ASSESSMENT OF OPTIMUM YIELD OF GROUNDWATER WITHDRAWAL IN THE YOGYAKARTA...

J. SE Asian Appl. Geol., Jan–Jun 2013, Vol. 5(1), pp. 41–49

ASSESSMENT OF OPTIMUM YIELD OFGROUNDWATER WITHDRAWAL IN THEYOGYAKARTA CITY, INDONESIA

Doni Prakasa Eka Putra∗1, Muhammad Iqbal1, Heru Hendrayana1, and Thomas TriadiPutranto2

1Geological Engineering Department, Faculty of Engineering, Gadjah Mada University, Yogyakarta, Indonesia2Institute of Hydrogeology, RWTH Aachen, Germany

Abstract

The high demand of clean water supply and ground-water usages in Yogyakarta City, Indonesia hascaused the lowering of groundwater table of about 3m in average since 1984. This condition is caused bythe increase number of groundwater pumping in thiscity relate to the rising of the urban population, eco-nomics condition, education and cultural activities,change of land use and increase number of hotels onthis famous tourism city in Indonesia. Therefore, itbecame necessary to assess the optimum yield fromthe groundwater system beneath this city which canstill preserve the recent level of shallow groundwater.This is important because most of the people in thiscity depends their daily water supply from shallowdug well. In order to assess the safe yield, a ground-water modeling is conducted. The data used onthis modeling was taken from previous research andalso primary data collected during this research suchas; aquifer geometry, aquifer characteristics, shallowgroundwater level, water usage/pumping wells dis-charge, river discharge and surface water level, andclimate data. Prediction of the impact of ground-water pumping was made by increasing the waterusage/pumping wells discharge on several scenarios.The result of this modeling shows that the optimumyield of the aquifer beneath Yogyakarta City area isabout 125,000 m3/day.Keywords: Optimum yield, groundwater usage,

∗Corresponding author: D. P. E. Putra, Department ofGeological Engineering, Faculty of Engineering, GadjahMada University, Jl. Grafika 2 Yogyakarta, 55281, Indone-sia. E-mail: putra [email protected]

groundwater table, pumping, urban population,shallow dug well.

1 Introduction

Yogyakarta is the famous education, culturaland tourism City in Indonesia. It’s located inthe lower slope of Merapi Volcano of Java Is-land (see Figure 1), and has about one millioninhabitants. In this city, the daily water sup-ply for each usage is taken from the ground-water resources despite there are four parennialrivers flowing near and through the city. Therecent amount of groundwater withdrawal inthis city is known to be about 45.000 m3/day(Iqbal, 2013) and its already cost the decreaseof groundwater level of about 3 m in averagecompare to the groundwater level in year 1984.Therefore, in order to sustain and conserve thegroundwater resources in this city, it is impor-tant to assess how much actually the optimumyield of groundwater withdrawal which can beapplied in Yogyakarta City without causing un-acceptable decrease of recent groundwater ta-ble. To answer the question, a groundwatermodel can be used to simulate several scenariosof groundwater withdrawal and the optimumyield of groundwater abstraction can be definedfor Yogyakarta City.

41

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Figure 1: The location of the study area.

2 Literature study

In term of groundwater basin, YogyakartaCity is located in the central part of theYogyakarta-Sleman Groundwater Basin, whichis a part of groundwater-province fromMerapi-Stratovulcan (Hendrayana, 1993). TheYogyakarta-Sleman Groundwater basin ismainly composed by the quarternary fluvial-volcanoclastic sediments. The term fluvialrelate to the streams/rivers deposition pro-cess, and the term volcanoclastic is used be-cause these quarternary sediments are largelyattributable to Merapi Volcano deposits. Ra-hardjo et al. (1977) named these quarternaryfluvial-volcanoclastic sediments as Young Mer-api Alluviums. MacDonald and Partners (1984)differentiated the Young Merapi Alluviumsinto two types of Formation; Yogyakarta andSleman Formation. The Yogyakarta Formationforms the surface outcrop throughout most ofthe lowland area of the Yogyakarta Basin fromthe Merapi Middle Slopes to the coast. It con-stitutes the upper part of the former YoungerMerapi Volcanics. On the other hand, The Sle-man Formation is designated as a lower part ofthe former younger merapi alluviums. The for-

mation extends from the Merapi Upper Slopesto south of Bantul. This quarternary sedimentsuccession built an aquifer system, which isnamed as Merapi Aquifer System (MacDonaldand Partners, 1984; Hendrayana, 1993).

The Merapi Aquifer System is differentiatedinto two main aquifers; Yogyakarta Formationpredominantly as the upper aquifer and SlemanFormation predominantly as the lower aquifer(MacDonald and Partners, 1984). The base ofthe aquifer system is developed by compactedbreccia and lava layers of Old Merapi Volcanic-deposits in the north. In the central and southof the Yogyakarta City, limestone, marl andtuff of Sentolo Formation lye under the MerapiAquifer System. In much of the region, the baseof the system is in low-permeability rock (Mac-Donald & Partners, 1984) and commonly des-ignated as aquiclude (Hendrayana, 1993) hencesome borehole data show that in some areas,the base of the system acts rather as aquitardthan aquiclude. The system consists of sedi-ment units varying in age from lower Holoceneto recent time and is composed by interbeddedmiddle to coarse sands, gravels, silts, clays andlenses of breccia. As the source of sedimentsoriginates mainly from the eruption materials

42 c© 2013 Department of Geological Engineering, Gadjah Mada University

ASSESSMENT OF OPTIMUM YIELD OF GROUNDWATER WITHDRAWAL IN THE YOGYAKARTA CITY

of Merapi Volcano, andesitic-siliciclastic sedi-ments are dominant. In the upper aquifer, sandand gravels are to be more prevalent in the top10 to 25 m (MacDonald and Partners, 1984). Butregionally, the lower aquifer consists of coarserdeposits than the upper aquifer (Hendrayana,1993). The thickness of the Merapi AquiferSystem is distributed differently depend on thegeological condition. Regionally, it increasesfrom north-boundary of the aquifers to the cen-tral part of Yogyakarta City, and then decreaseswhen exceeding the Yogyakarta graben.

Putra (2007) mention that the results of thelithostratigraphy correlation from boreholedata within the study area show there are ac-tually five quarternary layers or successions,which built the multilayer aquifers of the Mer-api Aquifer System beneath Yogyakarta City(Figure 2). Each layer consists of a heteroge-neous composite of gravel, sand, clayey sand,and clay facies, and they are separated by lat-erally uncontinuous sandy silt to clay layers.The laterally uncontinuous semi-permeable toimpermeable layers make incomplete separa-tion between the aquifers and cause hydraulicwindows. As a consequence, the aquifers ofthis multilayer system are connected directlyto each other in some places. The existence ofhydraulic windows in the aquifer system inthe study area can have an important role. Thehydraulic windows allow mixing process ofgroundwater from different aquifers. Its meanthat groundwater withdrawal in the loweraquifer system can also affected the upperaquifer and vice versa.

According to the constructed hydrogeologi-cal section (Figure 2), the thickness distributionof the Merapi Aquifer System is controlled bythe basement morphology. The thickness ofthe system increases slowly from northern-part of the study area (+50 m) to the northfootwall (FW) of Yogyakarta graben (+80 m)and then increases abruptly in the central-partof Yogyakarta City (+150 m), attributable tothe hangingwall (HW) of Yogyakarta graben.When exceeded Yogyakarta graben, the thick-ness of the system decreases, abruptly in thesouth footwall of Yogyakarta graben (+50m) and then decreases continuously to the

southern-part of the study area (+30 m). Fromthe total thicknesses of the system, thicknessessof the upper aquifer in the study area variesfrom about 25 m in the northern-part of thestudy area to about 45 m in the centralpart ofYogyakarta City and then decreases to about10 m in the southern-part. The upper aquiferis composed mainly of middle to coarse sandsand gravels. According to MacDonald andPartners (1971), the average hydraulic conduc-tivity of this upper aquifer under YogyakartaCity is reported as about 7.8 m/d (Putra, 2007).The storativity of the aquifer under the Yo-gyakarta city ranges from 0.03 to 0.20, andbased on the measurement from Alpin Con-sult (1989), the average specific yield of theupper aquifer is reported to be about 20%. Thethicknessess of the lower aquifer in the studyarea ranges between 20 m (southern-part of thestudy area) and 105 m (central-part of the studyarea), and is composed mainly by coarse sandwith gravels and blocks, lenses of breccia andalso lava layers.

3 Theoritical background

Considering a unit volume of porous media inthe study area, the law of conservation of massfor a steady state condition is applied. It re-quires that the rate of fluid mass flowing intothe aquifer system must be equal to the rate offluid mass flowing out of the aquifer system.The equation of continuity that translates thislaw into mathematical form can be written as(Freeze and Cherry 1979):

∂

∂x

(

Kx∂h

∂x

)

+∂

∂y

(

Ky∂h

∂y

)

+∂

∂z

(

Kz∂h

∂z

)

= 0

(1)Where:

x, y, and x the Cartesian coordinates,

Kx, Ky, Kz the hydraulic conductivity compo-nents and

h the hydraulic head.

Based on the mathematical formulations above,the groundwater head is calculated by an itera-tive process in the numerical model. A numer-ical model is one of the methods used to solve

c© 2013 Department of Geological Engineering, Gadjah Mada University 43

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Figure 2: Concept of aquifer system underlying Yogyakarta City area (Putra, 2007).

groundwater problems in a hydrogeology sci-ence (Spitz and Moreno, 1996). It has an abil-ity to tie data and physical principles into com-prehensive and effective representations of thestudy area. The calculation will be ended assoon as the highest difference in heads from twoiteration steps falls below the error tolerance.

4 Methodology

There are actually three main sections in orderto achieve the objectives of this research; (1) un-derstanding the natural system, (2) developingthe hydrogeological model (conceptual model),and (3) applying the numerical groundwaterflow model. Figure 3 shows the methodologi-cal step of this research.

The study background about hydrogeologyand numerical models in this research wasfirstly developed by understanding the naturalsystem which contained many different tasks.Literature reviews were undertaken by compil-ing published hydrogeological and numericalmodelling guidelines and analyzing the previ-ous reports about geology, hydrogeology, hy-drology of Yogyakarta and numerical models

which were already created in the past. Theoutcomes of the literature reviews led to thepreliminary understanding of the natural sys-tem and enable us to define the step of data ac-quisition for the site investigation. Then, theprincipal task on data acquisition was combin-ing different data sources to obtain homoge-nous data set from several sources.

Developing the hydrogeological model con-ducted by combining results of literaturestudy and the field investigation. Definingthe boundary conditions of the study area wasthe main focus of hydrogeological condeptualmodel. Well distribution, borehole logs, piezo-metric head measurements from observationwells, groundwater abstraction from produc-tion wells, pumping tests, aquifer parameterssuch as hydraulic conductivity, effective poros-ity, and storage compressibility, surface wa-ter packages (e.g. surface water level, riverbeds thickness, and its hydraulic conductivity),and climatology data (e.g. precipitation, hu-midity and temperature) were collected fromsecondary data. On this stage, calculation ofgroundwater recharge was conducted by ap-



plying some existing hydrological equations, asexplained below.

Regarding the climate condition, the averageannual rainfall in Yogyakarta City is about 2450mm/year, with the annual mean temperature inYogyakarta City is about 27.3◦C (Iqbal, 2013).The estimation of the groundwater rechargewas conducted based on simple water balanceequation:

U = P − Etr − Ro (2)

where:

U Groundwater recharge (mm/year)

P Annual Precipitation (mm/year)

ETr Evapotranspiration Real (mm/year)

Ro Surface Runoff (mm/year)

ETr was calculated by applying TURC (1954)Equation (Singh, 1989), as written below:

ETr =P

√

0.9 +P2

(300 + 25 · Tm + 0.05 · T3m)2

(3)where:

Ro Runoff (cm/a)

P Rainfall (cm/a)

Tm Annual mean temperature (◦C)

A Watershed (km2)

Surface runoff was calculated by applying em-pirical runoff equation from Department ofAgriculture of India (1990) which is proven tobe suitable for Java Island – Indonesia (InstitutTeknologi Bandung, 2001). The run off estima-tion based on this method is written as the fol-lowing below:

Ro =1.511 × P1.44

T1.34m × A0.0613

(4)

Applying the above equations, the ground-water recharge in the research area is found tobe about 400 mm/year.

The numerical model is based on a programcode which solves the groundwater equations.It has the general purpose to simulate a vari-ety of specific hydrogeological model by vary-ing input data. In this study, Visual Modflowsoftware was used to solve the numerical prob-lem. The program uses finite difference analysisto solve the groundwater flow equation in bothsaturated and unsaturated conditions. Thisstudy was focused on the simulation of ground-water flow model under saturated condition inboth non time-dependent (steady state) models.Prediction of the optimum yield was conductedby comparing the effect of several groundwaterabstraction values to the recent groundwater ta-ble condition.

Numerical models consist of governing equa-tion, boundary conditions and initial con-ditions. Correct selection of the boundaryconditions is a critical step in model design.In steady-state simulations, the boundarieslargely determine the flow pattern. The modelboundaries within the model domain selectedfor this model include few assumptions whichare necessary to simplify the model numer-ically. Some assumptions have to be madefor the model. The main assumptions are asfollow: (1) the water table in the north andsouth boundaries is assume to be constant withtime, (2) recharge rates used are assumed netlong-term values, (3) the river stages are de-rived based on surface topography and fieldobservations, (4) Hydraulic conductivities usedare partly estimated and averaged over a largearea based on pumping test well and field ob-servations, (5) topographical information issufficiently accurate, and (6) Interpolation andextrapolation of data is acceptable.

5 Results

The model domain selected measures about90 km2 extending from 426500 mE to 436500 mEand 9133000 mN to 9142000 mN as shown inFigure 4. The hydrostratigraphic units whitinthe model domain are discretized into 100 gridcell in the x-direction dan 90 grid cell in they-direction. Each grid block consists of a ho-mogeneous entity representing a sub-unit of


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Figure 3: Flowchart of this research methodology.

Figure 4: Discretization of the model area.

the hydrostratigraphic unit. Regular grid sizesare used in the model in order capture thefull model domain and provde sufficient detailsand numerical stability in the areas of interest.Grid size measure 100m x 100m. The modelconsists of 7 layers which are bounded on toplayer by topographical surface and on bottomlayer by tertiary rocks (Sentolo Formation).

The hydrogeological boundaries in the cur-

rent model are constant heads, recharge, andrivers boundaries as shown in Figure 5. Theboundary of constant heads assigned to thenorthern and southern boundary of the modelto represent constant water table boundaries.Recharge is assigned to be about 400 mm/yearfor whole area. Recharge to the model is es-sentially from rainfall infiltration; however inthe model recharge to the aquifer system canalso occur when the groundwater level is lowerthan the river stage. There are five river bound-aries apply to the model; Bedog river, Winongoriver, Code river, Gadjah Wong river and Tam-bak Bayan river. The surface water stages weremeasured in the upstream and downstreamduring dry and wet season of 2012 (Table 1).

In order to answer the objectives of the re-search, the influence of existing water well isconsidered in the model. Pumping test andfield observation of existing wells indicated thatdrawdown caused by well is small due to thehigh permeability of the water bearing layers.Hydraulic conductivity of the aquifers in themodel determines the ease with which ground-water moves within the aquifer systems. Basedon the conceptual hydrogeological model, thehydraulic conductivity zones are identified andassigned to the model as shown in Figure 6. The



Table 1: Rivers condition in Yogyakarta city area.

River Bedog Winongo Code Gajahwong Tambak Bayan

US DS US DS US DS US DS US DS

River stage (m) 131 62 130 67 130 73 136 60 135 67

Riverbed bottom (m) 130.2 61 126.5 66 127 71 134.5 58.7 133 66.1

Note: US = upstream; DS = downstream

Figure 5: Boundaries assigned for the ground-water model.

vertical hydraulic parameters assigned same asthe horizontal conductivity values. Aquiferstorage parameters are not required in steady-state simulation. Result of the modeling isshown in Figure 7.

Due to understand the effect of groundwaterwithdrawals to the changes of water table in Yo-gyakarta city, a series of predictive sensitivityanalysis are conducted in addition to the recentgroundwater withdrawal scenario. The predic-tive sensitivity scenarios include:

Base case: groundwater withdrawals 43,472m3/day

Scenario 1: groundwater withdrawals 125,712m3/day


Figure 7: Simulated water table of Yogyakartacity.



Scenario 5: groundwater withdrawals1,137,888 m3/day

Figure 8 shows the average of water table de-crease in Yogyakarta city due to those severalscenarios. The models show that the averagedecreases to 3 m of water table occur for base-case to scenario 3 compare to the water table inyear 1984. However, if the existing groundwa-ter withdrawals result only 8 m maximum de-crease of water table, all the groundwater with-drawals of scenario 1 to 3 reveal maximum de-crease of water table up to 11 m. Scenario 4 and5 shows merely worst effect to the decrease ofwater table than the other scenarios.


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Figure 6: Hydraulic conductivity zones applied in the model.

Figure 8: Total groundwater withdrawals ver-sus decrease of water table.

6 Discussion and conclusion

Commonly, the optimum yield is calculatedbased on their effect to the drawdown of watertable. The allowable drawdown of water tableis up to 60% from the total thickness of aquifer.Considering that most of the people in the Yo-gyakarta City are still depends their water sup-ply from shallow dug wells, it is wise if the op-timum yield is calculated based on the commondeep of dug wells (less than 12 m or about 25–30% from the total thickness of upper aquifer).

Based on the results of modelling, ground-water withdrawals of about 125,712 m3/dayfrom the aquifer of Yogyakarta City is alreadycause the decrease of water table to 3 m deeper

than the recent condition (maximum draw-down). Therefore, for the case of YogyakartaCity, it can be concluded that the optimum yieldof aquifer beneath this city should be about125,000 m3/day. This conclusion can be arguedas the similar maximum drawdown are also oc-cur for groundwater withdrawals up to 628,250m3/day, however it should be bear on mindthat there are some limitations to the model,such as a constant head in the north and southboundaries which in the future may be changedue to the expansion of city and human activi-ties.

References

Freeze, R.A., and Cherry J.A. (1979) Groundwater,604 p, Prentice-Hall, Inc., Englewood Cliffs, NJ.

Hendrayana, H. (1993) Hydrogeologie und Ground-wassergewinnung Im Yogyakarta Becken, In-donesien, Dissertation, RWTH-Aachen.

Institut Teknologi Bandung (2001) PenelitianCekungan Air Bawah Tanah Mojokerto, JawaTimur, Dinas Pertambangan dan Energi PropinsiJawa Timur.

Iqbal, M. (2013) Pemodelan Aliran Airtanah diWilayah Kota Yogyakarta, Skripsi, Jurusan TeknikGeologi, Fakultas Teknik, Universitas GadjahMada.

MacDonald and Partners (1971) Kali Progo BasinStudy – Part B: Groundwater, 95 p, Directorate



General of Water Resources Development Project,Government of The Republic of Indonesia.

MacDonald and Partners (1984) Greater Yo-gyakarta Groundwater Resources Study Volume3: Groundwater, 116 p, Directorate General ofWater Resources Development Project (P2AT),Ministry of Public Works, Government of TheRepublic of Indonesia.

Putra, D.P.E (2007) The Impact of Urbanizationon Groundwater Quality, A Case Study in Yo-gyakarta City – Indonesia, Mitteilungen zur Inge-nieurgeologie und Hydrogeologie, Heft 96, 148 S,Okt 2007, Herausgegeben vom Lehrstuhl fuer In-

genieurgeologie und Hydrogeologie, Univ.-Prof.Dr. R. Azzam, RWTH Aachen, Germany

Rahardjo, W., Sukandarrumidi and Rosidi, H.M.D.(1977) Geological map of Yogyakarta Quadrangle,Geological Survey of Indonesia, Bandung.

Singh, V.P. (1989) Hydrologic Systems Volume 2:Watershed Modeling, 320 p, Prentice-Hall Inc,New Jersey.

Spitz, K. and Moreno, J. (1996) A Practical Guide

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