物理侵蝕速率對(duì)溫度變化并不敏感。溫度的作用體現(xiàn)在冬季地表冰凍和春季的解凍過(guò)程,這會(huì)產(chǎn)生巖石和土壤的破碎松動(dòng),使得地表物質(zhì)容易被侵蝕出來(lái)。
圖2 布哈河2007年和沙柳河2009年的水文氣象參數(shù)的日變化Fig.2 Daily variations of air temperature, precipitation, water discharge and suspended particulate material (SPM) content in the Buha River in 2007 and in the Shaliu River in 2009
圖3 布哈河2007年和沙柳河2009年的物理侵蝕速率與氣溫的日變化Fig.3 Daily variations of air temperature and physical erosion rate (PER) in the Buha River in 2007 and in the Shaliu River in 2009
圖4 布哈河2007年和沙柳河2009年的物理侵蝕速率與氣溫(a, b)和河水流量(c, d)的關(guān)系Fig.4 The relationship of daily physical erosion rate (PER) vs. temperature (a, b) and water discharge (c, d) in the Buha River in 2007 and in the Shaliu River in 2009
3.2.2 物理侵蝕速率與徑流和降雨之間的關(guān)系
大氣降雨會(huì)產(chǎn)生地表沖刷,侵蝕地表物質(zhì)。雖然大氣降雨是偶然發(fā)生的,但是降雨會(huì)產(chǎn)生地表徑流,這個(gè)過(guò)程會(huì)長(zhǎng)于降雨過(guò)程。因此,對(duì)河水流量的監(jiān)測(cè)是一個(gè)連續(xù)的過(guò)程,河水中攜帶的侵蝕物質(zhì)也能被連續(xù)監(jiān)測(cè)到。徑流因子從某種程度上講,是對(duì)大氣降雨的一個(gè)體現(xiàn),可以等同于降雨。圖4c和圖4d顯示沙柳河和布哈河流域的物理侵蝕速率和流量呈現(xiàn)良好的指數(shù)正相關(guān),相關(guān)系數(shù)分別為0.77和0.68。這說(shuō)明兩個(gè)流域的地表物理侵蝕速率受到徑流或者降雨的主要控制。此外,在相同河水流量條件下,雨季前期和初期(5—6月)的物理侵蝕速率要高于雨季中、后期(7—10月),這說(shuō)明:(1)5月前的冰凍和冰融過(guò)程產(chǎn)生了大量易侵蝕細(xì)顆粒物質(zhì);(2)青海湖流域春季頻繁的塵暴事件累積了一定量的大氣降塵在地表,這兩個(gè)過(guò)程,在雨季來(lái)臨時(shí)被快速?zèng)_刷出來(lái)。在本文的研究中,冬季氣溫變化從-10℃到-25℃,此時(shí)地表被冰雪覆蓋而處于冰凍狀態(tài)。在這個(gè)過(guò)程中,冰凍侵蝕產(chǎn)生的細(xì)顆粒物質(zhì)最終在季風(fēng)降雨來(lái)臨被輸出到河流系統(tǒng),導(dǎo)致5—7月高的河流沉積物通量。關(guān)于冰凍過(guò)程產(chǎn)生巖石的機(jī)械破碎并產(chǎn)生大量細(xì)顆粒物質(zhì)的現(xiàn)象此前也有學(xué)者在全球其他區(qū)域觀察到(如Galliardet et al,1999;Hun and Edmond,1999)。
此外,總體而言,布哈河和沙柳河流域河水流量越大,物理侵蝕速率越高(圖4c和圖4d),反應(yīng)了氣候因素中降雨產(chǎn)生的徑流對(duì)地表侵蝕的控制作用。但是降雨量與物理侵蝕速率的關(guān)系并不是一個(gè)簡(jiǎn)單的正相關(guān)關(guān)系。圖5a和圖5b中顯示兩個(gè)流域均有一些異常高的物理侵蝕速率值(紅色方框標(biāo)記),而每個(gè)異常高值分別對(duì)應(yīng)著一次強(qiáng)降雨。例如:在沙柳河流域,2009年7月15日5小時(shí)內(nèi)發(fā)生的一次26 mm強(qiáng)降雨產(chǎn)生了21078噸的河流懸浮物通量,本次短時(shí)間降雨僅占全年降雨總量的6.07%,卻產(chǎn)生了高達(dá)全年懸浮物通量的35.2%。相比較而言,隨后整個(gè)8月份產(chǎn)生的92 mm降雨卻僅產(chǎn)生了6937噸的懸浮物通量,低于單次26 mm強(qiáng)降雨產(chǎn)生懸浮物總量的1/3。同樣,在布哈河流域2007年7月18日產(chǎn)生的一次19.7 mm降雨(占全年降雨量的5.05%)產(chǎn)生的懸浮物通量高達(dá)全年通量的22.5%。圖5b中其他用紅色方框標(biāo)記的也各自對(duì)應(yīng)了一次瞬時(shí)強(qiáng)降雨事件。
圖5 布哈河2007年和沙柳河2009年的物理侵蝕速率與降雨的日變化Fig.5 Daily variations of precipitation and physical erosion rate (PER) in the Buha River in 2007 and in the Shaliu River in 2009
這些結(jié)果表明,在干旱半干旱的青海湖流域,地表侵蝕過(guò)程受到降雨和徑流的影響,但是降雨因素中,降雨的強(qiáng)度是決定侵蝕速率的一個(gè)非常重要的因素,一次短時(shí)高強(qiáng)降雨時(shí)間可能產(chǎn)生全年侵蝕通量的30%以上。
本文通過(guò)對(duì)青海湖兩條最大河流布哈河和沙柳河流域日氣溫、降雨、流量以及懸浮物濃度持續(xù)一年的監(jiān)測(cè),分析了氣候因素與干旱/半干旱區(qū)物理侵蝕速率的關(guān)系。結(jié)果表明,在氣候因素中,物理侵蝕速率主要受降雨控制,而氣溫的影響較弱。更重要的是,單次的瞬時(shí)降雨強(qiáng)度而非總降雨量對(duì)地表侵蝕速率的主導(dǎo)作用非常明顯。這個(gè)過(guò)程對(duì)于理解青海湖湖泊沉積物鉆孔中沉積物速率與古氣候之間的關(guān)系提供了重要的實(shí)測(cè)數(shù)據(jù)支持。
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Response of physical erosion rates to climate changes within the Lake Qinghai catchment
ZHANG Fei1, JIN Zhangdong1, XIAO Jun1, GAO Qiang2, SHI Yuewei2
(1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China; 2. Bureau of Hydrology and Water Resources of Qinghai Province, Xining 810001, China)
Background, aim, and scopeWhether climate controls erosion rates is still controversial. Some found significant correlations between erosion rates and climate (e.g. precipitation, discharge), and others didn't. In addition, Previous studies were mainly focused on high precipitation and tectonic active regions, such as the Taiwan, Alps, Andes and Himalayas, whereas the study in low precipitation area remains scarce. Lake Qinghai catchment, located in the NE Tibetan Plateau, is quite sensitive to climate changes. The detailed hydrological and meteorological data in the Lake Qinghai catchment provides a good opportunity to in depth understand the effects of climate factors on erosion rate in arid/semi-arid regions.Materials and methodsThe daily air temperature, water discharge, precipitation and suspended particulate material (SPM) were monitored over a whole year period in the Buha River hydrological station in 2007 and the Shaliu River station in 2009. The water discharge was monitored twice a day, and when in flood periods, the monitoring frequency was increased to 4—5 times one day. The daily SPM concentration was measured only from theperiod of May to October, since the SPM was extremely low during dry seasons. The physical erosion rates (PER) were calculated by daily SPM concentration multiplied water discharge.ResultsThe results showed that the courses of air temperature, water discharge, precipitation and suspended river sediment in the Buha and Shaliu Rivers exhibited great seasonal variations with their high values during the monsoon seasons and low values during the dry seasons. In the Buha River catchment, the total precipitation was 389.8 mm in 2007, and the mean daily air temperature and water discharge during a year ranged from -20.5℃ to 16.5℃and from 2 m3· s-1in February to 220 m3· s-1in August, respectively. In the Shaliu River catchment, the annual precipitation was 431 mm in 2009, with the mean daily air temperature and water discharge ranging from -25.5℃ to 12℃ and 0.1 m3· m-1in February to 107 m3· s-1in July, respectively. It is observed that the daily SPM concentrations vary from <0.001 kg · m-3in the dry season to 2.280 kg · m-3in the monsoon in the Buha River, and from <0.003 kg · m-3to 2.350 kg · m-3in the Shaliu River. Seasonal PER exhibits signifi cant changes in the Buha and Shaliu Rivers. The SPM fluxes vary from <1 tons · day-1in the dry season to 23155 tons · day-1in the monsoon in the Buha River, and from <1 tons · day-1to 21078 tons · day-1in the Shaliu River.DiscussionThe relationships between temperature and erosion rates were weak (R2is 0.22 and 0.17 in the Shaliu and Buha River, respectively), whereas the water discharge showed good correlation with erosion rates (R2is 0.77 and 0.68 in the Shaliu and Buha River, respectively). In addition, some samples with abnormally high physical erosion rates were corresponded to heavy rain events. In the Shaliu River, a 26 mm rainfall within 5 hours on 15th, July generated 21078 tons of SPM, accounting for 35.2% of the total year fl ux in 2009, whereas totally 92 mm rainfall in the entire August only contributed 6937 tons. Similarly, a highest daily rainfall (20 mm) in the Buha River brought in a highest SPM fl ux on 18th July, and produced 22.5% of the total year fl ux in 2007. Meanwhile, it was also observed that at a given discharge, the suspended sediment fl ux was generally higher from May to June than from July to October in both river catchments, indicating that freezing processes before monsoon and input of eolian dust in spring season provided large volumes of fi ne materials which was fi nally fl ushed out at the onset of monsoonal rainfall resulting in the high suspended sediment fl uxes during the pre-monsoon period.ConclusionsIn the Lake Qinghai catchment, water discharge and precipitation controls erosion rates, but temperature shows only a weak control. The most important is that precipitation intensity rather than total rainfall volume or discharge fi nally determines the erosion rates. One storm rain event can produce more than 30% of the total yearly suspended river sediment flux. Higher erosion rates during pre-monsoon period than mid- and post-monsoon are ascribed to freezing erosion and eolion dust input.Recommendations and perspectivesWe firstly detailed discussed whether temperature, discharge and precipitation control physical erosion rate in the two largest rivers in the Lake Qinghai catchment, which are critical to explore their relationships at arid/semi-arid climate condition. Also, this study is signifi cant for understanding the relationship between deposition rate of Lake Qinghai sediment and paleoclimate changes over long-time scale.
physical erosion rate; precipitation; water discharge; temperature; Lake Qinghai
ZHANG Fei, E-mail: zhangfei@ieecas.cn
10.7515/JEE201606006
2016-07-08;錄用日期:2016-10-26
Received Date:2016-07-08;Accepted Date:2016-10-26
國(guó)家自然科學(xué)基金項(xiàng)目(41403111);中國(guó)科學(xué)院西部之光項(xiàng)目
Foundation Item:National Natural Science Foundation of China (41403111); West Light Foundation of Chinese Academy of Sciences
張 飛,E-mail: zhangfei@ieecas.cn