The tank cascade systems of Madurai
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:
Water availability in the tanks depended on rainfall patterns: The availability of water in the tanks across the seasons was uncertain and mostly dependent on rainfall. A reduction of 11.2 percent in the north east monsoon and 10.6 percent in the south west monsoon was observed in the region which affected water storage in the tanks.
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.
Rural catchments were the least affected by urbanisation: This was because villagers actively participated in tank based agricultural development programs and had their own Water User Associations (WUAs) headed by Sarpanch (or Talaivar). Also, villages were located far away from the urban regions and hence were not much influenced by urbanisation.
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.
Evaporation and evapo-transpiration affected water storage in the tanks: Evaporation was dependent on the water availability in the tanks and was higher during monsoon seasons and lower during summer and winter seasons. Evaporation was higher for rural areas due to more surface water availability within the catchments. Evapotranspiration was higher for peri-urban catchments followed by urban catchments across summer, south west monsoon, and north east monsoon, whereas it was lesser in rural areas.
Groundwater storage was high during the non monsoon season: The volume of groundwater was about 250 percent higher during the non-monsoon seasons as compared to the monsoon season. This was because the tanks got full during the north east monsoon resulting in maximum percolation aiding recharge during winter. In contrast, infiltration was high during summer rainfall as unsaturated and dry soil conditions led to more percolation during initial showers. Poor maintenance and siltation of tanks in urban and periurban areas also acted as a barrier for early groundwater recharge.
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:
- Field level measures such as revival, restoration, and rehabilitation through provision of dead storage in tanks for water storage during non monsoon seasons; sedimentation tanks to keep a check over the intrusion of silt transported from upstream catchments; and regular de-siltation of tanks and water hyacinth removal to enhance water storage and improve water quality.
- Measures to aid groundwater recharge in urban catchments, especially in the tank water spread areas using techniques such as Managed Aquifer Recharge (MAR) through well, shaft, or borehole recharge techniques, spreading methods, induced bank infiltration techniques, in-channel modifications, and runoff harvesting methods.
- Introduction of less water-consuming crops to balance the demand and water availability. Improving irrigation facilities using measures like leveling of the field, mulching, ridging, and compartmental bunding for active irrigation tanks
- More involvement of local communities, water user associations, policymakers, and village and district administrations and introduction of public participatory approaches for conservation and resource mobilization
The paper ends by identifying areas for future research
- Using advanced hydrological models to develop future scenarios considering present and future climate change impacts.
- Improving methodologies to reduce uncertainty errors in the water balance approach.
- More focus on understanding the impacts of individual factors related to land use and land cover changes such as vegetation cover, barren land, and the forest (cover) leaves etc on water storage in the tanks.
- Understanding the correlation between tank sedimentation, buffering capacity of the cascading tanks and their impacts on agricultural productivity.
- Devise methodologies to measure the volume of rainfall obtained under the residential rainwater harvesting schemes, which can be useful to generate additional storage within each catchment and groundwater recharge in the tanks which would help in desiltation of existing tanks to increase groundwater percolation.
- The development of open-source, user friendly web-application based water budgeting tools to calculate the water balance of the given catchment through minimal inputs to aid and empower people and communities calculate and understand their water use.