t. Rainwater harvesting (RWH), the small-scale collection
and storage of runoff for irrigated agriculture, is recognized
as a sustainable strategy for ensuring food security,
especially in monsoonal landscapes in the developing world.
In south India, these strategies have been used for millennia
to mitigate problems of water scarcity. However, in the past
100 years many traditional RWH systems have fallen into
disrepair due to increasing dependence on groundwater. This
dependence has contributed to accelerated decline in groundwater
resources, which has in turn led to increased efforts at
the state and national levels to revive older RWH systems.
Critical to the success of such efforts is an improved understanding
of how these ancient systems function in contemporary
landscapes with extensive groundwater pumping and
shifted climatic regimes. Knowledge is especially lacking regarding
the water-exchange dynamics of these RWH tanks at
tank and catchment scales, and how these exchanges regulate
tank performance and catchment water balances. Here, we
use fine-scale, water-level variation to quantify daily fluxes
of groundwater, evapotranspiration (ET), and sluice outflows
in four tanks over the 2013 northeast monsoon season in a
tank cascade that covers a catchment area of 28 km2
. At the
tank scale, our results indicate that groundwater recharge and
irrigation outflows comprise the largest fractions of the tank
water budget, with ET accounting for only 13–22 % of the
outflows. At the scale of the cascade, we observe a distinct
spatial pattern in groundwater-exchange dynamics, with the
frequency and magnitude of groundwater inflows increasing
down the cascade of tanks. The significant magnitude of return
flows along the tank cascade leads to the most downgradient
tank in the cascade having an outflow-to-capacity ratio
greater than 2. At the catchment scale, the presence of tanks
in the landscape dramatically alters the catchment water balance,
with runoff decreasing by nearly 75 %, and recharge
increasing by more than 40 %. Finally, while water from the
tanks directly satisfies ∼ 40 % of the crop water requirement
across the northeast monsoon season via surface water irrigation,
a large fraction of the tank water is “wasted”, and more
efficient management of sluice outflows could lead to tanks
meeting a higher fraction of crop water requirements.