GeoLog

erosion

Imaggeo on Mondays: Soil and water conservation in the Dogon Plateau, Mali

Velio Coviello, a scientist from the Research Institute for Hydrogeological Protection, Italy, and one of the winners of the EGU 2014 Photo Contest, brings us this week’s Imaggeo on Mondays. He sheds light on his winning image and the problems associated with conserving soils and water in Western Africa… 

This picture was taken on Mali’s Dogon plateau during the dry season, in the course of a late sandstorm day. Between November and March, a hot, dust-laden Harmattan haze frequently persists over the whole of  Western Africa. The Harmattan is a hot, dry wind blowing from the Sahara, carrying large amounts of dust and transporting it for hundreds of kilometers. Here, we see two men drawing water from a deep and narrow well excavated by hand. This latter is a task commonly carried out by children, who climb down to dig the well bottom.

Men and children drawing water for irrigation in the Dogon plateau during a sandstorm. (Credit: Velio Coviello via imaggeo.egu.eu)

Men and children drawing water for irrigation during a sandstorm. (Credit: Velio Coviello via imaggeo.egu.eu)

Mali has a low population density, most settlements are concentrated in the southern part of the country and along the Niger River, where the climate is less harsh and water availability is higher. In the north, Mali is arid and only those who raise livestock can make a living.

One of the most important tourist attractions in Mali is the Dogon Plateau, which sits in the central part of the country, east of the Niger River. The plateau gently descends westward to the river valley and ends in abrupt cliffs on the southeast. These cliffs reach an elevation approaching 1,000 meters at Bandiagara, the main village of the Pays Dogon (Land of the Dogon). These geological, archaeological and ethnological interests, together with the striking landscape, make the Dogon Plateau one of West Africa’s most impressive sites.

Ensuring the population has safe and sustainable access to water is one of the major challenges in the Sahelian region. Facing recurring drought events and encroaching desertification, Sahelian countries are currently heavily affected by climate change. Extreme rainfall events and high rainfall intensity are the main cause of soil erosion and land degradation. Consequently, high rates of soil transport can lead to reservoir siltation and the reduction of water availability for agriculture. To cope with these issues, traditional soil and water conservation (SWC) measures like hillside terracing, permeable rock dams, stone lines, earth basins, planting pits and earth mounds have been regularly employed in the Sahelian area. The Dogon Plateau is home to a broad variety of these measures, implemented to deal with the acute shortage of soil and water. As the population urgently needs support to preserve soil fertility and reduce soil erosion, SWC measures need to be improved and adopted more widely. However, most donors fund short-term projects without considering the maintenance that is needed to ensure SWC measures remain effective long-term.

The first lesson is that there is much to learn from the traditional ways of doing things and SWC projects should always begin by looking at what the people are doing for themselves. Secondly, the international cooperation actors should set up long-term funding programs improving the participation and inclusion of local communities. The final goal would be to ensure the stakeholders are not permanently dependent on international aids.

by Velio Coviello, Research Institute for Hydrogeological Protection (IRPI) and Italian National Research Council (CNR)

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

GeoTalk: Veerle Vanacker on land use, degredation and the potential of revegitation

Today in GeoTalk, we’re talking to Veerle Vanacker, and eminent geomorphologist and winner of the EGU Division Outstanding Young Scientist Award last year. She tells us about her breakthroughs in modelling land use change and erosional processes…

First, could you introduce yourself and tell us a little about what you are currently working on?

I currently work as a lecturer in geomorphology at the University of Louvain (UCL, Belgium). My research focuses on the interactions between human activities and earth surface processes. After graduating from university (KULeuven, Belgium) in 1998, I started to conduct fieldwork in the Ecuadorian Andes in the scope of an inter-university cooperation project. My PhD research aimed to improve our understanding of the impact of land use change on geomorphic processes in tropical mountain environments. Remote sensing was then increasingly used to extract physical land properties. In 2002, I started as a post-doc with Eric Lambin, who chaired the IGBP Land Use Land Cover Change project at the time. Later, I relocated to the University of Hannover (Germany) to learn about new geochemical tracers to track erosion and sedimentation rates.

Veerle Vanaker out in the field (literally!). (Credit: Veerle Vanaker)

Veerle Vanaker out in the field (literally!). (Credit: Veerle Vanaker)

Last year, you received a Division Outstanding Young Scientists Awardfor your work on how land use change can influence erosion rates in mountainous regions. Could you tell us a bit more about your research in this area?

In mountainous regions, we commonly observe high erosion rates and elevated sediment fluxes. They are not directly a sign of increased human disturbances, as natural erosion rates in mountainous sites can be high due to steep slopes, tectonic activity, and the erosive climate. How to separate these processes from the impact of land use on sediment flux was a question that urgently needed answering. By combining spatial information (from remote sensing) with sediment flux data and geochemical tracers, we were able to quantify the changes in erosion rates due to anthropogenic disturbance.

The relationship between land use and erosion has been a subject of much debate, something your methods have helped resolve. Could you describe their key points?

Previous studies mostly used a time series of river sediment fluxes to analyse how erosion and sediment yield varied over time and space, but modern sediment fluxes don’t provide a good baseline for assessing human impacts on sediment flux as they are biased by both short records and long-term human occupation. Instead, we need natural benchmarks that we can compare disturbed areas to. In our studies, we established natural rates of sediment flux by looking at the concentration of terrestrial cosmogenic nuclides in river sediments. The beryllium isotope, 10Be, allows long-term catchment-wide denudational flux to be quantified. Modern sediment fluxes can then be compared to these rates to assess the impact of anthropogenic disturbance.

Vegetation on a degrated slope. Vegetation is a major control of runoff, erosion and sediment mobilization in highly degraded catchments. (Credit: Veerle Vanaker)

Vegetation on a degrated slope. Vegetation is a major control of runoff, erosion and sediment mobilization in highly degraded catchments. (Credit: Veerle Vanaker)

What activities have the greatest influence on sediment production and how can we reduce their impact?

Our studies showed that the conversion from native forests to agricultural land causes soil erosion to accelerate by up to two orders of magnitude. The main driver for this is the decrease in the protective ground vegetation cover. Such a loss of land cover can trigger shallow landslides, which present a major threat to human life, property and infrastructure.

Through our work in experimental catchments in the Andes, we were able to show that revegetation programs coupled with bioengineering significantly reduces sediment production and mobilisation. Revegetating active gully channels is a particularly efficient way of enhancing sediment trapping and infiltration of runoff water in gully channels, rather than transporting it downslope.

Lastly, what are your research plans for the future?

Human pressure on the land is globally increasing, which poses a serious threat to environmental sustainability. I’m highly interested in the interactions between soil systems and water, and what will happen to sediment and nutrient fluxes under changing human pressure. We observe that soil processes play a major role in the resilience of an ecosystem to human disturbance. To understand these interactions, we need new data on the rates of soil formation, chemical weathering and erosion fluxes for both undisturbed and anthropogenically disturbed sites. By combining data from different geochemical techniques, such as terrestrial and meteoric cosmogenic nuclides, with remote sensing data from very high resolution images, we should be able to make major advances in these research topics.

If you’d like to suggest a scientist for an interview, please contact Sara Mynott.

 

Imaggeo on Mondays: Beautiful Badlands

This week’s Imaggeo on Mondays is brought to you by Gert Verstraeten, who takes us through the formation of some of the striking landscapes in the Mediterranean – the badlands.

For more than 7 years I have been performing research on reconstructing historic erosion rates in the Taurus Mountains in Southwest Turkey. Badlands are heavily eroded landscapes, where soft, clay-rich rocks and soils have been cut back by the action of wind and water.

As in many other Mediterranean environments, intense erosion in can often be related to former human impact on the landscape. Consequently, many degraded landscapes, and especially badlands regions in semi-arid environments are often explained as the result of anthropogenic soil degradation.

In most cases, though, Mediterranean badlands are simply the result of natural processes.

“Badlands of Cappadocia” by Gert Verstraeten, distributed by the EGU under a Creative Commons Licence.

After my last field campaign in August 2012, we travelled to Cappadocia on a small holiday trip to visit the badlands landscape around Göröme.  The badlands of Cappadocia offer a spectacular landscape that is the result of natural erosion processes that have been in operation for tens of thousands of years. The badlands are developed in volcanic deposits (lava flows and more importantly ignimbrites) dating roughly between 10 and 5 Million years ago.

Fluvial (river), lacustrine (lake) and aeolian (wind-blown) deposits intersperse the volcanic ones, creating a (sub)horizontal stratification of medium soft to very soft deposits, together with lava flows and pumice deposits, which are more resistant.

A Hoodoo, also known as a ‘fairy chimney’ in the Grand-Staircase Escalante National Monument rim-rocks, Utah (credit: Wikimedia Commons user Ciar).

During the Pliocene and Pleistocene the Central Anatolian Plateau was incised by up to 400 metres  through regressive erosion of the Kizilirmak River from the north. This triggered the formation of badlands in the relatively soft sediments that are easily eroded.

Different landforms developed due to heterogeneities in the stratigraphy. When more resistant deposits (lava flows, pumice deposits, welded ashes) cover softer ones (non-welded ashes, fluvial and aeolian fine-grained deposits), erosion results in the formation of the typical cap-topped ‘fairy chimneys’ for which Cappadocia is so famous. But where only soft ash flow deposits are present, typical so-called ‘sweeping curves’ are formed as seen in the badlands of Cappadocia.

Here, relatively fine-grained ignimbrites are outcropping, immediately northwest of the town of Uçhisar. Exploring the region around Göröme, whether on foot, by car or in a hot air balloon in the early morning, makes discovering all the different erosion features possible. Humans have been occupying this region of natural beauty for millennia but especially since the Byzantine period when villages, churches and monasteries have been carved into the soft rocks.

By Gert Verstraeten, University of Leuven

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their images to this repository and since it is open access, these photos can be used by scientists for their presentations or publications as well as by the press and public for educational purposes and otherwise. If you submit your images to Imaggeo, you retain full rights of use, since they are licensed and distributed by the EGU under a Creative Commons licence.

GeoTalk: Simon Mudd

Today in GeoTalk, we’re talking to Simon Mudd, an exceptional and forward-thinking geomorphologist.

First, could you introduce yourself and let us know a bit about what you are currently working on?

I am lecturer in Landscape Dynamics at the University of Edinburgh, where I have been since 2007. Before that I was a post-doc at Vanderbilt University, in Nashville Tennessee. I received my PhD from Vanderbilt in Environmental Engineering in 2006. My work is split between salt marshes, where I construct numerical models to examine the critical rates of sea level rise beyond which marshes are likely to die, and mountainous landscapes where I examine the interaction between erosion rates and the geochemical evolution of soils. Both of these research avenues have implications for the global CO2 budget: marshes are extremely efficient at burying organic carbon, and the chemical weathering of silicate minerals consumes atmospheric CO2.

During the EGU General Assembly, you received an Arne Richter Award for Outstanding Young Scientists for your research into how climate change and tectonic forcing influence hill-slope morphology and soil thickness. Could you summarise how these processes interact?

Broadly speaking, faster tectonic uplift leads to faster erosion rates, which result in steeper hillslopes, sharper hilltops and thinner soils. This means that tectonic uplift should leave an imprint on upland topography and upland soils. One of my aims is to try and understand how topography and soil thickness responds to tectonic forcing well enough to be able to quantify tectonic history

The man himself – Simon Mudd. (Credit: Simon Mudd)

How do chemical weathering and physical transport processes influence landscape dynamics?

Chemical weathering often results in a mass loss from weathered materials. If mixing processes maintain constant regolith (loose rocky material) or soil density, and there are lines of evidence to suggest this to be the case in many landscapes, this mass loss will lower the surface of the Earth. There are places on Earth where chemical weathering accounts for over 50% of total denudation and in these places chemical processes can be just as important as physical processes in determining the geometry of the land surface.

I think this is underappreciated by geomorphologists: most landscape evolution models do not include a chemical component. In addition, chemical weathering sets both the particle size and mineralogy of soils. Particle size is theorized to control how rapidly soil particles will move downslope given a fixed input of disturbances (from burrowing animals, for example). Particle size and mineralogy are primary factors in determining soil hydraulic conductivity, which controls the biogeochemical functioning of soils. They also control cohesion, which determines the stability of a slope.

What role does vegetation have to play in the evolution of geomorphic systems?

Many sediment transport phenomena are biologically mediated. On salt marshes, marsh plants trap sediment and help maintain the marsh elevations relative to mean tide in the face of rising sea levels. On hillslopes, plant roots add cohesion to the soil and reduce landslide risk. They also protect the soil from erosion by rainsplash and overland flow. Root growth and tree toppling can also move sediment down slope. In many landscapes there is evidence to suggest that the dominant sediment transport mechanisms are biological, and if we are going to accurately predict erosion rates in the face of changing climate we are going to have to link sediment transport with changing plant biomass.

Out in the field with some budding geomorphologists! (Credit: Simon Mudd)

Finally, what are your plans for the future?

It is an incredibly exciting time to be a geomorphologist because there are not one, but two major advances that are revolutionising our field. The first is cosmogenic nuclides, which provide information on erosion rates averaged over hundreds to thousands of years – long enough to average over the probability distribution of storms. We now can quantify how fast landscapes are eroding with relative ease.

The second advance is high resolution topographic data that is now being widely collected and often is freely available. Light Detection and Ranging technology (LiDAR) returns clouds of data on centimetre scale resolution, allowing us not only to retrieve extremely high resolution maps of the Earth’s surface, but also information about plant canopies. The convergence of these techniques allows us to collect information about both fine resolution surface processes and plant populations simultaneously. With this data we will be able to examine how erosion rates modulate soil chemistry and thickness, how this influences plant productivity, and how these plants modulate sediment transport. These are the topics I’m most excited about working on in the future.

If you’d like to suggest a scientist for an interview, please contact Sara Mynott.