application of simple hydrologic model for...
TRANSCRIPT
1
APPLICATION OF SIMPLE HYDROLOGIC MODEL
FOR RECALCULATING WATER BALANCE OF
CACABAN DAM IRRIGATION SYSTEM 1
Sukirno Faculty of Agricultural Technology, Gadjah Mada University, Yogyakarta.
Sahid Susanto Faculty of Agricultural Technology, Gadjah Mada University, Yogyakarta.
A B S T R A C T
The aim of study was focused in application of simple hydrologic model of Mock to recalculate water
balance at Cacaban dam irrigation system. The model is basically rainfall-run-off relationship
model. It contains three tanks arranged in vertical position. Six parameters in the model were
optimized by trial and error. The Cacaban dam irrigation system is located at Central Java. The dam
was constructed mainly for supplying water to irrigate 6628 ha of land.
Recalculating water balance was predicted by supply-demand approach. For predicting water
availability at the dam as supply side was predicted by the hydrologic model. Predicting water
irrigation as demand side was calculated based on existing various cropping pattern. Hydrologic
data in the year of 2007 and 2008 were used as calibration and verification model, respectively.
These two years of hydrologic data then were applied also to simulate water availability at the dam,
and for calculating irrigation water demand as well. The calibration and verification results then
were compared with calibration and verification results at the upper watershed of Sempor and
Wadaslintang dam which was applied two years hydrologic data of 1992-1993 and 2000-2001,
respectively. Generally, the result shows that the simple hydrologic model can perform well. The
different forest area occupied at each watershed can be reflected well by the parameter of model.
High rainfall intensity at the tropical monsoon climate can also be well simulated as discharge by
the model.
The model then was applied to recalculate water supply of water balance from the dam using the
year of 2008. Recalculating water demand for irrigation was used at group-1 with the first planting
season at the second week of October. Using yearly basis, the result proved that the ability of
Cacaban Dam for supplying water irrigation was not enough.
Key words: simple hydrologic model, tropical monsoon, upper watershed of dam, water balance
1. INTRODUCTION
Cacaban dam irrigation system is located at Tegal district Central Java Province (Figure 1).
The dam is the strategic asset that play a crucial role in providing water for supplying water
irrigation (to irrigate 7439 ha of land), flood control and fisheries. Total area of the upper
watershed of Cacaban dam is 60.66 km2. Storage capacity of the dam is 90 million m3.
1 This paper is to be presented at the 82th Annual Meeting of International Commissions on Large Dams
(ICOLD) on June 1-6, 2014 at Bali Nusa Dua Conference Center, Bali
2
The current condition, land degradation in the upper area of the dam has increased. The
degradation caused by excessive farming activity. Sedimentation flowing from the upper
watershed to the dam has increased significantly. It implies water storage capacity of the
dam become decreasing. In the same time, water demand for irrigating land is getting high
especially in the second planting date (PD-2) where the season is moving from the wet
season to the dry season and in the third planting date (PD-3) in dry season.
2. METHOD
2.1. The Model Structure
Simple hydrologic model of Mock was used to recalculate water balance at Cacaban dam.
The model is basically of rainfall-run-off model that containing three tanks (Mock, 1973).
Figure 2 shows the model structure.
Figure 2. Structure of simple hydrologic model of Mock
AET
P
WS DRO (WS-I)
∆SM
ISM SMC
GWS IGWS
∆S
BF=I-∆S
I
AET = CF x Eto GWS = 0.5x(1+k)xI+(kxIGWS)
ER = P - AET IGWS = GWSI-1
WS = ER - ∆SM BF = I - ∆S
ISM = SMI-1 ∆S = GWS - IGWS
I = IC x WS QTot = DRO + BF
Where:
P = rainfall (mm)
CF = crop factor
Eto = potential evapotranspiration (mm)
AET = actual evapotranspiration (mm)
ER = excess rainfall (mm)
DRO = direct runoff (mm)
∆S = change of ground water volume
∆SM = change of soil moisture (mm)
SMC = soil moisture capacity (mm)
ISM = initial soil moisture (mm)
WS = water storage (mm)
IGWS = initial ground water storage
GWS = ground water storage (mm)
IC = infiltration coeficient
BF = base flow
I = Infiltration
Figure 1. Location of Cacaban Dam
Using high variety of rice by the
farmers with shorter age and
higher water consumptive use
comparing the usual variety of
rice gives contribution in
increasing water demand of
irrigation. Therefore, recalcu-
lation of water balance of the
Cacaban dam irrigation system
is needed. The aim of study was
focused in application of the
model to recalculate water
balance at the Cacaban dam.
Upper watershed of dam
Irrigation Common Area Cacaban Dam
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The model contains six parameters. Those are infiltration in rainy season (ICw), infiltration
in dry season (ICd), initial soil moisture (ISM), soil moisture capacity (SMC), initial
groundwater storage (IGWS), and groundwater recession constant (K). The model does not
installed instrument for optimizing parameter model. Therefore trial and error approach is
applied in the optimization process. Graphical and simple statistical measures (correlation
coefficient and volumetric error) are used to prove the model performance. Calibration and
verification is needed to know model performance.
2.2. Calibration and verification
One year monthly basis was applied for calibration and verification process to get optimum
value of parameters model. Accuracy of model performance was calculated statistically
using correlation coefficient (R) and volumetric error (VE). Scatter and time series diagram
were also applied to describe optimal model performance.
2.3. Recalculating water balance
Calculation of water balance in the dam is used with following simple formula:
Water storage in i period is calculated by:
Where:
Ii : discharge inflow in i period
Vi : water reservoir volume in i period
Vi -1 : water reservoir volume before i period
Oi : water supply in irrigation area from the
dam in i period
Pi : percolation in reservoir inundation
in i period
Ei : evaporation in i period
Lpi: excess water on spillway in i period
Hi : rainfall in i period
3. RESULTS AND DISSCUSSION
3.1. Calibration process of hydrologic model
Calibration and verification of the model was conducted by using monthly rainfall and
discharge data of 2007 and 2008, respectively. Calibration process gave optimal six
parameters model (Table 1).
Table 1. Optimal parameter result of the model from calibration process
at the Upper watershed of Cacaban dam
Parameter Unit Symbol Range Optimal
parameter Min Max
Infiltration coefficient in rainy season - CWS 0.5 1.0 0.5
Infiltration coefficient in dry season - CDS 0.5 1.0 0.7
Initial soil moisture (mm) ISM 10 100 150
Soil moisture capacity (mm) SMC 150 500 180
Initial groundwater storage (mm) IGWS 100 250 300
Groundwater recession constant - K 0.5 1.0 0.7
Ii = Vi – Vi -1 + Oi + Pi + Ei + Lpi – Hi ............................................................... (1)
Vi = Vi -1 + Ii + Hi - Pi - Ei - Lpi - Oi ................................................................ (2)
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Statistically, good performance of the model indicated from the value of correlation
coefficient (R) and volumetric error (VE) was 0.895 and 0.032, respectively. In verification
process gave the value of R and VE was 0.833 and 0.10, respectively. With the value
obtained in calibration and verification process prove that the model is sensitive enough to
simulate the discharge flowing to the dam. Figure 3a and Figure 3b show the result of
calibration and verification in time series diagram, respectively.
Calibration and verification in the form of scatter diagram are presented in Figure 4a and
Figure 4b, respectively.
Figure 3b. Time series diagram of verification result in 2008
Figure 3a. Time series diagram of calibration result in 2007
Figure 4. Scatter diagram: of calibration (a) and verification (b)
(a) (b)
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3.2. Application of the model in other watershed
In order to prove that the model give also well performed to other location, the calibration
and verification result are compared with two other watersheds, namely upper watershed of
Sempor and Wadaslintang dam. Considering that upper watershed of Cacaban dam
geologically is similar in giving effect to hydrologic response with the two watersheds above,
discussion is directed to prove that dominated area of forest has significant affect to
temporary storage function. The significant effect can be detected from the value of
parameter model.
Occupation of forest area at the three upper watersheds of dam are presented using the ratio
of forest area to total watershed area (%). Forest area at the upper of Cacaban dam is more
dominated than the two other watersheds. Table 2 shows occupation of forest at the upper
of watershed of Cacaban dam is 48.3%, followed by upper watershed of Wadaslintang dam
(7.9%) and Sempor dam (3.7%). Meanwhile, parameter results of hydrologic model in
calibration process for three upper watersheds are presented in Table 3.
Table 2. Land use pattern in each upper watershed of dam
Land Use
Upper watershed of
Cacaban dam Sempor dam Wadaslintang dam
Open cultivated
upland crop, ha (%)
2,267
(37.4)
3,718
(84.2)
13,936
(72.4)
Forest land, ha (%) 2,931
(48.3)
164
(3.7)
1,524
(7.9)
Settlement, ha (%) 207
(3.4)
278
(6.3)
2,398
(12.5)
Water storage,
ha (%)
660
(10.9)
256
(5.8)
1,395
(7.2)
Total (ha) 6,007 4,416 19,253
From Table 3 show that the parameter model of CDS, ISM and SMC for the upper watershed
of Cacaban dam are higher that the two other watersheds. It reflects occupation of forest
area in related with hydrologic response.
Table 3. Optimal parameter of the model in each upper watershed of dam
Parameter
Unit Symbol Optimal Parameter at Upper Watershed
Cacaban
dam
Sempor
dam
Wadaslintang
dam
Area km2 A 60.66 44.15 192.53
Infiltration coefficient in rainy
season - CWS 0.5 0.5 0.1
Infiltration coefficient in dry
season - CDS 0.7 0.5 0.4
Initial soil moisture (mm) ISM 150 200 100
Soil moisture capacity (mm) SMC 180 200 250
Initial groundwater storage (mm) IGWS 300 1500 1500
Groundwater recession constant - K 0.70 0.99 0.94
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3.3. Application of hydrologic model for recalculating water flowing to the dam
In reference to the formula for recalculating water balance as mentioned above, discharge
flowing to the dam as supply side was simulated by the model using optimal parameter result
in calibration process. In demand side, water demand for irrigation was calculated by
common formula. Data in the year of 2008 was used for this calculation. Figure 5 shows
observed and simulated discharge flowing to the dam and their cumulative. In the end of the
year, cumulative discharge gives 60 million m3. Design water storage capacity of the dams
is 90 million m3. It is clear that the actual water storage capacity is less than the designed
capacity. This fact proved that the water flowing to the dam never been reached overflow
through spillway since the dam was constructed in 1958.
3.4. Water balance
As mentioned above that a good performance of the model was shown in calibration and
verification process. It reflects that the model is sensitive enough to simulate discharge at
the upper watershed flowing to the dam which is located in tropical monsoon area. In order
to calculating water balance of Cacaban dam irrigation system, the model was applied to be
a tool as part of component in water balance formula. The result gave accurate enough in
predicting water flowing to the dam as water availability for irrigating the common areas of
Cacaban dam. Using hydrologic data of 2008, an example of recalculation water balance is
presented in Figure 6 (Sukirno, et.al., 2010).
It is well known that, there are Four Group of planting date with each group has three
different planting season at the Cacaban irrigation system. The first planting season started
second week of October. The second planting season started from first week of November
then followed by third planting season started from third week of November. Figure 6 as
mentioned above gave an example recalculating water balance result for Group 1. It can be
understood from the figure that the water availability of Cacaban Dam is not enough for
supplying water at the total area of irrigation common area for the Group-1 with the first
planting season at the second week of October. There are two period of deficit of water (-)
from the second week of October (Oct II) to second week of December (Dec II) and from
the first week of March (Mar I) to second week of July (Jul II). Surplus (+) of water happens
Figure 5. Observed and simulated discharge flowing to the dam
Water release for irrigation
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from the first week of December to the first week of March. Recalculating of the water
balance was also conducted for other three Groups. The results also gave that water
availability at the dam still not enough to supply water for irrigating total common service
area of Cacaban dam system.
4. CONCLUSION AND RECOMMENDATION
4.1. Conclusion
1. Simple hydrologic model of Mock was applied to simulate discharge of watershed at the
tropical monsoon region. The upper watershed of Cacaban dam was used as study. The
result show that the discharge can be simulated well by the model.
2. In order to prove that values of parameter model has correlation with land use pattern,
the model was also applied at the upper watershed of Sempor and Wadaslintang dam for
comparison. Forest area was used as variable of comparison. The result show that the
parameter of the model is in line with occupation of forest area.
3. Application of the simple hydrologic model gives accurate enough for recalculating
discharge flowing the dam as part of water balance calculation
4. The result show that water availability of Cacaban dam is not enough to solve water for
irrigating total common area of Cacaban irrigation system, even for existing Four Group
of irrigation schedule.
4.2. Recommendation
Even though the hydrologic model of Mock is simple, however it proves that the model gives
good performance in simulating discharge at three location studies of the upper watershed
of dams. Therefore, the model is still possible to be applied to calculated discharge in other
watershed of tropical monsoon climate.
In case application of the model for predicting water availabilty at the Cacaban dam
irrigation system, it is recommended to increase water supply using inter watershed
management.
Figure 6. Water balance of 2008 for planting Group-1 of first planting season
(Source: Sukirno et.al., 2010)
(-) (-) (+)
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ACKNOWLEDGEMENT
This paper is part of research in the year of 2009 funding by the Regional Planning Agency
of Tegal district, Central Java Province, Indonesia. For that reason, our thank is addressed.
Our thank is also addressed to our colleagues namely Chandra and our two under graduated
students for their contribution in the research.
REFERENCES
Mock, F.J., 1973. Land Capability Appraisal Indonesia. Water Availability Appraisal.
Report Prepared for the Land Capability Appraisal Project. Bogor-Indonesia, 1973.
Sukirno, Sahid Susanto, Sri Haryono and Noviana, M., 2010. Recalculating Water Balance
Of Cacaban Dam System. Proceeding of the 6th Asian Regional Conference of
International Commission on Irrigation and Drainage (ICID), 10-16 October 2010,
Yogyakarta, Indonesia.