EVIDENCES OF CLIMATIC UNCERTAINTIES LINKED
GROUNDWATER VARIABILITY IN PERI-URBAN
AGRICULTURAL AREAS OF DELHI
P.S. DATTA and S.K. TYAGI
Nuclear Research Laboratory, Indian Agricultural Research Institute, New Delhi, India.
Urbanization, industrialization and population pressure induced changes in land use
significantly affect agriculture in peri-urban areas. In many such areas, associated with
this, inadequate availability of surface water supply, uncertain rainfall and environmental
changes induce indiscriminate groundwater use, leading to problems such as, decline in
groundwater levels and groundwater pollution in different parts. Nonetheless, climatic
uncertainties linked atmospheric carbon-dioxide assimilation, inter-cellular carbondioxide partial pressure and transformation processes also affect the water-use-efficiency
of plants. In this background, fusion of 18O stable isotope imaging of groundwater, 18O
and 13C isotopic record of water-plant-atmosphere interactions, superimposed on satellite
based land use images, etc., and other information systems provided an insight into many
long-term processes governing groundwater variability, which can be helpful in
determining groundwater management strategies under changing environmental
conditions and climatic uncertainties in Delhi region.
INTRODUCTION
Much of the water needs in the investigated area depend upon the hydrological cycle, and
the natural recharging of water resources. Urbanization, industrialization and population
pressure has significantly induced changes in land use in the peri-urban agricultural areas,
with challenge of sustaining production in a limited land area. Natural resources also
remain under increased pressure because of the use of land for clay pits, quarries, sewage
disposal tanks and garbage dumps, and water pollution from local industrial and urban
sources. Groundwater, which collects in the aquifers over thousands of years through
recharge, contributes to substantial quantity of water supply mainly because of the
insufficiency of the surface water supply share. In many parts, associated with this, due to
uncertain rainfall, environmental changes and rise in water demand for various purposes,
indiscriminate groundwater use and unplanned disposal of wastes, problems, such as,
lowering in groundwater levels, decline in productivity of wells, and groundwater
pollution with increasing trend are evident. Changes in the water availability as a result of
climatic stresses are likely to cause impacts, generally most detrimental in regions
already under existing stresses due to increases in the level of competition between
potential users for water.
Consequences of climatic uncertainties extend also to sharp reversals of weather
patterns, vegetative feedbacks and seasonal modifications in rainfall around the globe.
Climate change modifies total seasonal rainfall, evaporation, runoff, and soil moisture
storage and its pattern of variability. The present models of global atmospheric
circulation studied by Keeling et al [1] suggest that biospheric productivity is the primary
cause of the inter-annual fluctuations in the atmospheric CO2. Investigations by
Sieganthaler [2] indicate that the weather phenomenon, El-Nino, also brings a rise in CO2
exceeding the normal anthropogenic growth rate and lasts for about a year. Therefore, the
studies on the pattern of local variations in the isotopic composition of air CO 2 and
rainfall in urban areas are expected to provide important information on the changes in
environment on a regional scale. Internationally, aspects of environmental change have
been so far demonstrated using isotopic data mainly from temperate climates, and there is
limited understanding of the controlling factors in tropical regions. In this background, to
qualitatively assess the magnitude of the problem, temporal variations in the stable
isotopes (18O/16O and 13C/12C) signatures in groundwater, plant tissues and air-CO2
have been integrated with groundwater flow-pathways and contamination dynamics,
superimposed on satellite based land use images, GIS, etc., and other information
systems, through a case study in the Delhi region.
AREA PROFILE
The area is located in the path of the Indian southwest monsoon trough movement,
receives about 80% of the annual rainfall during July to September. This brings a
significant change in the environment and increase in plant biomass productivity.
According to Lintelo et al [3], the area beyond the urban conglomeration is still
predominantly agricultural, and within the NCT Delhi (in the north, northwest and west)
lie diminishing agricultural areas. Cultivation practices changed from storable crops such
as cereals and pulses to high-value commercial crops such as vegetables. Wheat, rice,
great and spiked millet and fodder crops are cultivated in most of the agricultural land.
Vegetable cultivation (such as cauliflower, cabbage, carrot, spinach, mustard leaves,
okra and tomato) is also popular. Agriculture in peri-urban areas may also include other
activities such as fisheries, poultry, goat rearing, horticulture floriculture, dairy farming,
cattle farming, etc.. The satellite image suggests the wide extent of agricultural land use.
About 44% of the land area is used for crop production, fallow land, plantation or
grassland while 17% consists of built-up areas.
The climate of the region is semiarid, with three well characterized seasons. The cold
season begins at the end of November, and extends to early July and continues upto
September. The hot summer extends from the end of March to the end of June. The
temperature is usually between 21.1°C to 40.5°C during these months. Winters are
usually cold and night temperatures often fall to 6.5°C during the period between
December and February. Based on the records over the period of 70 years, the average
annual temperature recorded in Delhi is 31.5°C. About 87% of the annual rainfall is
received during the monsoon months June to September. On an average, rain of 2.5 mm
or more falls on 27 days in a year. Of these, 21.4 days are during monsoon months.
EFFECTS OF ENHANCED CO2 AND HIGH TEMPERATURE ON PLANT WUE
Higher levels of atmospheric CO2 concentrations tend to increase the difference in partial
pressure between the air outside and inside the plant leaves and also induce plants to
close the leaf stomates. Thus, under CO2 enrichment crops may use less water even while
producing more carbohydrates. This is likely to improve water-use efficiency, which is
the ratio between crop biomass and the amount of water consumed. At the same time,
associated climatic effects, such as higher temperatures, changes in rainfall and soil
moisture, and increased frequencies of extreme meteorological events, could either
enhance or negate potentially beneficial effects of enhanced atmospheric CO 2 on crop
physiology.
Earlier studies by Datta et al [4] indicated that increasing biomass during the summer
monsoon rapidly consumes air-CO2 for photosynthesis, discriminating against 13C
isotope, causing associated changes in the isotopic ratios of carbon ( 13C/12C) and
oxygen (18O/16O) in the ground level air-CO2 and plant tissues. Vegetative feedback to
environmental changes was studied by Datta et al [4] in an agricultural farm during
March to September. Air-CO2 level ranged from 330 to 458 ppmv, with mean value of
357.8 ppmv, and the CO2 -δ13C value varied from -7.8 to -11.6‰, with mean of -8.9‰.
The air-CO2 level in the area over the last two decades increased considerably. While, the
pre-monsoon warmer months (March to June) had higher CO 2 levels (mean 366 ppmv),
relatively cooler and high (70-80%) humidity monsoon months had significantly depleted
level (mean 345 ppmv). During the monsoon months, due to rapid consumption of 12CO2
by the increased biomass productivity, δ13C is inversely related to air-CO2 level and
temperature, but directly related to temperature in the pre-monsoon period, affected by
wind direction. The CO2-δ18O value ranged from +38.7‰ to +50.9‰ (mean +41.2‰),
close to that of normal atmospheric CO 2 (δ18O=+41‰). During monsoon months,
equillibration of CO2 with relatively less enriched leaf water of the vegetation results in
depleted CO2-δ18O.
It has been also noted by Pande et al [5] and Wright et al [6] that water uptake by the
roots of a terrestrial vascular plant occur with negligible discrimination between H218O
and H216O and on a landscape scale, the 18O/16O ratio of leaf water influences the
18O/16O ratios of atmospheric carbon-dioxide and plant organic matter. Pande et al [5]
also observed that the isotopic enrichment phenomenon in wheat grain and peduncle
water is linked with development metabolism and water extraction capacity, with
possible modification by foliage fractionation and through enriched water contributed by
breakdown of soil organic matter. As observed by Datta et al [4], the characteristics of
13C discrimination in air-CO and lower C /C ratio in the months of May beginning and
2
i a
mid-June clearly suggest that during warmer months and under water limiting conditions,
which mainly restrict CO2 supply, the water-use-efficiency in plants adapted to low
water availability is expected to increase. Nonetheless, climatic uncertainties linked
atmospheric carbon-dioxide assimilation, inter-cellular carbon-dioxide partial pressure,
nutrient dynamics and transformation processes also affect the water-use-efficiency.
Reduced water supply is likely to place additional stress on agriculture, and the
environment. The demand for water for irrigation rises in a warmer climate, bringing
increased competition between agriculture--already the largest consumer of water
resources in semiarid regions--and urban users.
FACTORS INFLUENCING ISOTOPIC COMPOSITION OF RAINFALL
Analysis of the δ18O and δD data of New Delhi rainfall (IAEA Global Network Data
1961-96), made by Datta et al [7], indicated that although, the monthly mean values
range from -15.3 to +8.0‰ for δ18O and -120 to +55.0‰ for D and fall along the world
meteoric line, yet, the isotopic composition of rainfall events of less than 50 mm show
deviation form the meteoric line, with each summer months following different
evaporation line. Long-term mean rainfall and mean temperature together account for
about 80-95% of the long-term average variability of δ18O, with temperature alone
governing 80% variability. Although, depleted δ18O is generally associated with months
with heavy rainfall, yet, Datta et al [7] found a scattered correlation of monthly δ 18O with
rainfall, which reflect that atmospheric water vapour circulation patterns, distribution and
intensity of rainfall, composition and movement trajectory of the moist air-mass from
which the rainfall derives govern the temporal variability in δ 18O. However, in the
rainfall range 50-350 mm, monsoon months July, August and September and the El-Nino
years (1964-92) show different trends of amount effect. In any year, the summer months
May and June with maximum monthly temperature closely match with maximum
enrichment in monthly rainfall δ18O and are associated with higher air-CO2 level in some
of the years. El-Nino years and rainfall deficient years are generally associated with
relatively enriched δ18O in rainfall, as compared to the monthly rainfall δ 18O in the
normal monsoon years. Since, the Indian monsoon is a very complex phenomenon and
average monthly atmospheric circulation features in normal monsoon years differ
considerably from that in deficient monsoon years, it is difficult to ascertain whether ElNino affects India. Yet, studies reported earlier by Sieganthaler [2] found that El-Nino
years are associated with weak Indian summer monsoon.
GROUNDWATER RECHARGE AND CONTAMINATION CHARACTERISTICS
Investigations in the Delhi region, made by Datta et al [8,9] indicate that average
groundwater recharge from rainfall varies widely from region to region and within the
parts of a region, both in space and time, depending on climatic factors, such as,
frequency, intensity and distribution of rainfall and evaporation. Differences in recharge
(0.2 to 66.0%) from location to location result in wide range of spatial variations in the
stable isotope (18O) signatures of groundwater. Datta et al [10,11] observed that localised
recharge from high intensity rainfall, through stagnant water pools that are left in low
lying areas, and indirect recharge through lateral flow from surrounding areas in the west
are the main contributors to the groundwater along specific flow-pathways. Changes in
surface water availability influence the recharging of groundwater supplies and, in the
longer term, aquifers. Datta et al [10,11] observed that almost the whole peri-urban area
in the west, southwest, northwest and north is severely affected by nitrate (<20-1600
mg/l), fluoride (0.1-16.0mg/l) and heavy metals (Zn, Cu, Fe, Mn, Pb, Ni and Cd)
pollution of groundwater. The concentration of fluoride and nitrate in groundwater vary
spatially and temporally, governed by their concentration in the water recharging from
the unsaturated zone, different degrees of evaporation/recharge, adsorption/dispersion
processes in the soil zone, additions from groundwater flowing into the area from
surroundings and amounts of chemicals present in the soil. Intensified evaporation is
likely to increase the hazard of salt accumulation in the soil. Water quality may also
respond to changes in the amount and timing of precipitation.
GROUNDWATER VARIABILITY
From Figure 1and 2, a comparison of water levels from 1977 to 1983 and 1983 to 1995
indicates that during 1977, due to relatively higher annual rainfall (120 cm) and less
temperature anomaly, the water table was by and large within 6-8m below ground level
(bgl) in major parts of the territory, deepest being 23m bgl in Mehrauli Block. In 1983,
due to relatively less rainfall (~ 84 cm) the depth to water level declined to within 1014m bgl in major parts with the deepest level being 26m bgl at Mehrauli. In 1995, due to
further decrease in rainfall (~72 cm) the extent of area with water levels in the range of
10 to 20 m bgl has substantially increased and the deepest water level is about 35 m bgl
in Mehrauli block. During 1993 -1996, normal rainfall and less temperature anomaly kept
the water table relatively stable. In 1979and 1989, 1999 and 2002, due to very low
rainfall and high temperature anomaly, the water table declined considerably. During
1960–2000, the water levels declined by 4m or less in most parts of Delhi, rise being
confined to south western parts of Urban block. In the Central Najafgarh block and
south–eastern part in Mehrauli block, a fall of 4-12m is observed. This is mainly due to
intensive ground water development for irrigation. During 1977-1983, water table
declined by 4m or less in most parts of Delhi. Parts of Urban block (south-western) and
Mehrauli block exhibited a fall of 4m to 8m during this period possibly due to increased
pumpage for domestic purposes and farmhouses. During 1983–1995, water levels have
declined all over Delhi excepting a small area in northeast. Though in most parts of
Delhi, the water table decline has been less than 4m, significantly greater declines (4m to
>8m) have been recorded in areas in central Najafgarh block, both sides of the ridges in
southern Urban block and in the Mehrauli block. Ground water level in the Southern
parts is declining with rates of 1-4 m/yr.
It is not certain how individual water catchment areas respond to changing
evaporation rates and precipitation. It is likely however, that changes in temperature and
precipitation could cause relatively large changes in run-off. Arid and semi-arid regions
will therefore be particularly sensitive to reduced rainfall and to increased evaporation.
6
Deviation from Mean temperature (oC) at Delhi
(a)
4
2
0
-2
-4
-6
Jan-00
Jan-98
Jan-96
Jan-94
Jan-92
Rainfall pattern at IARI
Farm, New Delhi
1000
800
600
400
Drought Years
Rain deficit years
0
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
200
BAW ANA
(N)
BARW ALA
PUSA
Water Table b.g.l. (m)
(N)
(C)
-10
NAJAFG ARH
(W )
-20
BIJW ASAN
(c)
(SW )
N-Nothern, C-Central
W-Western, S-Southern
SW- Soth Western
-30
BHATTI
(S)
2003
2001
1999
1997
1995
1993
1991
1989
1987
1985
1983
1981
Decline in G roundwater Table at Some Places in Delhi
-40
1979
Rainfall (mm)
Jan-90
Jan-88
Jan-86
Average
Rainfall
729 mm
(b)
1200
Jan-84
Jan-82
Jan-80
Jan-78
Jan-76
-8
Year
Figure 1. Influence of temperature and rainfall on groundwater in Delhi
Figure 2. Map of Delhi region showing zonal disparity in water supply. Peri-urban areas
of north, north-west and south have 4-12 m decline in water table during 1960-2000
An increase in the duration of dry spells may not necessarily lead to an increased
likelihood of decline in groundwater levels, since increases in precipitation may be
experienced during other seasons. Changes in seasonal patterns of rainfall may affect the
regional distribution of both ground and surface water supplies. Changes to the
hydrological cycle as a result of climatic stress may interact with man-made changes in
land use, waste disposal and water extraction.
CONCLUSIONS
The evidences suggest that the decline in ground water levels may be due to the
following reasons which are associated with the climatic uncertainties: (1) Rainfall and
temperature anomalies that show two different declining trends during 1976 – 1987 and
1988-2002, (2) Rainfall deficiency and urbanisation induced reduction in recharge of
aquifers. Unplanned withdrawal from aquifers, not in commensurate with recharge and
increasing demand, during drought periods when all other sources shrink, and (3) Water
scarcity induced changes in cropping pattern in some stressed areas. Resource
characteristics and distribution differ from region to region and are geographically bound
entities, characterised by specificity of occurrence. Therefore, each area/region should be
treated separately, through proper choice of technology and research. Due to the
complexities of the hidden hydrogeologic system, systematic research is needed on
hydrogeologic characteristic of the groundwater flow field under natural and stressed
conditions, dynamics of groundwater contaminants, and its linkage with spatial and
temporal variability in concentration, depth variation in contaminants level in relation to
well structure and casing conditions, relative importance of vegetative uptake of
contaminants, denitrification potential of soil and geohydrology in limiting
contamination. Iso-concentration maps of contaminants levels in groundwater should be
prepared and revised time to time, in relation to the climate induced changes in landuse
pattern. Also, temporal variability in concentration should be monitored and linked with
climatic influences induced groundwater dynamics.
REFERENCES
[1] Keeling, C.D., Bacastow, R.B., Carter, A.F., Piper, S.C., Whorf, T.P., Heimann, M.,
Mook, W.G. and Roeloffzen, H., In: "Aspects of climate variability in the Pacific
and the Western Americas", Geophys. Monograph, 55: (1989)165-236.
[2] Sieganthaler, U., "El-Nino and atmospheric CO2", Nature, Vol. 345: (1990) 295-296.
[3] Lintelo, D.Te., Marshall, F. and Bhupal, D.S. Peri-urban Agriculture, fna/ana, 29:
(2001) 4-11.
[4] Datta, P.S., Bhattacharya, S.K., Tyagi, S.K. and Jani, R.A., "Vegetative growth and
ecophysiological significance of 13C and 18O composition of air-CO2", Plant Physiol
& Biochem. 22(1): (1995) 64-67.
[5] Pande, P.C., Datta, P.S., Bhattacharya, S.K., and Tyagi, S.K., Indian Journal of
Eperimental Biology, 33, (1995) 394-396.
[6] Wright, G.C., Hubick, K.T. and Farquhar, G.D., Aust. J. Plant Physiol., 15, 815-825
(1988).
[7] Datta, P.S., Tyagi, S.K. and Chandrasekharan, H., "Factors controlling stable
isotopic composition of rainfall in New Delhi, India", J. Hydrol., 128: (1991) 223236.
[8] Datta P.S., Bhattacharya S.K.and Tyagi S.K. “18O studies on recharge of phreatic
aquifers and groundwater flow paths of mixing in Delhi area.” J. Hydrol., 176,
(1996) pp.25-36.
[9] Datta P.S., Bhattacharya S.K. and Tyagi S.K. “Assessment of groundwater flow
conditions and hydrodynamic zones in phreatic aquifers of Delhi area using oxygen18.” Proc. Intn. Work. on Groundwater Monitoring and Recharge in Semi-arid
Areas, Hyderabad, IAH/UNESCO Publn. (1994) pp. S IV12 - S IV24.
[10] Datta P.S., Deb D.L.and Tyagi S.K. “Stable isotope ( 18O) investigations of the
processes controlling fluoride contamination of groundwater.” J. Contaminant
Hydrol., 24(1): (1996) 85-96.
[11] Datta P.S., Deb D.L.and Tyagi S.K. “Assessment of groundwater contamination
from fertilizers in Delhi area based on 18O, NO3- and K+ composition.” J.
Contaminant Hydrol., 27(3-4): (1997)249-262.