新研究在全世界的河川發現氣候留下的記號
英國布里斯托大學的科學家主持並發表在期刊《自然》(Nature)的新研究質疑了現有理論――他們發現氣候在全世界的河川都留下了鮮明的記號。
文章中發現不同氣候帶的河川縱剖面具有不同的特性。濕潤(左下)和半乾燥(右下)的河川顯示氣候造成他們的流量有所差異。圖片來源：布里斯托大學
地球科學家數十年來一直在試著找出氣候在河川形成過程中的影響，但目前為止還沒有得到系統性的證據。
如果你從河川的源頭往河口前進，路途中的海拔高度會一路降低。在某些河川中，這條路徑在高處時會下降得非常快，到了低處則變得相當平坦，結果便是形成一條凹口向上的高度剖面(又稱縱剖面)，就像是從一個碗的碗口畫到碗底得到的形狀。相較來說，直線形的縱剖面就像一道斜坡，從源頭走到河口途中不管哪個高度的下降幅度都一樣。
陳宣安等人進行的新研究顯示濕潤地區的河川傾向形成凹口向上的縱剖面，而越乾燥的地區河川縱剖面則會越來越直。
布里斯托大學地理科學院的陳宣安是主要作者，他說：「河川縱剖面是經過數千年到數百萬年逐漸形成的，因此它可以訴說當地氣候歷史的宏觀故事。我們推測氣候會影響河川縱剖面，因為氣候決定了河川中有多少水在流，以及水流沿著河床推動沉積物的相關力度有多強。」
在此之前科學家缺乏一個有系統的大型資料庫涵蓋全世界各個氣候帶的河川，使他們無法全面探討氣候跟河川形狀之間的關係。研究團隊運用NASA太空梭蒐集到的數據，以及共同作者倫敦瑪麗王后大學的Stuart
Grieve博士開發出的專門軟體，發展出新型且可以免費使用的河川縱剖面資料庫，其中涵蓋了超過33萬條世界各地的河川。
這篇研究首度證明全世界不同氣候帶的河川縱剖面有明顯差異，原因為乾燥在河川流量上呈現出來的影響。
濕潤地區的河川一般來說整年都有水在流動，因此可以持續搬運沉積物，並把河川縱剖面整體侵蝕成凹口向上的形狀。
隨著氣候逐漸變乾(從半乾燥、乾燥到超乾燥)，一年中只有下雨時河川會流動幾次，造成沉積物很少受到搬運。
此外，乾燥地區的河川也較容易經歷短期而強烈的暴雨，這種降雨類型不會讓整條河川都有水在流動。
作者在論文中利用數值模型模擬河川縱剖面的演化歷程如何受到流量特性影響，因而解釋了氣候、流量和縱剖面形狀彼此之間的關聯。
作者指出不論河川縱剖面是否還有其他可能的控制條件，流量特性對最終形成的剖面形狀具有最主要的影響。他們證明資料庫中不同氣候帶的剖面形狀差異，可以用氣候對流量造成的不同特性來解釋。
布里斯托大學地理科學院的Katerina
Michaelides博士主持了這項研究，她說：「數十年來教科書上的傳統理論，都將河川縱剖面描繪成會往凹口向上的形狀演化。在前人發表的研究中，濕潤地區的河川遠比乾燥地區的河川更常出現並受到深入探討，造成現有理論偏向從濕潤地區河川得到的觀察結果。」
「我們的研究證實世界上很多河川的剖面形狀並非是凹口向上，而直線形的剖面在乾燥環境中較為常見。」
「我認為乾燥地區的河川一直以來都沒有被好好研究並受到忽視，特別是考慮到乾旱地區其實占了全球陸地面積的40%。它們的流量特徵讓我們能從獨一無二的角度探討氣候對地表地形的影響。」
New research identifies a climate
signature in rivers globally
A new study, led by scientists from the
University of Bristol and published in the journal Nature, discovers a clear climatic signature on rivers globally
that challenges existing theories.
For decades geoscientists have been trying to detect
the influence of climate on the formation of rivers, but up to now there has
been no systematic evidence.
If you walk from a river’s source to its mouth, you
walk a path that descends in elevation. In some rivers, this path will descend
steeply out of the uplands, and then flatten out in the lowlands. This results
in an elevational profile (which we call the long profile) that has a concave
up shape, similar to the shape of the inside of a bowl as you trace it from the
inside rim to the bottom. In contrast, a straight long profile descends evenly
in elevation, like a ramp, along the path as you walk from the source to the
mouth.
The new research by Chen et al. shows that while
river long profiles tend to be concave up in humid regions, they become
progressively straighter in drier regions.
Lead author Shiuan-An Chen from the University of
Bristol’s School of Geographical Sciences, said: “The long profile is formed
gradually over tens of thousands to millions of years, so it tells a bigger
story about the climate history of region. We would expect climate to affect
the river long profile because it controls how much water flows in rivers and
the associated force of water to move sediment along the riverbed.”
Up until now scientists have lacked a large, systematic
dataset of rivers that spans the range of climate zones on Earth, enabling full
exploration of the links between climate and river form. The research team
produced a new, freely available, database of river long profiles, generated
from data originally collected by NASA’s space shuttle. They used specialist
software developed by co-author Dr Stuart Grieve at Queen Mary University
London to develop a new long profile database that includes over 330,000 rivers
across the globe.
The study shows for the first time at the global
scale that there are distinct differences in river long profile shapes across
climate zones, and that the reason behind these differences lies in the
expression of aridity in streamflow in rivers.
In humid regions, rivers tend to have flow in them
all year round which continually moves sediment and erodes the overall profile
into a concave up shape.
As the climate becomes progressively arid (from
semi-arid, to arid, to hyper-arid), rivers only flow a few times per year when
it rains, moving sediment infrequently.
Additionally, arid rivers tend to experience brief,
intense rainstorms, which do not create flow over the entire river length.
These links between climate, streamflow and long
profile shape are explained in the paper using a numerical model which
simulates the evolution of river profiles over time in response to streamflow
characteristics.
The authors show that regardless of all other
potential controls on river profiles, streamflow characteristics have a
dominant effect on the final profile shape. They demonstrate that the
differences in the climatic expression of streamflow explain the variations in
profile shape across climatic regions in their database.
Dr Katerina Michaelides, also from Bristol’s School
of Geographical Sciences, who led the research added: “Traditional theory
included in textbooks for decades describes that river long profiles evolve to
be concave up. Existing theories are biased towards observations made in humid
rivers, which are far better studied and more represented in published research
than dryland rivers.
“Our study shows that many river profiles around the
world are not concave up and that straighter profiles tend to be more common in
arid environments.”
“I think dryland rivers have been understudied and
under-appreciated, especially given that drylands cover ~40% of the global land
surface. Their streamflow expression gives unique insights into the climatic
influence on land surface topography.”
原始論文：S-A. Chen, K.
Michaelides, S. Grieve and M.B. Singer. Aridity is expressed in river
topography globally'. Nature, 2019 DOI: 10.1038/s41586-019-1558-8
引用自：University of Bristol. "Climate signature
identified in rivers globally."
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