This is the 5th lesson of the Three Day Residential Training Workshop on Cascade Water Resources Development and Management prepared for the Climate Resilient Integrated Water Management Project

1. Module Objectives:
After this training, the participants will be able to explain:
 The nature of Tank Cascade Systems in Sri Lanka;
 How the tank cascade system has evolved on dry zone landscape?;
 The important features of tank cascade systems;
 Present status of tank cascade systems in Sri Lanka;
 The contemporary issues found in cascade systems;
 What consequences and constraints experienced by village communities?;
 Why we need to plan water resources development in cascades?
 How we can prepare a water management plan for a cascade?
 Why we need participation of communities in cascade development and management? and
 Roles and responsibilities of implementing partners in cascade development and management
Three Day Residential Training Workshop on
Cascade Water Resources Development and Management
P.B. Dharmasena
Lesson 5 - Contemporary risks to cascades and probable impacts

2. Lesson 5 - Contemporary risks to cascades and probable impacts
Lesson 5 Objective:
After this training session, the
participants will be able to explain:
• The present situation of cascade
environs and contemporary issues;
and
• Possible solutions to address the
problems

3. Session 5 - Contemporary risks to cascades and probable impacts
CONTENT
Topic Description
1 Land use changes Increasing demand for land, encroachment, destruction of ecosystems,
mismanagement of land resource
2 Soil erosion and sedimentation Soil erosion from tank catchments, sediment flow into tanks, tank
capacity reduction, change in tank geometry
3 Depletion of groundwater Groundwater movement in the cascade, changes in the system water
balance, causal factors for groundwater depletion
4 Water pollution Soil erosion, use of chemical fertilizer, application of agro-chemicals,
threat to human health
5 Threat to bio-diversity Floral diversity, faunal diversity, Issues threatening the biodiversity
6 Human - elephant conflict What is human elephant conflict and why does it happen?
The best solution: Let them to enjoy their resources,
Other solutions: Traditional bio-fence with Palmyra trees; Bee hive
fence; Electric fence; Siren drone
7 Possible solutions A discussion

4. 1. Land use changes
Land use types:
1. Paddy fields
2. Scrub lands
3. Forest
4. Secondary forest
5. Home gardens
6. Cultivated lands
7. Tanks

5. Decline in forest
cover in Tank
cascade system
areas - 1992
1. Land use changes
Increasing demand for
lands due to:
• Population increase
• Increased value of lands
• Commercial cultivation
• Development activities

6. Decline in forest
cover in Tank
cascade system
areas - 2010
1. Land use changes
Increasing demand for
lands due to:
• Population increase
• Increased value of lands
• Commercial cultivation
• Development activities

7. 1. Land use changes
Encroachment of
lands for:
• Paddy cultivation
• Rain-fed
cultivation
• Agro-well farming
• New residential
areas

8. Encroachment of lands for new residential areas
1. Land use changes

9. 1. Land use changes
Destruction of
ecosystems
• Biodiversity
reduction
• Loss of ecosystem
services and
functions
• Loss of flora and
fauna species
• Drought and flood
due to ecological
imbalance
Home garden 148
Forest 76
Kattakaduwa 77
70
34
9
23
10
21
13
Ecosystem biodiversity

10. 1. Land use changes
Mismanagement of land
resource
• Land suitability for crops –
ignored
• Soil conservation – not
adopted
• Irrigation planning without
considering drainage
• Use of agro-chemicals and
chemical fertilizer
• Traditional practices – given
away

11. 2. Soil erosion and sedimentation
Soil erosion from tank catchments
Land use Soil loss
(t/ha)
Runoff
(mm)
Chena cultivation 11.73 388
Field with
conservation bunds
8.33 124
Scrub vegetation 0.25 7
Forest 0.04 7
Example: Maha 1989/90 season
Source: Dharmasena, 1994

12. 2. Soil erosion and sedimentation
Sediment flow into tanks – Results of a
tank sedimentation survey
Tank Year Catchment
(ha)
Capacity
(ha.m)
Sediment
(ha.m)
% silted
Paindikulama 1984 123 30.2 9.7 24.3
1987 123 28.0 11.9 29.8
1990 123 25.8 14.1 35.3
Siwalagala 1990 62 19.9 8.6 30.2
Marikaragama 1986 70 36.2 9.6 21.0
1990 70 35.3 10.5 22.9
Source: Dharmasena, 1992

13. 2. Soil erosion and sedimentation
Tank capacity reduction
• Tank water storage - reduced
• Paddy cultivation – affected
• Dead storage – disappeared
• Invasive weeds – covered the
water surface
• Fish population – affected
• Rain-fed farming - increased

14. Soil Erosion
Tank
Sedimentation
Rain-fed
Farming
Tank Capacity
Tank Irrigated
Farming
Vicious circle in tank-village farming
Population
Pressure
2. Soil erosion and sedimentation

15. Four strata of tank water body
Madakaluwa
Diyagilma
Vaangilma
Thawulla
2. Soil erosion and sedimentation
Change in tank geometry

16. Madakaluwa
Diyagilma
Thawulla
Vaangilma
2. Soil erosion and sedimentation
Change in tank geometry
Sediment deposited on tank bed

17. Geometry of water body affects the loss
y = 59.471x-1.3351
R2
= 0.786
30.0
40.0
50.0
60.0
70.0
80.0
90.0
100.0
0.7 0.9 1.1 1.3 1.5
Capacity/area (m)
Percent
water
loss
2. Soil erosion and sedimentation
Change in tank geometry
Tank water loss is
high if the capacity/
water spread area
ratio is low

18. 3. Depletion of groundwater
Groundwater movement in the cascade

19. 3. Depletion of groundwater
Groundwater movement in the cascade
Source: Muditha Perera, 2016

20. 3. Depletion of groundwater
Changes in the system water
balance:
• High surface evaporation from
tanks due to large water spread
area and invasive aquatic weeds;
• Quick surface runoff from chena
in the tank catchments lowers
the subsurface water movement;
• Silted up tanks cut off the
groundwater flow along the
cascade;
• Increased use of groundwater
through agro-wells

21. 3. Depletion of groundwater
Causal factors for groundwater
depletion:
• Extensive use of groundwater for
farming
• Forest removal of tank catchments
for rain-fed farming (chena)
• Changes in the water balance due to
soil erosion and sedimentation
• Changes in rainfall behaviour due
to climate change

22. Water is a very limited resource
0.03%
4. Water pollution

23. Environmental
pollution
Sound
pollution
Radioactive
pollution
Heat
pollution
Soil
pollution
Light
pollution
Electro
magnetic
pollution
Water
pollution
Air
pollution
4. Water pollution Water is the most disastrous pollution among
environmental pollutions of the world

24. Water Pollution in tanks – Example from Vavuniya

25. 4. Water pollution

26. 4. Water pollution

27. USE OF CHEMICAL FERTILIZER IN SOUTH ASIAN COUNTRIES
Country Use in farm lands
(kg/ha)
Sri Lanka 284.3
Bangladesh 164.5
Pakistan 163.3
India 153.5
Bhutan 9.0
Nepal 7.7
4. Water pollution

28. 4. Water pollution
Water Pollution in tank cascade
systems:
• Soil erosion from tank catchments – tank
water is polluted with eroded sediments
due to absence of godawala, perahana
and iswetiya
• Chemical fertilizer applied in upstream
cultivated lands – N, P, Cd dissolved in
water flowing into tanks
• Eutrophication affects aquatic life
• Weed infestation increases
evaporation
• Human health issues
• Agro-chemicals in water especially with
hard water
• Threat to human health

29. 5. Threat to biodiversity
Tank and environs
Cultivation
areas
Natural/semi-
natural areas
Habitat
Gasgommana
Kattakaduwa
Other
associated
area
Tank
water
body
Tank
bund
Chena
Home
gardens
Paddy
fields
Forests
Scrublands
Number of Flora
species recorded
201 153 68 40 153 55 42 17 123 32
Biodiversity in Tank cascade systems - floral diversity in Kapiriggama
tank cascade (2013-2015)

30. 5. Threat to biodiversity
Animal Group
Recorded Faunal Species
Total Endemic Migrant/ Exotic Domestic
Land snails 7 3 1 0
Dragonflies 20 1 0 0
Butterflies 46 1 0 0
Crabs 1 1 0 0
Fishes 25 4 3 0
Amphibians 13 1 0 0
Reptiles 26 3 0 0
Birds 117 8 14 0
Mammals 29 2 0 4
Total 284 24 18 4
Biodiversity in Tank cascade systems - faunal diversity in Kapiriggama tank cascade (2013-2015)

31. 5. Threat to biodiversity
Issues threatening the
biodiversity:
• Agricultural expansion into non-
target areas;
• Spread of Invasive Alien Species
(IAS);
• Unsustainable biomass extractions
and loss of vegetative cover;
• Pollution and siltation of water
bodies; and
• Lack of awareness among
communities

32. 6. Human – elephant conflict
What is human elephant conflict?
Human elephant conflict occurs
when elephants and people live
together in an area where they share
the same resources. Therefore, they
compete for the same food, space
and water.
Why does it happen?
HEC occurs when people
change elephant habitat into
people habitat by clearing
land to grow crops, building
villages and adding roads.

33. 6. Human – elephant conflict
The best solution:
Let them to enjoy their resources
Other solutions
• Traditional bio-fence with
Palmyra trees
• Bee hive fence
• Electric fence
• Siren drone