This document summarizes a presentation given at the International Water Conference in Sri Lanka on sustainable water management in tank cascade systems. Some key points: - Tank cascade systems are a traditional Sri Lankan method of water resources management that store, convey, and utilize water across interconnected tanks and streams. This helps mitigate droughts and floods. - The Village Tank Cascade System was declared a Globally Important Agricultural Heritage System by the UN's FAO for its sustainable practices. - Challenges include high amounts of water lost to the sea each year, lack of coordination between water agencies, and ignoring traditional management systems. - Tank cascades provide multiple benefits like food/water security, biodiversity,

1. Presentation made at International Water Conference Sri Lanka- 2022 on 16th December, 2022,
at the Centre of Excellence for Water & Sanitation, Ratmalana, Sri Lanka
Sustainable Water Management in Cascade
System: A model in Environmentally
Sensitive Area
P.B. Dharmasena – dharmasenapb@gmail.com

2. We searched for a Globally Important Agricultural Heritage System in Sri Lanka
During 2014-2017, we
investigated
• Traditional water
resources
management
• Traditional
agricultural practices
• Traditional culture
• Uniqueness of the
system
• Attractions to tourism

3. Globally Important Agricultural Heritage System (GIAHS)
Food & Agriculture Organization declared Village Tank Cascade System
(VTCS) as the first Globally Important Agricultural Heritage System
(GIAHS) on 19th April 2019 in Sri Lanka. It is 39th in the World.

4. • Introduction
• Origin of tank cascade concept
• Tank cascade systems of Sri Lanka: Anatomy, distribution and
definitions
• Importance of cascade approach
• Traditional tank-village ecosystem – adaptive capacity for
present situation
CONTENT

5. Present crisis of water
• High discharge to sea causing drought and flood while wasting
productive water – lost more than 50% of annual incidence
• Sea water intrusion especially in Mannar and Jaffna
• Ignorance of the ancient water resources management systems
• Less attention paid on restoration of tank cascade ecosystems
• Lack of effective coordination among water related agencies -
There are over 40 agencies dealing with water in one way or
another
• Lack of a National Water Policy – Submitted by a Cabinet
appointed committee in January 2021

6. Annual average rainfall: 2,000 mm
Land extent: 65,610 km2
Total volume of water: 131.22 bil. m3
Discharge to sea: 66.18 bil. M3 (>50%)
No. of river basins: 103
Major reservoirs and dams: 80
Small tanks: 16,476
Small anicuts: 14,468
Shortage or wastage of water?

7. River basins – Water losses to Sea
River Basin Total Basin
Area (km2)
River
lengths
(km)
Total Rainfall
(million m3)
Drainage to
Sea
Major/
medium
reservoirs
Minor tanks
(mil. m3) %
Deduru Oya 2,616 142 4,794 1,608 34.0 4 2,408
Kala Oya 2,772 148 4,424 587 13.0 3 1,015
Malwathu Oya 3,246 164 4,592 568 12.0 5 1,731
Yan Oya 1,520 142 2,269 300 19.0 2 746
Kelani Ganga 2,278 145 8,692 5,474 62.0 0 9
Gin Ganga 922 113 3,039 1903 62.0 0 0
Kalu Ganga 2,688 129 10,122 7,862 77.0 0 3
Walawe Ganga 2,442 138 9,843 2,165 22.0 12 750
Mahaweli Ganga 10,327 335 26,804 11,016 41.0 9 552
Meeoya 1,516 109 2,176 338 16 1 750

8. Ancient water resources management planning in Sri Lanka
Giant canals – from river basin to river basin
Giant canals – from river to reservoir
Giant canals – from reservoir to
reservoir
Within tank cascade systems
Tank-village system
Field water
management

9. • Single bank canal
• It runs along almost
contour
• Water is collected from
upstream
• Water is released to both
sides (upstream and
downstream)
• Canal moves along tank
up streams to protect
tanks from sedimentation
• Tank cascade systems are
fed and secured with
water
Beauty of Giant Canals in Sri Lanka

10. Mahakanadarawa
Nachchaduwa
Kalawewa-Balaluwewa
Dambulu oya
Malwathu oya
Kanadara oya
Kala Oya
From river basin to
river basin (860 AD)
From river basin to
Reservoir (860 AD)
From reservoir to
reservoir (540 AD)
From River basin to
river basin
From Kalaoya river basin to
Malwathuoya river basin
From river basin to
reservoir
From Malwathuoya to
Mahakanadarawa reservoir
From reservoir to
reservoir
From Kalawewa reservoir to
Nachchaduwa reservoir
Giant Canals to fulfill the water requirement in water deficit rivers
and reservoirs

11. Kalawewa-Balaluwewa
Thisawewa
Siyambalangamuwa
Nachchaduwa
Nuwarawewa
Mahavilachchiya
Basawakkulama
Malwathu oya
Thalawa oya
Kala oya
From
reservoir to
reservoir
From Kalawewa
reservoir to
Thisawewa
reservoir
From
Nachchaduwa
reservoir to
Nuwarawewa
reservoir
Giant Canals to fulfill the water requirement in water deficit rivers
and reservoirs

12. Left bank
Sedimentation
Diyakaliya
Natural rock
Soil mound
Mahailluppallama
Amunukole
Diyakaliya – Tank with
Turbulance
Source: Dr. Ashoka Karunaratne, 2021
A Special Type of Tanks along Giant Canal to Ensure Water Security
Rainwater inflow

13. Kendewa cascade
Kapiriggama cascade
Bandara Rathmale cascade
Kallanchiya tank
Tank Cascade Systems of Sri Lanka

14. Network of tanks and streams in
Anuradhapura District
No flood and no
drought
Malwathuoya
river
Kalaoya river
Yanoya river
This enhances groundwater status in the river basin area mitigating natural disasters such
as drought, flood etc.

15. Nachchaduwa
Kanadarawewa
Vilachchiya
Kalawewa and
Balaluwewa
Huruluwewa
Network of tanks and streams in the form of cascades
Main factors for cascade formation:
1. Morphology (landscape)
• Lower elevation,
• Moderate undulation
2. Rainfall
• Bimodality
• ~1500 mm/ year
3. Soil and geology
• Regolith aquifers
• Low infiltration
• RBE – LHG

16. Tank clusters or cascades
Hydrology based definition
• A ‘cascade’ is a connected series of tanks organized within a micro-catchment (meso
catchment) of the dry zone landscape, storing, conveying and utilizing water from an
ephemeral rivulet’. – Madduma Bandara, 1985
Ecology based definition
• Tank cascade is an ecosystem,
where water and land resources
are organized within the micro-
catchments of the dry zone
landscape, providing basic needs
to human, floral and faunal
communities through water, soil,
air and vegetation with human
intervention on sustainable basis’.
– Dharmasena, 2017

17. What is an Environmentally Sensitive Area?
• It is an area in which
plant or animal life or
their habitats are
either rare or
especially valuable
because of their
special nature or role
in an ecosystem and
which would be
easily disturbed or
degraded by human
activities and
developments
Example of Bat
• Good planter and
pollinator – Mango,
Cashew, Avocado etc.
• Pest controller – One bat
consumes about 600
insects during one night
• The bat supplies
Nitrogenous fertilizer
• Habitats of the bat -
Terminalia arjuna,
Madhuca longifolia
• People are removing
habitats

18. Distribution of tank
cascades in Sri Lanka
North and North
Central cascade zone
North Western
cascade zone
South and South
Eastern cascade zone
• Small tanks – 14,421
• About 80% of these
tanks are in form of
cascade systems
• 1262 tank cascade
systems have been
identified

19. Productivity Potential of Tank Cascade System
• Physical productivity potential:
• food, water, air, timber, fuel wood, medicine, raw materials for handicraft, …..
• Economic productivity potential:
• agriculture, fishery, livestock, handicraft, ………..
• Ecological productivity potential:
• habitats, biodiversity, aesthetic beauty, nutrient cycling, …………..
• Scio-cultural productivity potential:
• living environment, local knowledge, tourism, education, human health, rituals,
……………..

20. Why cascade systems again?
1. Drought and flood events can be expected more
due to climate change
2. High prevalence of poverty in tank cascade
areas
3. Mal-nutrition can be greatly reduced by tank
fishery and animal husbandry
4. Better land use under cascade principles –
addresses ecological issues such as clearance of
village forests, unsustainable land use etc.
5. Improve living conditions
6. Health issues – drinking water, contamination
of agro-chemicals and heavy metals leads to
kidney failures, malaria etc.
7. Most importantly ……….

21. • Rainwater harvesting model;
• Rich subsurface flow – Wet areas,
groundwater availability;
• Upstream water holes for
sediment trap, upstream soil
ridges for siltation control,
filtering action of the meadow;
• Rich biodiversity with well
maintained land uses;
• Farming activities that promote
social cohesion;
• Sharing land and water resources
during droughts;
• Food security through animal
husbandry and fishery
………It is an answer to drought hazards

22. Sanitary places of
women and kids
Sanitary places of men
Faeces as a nutrient source to paddy fields
A Self sustained System based on precise management of water
Source: Dharmasena, 2010

23. Godawala
Perahana
Gasgommana
Iswetiya
Tank water – no pollution, no sedimentation: safe drinking water, good storage
Source: Dharmasena, 2010

24. Landa
Thisbambe
Gangoda Kattakaduwa
Kiul-ela
Paddy field - no salinity, good fertility: High productivity
Source: Dharmasena, 2010

25. Agro-silvopastoral systems – Free roaming model
Traditional village ecosystem – Year around cattle grazing areas
paddy lands with
fallow period
Shrub land for free
grazing
Rainfed farming with
fallow period

26. System Sustainability
• Policy issues – Administrative
and hydrological boundaries
• Tank Cascade Management
Committee - Failed due to lack
of policies and Acts.
• Cascade Management Fund
• Education and training
• Strict land use plans essentially
through participatory approach