Desiccation of The Aral Sea

The Aral Sea is a closed basin situated in Uzbekistan and Kazakhstan with an extremely large catchment area. The desert landscape receives less than 90mm of rainfall per annum and exhibits a strong continental climate characterised by extreme temperatures in the summer and winter. The sea has no outflow but two rivers the Amu Dar’ya and Syr Dar’ya feed the basin with waters of snowfield and glacial origin (Laity 2008).

The Aral Sea once extended 66 100 km² but problems developed with the diversions of its premiere inputs in the 1960s and 70s. By 1987, 60/70% of the Aral Sea’s volume had been lost, with a water level reduction of 14m (Glantz, 2007). The Aral Sea became split into two bodies of water, the Large Aral Sea and the Small Aral Sea (Sorrel et al, 2006). Water levels had been known to fluctuate over the Holocene with marine fossils and relict shore terraces providing evidence of 20-40m oscillations in response to changes in the climate system and the subsequent implications for river discharge. Current changes are far from natural.

     

The affects of hydrological change in the Aral Sea are wide ranging. Diversion for the purpose of cotton and rice field irrigation has seen huge increases in the Aral Sea’s salt concentration, decimating the local fishing industry. Sorrel et al (2006) report surface water salinity rose from 10.4 g kg^-1 in 1960 to more than 80 g kg^-1 in 2003. In addition, strong winds transported toxic dust onto farms several hundred kilometers downwind from sediments that had once been covered under water. Life expectancies of approximately 3.5 million people have been cut significantly as a result of exposure to toxic chemicals. Rates of disease among children is increasing with intoxication of heavy metals causing renal tubular dysfunction, in addition to increased number of cases of tuberculosis, malignancies and psychiatric disease  (Kaneko et al, 2011; Matsapaeva et al, 2010). Desiccation of the Aral Sea has also had extensive climatic impacts. Small et al (2001) illustrate the changes in surface air temperature since the disruption of the Aral Sea’s inputs. Mean, maximum and minimum temperatures near the Aral Sea have changed by up to 6^o C. The magnitude of change decreases with distance from the 1960s shoreline.

This example demonstrates the enormous control humans can have on the environment, potentially causing irreversible damage. Not only do we have the power to change the environment, but also threaten the well being of the contemporary society, ecological communities, and even regional-scale climatic outputs. The scale of the damage caused in the Aral region is testified by the 300 projects proposed to alleviate crisis. Furthermore, responses are aimed towards minimising the damage, not restoring to former characteristics. Small et al reinforce the recklessness of many contributions by suggesting, “If every expert brought a bucket of water, the Aral Sea would be filled again”…  

References

Laity, J. (2008) Deserts and Desert Environments, Chichester: Willey-Blackwell

Alternative Uses of the Desert Environment!

My next blog will draw together many of the components involving climate and water resources in deserts mentioned beneath by focusing on a case study of the Aral Sea. But until then here's what else goes on in deserts!

Branson launches desert spaceport...

Humans and Water Resources

It has previously been noted that climatic shifts can have a significant impact on desert hydrology and in turn human occupation of the landscape. In addition to the climatic component of hydrological change there is an increasing anthropogenic component. There are four major ways in which humans are interfering with desert water resources (Laity, 2008):

1: Groundwater withdrawal. Groundwater is increasingly used for the maintenance of human settlements and plant and animal habitats. Its withdrawal by far exceeds natural rates of replenishment.

2: River flow alteration. Rivers are being diverted away from their natural cause for the purpose of land irrigation and the construction of dams to regulate flow.

3: Salinisation. Contemporary irrigation technologies have resulted in increased salinity as extremely high evaporation rates cause mineral salt precipitation. The land is severely degraded as a result and fertility is much reduced. In turn, crop yields reduce.

4: Lake disturbance. Pollution e.g. disturbances of geochemical balances through processes including eutrophication or toxic dust absorption can have long-standing impacts on closed or semi-closed basins.

References

Laity, J. (2008) Deserts and Desert Environments, Chichester: Willey-Blackwell

Hydrological Change and Human Occupation in Deserts

Viewing the diagrams incorporated within the mentioned sources enhances this post.

Hydrological shifts in desert regions have had significant impacts on the viability of the landscape for inhabitation. As a consequence, populations of deserts have fluctuated dramatically. One major controlling factor of the availability of water in deserts is climate. I would like to provide two examples of the influence of climatic change on desert hydrological regimes.

The western coast arid zone of South America includes the Peruvian and Atacama deserts. Initiated by uplift of the Andes and the development of the Antarctic bottom waters and the Peruvian-Humboldt current, the Atacama is generally accepted as being very old (Goudie, 2002). Hyper-aridity has predominated since the middle to late Miocene. Betancourt et al (2000) present evidence of hydrological changes over the last 22,000 years from the Atacama. Vegetational and groundwater change were reconstructed from radiocarbon dated fossil rodent middens and wetland deposits. Rodent middens provide high taxonomic resolution of past vegetation and are used in this study in terms of rainfall seasonality. Diatomaceous wetland deposits provide a more continuous record. Data reveals increasing summer precipitation, grass cover and groundwater from 16.2-10.5ka. Summer-flowering grasses spread to regions now lacking water and vegetation. In addition, another pluvial period occurred from 8-3ka. Betancourt et al suggest teleconnections or insolation forcing to be attributable.

Moving from South America to Africa, Damnati (2000) investigated lake fluctuations as a measure of palaeohydrological and palaeoclimatic change in the Sahara and Sahel. Lake status can be derived from a variety of data including stratigraphical, geochemical and palaeoecology, but also archaeological data since lakes are an important source of food and water in arid and semi-arid regions. It is clear from studying Damnati’s maps of lake status that there was a major pluvial phase 10 to 7ka. At 7 to 6ka many of the lakes shifted to an intermediate or low water balance. From 5ka to the present, lake levels in the Sahara and Sahel have continuously deteriorated to a point where over 95% of the sample have low or intermediate lake levels.

Climatic shifts determining water resources inherently influence occupation of arid and semi-arid environments. Kuper and Kropelin (2006) describe human occupation of the Sahara during the Holocene. Notable occupation events seem to correlate with Damnati’s findings. A major reoccupation of the Sahara occurred at 8.5 to 7ka as monsoonal rains transformed the landscape into a savannah-like environment. The Formation phase 7 to 5.3ka ended abruptly in congruence with diminishing lake levels. During this phase, however, there was the introduction of domestic animals including sheep and goats The Regionalisation phase saw retreat to highland refugia with greater precipitation or temporary lakes. Refugia included the Gilf Kebir and the Sudanese plains. Finally, 3.5 to 1.5 ka, human activities were restricted to northern Sudan. This is known as the Marginalisation phase where rains ceased even in the ecological niches. 

References

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.

Water in Deserts

Water is a critical resource in Desert Environments. Hydrological processes in deserts are particularly important for ecological potential and human inhabitation. The history of water in deserts is rich. More to follow!!!

Past Monsoonal Changes and Anthropogenic Impact in the Thar.

The Thar Desert, also known as the Great Indian Desert or the ‘place of death’ by locals, occupies around 2.34 million km² of the northwest of the Indian subcontinent and the adjoining area of Pakistan (Laity, 2008). The desert has a number of striking features such as its large covering of shrubs, bushes and trees, frequent scatterings of civilization, and intensive farming of cattle, goats and camels. The high desert population, 100 per km² in Rajasthan, coupled with its association with the southwest monsoon, has placed anthropogenic and climatic pressures on the landscape (Goudie, 2002).

Southwest monsoon variability has played a critical role in shaping the characteristic features of the Thar Desert. Fluvial, aeolian and lacustrine deposits found in the Thar region reflect the sensitivity of the environment and illustrate the influence climate has had on geomorphic processes and systems (Kar et al, 2001). The southwest monsoon is important in determining the amount of rainfall that the Thar receives. In turn the amount of rainfall plays a critical role in the spatial and temporal nature of desert margin shift. Substantially regulated by changes in solar insolation and the temperature gradient found between the land and sea - precipitation, evaporation and wind changes can be identified from the sedimentary record. Juyal et al (2006) used sedimentology and luminescence dating to suggest the existence of a river system as a result of enhanced monsoon conditions 130-120 ka. Since then changes in the characteristics of the sediment indicate fluctuating monsoonal strength at 120-100 ka, 100-70 ka, 70-60 ka and 60-30 ka with periods of intense and reduced precipitation. Aeolian sedimentation post 30 ka suggests much drier conditions.

The fluctuation between aeolian and humid conditions provides great scope from congruent changes in vegetation. Arid and hyper-arid desert conditions provide a biologically stressful environment and species are forced to make adaptations or die. Water is usually the most important factor governing plant growth. Hence the monsoon is important in determining the type of vegetation found in the Thar through time. Various studies using values of δ¹³C of organic carbon in lakes have shown the relative abundance of C₃ versus C₄ vegetation during periods of increased precipitation or strengthening of the monsoon (Enzel et al, 1999).

High desert population has permitted extensive anthropogenic influence on the environment. Yadav and Rajamani (2004) have studied the geochemistry of aerosols in the Thar and stressed their importance in terms of climate change, nutrient dynamics and environmental health. Anthropogenically produced aerosols have been shown to have the potential of contributing to ongoing desertification. Wada et al (1995) also link human activities to desertification in the Thar suggesting that overgrazing by large mammals can significantly alter natural plant succession and ultimately reduced vegetative cover. As Charney’s hypothesis suggests, this can trigger extensive feedback mechanisms. Subdivision and fragmentation of land holdings have also been identified as a major cause of desertification in the Thar Desert. Shrinking land holdings have induced constant cultivation resulting in soil infertility and reduced long term productive potential (Ram et al, 1999).

References

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.

Laity, J. (2008) Deserts and Desert Environments, Chichester: Willey-Blackwell

The Thar!

Some basic info on the location of my next blog:

The Thar

Welcome!

Welcome to Deserts and Environmental Change! The blog will discuss a number of issues regarding deserts and environmental change. Deserts, depending on definition, occupy 49 million square kilometres of the Earth's surface and are often characterised by their great aridity. Climate, weather, geomorphology, hydrology and ecology are just some of the components of the desert framework that have been subject to drastic change through time and are sensitive to future perturbations and environmental change. The blog will touch upon some of these aspects highlighting case studies from around the world, ask challenging questions, and invite contributions to media articles, videos, fieldwork and academic publications. There will be particular attention paid to the anthropogenic influence on the desert system.