Arid and semi-arid regions of Southern-Indian peninsula are known to experience frequent droughts and the watersheds in these regions are characterised by hot climate, scanty water availability and erratic rainfall.
Many water recharge structures, such as tank cascade systems were constructed historically to take care of the water needs of the region, which helped in cultivation of paddy during monsoon and the dry seasons, provided water for domestic and livestock consumption, and protected the surrounding flora and fauna thus maintaining the biodiversity of the region.
However, many of these tanks have now started to deteriorate and under perform in terms of irrigation and groundwater recharge due a number of climatic, environmental and social factors, many of which continue to be unexplored.
This study titled 'Water management using traditional tank cascade systems: a case study of semi‑arid region of Southern India' published in the journal SN Applied Sciences aims to explore the factors affecting the water balance of the region and their impacts on the Vandiyur tank cascade system (VTCS) in the city of Madurai, India.
Land use/land cover maps were developed to understand the significance of using a water balance approach in understanding the behaviour of hydrological components governing the water budget of the catchment.
The study finds that:
Unplanned urbanisation led to degradation of the tank system
: Unplanned urbanisation by up to 300 percent in peri-urban and urban regions led to high catchment runoff (40–60 percent of the rainfall) leading to irregular seasonal availability of water in the tanks, with summer having the least tank storage.
Higher catchment runoff led to poor water storage
: The catchment runoff was high due to the geology of the region that included rock formations that ranged from ancient Archean to recent alluvium and geologically classified into sedimentary formations and hard rocks. The hard rocks included granite which had poor groundwater potential and were located at shallow depths of 1 to 8 m below ground level. Shallow depths leading to thin soil layers on the top led to more runoff as the soil underwent early saturation. Urbanisation led to further increase in runoff leading to low water storage.
Tank irrigation was better in rural catchments
: More groundwater and surface water was available in rural as compared to urban catchments. This was because monsoon seasons resulted in a higher outflows in peri-urban and urban catchments. Even though collective water availability in the urban catchments was observed higher than rural catchments, especially during winter and summer, urbanisation and encroachment of the tank water spread area, deteriorated the tank irrigation capacities in urban and peri urban areas.
The primary functions of tanks such as provision of water for irrigation, groundwater recharge, protection from natural hazards such as floods and droughts were negatively affected due to the impact of urbanisation and related factors such as evaporation, evapotranspiration and increased runoffs leading to decreasing tank functionality. This led to depleted recharge in the wells in the command region, declining green cover near the tank region, and threats to aquatic life forms leading to increase in invasive species.
The study recommends:
The paper ends by identifying areas for future research