Rajasthan’s Thar Desert and its Environment by Bob Cranwell
Much of Northern India is made up of huge deposits of alluvium, extending to several thousand feet in thickness in places. It is composed of material washed and eroded down from the Himalayas and from the Northern edges of the Deccan plateau, via glaciers, rivers, landslides etcetera.
It stretches from Karachi in Pakistan, up through the Punjab and around to Delhi, then Eastwards to Bengal, and where adequate water can be guaranteed, they can be amongst the most productive in Asia,Soils are made up of three differing groups of size particles, sand, silt and clay, in descending order of size. Sandy soils can be agriculturally useful, if enough water can be found – they have rapid drainage, and high evaporation rates, too, and can often be lacking in essential minerals.
Clay soils on the other hand hold water well, but can be difficult to work. Clays are the finest particles of all in soils, and water may adhere by molecular attraction to individual particles. They are often found where the sites of old lake beds have been, where the smallest particles have had long enough to settle out in the still water
Silty soils come between the two, they are freely drained, but have a good ability to both take in and hold water; they are also characteristically fertile soils. Alluvium is mainly composed of silt-sized particles.
In Rajasthan in particular, the border areas of the Thar desert can produce good crops. Cotton, millet, wheat, oilseed, pulses and sugarcane are all grown with some success where water is available.
This tends to be where the desert is bounded by rivers, such as the Indus and the Sutlej in the West and North, and to an extent in the South too, where the Rann of Kutch is periodically flooded, even though saline pans are often the only result of such flooding.
To the East and South, the desert is bounded by the Aravalli hills, which although dry, are able to hold somewhat better supplies of groundwater. As little as 2000 years ago this area was covered with woodland and jungle, which has died out for a combination of reasons, woodcutting, overgrazing and some alteration in the general climate.
The most significant changes in climate though, have been fairly local ones, due to the changes in vegetation. Natural or managed woodlands have broadly similar effects on local climates and soils. Mainly, they act as water reservoirs by providing shade, which reduces water loss; they also cut down windspeeds, which does the same.
They provide humus rich soil, which helps to both absorb and store moisture, and by a combination of these factors produce a beneficial environment for new growth to continue. Part of the problem for Northern India is the extreme seasonality of the rainfall.
It’s said that the monsoon hits Bombay within a few days of July the 6th each year. In Northern India generally this spells the time when the only rains of the year will fall, and it all comes in the space of a couple of months, with virtually nothing falling in the rest of the year, including the hottest periods.
Both evergreen and broadleaved woodland can withstand a high degree of seasonality, but other factors are at work. Where overgrazing has occurred over extended periods, vegetation suffers. Goats in particular, will take every scrap of vegetation available, especially the young trees and new leaves. Goats can be found even in the topmost branches of trees, in areas adjacent to deserts all over the world, slowly killing the remaining tree cover, once they have eaten the ground bare.
Coupled with this is the most serious fuel shortage in the world, that of wood. Much of ‘the 3rd world’ depends on wood for its heating and cooking; few third world villagers have access to, or can afford to use electricity or gas. Labour, though, is cheap and both children and adults spend long hours collecting wood.
Perhaps a couple of days each week for some family members is spent this way, ranging over an ever increasing area. In the quest for fuel, I have seen people literally digging up perfectly good trees by the roots, and these trees offered the only source of shade for some distance around their courtyard. Once the tree cover is thinned out, and the ground vegetation depleted, the sparser woodland dries out more easily, the shade having been removed, and the drying winds able to circulate more thoroughly in the brush.
Winds begin picking up the silt-sized particles, too, since clay sticks together, and sand is too heavy except for the stronger winds. Such water as there is moves more freely through the soil, the lack of organic matter means it dries out more quickly.
The lack of vegetation means that sudden rains tend to wash over the soils surface, since it cannot soak into an extremely dessicated soil. Often a pan will form, sometimes on the surface, composed of baked mud, or a foot or two under the surface, in the form of clays washed through the upper layers of the soil, which now form a waterproof barrier.
Both of these can accelerate topsoil erosion too. This is how the Thar desert has formed, and the area of totally unusable land continues to increase each year.
The resulting arid region receives less than 10 inches of rainfall in a year, which poses some problems for the creatures which live within it, of which there are surprisingly many. Both insects and reptiles are well represented here, lizards, beetles, scorpions. Then a range of mammals such as gerbils, or the long-eared hedgehog, through to the larger creatures such as the Asiatic wild ass and the camel.
What they all must live on, ultimately, though is the vegetation available in such an area, which itself possesses some interesting features. Most desert vegetation can be classed in two groups. Some such as Mimosa and Cassia and Acacia are equipped with very tough bark to prevent the loss of moisture, even the leaves may have a waxy cuticle.
Others, notably cacti and spurges have very fleshy bodies, but still have a leathery, impermeable skin. Both often show spines in the place of leaves, thus reducing their surface area, and further reducing water loss by transpiration. Some plants protect themselves further by producing a poisonous, acrid sap to deter browsing animals.
Unfortunately, there are some beetles adapted to live on such plants.and they too have inherited the foul-tasting characteristic as their own protection. So plants try to stop water loss, but they must also find new water, because to live, they must lose some of that water in photosynthesis. Very dry soils may only hold tiny amounts of water, adhering to clay particles by magnetic forces.
The power required to gain this water may be phenomenal, and some plants can exert a suction of 150 pounds per square inch, about one and a half times the air pressure in a truck tyre. They do this by concentrating minerals in the root sections of the plant, which causes what is known as a diffusion gradient.
Because forces in nature tend to balance one another out, there is a tendency for things to flow from a weaker to a stronger solution (thereby making the weak stronger, and the strong, weaker !). Osmosis, its called.
Other measures to reach water can be just as dramatic, such as the huge tap roots that Acacias can send down, over 100 feet to any sources of permanent moisture. Yet another strategy can be found in many grasses and herbs, which spread shallow roots over a wide area to take advantage of any dew moisture, which would otherwise evaporate and never reach other plants roots.
In the long term, the survival of any plant species depends on how well it reproduces, and this is of particular importance to desert species. Some may be perennials, in which case they may grow out in colonies of themselves, able to withstand a couple of bad years on their resources, and the main plant can die back, a little, to flourish again when water is more plentiful.
When they seed, they must either produce fewer, larger seeds, which contain enough resources for the plant to become established, or, make the seed in such a way that it only opens when there is water about. Such a seed may remain viable after many years.
Animals which live in the desert show a similar wide range of adaptations, although the changes are related to the sort of animal. Birds, for instance, can travel a long distance in search of water. Sandgrouse will flock once daily at a water-hole, often travelling 50 km in search of it.
Legend had it that this bird carried water back to its young in soaked breast-feather, despite the problem of evaporation on the flight home. Closer study has revealed that the young receive their water in a much more mundane fashion, via regurgitated food.
Most desert birds are small, ground living creatures, often spending large amounts of time in the shade of a rock during the day. They also have to take care when breeding to ensure the eggs or the chicks themselves do not overheat.
Larger birds suffer less from the heat, too, since it is easier for them to circle up to the cooler air on thermal updraughts. One large bird which prefers to run along the ground, though, is the great Indian Bustard, which has well adapted and horny feet for getting about on the stony ground.
Unlike many endangered species, which are rapidly losing their habitat, the irony for the bustard is that its habitat is increasing greatly, and it is predation by humans, often sport hunting which threatens their survival.
Generally speaking, the smaller animals have greater difficulty dealing with overheating, since they have a greater surface area relation to body mass, but if its a reptile, then heat poses no real problem until it becomes extreme, and then they merely seek shade.
Small mammals are often not equipped to lose heat via sweat glands or panting, and so they will rarely venture out in the daytime. They stay in burrows where the temperature one metre underground stays around an even 30 C.
Another method to lose heat might be by stilting – lifting themselves above ground, as scorpions do, to allow air to circulate. Or they may be equipped, like the long eared hedgehog with two radiators on its head to lose heat with.
They can usually gain all the water they need from the vegetation they eat. Gerbils in fact can survive on a diet of completely dry food, since they possess the ability to utilise metabolic water. As carbohydrates are digested, they break down to produce carbon dioxide which is expelled in respiration, and water, which is reabsorbed.
The smallest animals, the insects, require virtually no access to water as such, since they lose very little through their hard exoskeleton. Predators can get sufficient water from the bodies of their prey, for instance, or they can get it from the vegetation they consume if they are herbivorous.
Larger warm blooded animals find less of a problem in dealing with heat, since they possess both sweat glands and the ability to pant and lose heat this way. Unfortunately, both methods use large quantities of water which must be replaced.
To an extent this can be done if the animal has access to large amounts of green fodder, as they may occasionally have after a monsoon shower gets as far North as the Thar, but it normally visits a water hole at frequent intervals.
Camels of course are legendary in their ability to go without water. This is achieved by their ability to tolerate very high levels of dehydration – they may lose 40% of their body weight before suffering ill effects, whereas a loss of 20% would kill most mammals.
The camel can also consume huge quantities of water, (one third of body weight in ten minutes), without ill effect when seriously dehydrated, by an adaptation of the red blood cells. Such sudden and heavy dilution of the blood of a human, for instance, would rupture the red corpuscles, often with fatal effect. Other animals found in the Thar desert include blackbuck, nilgai – or blue bulls, desert fox and wild cat.
Bob Cranwell is a writer and has the website Amateur Emigrant which shares travel memoirs, ways of seeing, adventure travel, and philosophy about the many places he has journeyed over the years.