Surface Water Mapping for the Management of Ancient Tank Cascade Systems in Sri Lanka
Region Asia
Summary
Sri Lanka's ancient tank cascade system, dating back over 2,000 years, comprises more than 30,000 earthen reservoirs known as 'tanks' or 'wewa' locally which capture rainwater and runoff to irrigate rice paddy fields across the northwest region of the island. This network is recognized as a priority in Sri Lanka's National Adaptation Plan and has long sustained smallholder rural communities through droughts and floods alike. Over time, silt accumulation, ageing infrastructure and fragmented maintenance have degraded many tanks, reducing storage capacity and threatening water security.
In 2025, the International Fund for Agricultural Development (IFAD), the IFAD-funded Smallholder Agribusiness and Resilience Project (SARP) and the European Space Agency's Global Development Assistance (GDA) water resources programme, collaborated on a project that provided long-term water dynamics maps to support evidence-based tank rehabilitation and management. As part of the GDA, GeoVille, an Austrian company, produced the data.
Key outputs from the project include:
- A set of 104 monthly surface-water dynamics maps at a resolution of 10 m, covering a nine-year period from 2017 to 2025 across the six districts of interest. These maps cover over 10,000 water bodies and enable the calculation of the frequency of annual seasonality, as well as the maximum and minimum extent, for each of these tanks. This provides detailed historical insights into tank usage. This data can be used to guide restoration efforts and can serve as a key parameter in hydrological modelling. It can also be used for research purposes and integrated into national databases of Sri Lankan water authorities.
- Statistics for 189 tanks targeted by the SARP project to support their restoration efforts.
In the course of 2026, IFAD, SARP and ESA are promoting these datasets to encourage their usage for national water security and sustainable agricultural development.
Value Chain Diagram
Decide
- Supporting management and restoration of the ancient tank cascade system to strengthen the resilience of smallholder rural communities.
Apply
- Applications of water extent information for more than 10,000 irrigation water bodies over nine years at 10-meter resolution:
- Targeting and impact measurement of tank restoration.
- Input parameters into hydrological modeling.
- Integration into national tank databases.
Data production
- Surface water frequency and seasonality.
- Minimum and maximum water extent.
- Statistics for 189 targeted tanks.
- Regional metrics.
Sense
- Sentinel 1 radar and Sentinel 2 optical combined.
- Height above nearest drainage (HAND) and topographic wetness index (TWI) combined to obtain the monthly composites.
Decision-Making Context and Needs
Sri Lanka’s tank cascade system is a uniquely engineered landscape that covers the northern part of the island. Farming communities use the linked reservoirs to capture and distribute runoff for rice paddy cultivation during monsoon seasons. Tanks require maintenance and many have gradually lost storage capacity due to silt accumulation and ageing hydraulic structures. Management and restoration responsibilities such as maintenance of embankments, repairs of spillovers and gates and canal networks are shared between the Department of Agrarian Development (DAD), the Irrigation Department and Provincial Irrigation Departments, as well as local farmer organisations.
Because water moves from one tank to another, conditions at one site can influence other parts of the cascade. Interventions planned without considering these connections may lead to unintended effects upstream or downstream. Government authorities and farmer societies need information on tank usage to identify where the most relevant storage losses occur in order to prioritize support effectively.
Earth Observation Data Products and Information
The surface water dynamics products were generated using a combination of European satellite missions and complementary geospatial datasets. The input data used to detect areas covered by water includes monthly composites and time-series derived from Sentinel-1 synthetic aperture radar and Sentinel-2 optical imagery. The Height Above Nearest Drainage (HAND) and the Topographic Wetness Index (TWI) hydrological indices were used to filter out signals from areas unlikely to contain water. This resulted in 104 monthly water-extent maps at 10-metre resolution covering the period from January 2017 to August 2025. The following aggregate products were computed based on the monthly water maps:
Water frequency: Indicates how often each pixel is classified as water from 2017 to 2025, identifying areas with persistent or infrequent water presence.
Water seasonality: Shows the number of months per year in which water is detected and provides insight into seasonal patterns in water availability.
Minimum and maximum surface water extent: Describes the annual minimum (dark blue) and maximum (light blue) areas covered by water. Maximum water extent is defined by water presence during at least one month within one year. Minimum water extent describes the area in which water presence is detected in all months within one year.
Targeted Tank Metrics
This initiative provided SARP with monthly surface area measurements for the 189 tanks selected for restoration by the project. These records show how each tank expanded during the rainy season and contracted during dry periods, capturing nine years of responses to monsoons, droughts and seasonal variability.
These long-term insights show how each tank has been used, indicating where full capacity has been attained and where it has not. This time series serves as a basis for analyzing the influence of rehabilitation efforts on tank behavior and resilience across the cascade.
Underlying Observations and Data Sources
Surface water dynamics were produced using a combination of satellite earth observation data and complementary geospatial datasets. The main observations and input include:
- Sentinel-2 optical surface reflectance (10–20 m), used to distinguish water from vegetation and soil under clear-sky conditions.
- Sentinel-1 radar backscatter (10 m), enables mapping of water extent independent of cloud cover, providing reliable observations during the monsoon and wet seasons.
- Height Above Nearest Drainage (HAND), a topographic index used to support interpretation of hydrological behavior across tank systems.
- Topographic Wetness Index (TWI), derived from terrain data to help identify areas with higher potential soil moisture and surface saturation.
Impacts and Benefits
The surface water dynamics dataset provides SARP and national institutions in Sri Lanka with historic and long-term insights to guide the management and restoration of tanks within cascade systems. The project demonstrated several benefits and potential applications:
- Improved insights of tank conditions over time: monthly observations help institutions see how tanks respond to wet and dry periods, providing evidence of reduced storage capacity, seasonal variability, or changes in water availability.
- Linking water behaviour with agricultural activity: when compared with cropland and paddy-extent products, the data help illustrate how water availability aligns with cultivation patterns, supporting agricultural monitoring efforts.
- Basic indicators of climate response: evaluating changes in water presence alongside rainfall offers initial insights into how tanks react to variable precipitation and extreme events.
- Reference for rehabilitation efforts: historical measurements establish a baseline against which changes following desilting or structural improvements can be assessed.
Beneficiaries and operational use:
- Government agencies such as the Department of Agrarian Development (DAD), Irrigation Department and Provincial Irrigation Departments can use the information to update tank inventories, guide restoration priorities and assess the effects of restoration.
- Water resource planners can use water extent data as an important variable in hydrological or land-use assessments.
- Local farmer organisations can gain a clearer understanding of past tank behavior and seasonal water patterns relevant to their agricultural planning.
- Development programmers, such as SARP, can use the dataset to target interventions and monitor improvements over time.