Presentation made at the seminar organized by IEPSL on 19th August 2011 at Waters Edge, Colombo to explain the importance of tank cascade management in sustainable water resources management

1. Managing Tank Cascades in the Dry Zone
for Optimum Water Supply
19th
August 2011P.B. Dharmasena
Presentation made at the seminar organized
by IEPSL on 19th
August 2011 at Waters Edge, Colombo

2. Sustainable Eco-system
Rainwater harvesting
Flood control
Drought mitigation
Stable land use
Rich bio-diversity
Optimum supply and Optimum use of water resource
Balanced food production system
Sustainable land management
Reduced effects of cyclone
Reduced epidemic threats

3. Nachchaduwa
Kanadarawewa
Vilachchiya
Kalawewa and Balaluwewa
Huruluwewa
Main factors:
1. Morphology (landscape)
• Lower elevation,
• Moderate undulation
2. Rainfall
• Bimodality
• ~1500 mm/ year
3. Soil and geology
• Regolith aquifers
• Low infiltration
• RBE – LHG
Cascades formed on the dry
zone landscape

4. Tank Cascade SystemsTank Cascade Systems

5. 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

6. Malnutrition
• Low nutritional status of children
and mothers
– 22% of Lankan children are
underweight.
– In some districts over 30% of children
between 5-10 years are underweight
– 21 % anemic.
– 18 % of children are stunted
– 15 % of children showing signs of
wasting
– 30.3 % of mothers are anemic.
(Demographic and Health Survey of 2006/7)

8. Main reasons for less impact of
rehabilitation programs
 Limiting the development
program to repairs of
structures and infrastructure
development such as access
roads, community centers etc.
(hardware);
 Rehabilitation of individual
tanks, without giving adequate
consideration on entire
cascade; and
 Less attention paid to restore
the tank bed and its
surrounding eco‑system.

9. Soil erosion
Tank
sedimentation
Rainfed farming
Tank capacity
Tank irrigated
farming
Vicious circle in tank-village
farming
Population pressure
and increased needs

10. Consequences of rehabilitation programs
 Inundation of upstream paddy
lands;
 Development of salinity
conditions in the upper area;
 Increase of tank water losses;
 Disappearance of the tree
strips in the high flood region
(Gasgommana) and the grass
cover (Perahana) underneath;
 Disappearance of some
indigenous fish species, due
to diminishing favourable
breeding and living
environment created by
shallow waters.

11. Strategies to regain the sustainability
1. Crop diversification supported by supplementary groundwater.
2. Intensive agriculture using less water consuming and drought
resistant crops integrated with livestock
3. Desilting tanks followed up with a strong effort to protect
upstream tank catchments to prevent subsequent
sedimentation.
4. Feeding the tank cascade systems wherever possible than
destroying them as attempted earlier in the Mahaweli Project
area
5. New and effective institutional mechanism to small tank
system management.
Madduma Bandara, C.M., 2004. Small tank cascade systems: their relevance
for minor irrigation rehabilitation. Proc. of the Symposium on Small Tank
Settlements in Sri Lanka, held on 21st
August 2004, organized by HARTI, Colombo.

12. W
ellDrained,
RBE,<30cmWellDrained,
RBE,30-60cmWellDrained,RBE
>60cm
ModeratelyDrained,
RBE,>60cmImperfectlyDrained
RBE
PoorlyDrained
LHG
Model Cross Section
Village
Forest
Fruit
Forest
Rainfed
Homestead
Irrigable land

13. Main irrigation canal
Natural drainage
Irrigable land
Homestead
Other field crops
Village forest
Main road
Village road
Farm tract
Communal
lands
Tank
Lower Canal
Upper Canal
Fruit forest gardens
Upstream reservation
Downstream reservation

14. Geometry of water body affects the loss
y = 59.471x
-1.3351
R
2
= 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)
Percentwaterloss

15. PARTIAL DESILTING CONCEPT
• Reduces water losses
• Makes upstream area free of water
• Reduces salinity in the upstream area
• Allows to re-establish gasgommana
and perana
• Avails water during dry period
• Increases cropping intensity

17. Ancient Yodha Ela Water Conveyance Model

18.  Who is responsible for water resources management at
local and national level?
 Can groundwater extraction really be controlled by any
existing regulations? Will anyone enforce unpopular
restrictions?
 Do current water laws really cover the whole water
sector? What about the gaps?
 What is needed to make inter-departmental cooperation
in the water sector possible?
 What are the chances of IWRM succeeding under
present legal and administrative environment?
(Administrative boundaries do not follow watershed
boundaries)
Need of an Institutional Framework

19. Anuradhapura
District (54%)
Matale
District (18%)
Kurunegala
District (13%)
Puttalam
District (15%)
One Watershed in 4 Administrative
Districts - Kala Oya Watershed
Total Extent: 2,870 km22
Irrigable Area:Irrigable Area: 61.368 ha61.368 ha
No. of small tanks:No. of small tanks: 600600
Dewahuwa, Kandalama, Dmbulu oya,Dewahuwa, Kandalama, Dmbulu oya,
Kalawewa, Rajanganaya, UsgalaKalawewa, Rajanganaya, Usgala
Siyambalangamuwa, Katiyawa,Siyambalangamuwa, Katiyawa,
Angamuwa