青藏高原納木錯(cuò)流域冰雪融水徑流量估算

陳飛,蔡強(qiáng)國(guó),孫莉英?

(1.中國(guó)農(nóng)業(yè)大學(xué)水利與土木工程學(xué)院,100083,北京;2.中國(guó)科學(xué)院地理科學(xué)與資源研究所陸地水循環(huán)與地表過(guò)程重點(diǎn)實(shí)驗(yàn)室,100101,北京)

青藏高原納木錯(cuò)流域冰雪融水徑流量估算

陳飛1,2,蔡強(qiáng)國(guó)2,孫莉英2?

(1.中國(guó)農(nóng)業(yè)大學(xué)水利與土木工程學(xué)院,100083,北京;2.中國(guó)科學(xué)院地理科學(xué)與資源研究所陸地水循環(huán)與地表過(guò)程重點(diǎn)實(shí)驗(yàn)室,100101,北京)

冰川積雪是寒區(qū)固體水資源的重要組成部分,在全球氣候變暖背景下,進(jìn)行冰雪融水量的計(jì)算具有重要意義。本文以青藏高原納木錯(cuò)流域?yàn)檠芯繉?duì)象,近40年,納木錯(cuò)流域氣溫增幅達(dá)到0.04℃/a,所對(duì)應(yīng)的是流域冰川年退縮率達(dá)到1.12 km2/a,湖泊面積擴(kuò)張速率達(dá)到2.1 km2/a。借助第1、2次中國(guó)冰川編目數(shù)據(jù)和MODIS遙感影像資料,基于水量平衡建立冰川體積-融水徑流量經(jīng)驗(yàn)公式(包括冰川區(qū)降水量、冰川消融量、蒸發(fā)量和融水徑流量等參數(shù)),并采用SRM積雪消融模型(積雪覆蓋衰減率、氣溫直減率、度日因子值和徑流系數(shù)等參數(shù)),分別對(duì)納木錯(cuò)流域內(nèi)的冰川和積雪融水徑流量進(jìn)行估算,從而實(shí)現(xiàn)大尺度稀缺資料的高寒地區(qū)水文模擬。結(jié)果表明:納木錯(cuò)流域年均冰川融水徑流量是2.99億m3/a,積雪融水徑流量是8.10萬(wàn)m3/a,冰川融水量約是積雪融水量的38倍。通過(guò)比較納木錯(cuò)流域氣溫升高、冰川退縮和湖泊擴(kuò)張之間的關(guān)系,納木錯(cuò)湖泊增加水量約為流域內(nèi)冰川融水徑流量的80%,遠(yuǎn)高于季節(jié)性積雪融水的補(bǔ)給量;因此,可以推測(cè)隨著氣候變暖,納木錯(cuò)流域東南側(cè)念青唐古拉山大面積冰川的急劇消融,是造成納木錯(cuò)湖泊擴(kuò)張的重要原因之一。

冰川;積雪;中國(guó)冰川編目;積雪消融模型;融水徑流量

隨著全球氣候變暖形勢(shì)加劇,寒區(qū)冰川積雪大量消融,導(dǎo)致冰雪融水徑流增加,給生態(tài)環(huán)境帶來(lái)嚴(yán)重威脅[1]。青藏高原被稱作“亞洲水塔”,區(qū)域內(nèi)冰川積雪分布廣泛,固體水資源量相當(dāng)豐富。據(jù)中國(guó)第2次冰川編目統(tǒng)計(jì)資料,2008年青藏高原冰川面積達(dá)4萬(wàn)5 045.2 km2[2]。2003—2010年期間,青藏高原常年積雪分布面積約占整個(gè)青藏高原的13.3%[3]。青藏高原作為氣候敏感區(qū)和生態(tài)脆弱區(qū),伴隨著氣溫升高,固體水資源變化產(chǎn)生的生態(tài)影響更為顯著。李治國(guó)[4]研究發(fā)現(xiàn),氣候變化背景下,近50年青藏高原冰川以退縮為主,而湖泊水量以增加為主。段水強(qiáng)[5]認(rèn)為氣溫、降水和蒸發(fā)等因子都會(huì)對(duì)湖泊的水量平衡產(chǎn)生重要影響。辛?xí)远琜6]對(duì)藏東南然烏湖流域,1980—2005年冰川和湖泊變化間關(guān)系進(jìn)行分析,認(rèn)為湖泊加速擴(kuò)張,主要受到冰川退縮、融水徑流量加大的影響。而青藏高原引起湖泊擴(kuò)張的主要驅(qū)動(dòng)力尚未清晰;因此,研究冰川融水與湖泊擴(kuò)張的關(guān)系,對(duì)于揭示氣候變暖背景下,寒區(qū)冰雪消融規(guī)律具有非常重要的意義。

由于青藏高原環(huán)境惡劣,針對(duì)冰川積雪融水徑流量的估算,傳統(tǒng)定點(diǎn)實(shí)測(cè)方法局限性較大,一般采用寒區(qū)水文模擬的方法。寒區(qū)水文模型按照研究對(duì)象,可以分為冰川消融模型[7]、積雪消融模型[8]和凍土水文模型[9];按照數(shù)據(jù)處理方式,可以分為基于氣象因子的統(tǒng)計(jì)模型[10-12]和基于物理機(jī)制的能量平衡模型[13-14];按照水文過(guò)程,可以劃分為融水產(chǎn)流模型[15-16]和融水匯流模型[17]。但由于青藏高原冰雪融水模擬研究,面臨的主要問(wèn)題是缺乏全面、同步的觀測(cè)數(shù)據(jù),較難實(shí)現(xiàn)大尺度的寒區(qū)水文模擬及驗(yàn)證[18]。在地理信息技術(shù)的支持下,借助遙感影像資料,進(jìn)行冰凍圈宏觀區(qū)域的研究,非常高效便捷[19]。隨著地理信息系統(tǒng)的發(fā)展,在集總式水文模型的基礎(chǔ)上,又產(chǎn)生了分布式冰雪消融模型[20]。冰川消融一般借助多時(shí)相遙感影像資料,通過(guò)統(tǒng)計(jì)冰川規(guī)模參數(shù)變化來(lái)反映氣候變化。例如,馮童等[21]和孫美平等[22]分別基于第1、2次中國(guó)冰川編目數(shù)據(jù),對(duì)近半個(gè)世紀(jì)葉爾羌河流域和祁連山的冰川變化進(jìn)行了分析。而針對(duì)積雪消融應(yīng)用較為廣泛的就是SRM積雪消融模型。SRM模型首先由Marrtinec于1975年提出,1983年由NASA組織正式公布,已經(jīng)在全球112個(gè)流域(面積0.76～91萬(wàn)7 444 km2,海拔346～7 690m)成功通過(guò)了世界氣象組織評(píng)價(jià)測(cè)試(R2＞0.8)[23]。SRM模型以MODIS積雪遙感資料為主要輸入?yún)?shù),在青藏高原氣候環(huán)境惡劣,數(shù)據(jù)資料稀缺地區(qū)存在應(yīng)用優(yōu)勢(shì)。

本文選取青藏高原納木錯(cuò)流域?yàn)檠芯繉?duì)象,針對(duì)研究區(qū)域范圍較大、冰雪資源豐富、監(jiān)測(cè)數(shù)據(jù)較少的情況,采用中國(guó)冰川編目數(shù)據(jù)和MODIS遙感影像資料,估算大尺度資料稀缺的高寒地區(qū)冰雪融水徑流量。在此基礎(chǔ)上,進(jìn)一步探討納木錯(cuò)流域氣溫升高、冰川消融與湖泊擴(kuò)張之間的關(guān)系,從而深化對(duì)全球氣候變暖形勢(shì)下高寒區(qū)冰雪消融規(guī)律的理解和認(rèn)識(shí)。

1 研究區(qū)概況

納木錯(cuò)流域位于青藏高原中南部(E 89°30＇～91°25＇,N 30°00＇～1°10＇),念青唐古拉山脈西段的北麓,流域面積1萬(wàn)610 km2。流域內(nèi)具有開(kāi)展地表過(guò)程及環(huán)境監(jiān)測(cè)的各類高寒區(qū)環(huán)境介質(zhì),例如冰川、季節(jié)積雪、高山凍土、湖泊、高寒草原(草甸)和濕地等。作為高原亞大陸型冰川集中分布區(qū)之一,流域內(nèi)冰川分布較廣,并以小規(guī)模冰川(0.01～2.74 km2)為主,冰川分布總面積為182 km2。近40多年,納木錯(cuò)流域氣溫增幅達(dá)到0.04℃/a,所對(duì)應(yīng)的是流域冰川年退縮率達(dá)到1.12 km2/a,湖泊面積擴(kuò)張速率達(dá)到2.1 km2/a[24]。另外,流域年均積雪覆蓋約為20%,高海拔區(qū)域季節(jié)性積雪,在春末夏初,積雪消融產(chǎn)生大量融水,匯入納木錯(cuò)湖泊。流域太陽(yáng)輻射強(qiáng),年平均氣溫低于0℃。全年降水量主要集中在夏季6—10月份(92.7%),降水形態(tài)多以固體型降雪為主。

2 數(shù)據(jù)與方法

2.1冰川融水量估算

中國(guó)2次冰川編目數(shù)據(jù)均以遙感影像資料的提取為主,詳細(xì)記錄冰川的面積、儲(chǔ)量和長(zhǎng)度等重要參數(shù),為進(jìn)行冰川體積-消融量的模擬估算提供數(shù)據(jù)支持。劉時(shí)銀等[25]根據(jù)第1次冰川編目資料,擬合了納木錯(cuò)流域253條冰川面積和冰儲(chǔ)量之間的關(guān)系:

式中:S為冰川投影面積,km2;V為冰川儲(chǔ)量或體積,km3。

式中:Qg為冰川消融量,mm;Vg為冰川變化體積, km3;取冰的密度ρ=0.9 g/cm3。由于冰川表面強(qiáng)烈消融,幾乎所有的下滲水(降雨和消融水)都以融水徑流的形式流出,而且由于納木錯(cuò)流域的冰川普遍短小,冰川融水流經(jīng)短距離的山前地帶,直接注入納木錯(cuò)湖泊;因此,假定融水徑流過(guò)程入滲量可以忽略?；诹饔虮▍^(qū)水量平衡方程式

式中:P為冰川區(qū)降水量,mm;Qg為冰川消融量, mm;Qw為冰川區(qū)地表徑流深,mm;E為冰川表面蒸發(fā)量,mm。

通過(guò)獲取納木錯(cuò)流域冰川區(qū)年降水、冰川表面蒸發(fā)等參數(shù),最終估算得到納木錯(cuò)流域總徑流量。最后,依據(jù)朱立平等[26]獲取納木錯(cuò)流域冰川融水補(bǔ)給比例,得到流域的年均冰川融水徑流量。

2.2積雪融水量估算方法

SRM模型是一種使用度日因子法的概念性水文模型,用流域積雪覆蓋率變化,控制產(chǎn)流過(guò)程,用氣溫因子控制其融雪過(guò)程,一般用于和實(shí)測(cè)徑流的模擬對(duì)比、短期徑流預(yù)測(cè)和氣候變化的潛在影響評(píng)估3個(gè)方面,并在模擬和預(yù)測(cè)徑流量上有著很好的精度[27]。SRM模型的變量主要有積雪覆蓋面積變化、氣溫和降水;而模型的輸入變量主要包括積雪度日因子值、溫度直減率、氣溫臨界值、徑流系數(shù)、退水系數(shù)和融水徑流滯后時(shí)間等。

納木錯(cuò)流域年均積雪覆蓋超過(guò)20%,尤其是在流域東南側(cè)念青唐古拉山脈沿線,分布著空間上集中、時(shí)間上持久的季節(jié)性積雪,春末夏初積雪消融產(chǎn)生大量融水匯入納木錯(cuò)湖泊。2005年中科院青藏高原研究在此建站,已經(jīng)初步積累一批氣象和水文觀測(cè)數(shù)據(jù),便于SRM模型應(yīng)用基本參數(shù)(度日因子、徑流系數(shù)等)的提取;同時(shí),MODIS遙感影像資料又可以為該區(qū)域的積雪變化提供數(shù)據(jù)支撐。針對(duì)納木錯(cuò)流域范圍較大(＞1萬(wàn)km2),位于高寒地區(qū)監(jiān)測(cè)數(shù)據(jù)偏少的狀況,可選擇SRM模型,進(jìn)行整個(gè)流域積雪融水徑流量的模擬估算。本研究所應(yīng)用SRM軟件版本是WinSRM Version 1.11。

2.2.1不同高度帶的積雪面積衰減曲線 遙感數(shù)據(jù)提取的積雪面積是SRM模型非常重要的輸入變量。很多國(guó)內(nèi)外研究[28-31]表明,采用MODIS數(shù)據(jù)到SRM模型,具有非常好的模擬效果。筆者選擇8 d合成的MODIS積雪數(shù)據(jù)MOD10A2作為主要輸入數(shù)據(jù)源。MOD10A2 8 d合成數(shù)據(jù)的空間分辨率500m,積雪識(shí)別率高達(dá)87.5%,便于使用該數(shù)據(jù)對(duì)積雪消融變化進(jìn)行空間提取和分析[32]。采用納木錯(cuò)流域2004—2013年共10年的MODIS積雪覆蓋資料(460張柵格圖),通過(guò)統(tǒng)計(jì)MOD10A2影像,在納木錯(cuò)流域融雪季節(jié)的積雪覆蓋變化,確定流域的積雪覆蓋率衰減曲線。將流域根據(jù)海拔高度劃分為5個(gè)高程帶,分別是A(4 718～4 730 m),B(4 730～5 000m),C(5 000～5 300m),D(5 300～5 600m)和E(5 600～6 612 m)高程帶。各個(gè)高程帶的積雪覆蓋率曲線如下圖1所示。

圖1　納木錯(cuò)流域融雪期積雪覆蓋衰減曲線Fig.1 Depletion curves of the snow coverage in different zones of the Nam Co basin

由圖1可見(jiàn),納木錯(cuò)流域海拔越高,積雪覆蓋比率越大,但隨著時(shí)間推移,各高程帶積雪逐漸消融。其中,消融量最大的時(shí)間段是5月中旬。另外,海拔4 718～4 730m的A高程帶主要為納木錯(cuò)湖泊,不存在長(zhǎng)期積雪,因此,未進(jìn)行統(tǒng)計(jì)和顯示。

2.2.2氣溫和降水 溫度在融雪徑流過(guò)程中起著十分重要的作用,通過(guò)氣溫直減率,將實(shí)測(cè)數(shù)據(jù)推算到各高程帶的平均高度處。根據(jù)謝健等[33]在念青唐古拉山脈的研究成果,確定納木錯(cuò)流域氣溫直減率取值為0.65℃/hm。設(shè)置氣溫臨界值為2℃,通過(guò)降雨貢獻(xiàn)面積的控制選項(xiàng),確定降水形態(tài)在徑流估算中的影響作用。

2.2.3度日因子計(jì)算 度日因子被認(rèn)為是SRM模型中最為敏感的參數(shù)之一。據(jù)萬(wàn)欣等[34]的研究,納木錯(cuò)流域積雪密度為0.15～0.20 g/cm3,度日因子按照公式4計(jì)算獲取;另外,通過(guò)納木錯(cuò)流域物質(zhì)平衡花桿實(shí)測(cè)結(jié)果,該區(qū)域度日因子值設(shè)置為5.3 mm/(d·℃)。

式中:a為度日因子值,cm/(d·℃);ρs為積雪密度, g/cm3;ρw為水的密度,g/cm3。

2.2.4徑流系數(shù) 徑流系數(shù)α是一定匯水面積內(nèi),徑流深度與降水量的比值(0＜α＜1)。根據(jù)孜來(lái)布·阿不來(lái)提[35]的研究,冰川融水徑流系數(shù)值要明顯高于降水,普遍可達(dá)0.6～0.8,結(jié)合曲嘎切流域水文觀測(cè)資料,確定了納木錯(cuò)流域融水徑流系數(shù)取值為0.7。

2.2.5退水系數(shù) 退水系數(shù)一般是用無(wú)降水或融雪時(shí)期,后一天徑流量與前一天的比值,能夠反映每日融水直接補(bǔ)給到徑流量中的貢獻(xiàn)比例。

式中:k為退水系數(shù);Q為日均流量,m3/s;x和y為根據(jù)Qn和Qn+1的雙對(duì)數(shù)散點(diǎn)圖確定的2個(gè)常數(shù)。

表1列舉中國(guó)寒區(qū)流域SRM積雪融水徑流模擬過(guò)程中的退水系數(shù),參照該系數(shù)的取值范圍,本研究選取了寒區(qū)多個(gè)流域的融水徑流退水系數(shù),通過(guò)計(jì)算其算術(shù)平均值,最終確定了納木錯(cuò)流域的退水系數(shù)方程中的2個(gè)常數(shù)x=1.05,y=-0.05。

3 結(jié)果與討論

3.1納木錯(cuò)流域冰川融水徑流量估算

采用ArcGIS 10.0統(tǒng)計(jì)2次冰川編目數(shù)據(jù),納木錯(cuò)流域冰川總面積由195.89 km2縮減為182.45 km2,退縮速率達(dá)到1.12 km2/a,代入式(1)可得冰川總體積由53.99 km3變?yōu)?9.03 km3,再代入式(2),估算冰川年均消融量達(dá)到0.41 km3,即4.10億m3/a,再除以冰川區(qū)分布面積,則換算成冰川消融年均水當(dāng)量49.15mm/a。通過(guò)查閱文獻(xiàn),獲得納木錯(cuò)流域扎當(dāng)冰川(5 400 m)區(qū)域的降水量和冰川表面蒸發(fā)量,并用來(lái)代表整個(gè)流域冰川區(qū)水量平衡方程中降水量和蒸發(fā)量的參數(shù)取值,結(jié)合冰川消融量代入式(3),得到流域冰川區(qū)地表總徑流深為355.35mm(表2)。朱立平等[26]的研究結(jié)果是,納木錯(cuò)流域1971—1991年和1992—2004年,冰川融水補(bǔ)給比例分別是8.55%和11.48%,流域冰川融水補(bǔ)給比例平均為10.02%,據(jù)此,推算納木錯(cuò)流域冰川年均徑流深是35.61mm,換算成融水徑流量是2.99億m3/a。

表1　SRM積雪融水徑流模擬退水系數(shù)Tab.1 Simulated recession coefficients of SRM in different basins

表2　納木錯(cuò)流域冰川區(qū)水量平衡方程參數(shù)Tab.2 Water balance factors in the glacier area of Nam Co basin

針對(duì)納木錯(cuò)流域冰川消融的研究大都借助于多時(shí)相遙感影像資料,通過(guò)統(tǒng)計(jì)冰川規(guī)模參數(shù)的變化來(lái)反映氣候變化的影響,研究?jī)?nèi)容主要包括冰川自動(dòng)提取方法、冰川變化特性、冰川消融對(duì)氣候變化響應(yīng)規(guī)律、冰川融水量與湖泊水量的平衡關(guān)系等4個(gè)方面[39-41]。在全球氣候變暖背景下,納木錯(cuò)流域冰川退縮加劇,并對(duì)該區(qū)域的整個(gè)環(huán)境產(chǎn)生較大影響;但由于缺乏足夠的站點(diǎn)數(shù)據(jù),這些研究只關(guān)注了納木錯(cuò)流域冰川消融量,針對(duì)冰川融水徑流量的研究較少,僅在流域中的扎當(dāng)冰川(＜2 km2)進(jìn)行了長(zhǎng)期、系統(tǒng)的觀測(cè)和模擬,包括冰川物質(zhì)能量平衡的消融規(guī)律監(jiān)測(cè)[42],夏季冰川融水徑流規(guī)律模擬[43-44]等。筆者充分利用2次冰川編目數(shù)據(jù),通過(guò)建立水量平衡的方法,估算納木錯(cuò)流域的冰川融水量,為青藏高原稀缺資料地區(qū)冰川消融研究,提供一種簡(jiǎn)單方法。通過(guò)對(duì)比納木錯(cuò)流域的冰川退縮率和年均融水徑流量,可以推算出在現(xiàn)有氣候環(huán)境等條件不變的情況下,納木錯(cuò)流域冰川將在200年內(nèi)消融殆盡。

3.2納木錯(cuò)流域積雪融水量估算

通過(guò)輸入SRM積雪消融模擬軟件各參數(shù)(圖2),對(duì)納木錯(cuò)流域積雪融水徑流量進(jìn)行估算。SRM積雪消融模型的模擬結(jié)果顯示,納木錯(cuò)流域年均融雪徑流量模擬預(yù)測(cè)值達(dá)到80.95萬(wàn)m3。每年春末夏初(5月份),季節(jié)性積雪提供數(shù)量可觀的融水, 7—8月份盡管整個(gè)流域融水徑流量最大,但主要是由冰川消融補(bǔ)給為主,此時(shí)積雪融水量相對(duì)較少。對(duì)比流域內(nèi)冰川年均融水徑流量2.99億m3/a,是季節(jié)性積雪年均融水徑流量的38倍。

圖2　SRM模擬軟件參數(shù)設(shè)置Fig.2 Parameters of SRM

國(guó)內(nèi)外學(xué)者針對(duì)納木錯(cuò)流域積雪消融的研究,目前主要集中于季節(jié)性積雪分布變化[45-46]和消融特性[47],較少涉及到積雪融水量的估算。筆者借助于MODIS遙感影像資料和氣象水文數(shù)據(jù),結(jié)合高寒區(qū)其他流域SRM模型應(yīng)用參數(shù),從而實(shí)現(xiàn)了青藏高原大尺度的積雪融水徑流量估算;但由于納木錯(cuò)流域范圍較大,缺少水文徑流數(shù)據(jù),目前,只進(jìn)行流域季節(jié)性積雪融水徑流量的模擬預(yù)測(cè),并未與整個(gè)流域積雪融水徑流的實(shí)測(cè)數(shù)據(jù)進(jìn)行對(duì)比驗(yàn)證。

3.3納木錯(cuò)湖泊擴(kuò)張與流域冰雪融水量關(guān)系

基于不同時(shí)期衛(wèi)星遙感影像,對(duì)比納木錯(cuò)流域湖泊和冰川面積的動(dòng)態(tài)變化,進(jìn)一步討論氣候變化背景下,冰雪融水量對(duì)地表水資源變化的影響。查閱資料,獲取納木錯(cuò)流域近半個(gè)世紀(jì)湖泊面積變化情況(表3)。

近半個(gè)世紀(jì)納木錯(cuò)流域湖泊面積持續(xù)增加,尤其近幾年,湖泊擴(kuò)增率明顯加快。一些研究認(rèn)為,冰川消融加劇、高山凍土退化及降水量增加,均可引起納木錯(cuò)湖泊的擴(kuò)張[50]。納木錯(cuò)是典型的內(nèi)陸封閉湖泊,Zhou Shiqiao等[51]采用水量平衡方法,將納木錯(cuò)流域劃分為冰川區(qū)和非冰川區(qū),2008年基于2個(gè)區(qū)域不同的徑流系數(shù),對(duì)湖泊水量進(jìn)行估算,結(jié)果表明:降水和冰川融水分別占到入湖總水量的23%～28%和7%～22%。吳艷紅等[48]估算納木錯(cuò)湖泊總儲(chǔ)水量為816.85億m3,2007年單純季節(jié)性積雪消融量與湖泊總水量大概相差4個(gè)數(shù)量級(jí);再根據(jù)研究,納木錯(cuò)湖泊水量從1971—2004年,湖泊面積擴(kuò)增95.38 km2,而湖泊水量增加80.54億m3。比較同時(shí)段流域內(nèi)的冰川變化,冰川面積減少30.72 km2,固態(tài)水儲(chǔ)量減少111.73億m3。

圖3　納木錯(cuò)流域氣溫插值(克里金法)Fig.3 Air temperature interpolation in Nam Co basin (Kriging)

氣溫、降水和蒸發(fā)的變化,對(duì)湖泊的補(bǔ)給和消耗產(chǎn)生重要影響。其中,氣溫升高引起的冰川加速融化,是其中最為活躍的因子,而降雨和蒸發(fā)變化,則可以直接影響水量平衡的變化。納木錯(cuò)流域東南側(cè)念青唐古拉山冰川以小型冰川為主,對(duì)氣候變化的響應(yīng)更為敏感[52]。通過(guò)搜集流域臨近6個(gè)國(guó)家級(jí)氣象站點(diǎn)(班戈、當(dāng)雄、拉薩、那曲、申扎和日喀則)多年氣象數(shù)據(jù),在ArcGIS軟件中進(jìn)行插值處理,獲得納木錯(cuò)流域年均溫?cái)?shù)據(jù)(圖3)。從1970年開(kāi)始,納木錯(cuò)流域氣溫增幅達(dá)到0.04℃/a,尤其近年來(lái),增溫幅度更為明顯。而與之對(duì)應(yīng)的則是流域冰川退縮加劇(冰川退縮率1.12 km2/a),冰川融水徑流量將近3億m3/a,湖泊水量劇增(湖泊面積擴(kuò)增率2.1 km2/a),湖泊水量多年平均增加水量2.37億m3/a,由此粗略推算,納木錯(cuò)湖泊增加水量約為流域內(nèi)冰川融水量的80%,并遠(yuǎn)高于季節(jié)性積雪融水的補(bǔ)給量。高壇光等[53]采用氣溫-輻射融冰法和ANDERSIN與BRAUN融雪模擬方法,發(fā)現(xiàn)曲嘎切流域冰川消融補(bǔ)給占到整個(gè)流域的一半以上,隨著流域降水的增加,冰川消融量雖在逐年增加,但在總徑流的比例卻逐漸減少。張國(guó)帥等[54]通過(guò)徑流分割法,對(duì)曲嘎切流域內(nèi)冰川區(qū)和非冰川區(qū)產(chǎn)流進(jìn)行分割,結(jié)果表明,冰川融水對(duì)流域徑流的補(bǔ)給占到總徑流量的60%～80%;因此,冰川融水在納木錯(cuò)流域冰川分布區(qū)域占據(jù)著主要的徑流補(bǔ)給來(lái)源。GaoTanguang等[55]在J2000模型基礎(chǔ)上,設(shè)計(jì)納木錯(cuò)湖泊水位模擬的J2000水量平衡模型,情景模擬結(jié)果顯示,納木錯(cuò)湖泊水位上升是氣溫和降水雙重作用影響的結(jié)果。由此可以推斷,忽略降雨變化和積雪、凍土消融的補(bǔ)給,該區(qū)域冰川加劇消融所產(chǎn)生的大量融水徑流,是構(gòu)成納木錯(cuò)湖泊擴(kuò)張的重要原因之一;然而,在大尺度稀缺資料的高寒地區(qū),仍需要結(jié)合遙感影像資料,加大觀測(cè)工作,并進(jìn)一步深入探討氣候變暖背景下,冰川和湖泊的響應(yīng)關(guān)系。

4 結(jié)論

納木錯(cuò)流域冰川融水量采用冰川體積-徑流量經(jīng)驗(yàn)公式進(jìn)行估算,流域年均冰川融水徑流量是2.99億m3/a;積雪融水量采用SRM模型進(jìn)行估算,流域年均積雪融水徑流量是810萬(wàn)m3/a,冰川融水量約是積雪融水量的38倍。本研究實(shí)現(xiàn)了稀缺資料的高寒地區(qū)冰川積雪融水量估算,并通過(guò)對(duì)比納木錯(cuò)流域氣溫變化、冰川退縮和湖泊擴(kuò)張之間的關(guān)系,可以初步推測(cè)隨著氣候變暖,納木錯(cuò)流域東南側(cè)念青唐古拉山大規(guī)模冰川的急劇消融,是造成納木錯(cuò)湖泊擴(kuò)張的重要原因之一。

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Estimation ofmeltwater runoff from glaciers and snow cover in Nam Co basin,Tibetan Plateau

Chen Fei1,2,Cai Qiangguo2,Sun Liying2

(1.College ofWater Resources and Civil Engineering,China Agricultural University,100083,Beijing,China;2.Key Laboratory ofWater Cycle and Related Land Surface Processes,Institute of Geographical Sciences and Natural Resources Research,Chinese Academy of Sciences,100101,Beijing,China)

[Background]Glacier and snowcover are the key parts of solid water resources in high alpine region,quantitative calculation of meltwater is of significance.Based on the data from the First and Second Glacier Inventory of China,the glacier area in Nam Co basin has shrunk 13.44 km2at a rate of 1.12 km2/a in ten years.From the MODIS data(MOD10A2),the seasonal snow produced a large number ofmeltwater into Nam Co Lake.In the past forty years,the temperature increased at0.04℃/a, the solid water resources(glacier)reduced by 111.73×108m3,and the lake area increased nearly 100 km2at rate of 2.1 km2/a.However,the correlation between temperature rising,glaciermelting and lakeexpansion is not yet clear in Nam Co basin of the Tibetan Plateau,thus itwas selected as the study area, where is covered with glaciers(141.88 km2)and snow(20%),aiming to better understand the hydrological processes in the high alpine region under the background of global warming.[M ethods] The empirical equation was presented to quantify themeltwater amount from glacier on the basis ofwater balance theory.Empirical values were set for the variations in the equation,including the precipitation (406.70mm),the evaporation(100.50mm).The glacier ablation was calculated on the basis of the glacier volume variation.The Snowmelt Runoff Model(SRM)was used to calculate the runoff amount from snow melting in the Nam Co basin,using theWinSRM Version 1.11 software.The feasibility of the SRM was discussed and analyzed.The parameters of the SRM,including the degree day factor,runoff coefficient,lapse rate of temperature,recession coefficients and air temperature,were tested in Nam Co basin using the hydrological data,meteorological data and MODIS remote sensing data(MOD10A2). [Results]The results showed that the empirical equation and the SRM model could be used for the calculation of the meltwater runoff amount from glacier and seasonal snow in Nam Co basin.The calculated annual glaciermeltwater runoff was 49.15 mm/a(2.99×108m3/a),and the annual snow meltwater runoff was 8.10×106m3/a.The glacier meltwater runoff was almost 38 times that of the seasonal snow.Thismeant the glaciermeltwater was dominantwater resources in the Nam Co basin.The lake area was enlarged with the glacier shrinking when temperature increased.The increased water amountwith the lake expansion was nearly 80%of the calculated runoff amount from glaciermelting,far more than the calculated meltwater runoff from the seasonal snow.[Conclusions]It is deduced that the dramatic glaciermelting in the Nyainqentanglha Mountains at the southeast side of Nam Co basinmay be themost important drive factor for the lake expansion in the study area.However,more measurement should be conducted in the high alpine region to improve the accuracy of the calculated results by SRM. Moreover,the correlation between the glacier shrink and the lake expansion should be further investigated.

glacier;snow;Glacier Inventory of China;SRM;meltwater runoff

P343

A

1672-3007(2016)02-0127-10

10.16843/j.sswc.2016.02.017

2015-11-20

2016-01-24

項(xiàng)目名稱:國(guó)家自然科學(xué)基金重點(diǎn)項(xiàng)目“高海拔寒區(qū)融水土壤侵蝕機(jī)理與過(guò)程模擬研究”(41230746);國(guó)家自然科學(xué)基金“不同質(zhì)地黃土坡面水蝕動(dòng)力-輸沙過(guò)程耦合關(guān)系對(duì)侵蝕形態(tài)演化的響應(yīng)機(jī)制”(41471229),“坡面水蝕動(dòng)力與細(xì)溝發(fā)育形態(tài)的相互作用機(jī)制及其影響因素分析”(41271304)

陳飛(1986—),男,博士研究生。主要研究方向:流域融水侵蝕機(jī)理與模擬。E-mail:fei-27@163.com

簡(jiǎn)介:孫莉英(1978—),女,博士,副研究員。主要研究方向:水沙環(huán)境風(fēng)險(xiǎn)與控制等。E-mail:sunliying@igsnrr. accn