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The hydrological indicators of desertification
Journal of Arid Environments (1998) 39: 121–132 Article No. ae980403
The hydrological indicators of desertification
K. D. Sharma Central Arid Zone Research Institute, Jodhpur 342 003, India (Received 16 June 1997, accepted 12 December 1997) The lack of, development of, and mismanagement of water are the basic causes of environmental degradation and desertification in many arid regions. The signs, magnitude and severity of desertification can be established through such hydrological indicators as reduced area of water bodies, increased runoff and consequently decreased rainwater infiltration, accelerated soil erosion and sedimentation, and deteriorated ground-water resources. Case studies conducted in the Indian arid zone show that the interactions of humans, land and water have caused desertification in vast areas. ©1998 Academic Press Keywords: Indian arid zone; hydrological indicators of desertification; Nadi; Jodhpur District; arid zone hydrology
Introduction Desertification is attributed to natural phenomena and to human activities that degrade the land through mismanagement of the basic factors of land use systems, including soil, water and vegetation. Water is one of the main factors limiting production and settlement in drylands. Lack of, development of, and wasteful uses of water are fundamental causes of many problems of desertification and environmental degradation. The Mar del Plata Action Plan (UNWC, 1977) emphasized that many arid areas have a relatively large potential for water development, yet the real problem is not the lack of resources but the lack of an integrated water management policy to alleviate the current tragic conditions and prevent further desertification. Hydrological considerations are critically important in determining the magnitude and signs of desertification (Mageed, 1986). Many arid and semi-arid regions are confronted with overgrazed pasture land, vanishing forests and precarious agriculture subjected to uncertain rainfall, river flooding, drying up of surface water and depletion of ground-water. In such areas human use of natural resources needs to be in balance with the carrying capacity of these resources.
Hydroclimatic features of the arid zone Arid and semi-arid climates are characterized by a high level of incident radiation, high seasonal temperature variations, low humidity and strong winds with frequent dust 0140–1963/98/020121 + 12 $30.00/0
© 1998 Academic Press
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K. D. SHARMA
storms. Precipitation in these regions is generally intense and sporadic. High temporal and spatial variability of precipitation leads to a greater variability of short duration runoff, accelerated soil erosion by water and high sediment transport rates. In fact, the irregularities of the meteorological processes are the main cause of rainfall and streamflow variabilities from season to season and year to year. The hydrological balance is in a delicate state. A prolonged wet or dry spell may change the whole nature of the hydrology. There is no reliable relationship between seasonal and annual runoff to rainfall, and the magnitudes of the flood and the drainage basin areas are not directly related except in small basins (Sharma, 1992). The base flow is essentially absent while channel transmission losses are of critical importance. Further, a decline in soil structural stability with an attendant increase in surface crusting and surface runoff leads to reduced soil infiltration capacity and soil water storage. The ground-water and soil water in arid and semi-arid zones are subjected to relatively large storage changes (Jones, 1981). The hydrological cycle The hydrological cycle (Fig. 1) is the process through which water, driven by the sun’s energy, circulates from the earth to the atmosphere and back to the earth. Water moves through the cycle endlessly from the oceans to the atmosphere and as water vapour precipitates back onto the continents and into the oceans. Water that falls as precipitation on land is evaporated into the atmosphere directly or through the process of plant transpiration. Water returns to the oceans as surface runoff or percolates underground and eventually reaches the seas. Because oceans cover 70% of the earth’s Outgoing water vapour
Incoming water vapour ATMOSPHERIC WATER Precipitation
Evaporation
Evaporation
SURFACE STORAGE Evapo-transpiration
Overland flow Infiltration
SOIL WATER Interflow Capillary rise
Gravity water
GROUND-WATER Groundwater outflow Deep percolation
Figure 1. The hydrological cycle.
NETWORK OF CHANNELS, RIVERS, LAKES
Runoff
HYDROLOGICAL INDICATORS OF DESERTIFICATION
123
surface, the major part of precipitated water falls directly into them and does not contribute to potential water for human use. Distribution of fresh water resources in space and time over the globe and their magnitudes are characterized by great irregularities caused by climatic factors, variabilities and irregularities of the atmospheric motions governing the processes of water vapour transport, and precipitation and its accumulation on the ground surface. The consequences of climatic variability on the surface and subsurface water balance are among the prime causes of land degradation in drylands (Williams & Balling, 1996). The hydrological cycle in drylands may be expressed most conveniently as a simple water balance equation, adapted from Monteith (1991): Pd = Ed + Rd + Id + Sd + Gd
(Eqn 1)
where Pd is dryland precipitation, Ed is dryland evaporation (including evapotranspiration), Rd is net dryland water loss from runoff, Id is net dryland water loss from rainwater infiltration, Sd is net gain in dryland soil water content and Gd is net gain in dryland ground-water storage. Each of these amounts may be expressed as an equivalent depth of water per unit of time, such as millimeters per day, month or year. Many investigators have found that the overwhelming effect of desertification comes from disruptions in the hydrological cycle. In many cases removal of vegetation leads to increased runoff and potential evapo-transpiration rates resulting directly in a decrease in soil water content and a rapid decrease in amount of energy used to evaporate or transpire water into the atmosphere.
Hydrological indicators of desertification Since 1958 intensive hydrological studies have been conducted in the Indian arid zone to assess and monitor water resources. From these studies the hydrological indicators of desertification involving both the surface and ground-waters have been determined (Fig. 2), and their efficacy has been established through case studies.
Case studies The study area The case studies were conducted in the Jodhpur District (22,850 km2), extending between 25·2 to 27·5° N and 72·0 to 73·8° E with a Thorntwaite aridity index varying between 70 and 87 in the south-east to the north-west (Fig. 3). Mean annual rainfall over the past 95 years has been 307 mm, declining from south-east to north-west and varying considerably from year to year over a range of 65 to 846 mm. About 92% of the annual rainfall is received with relatively high intensity from June to September. July and August are the rainiest months. The area lies in a belt having an average minimum temperature of 10°C and an average maximum temperature of 33°C. Annual evaporation (3102 mm) far exceeds annual precipitation. Topographically the Jodhpur District has vast alluvial plains in the south and southeast, steeply rising hills in the south-east and centre, and dunes and interdunal plains in the north and north-west. The Luni River and its tributaries are ephemeral and form the area’s drainage system. The minor ephemeral streams emanating from the isolated
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hillocks feed internal drainage basins and infiltrate the deep windblown sand and alluvium. Area covered and turbidity of surface water The people of the Indian arid zone harness the meager precipitation in small earthen dugout/embanked ponds called Nadis (Fig. 4). This is an ancient practice, and the Nadis are the main source of drinking water (Sharma & Joshi, 1981). Nadis contain water for 2 to 12 months, depending upon the type of surface, drainage basin characteristics and amount and distribution of rainfall received. Nadis range from 1·5 to 12·0 m deep, have a storage capacity of 400 to 700,000 m3, and have drainage basins of various shapes and sizes (8 to 2000 ha). Nearly 80% of the Nadis have impounding areas smaller than 0·9 ha. In the Jodhpur District 87·4 km2 (0·4% of the area) are occupied by these water bodies. An overlay of maps of water bodies prepared from the Landsat Thematic Mapper data of 1986 on the Survey of India topographical maps of 1958 showed spectacular changes in the drainage basin area and the surface area of Nadis. Changes in the size and shape of water bodies largely depend on rainfall. But in this case a 1·8 to 2·4 times reduction in the drainage basin area over a 28-year period (1958–1986) resulted from such biotic interference as reclamation of drainage basins for cultivation, open-cast (open pit) mining, residential and industrial construction and the increasing trend of urbanization. The rainfall amount was almost the same, i.e. 254 and 237 mm during 1958 and 1986, respectively, with a similar distribution (Table 1). This decrease in the drainage basin area (Fig. 5) also caused a six to eight times corresponding decrease in the surface area of the water bodies. With this decreasing trend the Nadis may altogether disappear by the year 2020 (Anon., 1996).
Hydrological Indicators of Desertification
Ground-water
Surface water
Runoff
Area covered and turbidity of surface water
Infiltration
Evapotranspiration
Changes in water flow in water courses
Sequential changes in depth to water
Water quality
Sediment load
Water courses
Figure 2. Hydrological indicators of desertification.
Reservoirs and ponds
HYDROLOGICAL INDICATORS OF DESERTIFICATION
125
Flow changes in water courses The decrease in rainwater infiltration in the drainage basins and increase in water flow in the water courses associated with desertification (reduction in vegetation cover, surface crusting/sealing, eroded and compacted surfaces) may cause streambank erosion. Superimposing maps of water courses and floodplains prepared from the Survey of India topographical maps of 1958 and Landsat Thematic Mapper data of 1986 revealed that during 28 years the water courses increased in width by 1·8 times and their areal extent increased due to 2 to 4 m dissections of the floodplains (Fig. 6). During this period nine flash floods of moderate to severe intensity were recorded, and a 37·2 km2 area along the water courses was eroded. The eroded land was largely double cropland, whereas the new land consisted almost entirely of channel sand bars and point bars 1·0 to 1·5 m high, 1·5 to 2·0 m long and 0·8 to 1·2 m wide, having little economic value. The flash flood overtopped the banks and deposited riverine sediment in the floodplains in the form of sand sheets 0·2 to 0·8 m thick and mounds and ridges 1·0 to 2·5 m high, 150 to 950 m long and 120 to 350 m wide. At several places the floodplains have also been dissected into rills and gullies, forming badlands. The fresh sediment has 3 to 4% clay, 1% silt and 3 to 5% moisture retention capacity, as opposed to the 6 to 10% clay, 4 to 7% silt and 8 to 20%
250 mm
72°
73°
N
INDIA
JODHPUR District
27° Phalodi
300 mm
iR Mitr
250 mm
JODHPUR
LUNI R
LEGEND
26°
Sandy plain
0
Dune complex Younger alluvial plain Saline run
Hills 300 mm
10
20 km
Streams Isohyet
250 mm
Figure 3. Study area — the Jodhpur District in arid north-west India.
S. Dave
Figure 4. A representative water body (Nadi).
126 K. D. SHARMA
HYDROLOGICAL INDICATORS OF DESERTIFICATION
127
moisture retention capacity of the original sediment. Also, amounts of organic carbon (0·05 to 0·10%), phosphorus (12 to 35 kg ha–1) and potassium (104 to 219 kg ha–1) were lower in the fresh sediment than in the original sediment (0·15 to 0·23 %C, 13 to 66 kg ha–1 P and 317 to 495 kg ha–1 K), thereby indicating the depletion in soil fertility. About 8% of the area in the Jodhpur District is rocky, gravelly and stony wasteland. The dominant land use is grazing. The combined effects of trampling (which destroys the soil structure) and excessive grazing (which reduces vegetation cover) increase Table 1. Temporal changes in the water surface and drainage basin areas of Nadis
Water surface area (ha) Landform
Drainage basin area (ha)
1958
1986
1958
1986
3·86 3·50 6·36 3·66
0·72 0·46 0·92 0·94
116·4 208·9 527·7 148·6
72·2 137·5 231·9 76·1
Older alluvial plain Interdunal plain Rocky/gravelly pediment Buried pediment
0
1 km 1: 50,000
Sandy alluvial plain
Younger alluvial plain
Saline alluvial plain
Rocky/gravelly pediment
Figure 5. A sample of temporal changes in the areal extent of drainage basins between 1958 (– – –) and 1986 ( — ).
72° 55'
Diwandi
72° 55'
73° 0'
LUNI
73° 0'
73° 5'
Mogro Kalan
RF – 156,000
73° 10'
Lalki
73° 15'
73° 25'
73° 20'
Martuka
73° 30'
73° 35'
River course (1986)
River course (1958)
Flood plain
73° 40'
73° 45'
BILARA
Joswant Seger
Lawari
Chaukri Kalan
Bhawi
PIPAR
73° 35'
Older alluvial plain
LEGEND
Bala
i River Lun
Kaparda
Rian
73° 30'
Hungaon
73° 25'
Ramasni
Binawas
M
er
iv
iR
itr
Ramawas Kalan
73° 20'
Dantiwara
Bisalpur
73° 15'
Khejarli Khurd
Kakelas
73° 10'
0 1 2 3 4 5 6 7 8 9 10 km
73° 5'
Figure 6. Temporal changes in water courses between 1958 and 1986.
72° 50'
55' Dundora
25° Bhacharha
0'
26°
5'
26°
10'
26°
15'
26°
20'
26°
25'
26°
72° 50'
55'
25°
0'
26°
5'
26°
10'
26°
15'
26°
20'
26°
25'
26°
128 K. D. SHARMA
HYDROLOGICAL INDICATORS OF DESERTIFICATION
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runoff and can expose topsoil to accelerated wind or water erosion (Novikoff, 1983). In a case study, five contiguous rocky drainage basins of 0·8 to 2·2 ha with 3·6 to 8·0% slopes have been equipped to record rainfall, runoff and sediment concentration since 1979. The soils are shallow to moderately deep sandy loam covered with gravels and pebbles in places. Ecologically, the vegetation consists of thornscrub, with locally adapted desert grasses having 1·9 to 4·6% basal cover. The whole area has been continuously overgrazed and eroded for many years. The four treatments, introduced in the rainy season, were control (undisturbed, no grazing), and three grazing regimes: light grazing (one cow ha–1), moderate grazing (three cows ha–1) and heavy grazing (four cows ha–1) for 90 days. Both the peak flow and runoff yield increased with the severity of grazing (Table 2). Under the heavy and light grazing regimes the peak flow increased by about five and three times, respectively, over the control. Pereira (1979) also reported a five-fold increase in the peak flow in Eppalock Basin in Australia after the introduction of a heavy grazing regime. The runoff yield increased by about nine and two times under heavy grazing over the control and light grazing, respectively. Penetration of rainfall into the trampled soil was slight: 0·5 m as opposed to 1·25 m under the control. Reduced penetration may be attributed to the reduction in both magnitude and opportunity time for rainwater infiltration due to the destruction of soil structure by trampling. Soil loss was directly proportional to the severity of grazing. Sediment concentration under heavy grazing (643 g m–3) was almost twice that under light grazing (303 g m–3) and about 1·5 times that under moderate grazing (485 g m–3). On the other hand, increased soil erosion from introducing grazing in these drainage basins was more pronounced. Under heavy grazing soil loss increased by about 41 times over the control and 11 times over light grazing. This increase may be attributed to the creation of loose soil aggregates by cattle trampling and the kinetic energy of increased runoff rates. Sediment load — water courses Arid zone rivers have innately variable flow regimes, and both discharge and sediment yield are highly sensitive to any changes in the headwaters, especially long- or shortterm changes in vegetation cover. Devegetation of headwaters can increase sediment load and lead to sometimes dramatic changes from a sinuous suspension load river carrying a dominant load of clay and silt to a less stable, more seasonal, much coarser bedload river with a rapidly shifting set of braided channels in its downstream and aggrading reaches (Mageed, 1986). Based on 9 years (1979–1987) of data, the absolute values of sediment concentration in the water courses in the Jodhpur District vary between 0·2 and 53·5 g l–1 in the Table 2. Runoff and soil erosion under various grazing regimes
Grazing regime
Peak flow (1 s–1 ha–1)
Runoff (mm) (g m–3)
Sediment concentration (kg ha–1)
Sediment yield (kg ha–1)
Ungrazed (control) Light grazing Moderate grazing Heavy grazing
47·2 78·6 117·5 221·6
1·2 4·6 6·0 11·0
219 303 485 643
3·95 14·36 59·28 160·84
89·1
5·0
247
82·50
CD (5%)
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K. D. SHARMA
headwaters and 1·0 to 453·6 g l–1 in the downstream valley. These values are higher than the reported sediment concentration of 1·0 to 12·0 g l–1 in central Australian rivers and the 5·0 g l–1 average throughout the western United States of America (Mabbutt, 1977). The higher sediment concentration in the Indian arid zone may be attributed to the greater biotic interference in the headwaters. The increased biotic interference resulted in a 3 to 18 times increase in soil erosion over the control, depending upon the dominant landform within the drainage basins. But although the sediment was produced in the drainage basins, significant sediment delivery is limited to the major flood flows (Sharma & Murthy, 1993). This vast load of suspended sediment rapidly fills storage dams and severely limits the irrigation and power potential critical to developing community resources. Sediment load — reservoirs and ponds The processes of sedimentation are related to high variability and seasonality of arid and semi-arid streamflows. Many countries in arid and semi-arid regions, particularly developing countries, are experiencing severe reservoir capacity depletion and short life as a result of sedimentation (FAO, 1973), particularly in small reservoirs built across wadis and drainage channels for water supply (Jones, 1981). In the study area the storage capacity of Nadis declined by 1·9 to 7·8% annually in different landforms due to sedimentation (Sharma & Chatterji, 1982). As a result, costly dredging is required. This reduction is due to the removal of vegetative cover in their drainage basins by felling, burning and overgrazing, thus resulting in increased sediment production. The sediment yield increased by between 188 and 1680% in different landforms as a result of the devegetation of drainage basins (Table 3). Infiltration In the Indian arid zone a large area has deteriorated due to irrigation with high ( > 4 me–1 l) residual sodium carbonate (RSC) waters. The loamy fine sand and fine sandy soils in the Jodhpur District have acquired unusual hardness (Fig. 7) and appreciably reduced infiltration (Table 4). An estimated 2000 km2 will be degraded by irrigation with high RSC waters by the year 2020 (Anon., 1996). Ground-water Between 1966 and 1976 ground-water resources in the Luni Development Block in the Jodhpur District significantly deteriorated due to overexploitation (Mabbutt & Table 3. Effect of removal of vegetation on sediment yield
Sediment yield (m3 ha–1 year–1) Landform Sandy plain Dune complex Younger alluvial plain Older alluvial plain
With vegetation
Without vegetation
Increase (%)
0·6 0·8 0·5 1·2
3·6 2·3 8·9 4·2
500 188 1680 250
HYDROLOGICAL INDICATORS OF DESERTIFICATION
131
Floret, 1980). The water-table dropped beyond 12 m in 16% of wells, appreciably reducing the discharge potential of wells by 2 to 10 l s–1. Water quality also declined; 54% of wells fell into the highly saline category, i.e. electrical conductivity > 6000 µS cm–1. These results further show that within 10 years the area of groundwater overexploitation increased from 6 to 77%. 30
*
Infiltration rate (cm h–1)
20
*
* 10
*
* * 0
20
* 40 60 Time (min)
80
100
Figure 7. Effect of irrigation with high RSC waters on infiltration characteristics of sandy loam soils. (䊉) = 20 me l–1; (*) = 7 me l–1.
Table 4. Infiltration rate into soils irrigated with high RSC waters
Site
RSC (me l–1)
Infiltration rate (cm h–1)
I II III IV V
20.6 13.3 7.2 14.2 14.6
5.0 7.2 6.5 4.8 1.7
RSC=residual sodium carbonate.
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K. D. SHARMA
A recent case study, conducted between 1984 and 1994 shows that about 50% of the ground-water resource has already been used in the Indian arid zone. The watertable is dropping at a rate of 20–40 cm year–1 in about 75% of the area because groundwater withdrawal is exceeding annual recharge (Anon., 1996). As a result of this overexploitation, the ground-water quality is also deteriorating. With the present trend of ground-water use, by 2020 a major part of the Indian arid zone is expected to be devoid of economically viable ground-water (Anon., 1996). Conclusions The interactions of humans, land and water are causing environmental conditions in the arid zone to deteriorate and are resulting in the desertification of a large area of adjoining lands. The case studies confirm that within the Indian arid zone desertification has set in. The degree and severity of desertification can be determined through hydrological indicators — a developing sphere of research and monitoring in the field of arid zone hydrology. References Anon. (1996). Perspective Plan: 1996–2020. Jodhpur: Central Arid Zone Research Institute. FAO (1973). Man’s Influence on the Hydrological Cycle. Rome: Food and Agriculture Organization of the United Nations. 71 pp. Jones, K.R. (1981). Arid Zone Hydrology. Rome: Food and Agriculture Organization of the United Nations. 96 pp. Mabbutt, J.A. (1977). Desert Landforms. Canberra: Australian National University Press. 340 pp. Mabbutt, J.A. & Floret, C. (Eds) (1980). Case Studies on Desertification. Paris: UNESCO/ UNEP/UNDP. 279 pp. Mageed, Y.A. (1986). Anti-desertification Technology and Management. Nairobi: United Nations Environment Programme. 27 pp. Monteith, J.L. (1991). Weather and water in the Sudano-Sahelian zone. In: Shivkumar, M.V. K., Wallace, J.S., Renard, C. & Giroux, C. (Eds), Soil Water Balance in the Sudano-Sahelian Zone, pp. 11–29. IAHS Publication No. 199. Wallingford, UK: International Association of Hydrological Sciences. 40 pp. Novikoff, G. (1983). Desertification by overgrazing. Ambio, 12(2): 102–105. Pereira, H.C. (1979). Land Use and Water Resources in Temperate and Tropical Climates. London: Cambridge University Press. 232 pp. Sharma, K.D. (1992). Runoff and Sediment Transport in an Arid Zone Drainage Basin. Bombay: Indian Institute of Technology. Sharma, K.D. & Chatterji, P.C. (1982). Sedimentation in ‘Nadis’ in the Indian arid zone. Hydrological Sciences Journal, 27: 345–352. Sharma, K.D. & Joshi, D.C. (1981). ‘Nadis’ the vital water resources of the Indian arid zone. Journal of Arid Environments, 4: 247–251. Sharma, K.D. & Murthy, J.S. R. (1993). Sediment transport in arid zone drainage basins. Journal of Environmental Hydrology, 1(2): 20–27. UNWC (1977). Water development and management. In: Proceedings of the United Nations Water Conference, Mar del Plata, Argentina. Oxford: Pergamon Press. 2644 pp. Williams, M.A. J. & Balling, R.C. Jr. (1996). Interactions of Desertification and Climate. London: Arnold. 270 pp.
The hydrological indicators of desertification
K. D. Sharma Central Arid Zone Research Institute, Jodhpur 342 003, India (Received 16 June 1997, accepted 12 December 1997) The lack of, development of, and mismanagement of water are the basic causes of environmental degradation and desertification in many arid regions. The signs, magnitude and severity of desertification can be established through such hydrological indicators as reduced area of water bodies, increased runoff and consequently decreased rainwater infiltration, accelerated soil erosion and sedimentation, and deteriorated ground-water resources. Case studies conducted in the Indian arid zone show that the interactions of humans, land and water have caused desertification in vast areas. ©1998 Academic Press Keywords: Indian arid zone; hydrological indicators of desertification; Nadi; Jodhpur District; arid zone hydrology
Introduction Desertification is attributed to natural phenomena and to human activities that degrade the land through mismanagement of the basic factors of land use systems, including soil, water and vegetation. Water is one of the main factors limiting production and settlement in drylands. Lack of, development of, and wasteful uses of water are fundamental causes of many problems of desertification and environmental degradation. The Mar del Plata Action Plan (UNWC, 1977) emphasized that many arid areas have a relatively large potential for water development, yet the real problem is not the lack of resources but the lack of an integrated water management policy to alleviate the current tragic conditions and prevent further desertification. Hydrological considerations are critically important in determining the magnitude and signs of desertification (Mageed, 1986). Many arid and semi-arid regions are confronted with overgrazed pasture land, vanishing forests and precarious agriculture subjected to uncertain rainfall, river flooding, drying up of surface water and depletion of ground-water. In such areas human use of natural resources needs to be in balance with the carrying capacity of these resources.
Hydroclimatic features of the arid zone Arid and semi-arid climates are characterized by a high level of incident radiation, high seasonal temperature variations, low humidity and strong winds with frequent dust 0140–1963/98/020121 + 12 $30.00/0
© 1998 Academic Press
122
K. D. SHARMA
storms. Precipitation in these regions is generally intense and sporadic. High temporal and spatial variability of precipitation leads to a greater variability of short duration runoff, accelerated soil erosion by water and high sediment transport rates. In fact, the irregularities of the meteorological processes are the main cause of rainfall and streamflow variabilities from season to season and year to year. The hydrological balance is in a delicate state. A prolonged wet or dry spell may change the whole nature of the hydrology. There is no reliable relationship between seasonal and annual runoff to rainfall, and the magnitudes of the flood and the drainage basin areas are not directly related except in small basins (Sharma, 1992). The base flow is essentially absent while channel transmission losses are of critical importance. Further, a decline in soil structural stability with an attendant increase in surface crusting and surface runoff leads to reduced soil infiltration capacity and soil water storage. The ground-water and soil water in arid and semi-arid zones are subjected to relatively large storage changes (Jones, 1981). The hydrological cycle The hydrological cycle (Fig. 1) is the process through which water, driven by the sun’s energy, circulates from the earth to the atmosphere and back to the earth. Water moves through the cycle endlessly from the oceans to the atmosphere and as water vapour precipitates back onto the continents and into the oceans. Water that falls as precipitation on land is evaporated into the atmosphere directly or through the process of plant transpiration. Water returns to the oceans as surface runoff or percolates underground and eventually reaches the seas. Because oceans cover 70% of the earth’s Outgoing water vapour
Incoming water vapour ATMOSPHERIC WATER Precipitation
Evaporation
Evaporation
SURFACE STORAGE Evapo-transpiration
Overland flow Infiltration
SOIL WATER Interflow Capillary rise
Gravity water
GROUND-WATER Groundwater outflow Deep percolation
Figure 1. The hydrological cycle.
NETWORK OF CHANNELS, RIVERS, LAKES
Runoff
HYDROLOGICAL INDICATORS OF DESERTIFICATION
123
surface, the major part of precipitated water falls directly into them and does not contribute to potential water for human use. Distribution of fresh water resources in space and time over the globe and their magnitudes are characterized by great irregularities caused by climatic factors, variabilities and irregularities of the atmospheric motions governing the processes of water vapour transport, and precipitation and its accumulation on the ground surface. The consequences of climatic variability on the surface and subsurface water balance are among the prime causes of land degradation in drylands (Williams & Balling, 1996). The hydrological cycle in drylands may be expressed most conveniently as a simple water balance equation, adapted from Monteith (1991): Pd = Ed + Rd + Id + Sd + Gd
(Eqn 1)
where Pd is dryland precipitation, Ed is dryland evaporation (including evapotranspiration), Rd is net dryland water loss from runoff, Id is net dryland water loss from rainwater infiltration, Sd is net gain in dryland soil water content and Gd is net gain in dryland ground-water storage. Each of these amounts may be expressed as an equivalent depth of water per unit of time, such as millimeters per day, month or year. Many investigators have found that the overwhelming effect of desertification comes from disruptions in the hydrological cycle. In many cases removal of vegetation leads to increased runoff and potential evapo-transpiration rates resulting directly in a decrease in soil water content and a rapid decrease in amount of energy used to evaporate or transpire water into the atmosphere.
Hydrological indicators of desertification Since 1958 intensive hydrological studies have been conducted in the Indian arid zone to assess and monitor water resources. From these studies the hydrological indicators of desertification involving both the surface and ground-waters have been determined (Fig. 2), and their efficacy has been established through case studies.
Case studies The study area The case studies were conducted in the Jodhpur District (22,850 km2), extending between 25·2 to 27·5° N and 72·0 to 73·8° E with a Thorntwaite aridity index varying between 70 and 87 in the south-east to the north-west (Fig. 3). Mean annual rainfall over the past 95 years has been 307 mm, declining from south-east to north-west and varying considerably from year to year over a range of 65 to 846 mm. About 92% of the annual rainfall is received with relatively high intensity from June to September. July and August are the rainiest months. The area lies in a belt having an average minimum temperature of 10°C and an average maximum temperature of 33°C. Annual evaporation (3102 mm) far exceeds annual precipitation. Topographically the Jodhpur District has vast alluvial plains in the south and southeast, steeply rising hills in the south-east and centre, and dunes and interdunal plains in the north and north-west. The Luni River and its tributaries are ephemeral and form the area’s drainage system. The minor ephemeral streams emanating from the isolated
124
K. D. SHARMA
hillocks feed internal drainage basins and infiltrate the deep windblown sand and alluvium. Area covered and turbidity of surface water The people of the Indian arid zone harness the meager precipitation in small earthen dugout/embanked ponds called Nadis (Fig. 4). This is an ancient practice, and the Nadis are the main source of drinking water (Sharma & Joshi, 1981). Nadis contain water for 2 to 12 months, depending upon the type of surface, drainage basin characteristics and amount and distribution of rainfall received. Nadis range from 1·5 to 12·0 m deep, have a storage capacity of 400 to 700,000 m3, and have drainage basins of various shapes and sizes (8 to 2000 ha). Nearly 80% of the Nadis have impounding areas smaller than 0·9 ha. In the Jodhpur District 87·4 km2 (0·4% of the area) are occupied by these water bodies. An overlay of maps of water bodies prepared from the Landsat Thematic Mapper data of 1986 on the Survey of India topographical maps of 1958 showed spectacular changes in the drainage basin area and the surface area of Nadis. Changes in the size and shape of water bodies largely depend on rainfall. But in this case a 1·8 to 2·4 times reduction in the drainage basin area over a 28-year period (1958–1986) resulted from such biotic interference as reclamation of drainage basins for cultivation, open-cast (open pit) mining, residential and industrial construction and the increasing trend of urbanization. The rainfall amount was almost the same, i.e. 254 and 237 mm during 1958 and 1986, respectively, with a similar distribution (Table 1). This decrease in the drainage basin area (Fig. 5) also caused a six to eight times corresponding decrease in the surface area of the water bodies. With this decreasing trend the Nadis may altogether disappear by the year 2020 (Anon., 1996).
Hydrological Indicators of Desertification
Ground-water
Surface water
Runoff
Area covered and turbidity of surface water
Infiltration
Evapotranspiration
Changes in water flow in water courses
Sequential changes in depth to water
Water quality
Sediment load
Water courses
Figure 2. Hydrological indicators of desertification.
Reservoirs and ponds
HYDROLOGICAL INDICATORS OF DESERTIFICATION
125
Flow changes in water courses The decrease in rainwater infiltration in the drainage basins and increase in water flow in the water courses associated with desertification (reduction in vegetation cover, surface crusting/sealing, eroded and compacted surfaces) may cause streambank erosion. Superimposing maps of water courses and floodplains prepared from the Survey of India topographical maps of 1958 and Landsat Thematic Mapper data of 1986 revealed that during 28 years the water courses increased in width by 1·8 times and their areal extent increased due to 2 to 4 m dissections of the floodplains (Fig. 6). During this period nine flash floods of moderate to severe intensity were recorded, and a 37·2 km2 area along the water courses was eroded. The eroded land was largely double cropland, whereas the new land consisted almost entirely of channel sand bars and point bars 1·0 to 1·5 m high, 1·5 to 2·0 m long and 0·8 to 1·2 m wide, having little economic value. The flash flood overtopped the banks and deposited riverine sediment in the floodplains in the form of sand sheets 0·2 to 0·8 m thick and mounds and ridges 1·0 to 2·5 m high, 150 to 950 m long and 120 to 350 m wide. At several places the floodplains have also been dissected into rills and gullies, forming badlands. The fresh sediment has 3 to 4% clay, 1% silt and 3 to 5% moisture retention capacity, as opposed to the 6 to 10% clay, 4 to 7% silt and 8 to 20%
250 mm
72°
73°
N
INDIA
JODHPUR District
27° Phalodi
300 mm
iR Mitr
250 mm
JODHPUR
LUNI R
LEGEND
26°
Sandy plain
0
Dune complex Younger alluvial plain Saline run
Hills 300 mm
10
20 km
Streams Isohyet
250 mm
Figure 3. Study area — the Jodhpur District in arid north-west India.
S. Dave
Figure 4. A representative water body (Nadi).
126 K. D. SHARMA
HYDROLOGICAL INDICATORS OF DESERTIFICATION
127
moisture retention capacity of the original sediment. Also, amounts of organic carbon (0·05 to 0·10%), phosphorus (12 to 35 kg ha–1) and potassium (104 to 219 kg ha–1) were lower in the fresh sediment than in the original sediment (0·15 to 0·23 %C, 13 to 66 kg ha–1 P and 317 to 495 kg ha–1 K), thereby indicating the depletion in soil fertility. About 8% of the area in the Jodhpur District is rocky, gravelly and stony wasteland. The dominant land use is grazing. The combined effects of trampling (which destroys the soil structure) and excessive grazing (which reduces vegetation cover) increase Table 1. Temporal changes in the water surface and drainage basin areas of Nadis
Water surface area (ha) Landform
Drainage basin area (ha)
1958
1986
1958
1986
3·86 3·50 6·36 3·66
0·72 0·46 0·92 0·94
116·4 208·9 527·7 148·6
72·2 137·5 231·9 76·1
Older alluvial plain Interdunal plain Rocky/gravelly pediment Buried pediment
0
1 km 1: 50,000
Sandy alluvial plain
Younger alluvial plain
Saline alluvial plain
Rocky/gravelly pediment
Figure 5. A sample of temporal changes in the areal extent of drainage basins between 1958 (– – –) and 1986 ( — ).
72° 55'
Diwandi
72° 55'
73° 0'
LUNI
73° 0'
73° 5'
Mogro Kalan
RF – 156,000
73° 10'
Lalki
73° 15'
73° 25'
73° 20'
Martuka
73° 30'
73° 35'
River course (1986)
River course (1958)
Flood plain
73° 40'
73° 45'
BILARA
Joswant Seger
Lawari
Chaukri Kalan
Bhawi
PIPAR
73° 35'
Older alluvial plain
LEGEND
Bala
i River Lun
Kaparda
Rian
73° 30'
Hungaon
73° 25'
Ramasni
Binawas
M
er
iv
iR
itr
Ramawas Kalan
73° 20'
Dantiwara
Bisalpur
73° 15'
Khejarli Khurd
Kakelas
73° 10'
0 1 2 3 4 5 6 7 8 9 10 km
73° 5'
Figure 6. Temporal changes in water courses between 1958 and 1986.
72° 50'
55' Dundora
25° Bhacharha
0'
26°
5'
26°
10'
26°
15'
26°
20'
26°
25'
26°
72° 50'
55'
25°
0'
26°
5'
26°
10'
26°
15'
26°
20'
26°
25'
26°
128 K. D. SHARMA
HYDROLOGICAL INDICATORS OF DESERTIFICATION
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runoff and can expose topsoil to accelerated wind or water erosion (Novikoff, 1983). In a case study, five contiguous rocky drainage basins of 0·8 to 2·2 ha with 3·6 to 8·0% slopes have been equipped to record rainfall, runoff and sediment concentration since 1979. The soils are shallow to moderately deep sandy loam covered with gravels and pebbles in places. Ecologically, the vegetation consists of thornscrub, with locally adapted desert grasses having 1·9 to 4·6% basal cover. The whole area has been continuously overgrazed and eroded for many years. The four treatments, introduced in the rainy season, were control (undisturbed, no grazing), and three grazing regimes: light grazing (one cow ha–1), moderate grazing (three cows ha–1) and heavy grazing (four cows ha–1) for 90 days. Both the peak flow and runoff yield increased with the severity of grazing (Table 2). Under the heavy and light grazing regimes the peak flow increased by about five and three times, respectively, over the control. Pereira (1979) also reported a five-fold increase in the peak flow in Eppalock Basin in Australia after the introduction of a heavy grazing regime. The runoff yield increased by about nine and two times under heavy grazing over the control and light grazing, respectively. Penetration of rainfall into the trampled soil was slight: 0·5 m as opposed to 1·25 m under the control. Reduced penetration may be attributed to the reduction in both magnitude and opportunity time for rainwater infiltration due to the destruction of soil structure by trampling. Soil loss was directly proportional to the severity of grazing. Sediment concentration under heavy grazing (643 g m–3) was almost twice that under light grazing (303 g m–3) and about 1·5 times that under moderate grazing (485 g m–3). On the other hand, increased soil erosion from introducing grazing in these drainage basins was more pronounced. Under heavy grazing soil loss increased by about 41 times over the control and 11 times over light grazing. This increase may be attributed to the creation of loose soil aggregates by cattle trampling and the kinetic energy of increased runoff rates. Sediment load — water courses Arid zone rivers have innately variable flow regimes, and both discharge and sediment yield are highly sensitive to any changes in the headwaters, especially long- or shortterm changes in vegetation cover. Devegetation of headwaters can increase sediment load and lead to sometimes dramatic changes from a sinuous suspension load river carrying a dominant load of clay and silt to a less stable, more seasonal, much coarser bedload river with a rapidly shifting set of braided channels in its downstream and aggrading reaches (Mageed, 1986). Based on 9 years (1979–1987) of data, the absolute values of sediment concentration in the water courses in the Jodhpur District vary between 0·2 and 53·5 g l–1 in the Table 2. Runoff and soil erosion under various grazing regimes
Grazing regime
Peak flow (1 s–1 ha–1)
Runoff (mm) (g m–3)
Sediment concentration (kg ha–1)
Sediment yield (kg ha–1)
Ungrazed (control) Light grazing Moderate grazing Heavy grazing
47·2 78·6 117·5 221·6
1·2 4·6 6·0 11·0
219 303 485 643
3·95 14·36 59·28 160·84
89·1
5·0
247
82·50
CD (5%)
130
K. D. SHARMA
headwaters and 1·0 to 453·6 g l–1 in the downstream valley. These values are higher than the reported sediment concentration of 1·0 to 12·0 g l–1 in central Australian rivers and the 5·0 g l–1 average throughout the western United States of America (Mabbutt, 1977). The higher sediment concentration in the Indian arid zone may be attributed to the greater biotic interference in the headwaters. The increased biotic interference resulted in a 3 to 18 times increase in soil erosion over the control, depending upon the dominant landform within the drainage basins. But although the sediment was produced in the drainage basins, significant sediment delivery is limited to the major flood flows (Sharma & Murthy, 1993). This vast load of suspended sediment rapidly fills storage dams and severely limits the irrigation and power potential critical to developing community resources. Sediment load — reservoirs and ponds The processes of sedimentation are related to high variability and seasonality of arid and semi-arid streamflows. Many countries in arid and semi-arid regions, particularly developing countries, are experiencing severe reservoir capacity depletion and short life as a result of sedimentation (FAO, 1973), particularly in small reservoirs built across wadis and drainage channels for water supply (Jones, 1981). In the study area the storage capacity of Nadis declined by 1·9 to 7·8% annually in different landforms due to sedimentation (Sharma & Chatterji, 1982). As a result, costly dredging is required. This reduction is due to the removal of vegetative cover in their drainage basins by felling, burning and overgrazing, thus resulting in increased sediment production. The sediment yield increased by between 188 and 1680% in different landforms as a result of the devegetation of drainage basins (Table 3). Infiltration In the Indian arid zone a large area has deteriorated due to irrigation with high ( > 4 me–1 l) residual sodium carbonate (RSC) waters. The loamy fine sand and fine sandy soils in the Jodhpur District have acquired unusual hardness (Fig. 7) and appreciably reduced infiltration (Table 4). An estimated 2000 km2 will be degraded by irrigation with high RSC waters by the year 2020 (Anon., 1996). Ground-water Between 1966 and 1976 ground-water resources in the Luni Development Block in the Jodhpur District significantly deteriorated due to overexploitation (Mabbutt & Table 3. Effect of removal of vegetation on sediment yield
Sediment yield (m3 ha–1 year–1) Landform Sandy plain Dune complex Younger alluvial plain Older alluvial plain
With vegetation
Without vegetation
Increase (%)
0·6 0·8 0·5 1·2
3·6 2·3 8·9 4·2
500 188 1680 250
HYDROLOGICAL INDICATORS OF DESERTIFICATION
131
Floret, 1980). The water-table dropped beyond 12 m in 16% of wells, appreciably reducing the discharge potential of wells by 2 to 10 l s–1. Water quality also declined; 54% of wells fell into the highly saline category, i.e. electrical conductivity > 6000 µS cm–1. These results further show that within 10 years the area of groundwater overexploitation increased from 6 to 77%. 30
*
Infiltration rate (cm h–1)
20
*
* 10
*
* * 0
20
* 40 60 Time (min)
80
100
Figure 7. Effect of irrigation with high RSC waters on infiltration characteristics of sandy loam soils. (䊉) = 20 me l–1; (*) = 7 me l–1.
Table 4. Infiltration rate into soils irrigated with high RSC waters
Site
RSC (me l–1)
Infiltration rate (cm h–1)
I II III IV V
20.6 13.3 7.2 14.2 14.6
5.0 7.2 6.5 4.8 1.7
RSC=residual sodium carbonate.
132
K. D. SHARMA
A recent case study, conducted between 1984 and 1994 shows that about 50% of the ground-water resource has already been used in the Indian arid zone. The watertable is dropping at a rate of 20–40 cm year–1 in about 75% of the area because groundwater withdrawal is exceeding annual recharge (Anon., 1996). As a result of this overexploitation, the ground-water quality is also deteriorating. With the present trend of ground-water use, by 2020 a major part of the Indian arid zone is expected to be devoid of economically viable ground-water (Anon., 1996). Conclusions The interactions of humans, land and water are causing environmental conditions in the arid zone to deteriorate and are resulting in the desertification of a large area of adjoining lands. The case studies confirm that within the Indian arid zone desertification has set in. The degree and severity of desertification can be determined through hydrological indicators — a developing sphere of research and monitoring in the field of arid zone hydrology. References Anon. (1996). Perspective Plan: 1996–2020. Jodhpur: Central Arid Zone Research Institute. FAO (1973). Man’s Influence on the Hydrological Cycle. Rome: Food and Agriculture Organization of the United Nations. 71 pp. Jones, K.R. (1981). Arid Zone Hydrology. Rome: Food and Agriculture Organization of the United Nations. 96 pp. Mabbutt, J.A. (1977). Desert Landforms. Canberra: Australian National University Press. 340 pp. Mabbutt, J.A. & Floret, C. (Eds) (1980). Case Studies on Desertification. Paris: UNESCO/ UNEP/UNDP. 279 pp. Mageed, Y.A. (1986). Anti-desertification Technology and Management. Nairobi: United Nations Environment Programme. 27 pp. Monteith, J.L. (1991). Weather and water in the Sudano-Sahelian zone. In: Shivkumar, M.V. K., Wallace, J.S., Renard, C. & Giroux, C. (Eds), Soil Water Balance in the Sudano-Sahelian Zone, pp. 11–29. IAHS Publication No. 199. Wallingford, UK: International Association of Hydrological Sciences. 40 pp. Novikoff, G. (1983). Desertification by overgrazing. Ambio, 12(2): 102–105. Pereira, H.C. (1979). Land Use and Water Resources in Temperate and Tropical Climates. London: Cambridge University Press. 232 pp. Sharma, K.D. (1992). Runoff and Sediment Transport in an Arid Zone Drainage Basin. Bombay: Indian Institute of Technology. Sharma, K.D. & Chatterji, P.C. (1982). Sedimentation in ‘Nadis’ in the Indian arid zone. Hydrological Sciences Journal, 27: 345–352. Sharma, K.D. & Joshi, D.C. (1981). ‘Nadis’ the vital water resources of the Indian arid zone. Journal of Arid Environments, 4: 247–251. Sharma, K.D. & Murthy, J.S. R. (1993). Sediment transport in arid zone drainage basins. Journal of Environmental Hydrology, 1(2): 20–27. UNWC (1977). Water development and management. In: Proceedings of the United Nations Water Conference, Mar del Plata, Argentina. Oxford: Pergamon Press. 2644 pp. Williams, M.A. J. & Balling, R.C. Jr. (1996). Interactions of Desertification and Climate. London: Arnold. 270 pp.