Water Allocation Assessment and Optimization of Malabanban Watershed Using System Dynamic Approach

8/14/2019 Water Allocation Assessment and Optimization of Malabanban Watershed Using System Dynamic Approach

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1. Background of the Study

The integrity of water sources is very vital for the survival of

cities/towns that depends on them that the protection and the

way water is allocated must be maximized in the best

possible way. It is now accepted that water is considered a

finite resource even though the natural water cycle is still in process. Let us recall that the adage “Water, water

everywhere but not a drop to drink” is so very true since the

changing whether is already causing great changes in the

water cyle such as erratic floodings and significant drought

spells. While the water cycle is now erratic and we are

powerless to reverse this we can respond by developing

effective water governance policies. This aims to optimize

water allocation and to do this we have to know how water is

presently allocated and the how the people uses water in the

first place.

San Pablo City is an old city in the province of Laguna. It is

bounded by six municipalities and one city namely Calauan,Laguna in the northwest; Nagcarlan, Laguna in the northeast;

Alaminos, Laguna in the west; Rizal, Laguna in the east;

Lipa City, Batangas in Southeast and Tiaong and Dolores,

Quezon in the south (SPC CLUP 1998). It has a population

of (insert here) and it is abundantly blessed with abundant

water resources such as the famous seven lakes and the least

known but very important Malabanban watershed.

Malabanban watershed is a reserved area at the foot of Mt.

San Cristobal along the boundary of San Pablo City and

Rizal in the province of Laguna. It is characterized by lushed

vegetation at the lower part, grassland at the middle and

relatively barren at the top.

Soil maps produced by the Bureau of Soils and Water

Management (BSWM) generally consider this area as rocky

with little or no soil, a perfect area for catching water. There

are six springs in athe area of which four are within the

territorial limits of San Pablo City. In accordance with thelaw (PD 198) San Pablo City Water District (SPCWD)

maintained and control all spring sources and the trickle

down allocation was inadvertedly set up. The trickle down

allocation of water is also allowed by law provided that in

theory each entities should be aware of each others need

during times of plenty and also the awareness of who gets the

first priority when water supply is critically low.

This study aims to analyze the dynamics of water allocation

from the Malabanban watershed. Although classified as a

city, San Pablo has a vast track of agricultural land

occupying 18208.5 hectares which is about 85.09% of the

city's area. However only about 605 hectares of land is setaside for rice or about 3.35% of the total agricultural area.

The city is currently in deficit as far as rice production is

concerned and a devasting drought in 1999 have caused

further drop in rice production. Although the irrigated area is

about 100% it is obviously insufficient to address local

requirements.

At the same time, San Pablo City is overwhelmingly

dependent on the watershed for the supply of its drinking and

domestic water. More than 80% of the city is dependent on

the watershed for the water and with its drive to supply the

entire city, water demand is on the increase.

Water Allocation Assessment and Optimisation of Malabanban Watershed Using

System Dynamics Approach

Engr. Hadji Peejay U. Aranda1 Dr. Danilo C. Terante2

1Instructor – Technological Institute of the Philippines – Quezon City2Associate Professor – De la Salle University – Manila

Abstract

Water allocation for any region is normally regulated by governing institutions and normally taken for granted during times

of water plenty. However water is considered as a finite resource and therefore a sound allocation policy must be develop in

order to effectively utilize water whether there is a shortage or not. Another factor that necessitate the efficient use of water is

the aim of the City Government to increase rice production. The use of system dynamics is very helpful because it enables the

end users to clearly determine the inter relationship of various factors that can affect water distribution from the sources

towards the end users and test proposed policy outcomes before actual implementation were made and reap the consequences.

The model started out by developing the causal loop showing the effect of land alteration, precipitation, evaporation and

infiltration on the watershed and relate it to the downstream needs particularly rice farming. The causal loop is then the basis

of the model to be written in Vensim modeling tool to develop the feedback relationship. The developed model (named

MWAM or Malabanban Water Allocation Model) is a sub component of SPCWGOM or San Pablo City Water Governance

Optimization Model. The MWAM model have confirmed that the trickle down approach in water allocation is not sustainabledue to Malabanban watershed's vulnerability to rainfall variations. Although regulated allocation policies can be of help it

also shows that water end users must make the necessary adjutment in order to maximize the available water for the people

and to utilize new farming technologies and techniques.

Keywords: Dynamic modelling; Optimisation; Economic Water Use, Agricultural management; Water management, Crop allocation.



Figure 4.1 Modeling Approach in System Dynamics

The iterative nature of modeling in system dynamics is what

makes this research a worthy endeavor. Although tedious, the

the modeler/s will ultimately develop a greater understanding

of the hydrologic process that occurs in the study area. In this

paper, an understanding of the hydrologic process was made

in the context of the desired outcome: the need to determine

improvements that can be made in water governance.

System dynamics was applied in a similar study made in

Canada by Jutla (2005). In a controlled situation in which

even the materials were astificially set up as part of mine

rehabilitation. It was proven that, is set up properly, system

dynamics model will be very useful to assess the changing

conditions and provide a tool that would easily assess the

possible consequence of disruptions in the hydrologic

systems.

Figure 4.2 Causal Loop Diagram of the Malabanban

Watershed Allocation Model

The modeling process started out by conducting field work to

assess the surface hydrologic processess that occur in the

study area. This activities involves rainfall data, spring

production data. Influence diagrams were made to illustrate

the forces at work in the system which appear to be

connected to the phenomena underlying the concerns about

the subject matter. The previous steps did not produce

enough insight to solve the problem; the group then

proceeded to the construction of a simulation model.

Equations were written, and then revised and expansion of

the model was done. STELLA software was used for the

development of the mathematical model used in this study.

Test runs of the constructed model for existing condition

were done using hypothetical data and true data. Simulation

runs were then made (Codilla et al, 2009).

Figure 4.2 The Causal Loop Diagram for the Hydrologic

Process in Malabanban Watershed

Figure 4.2 shows the part of the causal loop that reflects the

hydrology at the catchment basin. Parameters such as the

water seepage area, covered area and grasslands are

adjustable parameters in which the consequence of

environmental enforcement can be adjusted the water

seepage area is adjustable such that the minimum size of the

protected area should be 230 hectares, which is presently the

size of the protected watershed area. The maximum value is

estimated to be at around 450 hectares. This is derived bt

tracing the area of the old lava field which was found by

visual inspection to be rocky and highly porous. For a rocky

watershed the amount of infiltrating water will nearly equalthat of the initial catchment during rainy months because the

saturated rocks will not absorb water anymore the infiltration

will be based on the geologic profile of the catchment area.



Figure 4.3 The Causal Loop Diagram for the water use and

demand of the San Pablo City Water District (SPCWD)

This causal loop diagram of the San Pablo City Water

District tells in a simple way the dynamics of the water

demand in San Pablo City. Being the sole entity mandated to

provide the domestic and the industrial needs of San Pablo

City it was given the primacy over any other else in the

allocation of water and naturally, the protection and the

integrity of the source - Malabanban Watershed. In this

model the Residential and Commercial Demand are

considered constant and data from SPCWD are needed for its

input.

The diagram states that the Total Water Demand of the

SPCWD service connection is the sum of the Residential and

Commercial/Institutional Demand. The controlling variables

are the SPCWD Spring Production and the SPCWD Well

Production. In normal operation, given good watershed

recharge at the Malabanban Watershed, the SPCWD Spring

Production normally outstrip the demand. However demand

varies from time to time and at the same time Spring

Production is affected by climatological variations.

Compensating for the short coming is the well production.

The well production is more expensive than spring since it

requires energy to function. Another concern here is the

quality of water in all except two of the wells (Sampaloc

Production Well and Reservoir and Central Production

Well) .

Figure 4.4 The Causal Loop Diagram for the water use and

demand of the agriculture area

Rice production is the weak spot in the Cityś drive for

agricultural sustainability. Way back in 1998 the city has

already institutionalize their goal of improving rice

production. In the city, water will be allocated between Water

Supply and Irrigation with water supply getting the bulk of

the water. Presently there are already several ways to

maximize rice production even when water and land are

scarce. In this model the net upstream flow is described as

the amount of water flowing from the streams away from the

watershed. The reason why it is described as the net

upstream flow is because SPCWD have already taken their

water allocation from the springs, thus any water that flows

downwards is left for irrigation or any other purposes. In this

diagram the area planted , low land precipitation and rice

variety requirements are the adjustable constants. The reason

will be the ability of the model to reflect changes in cropping

method as the need may set it.

The advantage of developing a system dynamics model is

that, once developed, it can accommodate myriads of

changes on several parameters as needed and it can also be

use to reflect possible consequences of any major policy shift

in governance or the adoption of innovative rice farming

techniques without actually experiencing the consequences.

It should be cautioned however that people should not be

overly reliant on models and that people should also exercise

care on improving the model based on scientific knowledge.

In the first place, a holistic system dynamic model should not

be done by engineering people alone but should also involve

people from other discipline in order for it to be more

reflective of the actual situation as much as possible.

5.0 The Simulation Runs

5.1 The existing condition under low rainfallThe simulation run for the existing condition was done using

VensimTM PLE modeling tool. The following parameters

were set based on the given data provided by various

government offices.

a. Seepage area = 470 hectares

b. total monthly rainfall = 145mm

c. grassland = 329 hectares

d. forest = 141 hectares

The rainfall volume that was used in this run was the lowest

monthly average for the last ten year rainfall period. The

seepage area reflects the approximate unconsolidatedmaterial that is very porous to the water. Of that approximate

area only about 239 hectares were accorded official

protection and have the benefit of annual tree planting

activities and other measures to spread vegetation.

Approximately 30 percent are forested and the rest are

grasslands. Areas lower than the spring elevations were

excluded as seepage area.

the following data were obtained after running the simulation



Figure 5.1 Net surface water infiltrating

Figure 5.2 Estimated Amount of water stored at the reservoir

aquifer

On the recharge side, the low monthly rainfall wouldnormally deliver an increase in the stored water at the

watershed. This is beneficial since it can now be said that the

watershed would be able to supply water to its stockholder.

There are several occasions in the ten year rainfall data that

there was no rainfall for a month or two. This has already

happened in 1998 (Jun and July). A run was conducted to

simulate water depletion

Figure 5.3 Estimated Water Depletion at Malabanban

watershed for June-July 1998

Even though at the end of July there was still a substantial

amount of water inside the aquifer the piezometric head was

already low such that only low amount of water will be ableto gush out of the spring. The subsequent spring yield from

the watershed was shown in the next figure below.

Figure 5.4 Spring yield for June-July 1998

This is the estimated spring yield for June-July 1998. The

flow was already critical when compared to the demand of

that period. Another thing in this situation only 4 spring

sources where being tapped at that time. While records of

spring production during that period were not around

anymore the data would still provide a good estimate as

corroborated by SPCWD staff who were already present

during that time.

Figure 5.5 SPCWD spring production June-July 1998

The decrease in production prompted Water district officials

to obtain water from the ground. Unfortunately, most of the

wells here contain iron which have to be filtered out in order

to be at par with spring water quality. Filtering water frommany of these wells added cost to overall water production

and obviously not beneficial to the water district in the long

run.

Perhaps the hidden problem that came out is the net outflow

towards the Makampongo and Calauan river have virtually

dried up. A potential issue in this water allocation setup is a

sudden need to provide trans-boundary water allocation since

at this occasion the water nearly dried up.



Figure 5.6 Net spring outflow from San Pablo City

5.2 The Existing Conditions under normal conditions

Under normal circumstances the available water underneath

the watershed will have an increase in its capacity. The

increase in piezometric head will also have an effect at the

spring yield as seen in figure 5.8

Figure 5.7 Estimated water recharge under normal rainfall

conditions

Figure 5.8 Spring Yield for one month under normal rainfall

conditions

The spring yield at normal rainfall is relatively flat. Although

the decline is basically the caused by the reduction of the Net

Available Subsurface Water basically the month to month

rainfall replenishes the aquifer thus providing adequate yield

from month to month at any given time. The effect on

SPCWD Spring Production is very significant since water

yield increases as a result of increased piezometric head and

increased spring yield.


conditions

An increasing spring production will yield to the decline of

SPCWD Well Production, which is actually beneficial

because it reduces the expenses of the water district.

Basically the increase in profit due to reduced well operation

is beneficial because additional resources can be diverted to

other aspects of operation.

A normal rainfall condition also provides substantial

downstream flow to the Calauan and Makampongo rivers.

The net positive outflow from San Pablo will definitely

benefit downstream users even though Malabanban

watershed is only just one of their tributaries


conditions

In the aspect of rice production, normal rainfall would

definitely reduce irrigation water demand and the effect is

shown in the graph below



Figure 5.11 Rice water requirement using common varieties

and normal rainfall conditions

This graph was made with the initial assumption of every

rice farmers use common rice varieties and water

requirement is based from data provided by San Pablo – City

Agriculture Office and the National Irrigation Administration

for estimating irrigation requirements. Consequentially, as

irrigation water decline net stream outflow from San Pabloalso increases downstream.

5.3 Changes in net streamflow when aerobic rice is adopted

Aerobic rice is now being promoted as a new variety of rice

to be adopted by farmers as a way to counter extreme

weather events brought by climate change. Basically the

water requirement is only about a third of what we farmers

use today. A run using low rainfall event was simulated with

data shown below.

Figure 5.12 Rice water requirement using common varieties

and normal rainfall conditions

Since Aerobic rice only requires soil to be moist, not

submerged, less water is diverted from the streams and

during rainy seasons no surface irrigation is required at all.

Figure 5.13 Net upstream flow without irrigation water

diversion

6.0 Conclusions and Recommendations

Conclusion

This study shows that system dynamics is very effective in

modeling hydrologic process. Although there is difficulty in

setting it up it would now be easier to improve it. In the case

of Malabanban Watershed Allocation Model it was shown

that rainfall is the major parameter that must be adequate to

get the system working in order. A drop in rainfall over time

would provide disastrous consequence particularly in the

aspect of water supply. While well production can mitigate

spring water supply it would obviously increase user cost.

The MWAM model have confirmed that the trickle down

approach in water allocation is not sustainable due to

Malabanban watershed's vulnerability to rainfall variations.

Although regulated allocation policies can be of help it also

shows that water end users must make the necessary

adjutment in order to maximize the available water for the

people and to utilize new farming technologies and

techniques.

Recommendations

In order to improve this model the following actions are

recommended:

1. Detailed Lithographic survey of the Malabanban

Watershed. Geologic survey is important in order to

fully determine the water bearing strata which

would improve accuracy in determining water

availability and further trace the recharge area for

acquisition and protection

2. Development of the San Pablo City Water District

Distribution Model. This model is required to

determine the non-revenue water that the water

district is experiencing right now. This will port

water production to water distribution in order to

integrate it in the overall water allocation picture.

3. This study also recommend the expansion of the

protected areas of Malabanban Watershed. Even

though the place is inaccessible for urban

development thus sparing it from land use

conversion even low density human occupation

would contaminate water infiltrating downward.



ACKNOWLEDGMENT

The authors would like to thank the following:

San Pablo City Water District, San Pablo City Government,

Our colleagues from TIP-QC and DLSU-Manila, Friends

from PICE-LNM and PICE Tarlac and PICE national for this

opportunity

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Water Allocation Assessment and Optimization of Malabanban Watershed Using System Dynamic Approach

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