DETERMINATION OF THE POTENTIAL WATER STORAGE CAPACITY
IN ANCIENT ABANDONED TANKS IN WALAWE RIVER BASIN, SRI
LANKA
1
H.L.K. Sanjaya, 1*T. Priyadarshana, 2N. Wijayarathna
1
Department of Limnology, Faculty of Fisheries and Marine Sciences & Technology, University of Ruhuna, Sri
Lanka.
2
Department of Civil Engineering, University of Moratuwa, Sri Lanka
Email: 1hlksanjaya@yahoo.com,1tilak@fish.ruh.ac.lk, 2tmnwijayaratna@gmail.com
ABSTRACT
Especially, in dry and Semi-arid areas of the world are suffering from lack of water due to ignorance of finding
new opportunities to overcome water scarcity. Similarly, Walawe river basin (WRB) which lies in dry and semiarid zone of Sri Lanka has a higher demand for water in Agricultural, Industrial and Domestic uses. More than
600 small scale tanks which have been constructed in the history are in the abandoned state, but, majority of
them has a potential for rehabilitation. However, storage capacity estimation of abandoned tanks for future
restoration purposes is still to be studied to understand its potential. Here, we studied the applicability of area
capacity relationship to fill the aforementioned gap. Required maps and data on capacity, dimensions of the
present working tanks were gathered from the government departments. Field works were conducted for
randomly selected 250 tanks for ground level verifications. Mapping and data interpretations were done with
Arc GIS 10.1 software. Data were statistically analyzed with SPSS 16.0 package. During the study,it was
identified about 527 abandoned tanks and out of them, 302 tanks were in scrub lands. All the abandoned tanks
are located at a slope of 0-4% in meter. A significant log area capacity relationship for small tanks (R2= 0.85)
was observed while log dam length and log capacity relationship for major and medium scale tanks (R2= 0.64)
were significant. According to the derived relationship, the estimated potential storage capacity of abandoned
small scale tanks was about 32Million Cubic Meter (MCM) which can increase the storage approximately by
3.27% in the basin, for improving the environment, agriculture and livelihood.
Keywords: Walawe river basin, Ancient Tank System, Area, Storage capacity, Rehabilitation
scarcity of water but because of the poor management
1. INTRODUCTION
of the water resources (Viala 2008). In most water
While the population is continuously growing the
scarce dry areas, store water as small tanks or
amount of fresh water on earth is constant and
reservoirs, having significant impact on lower reaches
therefore it creates a higher demand for water. The
and rarely considered as an important part of the
demand exceeds the availability of water in some
water resource system of a river basin (Sawunyama et
parts of the world. However, there is an upcoming
al. 2006).
water crisis in many parts of the world not due to the
1
Figure 1: The location map of Walawe river basin
It has been projected that the dry zone of Sri Lanka
an altitude of 2400m A.S.L. and travels 85km until it
will have to face an absolute water scarcity during the
meets the Indian Ocean at Ambalanthota (Smakhtin
next couple of decades due to the higher water
and Weragala 2005) . The precipitation is varied
demand with the increase of the population. Irrigated
within the basin as it comprises with Wet,
area will have to be increased by 42% (1011 MT) to
Intermediate, Dry and Arid zones according to the
cover the increased demand of rice production with
geographical variations of the area. The basin receives
the population growth (Amarasinghe et al. 1999).
both the South West and North East Monsoonal
Therefore studying on rehabilitation of existing water
precipitations and the South West precipitations are
resources in order to meet the future targets is a
received in upper 574km2 (Domroes and Ranatunge
timely requirement.
1993). The annual precipitation of this area is about
3680mm and the lower portion receives about
Walawe river basin is one of the largest river basin in
1120mm/yr (Arumugam 1969).
Southern Sri Lanka (Figure 1) which is started from
2
Approximately a half of the precipitation is converted
with the prevailing tank system to optimize the water
into surface runoff and the rest is evaporated, or
resources in the basin (Aloysius and Zubair 1999).
percolate into the ground. In general, 1.1 billion cubic
However, appropriate storage capacity determination
meters per year is discharged in to the sea by Walawe
of abandoned tanks for future restoration purposes is
River (Molle and Renwick 2005). The lower reach of
still to be studied.
2
the Walawe river basin is comprised of 1874 km land
Method for estimating the capacity of disappeared
area and possessed dry weather condition (Arumugam
tanks is not presently available for decision making
1969). As a major part of the basin is lay within the
process in water resource management to future water
dry zone and the end of the basin is having arid
demand in the area. But, predicting the storage
climatic conditions, ancient people have constructed a
capacity with GIS application is well documented in
chain of irrigation works to drench this dry soil in
literature (Sawunyama et al. 2006, Ran and Lu 2012,
lower reaches to fertile the land in this acrimonious
Venkatesan et al. 2012, Yang and Lu 2013). Many
earth.
studies have demonstrated the well-established area
Historical evidences reveals that there were more than
and storage capacity relationship at both regional and
600 tanks in dry zone of the basin (Molle and
global scale (Lehner et al. 2011, Ran and Lu 2012,
Renwick 2005) but a considerable proportion of these
Venkatesan et al. 2012). The method has been first
tanks are in abandoned state ( Disappeared due to log
developed by Meigh (1995) to estimate reservoir
term ignorance) under forest cover (Bandara 1987).
capacity by power relationship between area and
However all the tanks are not working tanks
capacity.
( Tanks which use for irrigation purposes) and
C = a 𝐴𝑏 ……………………… (1)
many of them could be identified as ground
water recharge and sediment trapping structures
Where, a and b are constants
with benefits to the ecosystem to mild the
This relationship has been derived through the
dryness of the region. Some tanks were capable of
information of 185 reservoirs and 118 lakes obtained
generating water springs which feeds the stream
from official documents of the Chinese government,
network in meso-catchments even during the dry
and previous studies. Furthermore, Meigh (1995) has
season (Bandara 1987).
used 1:50000 topographical maps to estimate the
storage capacity of small farm reservoirs with power
The impact of development activities during the
relationship in Botswana. According to Liebe et al.
British Period (1815 - 1948) has felt the region it
(2005),remotely sensed area data also have been
began to re-emerge as a popular resettlement region,
proven as a suitable method to calculate the storage
mainly catering poor agricultural communities. A
capacity of reservoirs. However, this relationship
notable development can be seen since the latter part
inherent to the area of interest as the Area Capacity
of 19th century, as a result of revitalization of ancient
power relationship depends on the geographical
tank systems and government sponsored colonization
features. The objective of the study was to derive a
schemes. Water balance studies have been carried out
model relationship to predict the potential storage
3
capacity of the abandoned tanks (Disappeared tanks)
2. MATERIALS AND METHODS
in Walawe river basin.
2.1. Data collection
the generated DEM map and percentage rise of the
Data collection was carried out from the government
slope map was set in meters.
departments
to
gather
information
on
present
functioning tanks and their storage descriptions. Data
2.3. Data Analysis and Interpretation
on functioning small tanks (water spread area; 4-50 ha
Tank capacity and area data of 101 tanks were
and depth; 1-4m) were obtained from the Department
transformed in to log forms in order to obtain the
of agrarian services and data for the major and
corresponding relationship (Venkatesan et al., 2012;
medium scale tanks were obtained from the
Sawunyama, 2006). Log capacity and Dam length
Department of irrigation, Sri Lanka. Hard copies of
relationship was also derived to predict the possible
Old 1:63000 maps and 1:50000 metrics maps were
storage capacity of major and medium scale tanks
collected from the department of Survey, Sri Lanka.
using present functioning 35 major and medium scale
Digital maps of 1:10000 covering the Walawe river
tanks. Statistical analysis was carried out to examine
basin with Elevation, Land use, Administrative
the suitability of the model parameters using SPSS
boundaries also were acquired. About 250 abandoned
16.0.
tanks were randomly visited and GPS locations were
taken for verification of their present status.
3. RESULTS
2.2. GIS Application
3.1. Tank Distribution
Along with old 1:63000 maps with abandoned tanks
Created elevation map for the entire Walawe river
data compiled during 1920s were scanned and
basin is given in figure 2. It indicates that all the
georeferenced. Disappeared tanks were then digitized
abandoned tanks are located in low elevations
to create the boundaries of the ancient abandoned tank
whereas the high elevated terrain has not been
map of Walawe river basin. Created tank map was
selected for tank construction. All together it was
differentiated in to layers according to the present
found that there are 527 tanks in abandoned state in
land use in their locations. Area of each tank was then
Walawe river basin and the abandoned tanks are
calculated with geometry calculator in Arc GIS 10.1.
located at low elevated area (200 MSL) (Figure 2). It
was found that about 31 of them were located
All together 68 elevation point grids (1:10000 scales)
covering both homesteads and paddy fields. It has
with 40km2 area were merged to generate the
been lost about 15 tanks due to tank expansion in
elevation point map of WRB. Digital Elevation Model
rehabilitation programs. The number of tanks in dense
(DEM) was generated with spatial analyst tool.
forest was 18 whereas 10 tanks were in rocky
Regularized spline method was used to generate the
substrates. About 73 tanks have been converted in to
DEM for Walawe river basin (Trần and Nguyễn
paddy fields and other 78 were in homesteads. The
2008). Slope raster map was created with the help of
remaining 302 tanks are abandoned in scrublands.
4
Figure 2: Abandoned (Disappeared tanks) tanks in Walawe river basin
The created slope map of WRB is illustrated in figure
percentage rise in some areas is about 130-350m.
3. Similar to elevations of the basin, the ancient
After the steep mountain range it meets the flat
abandoned tanks have been constructed at low slope
undulating terrain with a slope of 0-4 where almost all
terrain. The upper part of the basin is very steep. The
the tanks are located (Figure 3).
Figure 3: Abandoned Tank Distribution according to the slope of Walawe river basin
5
3.2. Storage Capacity Estimation
Where, C is the log capacity of the reservoir and the
The obtained relationship for functioning small tanks
A is the log area of the reservoir. a= 0.7868 in power
was
relationship and b= 1.1384 for small tanks in selected
catchment. The coefficient of determination for the
𝐶 = 0.7868 𝐴1.1384…………………………. (2)
model was R2 =0.85 (figure 4).
Figure 4: Log area capacity relationship of functioning small tanks in Walawe river basin
A regression analysis was carried out to observe the
capacity relationship for small scale tanks was strong
suitability of the derived model. Table 1 shows the
and significant (Table 1).
significance,
standard
error,
constants
and
R
(correlation coefficient) of the models. The log area
Table 1: Results Obtained from the Regression Analysis for Curve Estimation in Small Tanks in Walawe river basin
Coefficients
Unstandardized Coefficients
Standardized
Coefficients
B
Std. Error
Beta
Log Area
1.138
.048
.921
Constant
.787
.059
The dependent variable is log (Capacity).
6
t
Sig.
23.535
.000
13.226
.000
The relationship derived for major and medium scale
Where, C is the log capacity of the reservoir in acre
tanks was,
feet, and L is log length of the Dam in feet. The
𝐶 = 0.1772 𝐿2.2794……………………… (3)
Coefficient of determination for the relationship was
R2=0.64 (Figure 5).
Figure 5: Log dam length and capacity relationship of functioning major and medium scale reservoirs
The regression analysis carried out to observe the
tanks was not significant. However the relationship
suitability of the model is given in table 2. The area
obtained between log dam length and capacity
capacity relationship for major and medium scale
relationship was statistically significant (Table 2).
Table 2: Results obtained from the regression analysis for curve estimation functioning large scale reservoirs
Coefficients
Unstandardized Coefficients
B
Standardized
Coefficients
Std. Error
Beta
.799
Log Length
2.279
.299
Constant
.177
.067
t
Sig.
7.633
.000
2.637
.013
The dependent variable is log (Capacity).
With the obtained relationship, Potential tank storage
scrub lands and it was about 19.1 MCM. The total
capacities for abandoned small tanks were estimated
estimated storage capacity for all tank categories was
as the area capacity relationship was strong and
32.16 MCM (Table 3). This capacity is a 3.27%
significant. The estimated highest storage capacity
enhancement to the present storage in WRB.
was observed for the tanks which are abandoned in
7
Table 3: Estimated potential storage capacities of small scale, ancient, abandoned tanks according to their present land use.
Tanks at paddy lands and homestead
31
Potential storage
capacity(×106 m3)
2.56
Lost Tanks due to expansion
15
1.48
Tanks at Rocky areas
10
0.16
Tanks at Forests
18
2.7
Tanks at paddy lands
73
3.38
Tanks at homesteads
78
2.78
Tanks at scrublands
302
19.1
Total
527
32.16
Tank category according to present land use
Number of abandoned tanks
As studies have already proven that area capacity
4. DISCUSSION
relationship is applicable for Storage capacity
This study was carried out to fulfill the gap of
estimations (Meigh 1995, Sawunyama et al. 2006,
information on the potential storage capacity of
Ran and Lu 2012, Yang and Lu 2013), we used the
ancient abandoned tanks in Walawe river basin. The
aforementioned relationship after modifying for the
derived area capacity relationship is statistically
study area by assuming that the disappeared tanks will
significant (R2=0.85, p<0.001) and strong for low
have the same characteristics with the current
elevated, low slope area where almost all the tanks are
functioning tanks after rehabilitation. On the other
located. The irregularities of the terrain such as deep
hand, use of GIS based technique for assessment of
valleys can be a major deviator for this relationship
large amount of tanks is feasible rather than working
that can lead to unexpected high capacities in small
on each tanks physically (Sawunyama et al. 2006,
area. Therefore, slope and the elevation factors have
Venkatesan
to be concern before applying the model (Sawunyama
et
al.
2012).
Storage
Capacity
determination for large number of tanks at field level
et al. 2006). The slope of the terrain in this study was
studies is labor intensive and time consuming task.
0-4 in terms of percentage rise in meter and it was
acceptable for the relevant relationship. The area
As the lower part of the basin has dramatically
capacity relationship was not significant for major
developed during last few years, future planners and
and medium scale reservoirs; therefore they were
managers who need to concern on catering sufficient
checked for the dam length and capacity relationship.
amount of water for irrigation and industrial uses in
2
This relationship was less strong (R =0.64, P<0.001)
Walawe river basin will need to concern on
than the former relationship which was obtained for
rehabilitation
small scale tanks. This is due to the use of some
Therefore further field level verification for the model
reservoirs out of the low slope terrain due to the
is being undergone, to improve the accuracy as an
shortage of data.
appropriate tool for predicting the potential storage
potential
of
capacity in abandoned tanks.
8
disappeared
tanks.
the southwest monsoon in the wet zone of
Sri Lanka. Geografiska Annaler. Series A.
Physical Geography:137-148.
Lehner, B., C. R. Liermann, C. Revenga, C.
Vörösmarty, B. Fekete, P. Crouzet, P. Döll,
M. Endejan, K. Frenken, and J. Magome.
2011. High-resolution mapping of the
world's reservoirs and dams for sustainable
river-flow management. Frontiers in Ecology
and the Environment 9:494-502.
Liebe, J., N. Van De Giesen, and M. Andreini. 2005.
Estimation of small reservoir storage
capacities in a semi-arid environment: A
case study in the Upper East Region of
Ghana. Physics and Chemistry of the Earth,
Parts A/B/C 30:448-454.
Meigh, J. 1995. The impact of small farm reservoirs
on urban water supplies in Botswana. Pages
71-83 in Natural Resources Forum. Wiley
Online Library.
Molle, F. and M. Renwick. 2005. Economics and
politics and of water resources development:
Uda Walawe Irrigation Project. Sri Lanka'',
RR 87.
Ran, L. and X. Lu. 2012. Delineation of reservoirs
using remote sensing and their storage
estimate: an example of the Yellow River
basin, China. Hydrological Processes
26:1215-1229.
Sawunyama, T., A. Senzanje, and A. Mhizha. 2006.
Estimation of small reservoir storage
capacities in Limpopo River Basin using
geographical information systems (GIS) and
remotely sensed surface areas: Case of
Mzingwane
catchment.
Physics
and
Chemistry of the Earth, Parts A/B/C 31:935943.
Smakhtin, V. and N. Weragala. 2005. An assessment
of hydrology and environmental flows in the
Walawe River Basin. Sri Lanka.
Trần, Q. B. and T. T. Nguyễn. 2008. Assessment of
the influence of interpolation techniques on
the accuracy of digital elevation model.
Venkatesan, V., R. Balamurugan, and M. Krishnaveni.
2012. Establishing Water Surface AreaStorage Capacity Relationship of Small
Tanks Using SRTM and GPS. Energy
Procedia 16:1167-1173.
Viala, E. 2008. Water for food, water for life a
comprehensive
assessment
of
water
management in agriculture. Irrigation and
Drainage Systems 22:127-129.
Yang, X. and X. Lu. 2013. Delineation of lakes and
reservoirs in large river basins: An example
of the Yangtze River Basin, China.
Geomorphology 190:92-102.
5. CONCLUSION
The
derived power
relationship
using present
functioning tanks to predict the storage capacity of
abandoned tanks is statistically significant. All
together 527 abandoned tanks were identified from
Walawe river basin those should be somehow
rehabilitated to meet the future demand for water. The
potential storage can be stored in tanks which are
disappeared in scrub lands is 19.1 MCM (Million
cubic meters). The potential storage capacity of all
small scale abandoned tanks in Walawe river basin is
about 32 MCM. This will increase the storage
capacity approximately by 3.27% in the basin. Area
capacity relationship was found to be as a feasible
tool for future potential storage capacity estimation
with available data in Walawe river basin.
6. ACKNOWLEDGEMENT
The Authors would like to thank the TURIS project
(Grant
Reference
Number:
RUH/Pro.06)
Of
University of Ruhuna for supporting to accomplish
this work by giving financial support where necessary.
LIST OF REFERENCES
Aloysius, R. and L. Zubair. 1999. Simulation of
irrigation networks: Case of Walawe Basin in Sri
Lanka.
Amarasinghe, U. A., L. Mutuwatta, and R.
Sakthivadivel.
1999.
Water
scarcity
variations within a country: A case study of
Sri Lanka. IWMI.
Arumugam, S. 1969. Water resources of Ceylon.
Bandara, M. 1987. CM (1985) Catchment ecosystems
and village tank cascades in the dry zone of
Sri Lanka. Stratergies for river basin
development. Lundqvist J, Lohm U,
Falkenmark M (eds) Reidel Publishing
Company, Germany:99-113.
Domroes, M. and E. Ranatunge. 1993. Analysis of
inter-station daily rainfall correlation during
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