dc.description.abstract
Global groundwater resources are under stress due to high extraction rates, pollution, landuse
changes, and the impacts of climate change. In India, too, the decline in groundwater
level is a growing issue for water supply. This is well investigated in northern India,
but groundwater resources are also declining in parts of South India. While surface
water historically supplied agriculture, it has been largely replaced by groundwater
irrigation since the 1950s, resulting in the overexploitation of fragile aquifers. Population
growth, urbanisation, and rising food demand are further intensifying water needs.
Climate change adds uncertainty, with projections of higher temperatures, shifting
rainfall patterns, and greater drought risks.
The Sathaiyar basin, located in the South Indian state of Tamil Nadu, is marked by
locally declining and rising groundwater levels, providing a unique case to study these
dynamics. Half of the basin is covered by a cascade of irrigation tanks, which are small
surface water bodies that store excess river water and rainwater. The primary objective
of the present work is to assess the impact of i) natural groundwater recharge, as well
as ii) indirect recharge from irrigation tanks on groundwater. The quantification of
natural recharge was conducted using two approaches: the first involves simulating the
infiltration of rainwater through the unsaturated zone, and the second evaluates the
water level time series from an observation well. The local impact of tanks was assessed
by modelling the saturated-unsaturated zone processes of two separate irrigation tanks.
On the catchment scale, the effect of the tank cascade on groundwater was investigated
using hydrochemical and stable isotope tracers.
The natural groundwater recharge was simulated with three Hydrus 1D models. By
calibrating the models with stable water isotopes from soil water at different depths,
they provided reliable information on the quantity of recharge through different soils
while considering the influence of varying vegetation. Using only the soil water content
for model calibration would have resulted in a large range of recharge rates, which
emphasises the importance of using multiple data sources to build a reliable model. At the
site with the highest clay content of up to 80%, groundwater recharge is nearly negligible
0.01 cm from May to February, whereas at the site with only 20% clay, recharge amounts
to 31 cm for the same period. The results are in the order of magnitude of the recharge
estimated using the time series model Pastas, where the estimated annual recharge is up
to 4.7 cm yr−1. Analysis of the data from rainfall stations in the catchment indicated a
minor increasing trend for two stations, but no trend for the remaining locations. This
suggests that natural recharge has only a small impact on groundwater level rise and is
unlikely to increase substantially in the future.
In contrast, the recharge from irrigation tanks is high: the spatio-temporal trend
analysis of groundwater level time series shows that groundwater levels are mainly
increasing below the tank cascade. Most likely, the increasing trends are caused by the
iii
infiltration capacity of the tanks. The results of the two tank models show a significant
contribution of both tanks to groundwater recharge. As the tanks in the cascade are
filled with water for most of the time throughout the year, recharge is mainly limited
by low depth to groundwater and the resulting limited storage capacity of the aquifer.
Model results suggest that regular desilting of the tank bed would increase infiltration
rates. Another important parameter that strongly influences tank recharge efficiency
is the hydraulic conductivity of the weathered hard rock. In the present study, it was
constrained by calibration; future work could determine it directly using field experiments
or tracer studies.
The extent of the tanks’ impact was further investigated by using hydrochemical and
isotope tracers 𝛿2𝐻 and 𝛿18𝑂 on the catchment scale. Samples from groundwater and
tank water were taken before and after the monsoon season. The recharge effect of tanks
could be demonstrated even in deep wells (up to 230m depth) by means of stable isotopes.
An apparent difference was observed between the groundwater upstream of the tanks and
the groundwater influenced by the tanks. Besides the benefit of enhanced groundwater
quantity, a deterioration in its quality has been identified by hydrochemical analysis.
Groundwater near tanks receiving sewage inputs showed evidence of anthropogenic
pollutants, while agricultural activities additionally impacted the hydrochemical composition.
In the present study, the influence of anthropogenic activities, such as sewage
discharge and agrochemical application, could not be clearly distinguished from the
natural processes that influence the chemical composition of groundwater. Employing
tracers with higher specificity for human activities may enhance the resolution of future
studies.
The results of this work suggest that natural groundwater recharge is insufficient
to balance current human extraction rates in the catchment during the study period.
In contrast, the tank cascade provides sustained, indirect recharge and seems to be an
effective tool for local water resources management. This benefit, however, is tempered
by limited water quality, which emphasises the need to pair recharge management with
upstream sanitation and water quality monitoring.
en
