研究人員對山區集水區有更進一步的了解

原文網址：www.sciencedaily.com/releases/2015/10/151029190848.htm

Researchers advance understanding of mountain watersheds

研究人員對山區集水區有更進一步的了解

University of Wyoming geoscientists have discovered that the underground water-holding capacity of mountain watersheds may be controlled by stresses in Earth's crust. The results, which may have important ramifications for understanding streamflow and aquifer systems in upland watersheds, appears Oct. 30 in Science, one of the world's leading scientific journals.

懷俄明大學的地球科學家發現於山區集水區，土地的保水能力可能受控於當地地殼應力。此結果於10/30發表在世上頂尖的科學期刊《科學》(Science)，對我們了解山區集水區的地表和地下水系統來說可能有相當深遠的影響。

The scientists conducted geophysical surveys to estimate the volume of open pore space in the subsurface at three sites around the country. Computer models of the state of stress at those sites showed remarkable agreement with the geophysical images. The surprising implication, says Steve Holbrook, a UW professor in the Department of Geology and Geophysics, is that scientists may be able to predict the distribution of pore space in the subsurface of mountain watersheds by looking at the state of stress in Earth's crust. That state of stress controls where subsurface fractures are opening up -- which, in turn, creates the space for water to reside in the subsurface, he says.

科學家分別在美國三個不同地點進行地球物理探測，以估算它們地下的開放孔隙體積。各地得出的地球物理探測影像，和以電腦模型計算出的當地應力狀態互相比較，皆顯示兩者之間的關聯十分密切。懷俄明大學地質與地球物理系的教授Steve Holbrook說，這項驚人的發現代表科學家或許能利用地殼的應力狀態，來預測山區集水區的地下孔隙是如何分佈。他說應力狀態控制了哪些地下裂隙會連通，進一步確立了地下水應位於何處。

"I think this paper is important because it proposes a new theoretical framework for understanding the large-scale porosity structure of watersheds, especially in areas with crystalline bedrock (such as granite or gneiss)," Holbrook says. "This has important implications for understanding runoff in streams, aquifer recharge and the long-term evolution of landscapes."

「我認為這篇論文的重要性不言而喻，因為它提出了一套新的理論框架讓我們去了解大尺度下集水區的孔隙結構，尤其是在岩床為結晶岩層(如花崗岩或片麻岩)的地區。」Holbrook說。「這對河川流量、地下水含水層補注和地貌長期演化的研究皆有很大的啟發。」

James St. Clair, a UW doctoral student, is lead author of the paper, titled "Geophysical Imaging Reveals Topographic Stress Control of Bedrock Weathering." Holbrook, Cliff Riebe, a UW associate professor of geology and geophysics; and Brad Carr, a research scientist in geology and geophysics; are co-authors of the paper.

此篇題名為「地球物理影像法顯示地形應力控制了岩盤風化」(Geophysical Imaging Reveals Topographic Stress Control of Bedrock Weathering)的論文第一作者為懷俄明大學的博士生James St. Clair。共同作者則有Holbrook、Cliff Riebe(懷俄明大學地質與地球物理系的副教授)以及Brad Carr(地質與地球物理系的研究員)。

Researchers from MIT, UCLA, the University of Hawaii, Johns Hopkins University, Duke University and the Colorado School of Mines also contributed.

來自麻省理工學院、加州大學洛杉磯分校、夏威夷大學、約翰霍普金斯大學、杜克大學和科羅拉多礦業學院的研究人員也參與了此研究。

Weathered bedrock and soil together make up the life-sustaining layer at Earth's surface commonly referred to as the "critical zone." Two of the three study sites were part of the national Critical Zone Observatory (CZO) network -- Gordon Gulch in Boulder Creek, Colo., and Calhoun Experimental Forest, S.C. The third study site was Pond Branch, Md., near Baltimore.

風化岩床和土壤共同組成了位於地球表層，能供養生物生存的地帶，一般稱作「關鍵區」(critical zone)。三個研究地區中有兩個即屬於國家關鍵區觀測站網路的一部分，分別為科羅拉多州柏爾德河的Gordon峽谷，和南卡羅來納州的Calhoun實驗林。第三個地點則是馬里蘭州巴爾的摩附近的Pond河。

"The paper provides a new framework for understanding the distribution of permeable fractures in the critical zone (CZ). This is important because it provides a means for predicting where in the subsurface there are likely to be fractures capable of storing water and/or supporting groundwater flow," St. Clair says. "Since we cannot see into the subsurface without drilling holes or performing geophysical surveys, our results provide the means for making first order predictions about CZ structure as a function of the local topography and knowledge (or an estimate) of the regional tectonic stress conditions."

「這篇論文提供研究人員一條新的脈絡來瞭解關鍵區(CZ)的可通透裂隙會如何分佈。其重要性在於這讓我們有方法可以預測地底下可儲存地下水，且/或可供其流動的裂隙可能位於何處。」St. Clair說。「在沒有進行鑽井或地球物理探勘的情況下，我們是不可能看進地底深處的。但我們的研究結果讓我們有方法可以從當地地貌，以及對區域大地應力作用情形的認知(或估計)來建立方程式而初步預測CZ的構造。」

The research included a combination of geophysical imaging of the subsurface -- conducted by UW's Wyoming Center for Environmental Hydrology and Geophysics (WyCEHG) -- and numerical models of the stress distribution in the subsurface, work that was done at MIT and the University of Hawaii, Holbrook says.

Holbrook說此研究由兩部分結合而成，分別為地球物理影像技術及地下應力分佈的數值模型。前者由懷俄明大學的環境水文學與地球物理中心進行，後者則由麻省理工學院和夏威夷大學共同建立。

The team performed seismic refraction and electrical resistivity surveys to determine the depth of bedrock at the three sites, which were chosen due to varying topography and ambient tectonic stress. At the two East Coast sites, the bedrock showed a surprising mirror-image relationship to topography; at the Rocky Mountain site, the bedrock was parallel to topography. In each case, the stress models successfully predicted the bedrock pattern.

研究團隊選擇這三處地貌和環境大地應力皆有所差異的三個地點，並分別進行折射震測(seismic refraction)和地電阻探測以得知岩盤深度。結果顯示位於東岸的兩處(Calhoun實驗林和Pond河)，岩盤深度分佈與地貌呈現出令人驚訝的鏡像關係；而在洛磯山脈(Gordon峽谷)，兩者則幾近平行。另外，在每個案例之中，應力模型皆成功預測了岩盤分佈模式。

"We found a remarkable agreement between the predictions of those stress models and the images of the porosity in the subsurface with geophysics at a large scale, at the landscape scale," Holbrook says. "It's the first time anyone's really looked at this at the landscape scale."

「我們發現在大尺度，也就是以地景整體為尺度下，應力模型的預測結果，跟地球物理方法得出的地下孔隙分佈影像之間的關聯十分顯著。」Holbrook說。「這是第一次有人真正地從地景尺度出發來縱觀全局。」

St. Clair says he was fortunate to work with a talented group of scientists with an extensive amount of research experience. He adds the experience improved his ability to work with a group of people with diverse backgrounds and improve his writing.

St. Clair說他很幸運能與這群富含研究經驗且才華洋溢的科學家合作。他還說這次經歷增加了他與來自各式各樣背景的人們合作的能力，同時也增進了他撰寫論文的技巧。

"Our results may be important to hydrologists, geomorphologists and geophysicists," St. Clair says. "Hydrologists, because it provides a means for identifying where water may be stored or where the flow rates are likely to be high; geomorphologists, because our results predict where chemical weathering rates are likely to be accelerated due to increased fluid flow along permeable fractures; and geophysicists, because it points out the potential influence of shallow stress fields on the seismic response of the CZ."

「我們的結果對水文學家、地形學家和地球物理學家來說或許都相當重要。」St. Clair說。「對於水文學家，這提供了一套方法讓他們能辨識出地下水可能儲存於哪，或者地下水在哪裡會流得比較快；對於地形學家，由於液體會沿著可通透裂隙流動，故我們的結果能預測出化學風化速率可能會因此而在哪些地方增加；對於地球物理學家，這也許能說明淺層應力場變化在CZ對地震的反應中扮演了何種角色。」

Despite the discovery, Holbrook says there is still much work to be done to test this model in different environments.

儘管已經有了這項發現，Holbrook說還有許多研究工作尚待完成，以驗證這個模型能否運用在不同環境當中。

"But, now we have a theoretical framework to guide that work, as well as unique geophysical data to suggest that the hypothesis has merit," he says.

「但現在我們已經有個理論框架可以依循，而且這些特別的地球物理數據也顯示這項假說極富潛力。」他說。

引用自：University of Wyoming. "Researchers advance understanding of mountain watersheds." ScienceDaily. ScienceDaily, 29 October 2015.