PHILIPP AGRIC SCIENTIST
Vol. 95 No. 2, 199–208
June 2012
ISSN 0031-7454
Faulty Design Parameters and Criteria of Farm Water Requirements
Result in Poor Performance of Canal Irrigation Systems in Ilocos
Norte, Philippines
Wilfredo P. David1, Mona Liza F. Delos Reyes1, Manolo G. Villano1 and Arthur L. Fajardo2,*
1
Land and Water Resources Division, 2Agricultural Machinery Division, Institute of Agricultural Engineering, College
of Engineering and Agro-Industrial Technology, University of the Philippines Los Baños, College, Laguna 4031,
Philippines
*
Author for correspondence; e-mail: [email protected] , [email protected]; Tel.: +63 49 536 8746, +63
49 536 2792
Canal irrigation systems in the Philippines are characterized by their poor performance. For an insight
on the reasons for this situation, 10 canal irrigation systems were randomly chosen to assess the
soundness of the water balance parameters assumed in estimating their design crop and farm water
requirements. Paddy percolation losses were measured and were compared with the assumed values
during the project design stage. Farm ditch losses were also measured. Farm water requirements were
computed on the basis of the measured percolation and farm ditch losses and estimated net seepage,
evapotranspiration and other losses.
The ratio of the actual area served to the design service area was computed for each of the sample
irrigation systems. Data on actual area irrigated were determined based on the records of the farming
activities by season of the institutions or associations responsible for the operation and management
of the irrigation systems. The discrepancies between design and measured farm water requirements
were then related to the proportions of the actual area served to design irrigation service areas to help
explain, in part, the reasons for the poor performance of the irrigation systems.
On the average, only about 27% of the aggregate design service area of the sample irrigation
systems was actually irrigated during the dry season. This was mainly due to underestimation of the
assumed on-farm water losses during the planning stage of these systems. In fact, only one of the 10
systems studied had its value of the design farm water requirement within 100% of the values
computed by using the measured percolation and farm ditch losses.
Although the designs and operations of canal irrigation systems in the Philippines are carried out
by two agencies that are both under the Department of Agriculture, there is very limited interaction
between design and operation engineers. Such absence of a feedback mechanism could have resulted
in continuous use of the same flawed designs. Design shortcomings have not been corrected and the
same design faults can be found in most of the canal irrigations systems. Failure to properly identify
and rectify the design shortcomings could have been the major reason for insignificant increases in
rice cropping intensity even after massive rehabilitation efforts. It is, therefore, high time to give more
emphasis on the formulation of appropriate irrigation design criteria.
Key Words: canal irrigation systems, cropping intensity, design water requirements, irrigation system design
INTRODUCTION
The canal irrigation systems in the Philippines have been
performing below expectations (Ferguson 1987; David
2003, 2008). These irrigation systems, which include
national irrigation systems (NIS), communal irrigation
systems (CIS) and small water impounding projects
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
(SWIP), have an aggregate design service area of about
1.4 million ha of lowland rice farms at present. The total
area they actually irrigate is much less than their
aggregate design service area. On the average, less than
two-thirds of the design service area of a canal irrigation
system is actually served during the dry season (David
2003, 2008).
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Faulty Design Parameters and Criteria of Canal Irrigation Systems
The proportion of actual maximum areas served to
the design irrigation service areas of irrigation systems in
the Philippines has been the subject of a number of
studies. In her evaluation of representative NIS during
the period 1965–1983, Ferguson (1987) reported that the
maximum area irrigated during the dry season was only
75% of the design service area on the average. Larger
systems seemed to be less efficient than smaller systems.
What was striking was the rapid decline in the ratio with
vintage. Newer irrigation projects only served 56% of
their designed service areas in 1983 in contrast with the
high 94% served before 1965.
Analyzing the National Irrigation Administration
(NIA) statistics, David (2003) reported a very low
average annual irrigation intensity of 129% in the
combined NIS and CIS service area in 1990 and 1994. In
1998, the average irrigation intensity was only 118%.
Irrigation intensity is the ratio expressed in percent of the
actual area irrigated over the irrigation service area. The
annual irrigation intensity is the total of wet and dry
season irrigation intensity.
The dry season irrigation intensity is the more
important indicator of the quality of irrigation service
and, hence, of the performance of irrigation systems.
Low irrigation intensity in the dry season is usually an
indication of inadequate water supply or low water use
efficiency as a result of design mistakes and poor water
management.
Recently, David (2009) analyzed the irrigation
intensity in NIS and CIS service areas by season. His
analysis showed very little or insignificant improvement
in irrigation intensity even after the implementation of
the Agriculture and Fishery Modernization Act (AFMA)
of 1997 which was aimed primarily at accelerated
irrigation development in the country. For example, the
NIA reported average wet season and dry season
irrigation intensities in NIS and CIS service areas for the
pre-AFMA crop year of 1993–1994 were 68% and 54%,
respectively. For post-AFMA year 2003–2004, the
average wet and dry irrigation intensities were 68% and
61%, respectively. In crop year 2004–2005, the
corresponding wet and dry season irrigation intensities
were only 55% and 51%, respectively.
During the post-AFMA years, the dry season
irrigation intensity for both NIS and CIS irrigation
service areas fluctuated from about 50–61% depending
on the weather. On the average, 39–50% of the NIS and
CIS service areas cannot be supplied with irrigation water
during the dry season despite the fact that most of the
country’s gravity irrigation systems were designed for
80% water supply dependability.
David (2003, 2008 and 2009) reported not only the
very poor performance but also the rapid deterioration of
the gravity irrigation systems service area in the
Philippines. The deterioration rate of about 70,000 ha per
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Wilfredo P. David et al.
year in the total NIS and CIS service areas during the
pre-AFMA years (1992–1996) earlier reported by David
(2003) had increased to about 134,000 ha per year during
the post-AFMA years of 1998–2004 (David 2008, 2009).
This trend accounted for the very slow annual rate of
increase of only about 10,000 ha in the actual NIS and
CIS service areas despite massive efforts of rehabilitating
an average of 124,597 ha per year and constructing new
irrigation facilities at 19,285 ha per year during 1995–
2005.
Delos Reyes and Jopillo (1986) studied the
performance of CIS in terms of the proportion of the
design service area actually irrigated under participatory
and non-participatory systems of water management. The
results also revealed low ratios of 64% and 74% under
non-participatory and participatory systems of irrigation
water management, respectively.
Research findings in the Philippines and in other
Asian countries pointed to over-optimistic assumptions of
irrigation service area during the planning stage and
faulty and unrealistic design criteria as the main causes of
the poor performance of canal irrigation systems (World
Bank 1996; Rice 1997; Horst 1998: Plusquellec 2002;
David 2003, 2008). The design shortcomings include
unrealistic
assumptions
on
canal
hydraulics,
overestimation of surface water supply availability, and
underestimation of seepage and percolation.
A review of the performance of World Banksupported large-scale irrigation projects by Plusquellec
(2002) showed that the main cause of the lower-thanexpected performance of such projects was related to
over-optimistic assumptions regarding efficiency. The
impact of poor physical performance in terms of water
distribution and concurrent poor construction standards
on agricultural productivity was often overlooked.
Plusquellec (2002) also cited the results of a formal
evaluation of 21 irrigation projects by the World Bank’s
Operations Evaluation Department (OED) in 1990. For
the 21 projects, the estimated average economic rates of
return were 17.7% at appraisal, 14.8% at project
completion and only 9.3% at impact evaluation.
Another study (World Bank 1996) on the impact of
investments in 6 gravity irrigation systems in Thailand,
Myanmar and Vietnam showed that the economic rates of
return not only fell short of appraisal projections by a
substantial margin, but were all below 7%. The gap
between appraisal expectations and actual results was due
largely to excessively optimistic estimates of crop areas
served, irrigation project design faults and construction
inadequacy.
Horst (1998) and Ersten (2009) postulated that the
problems in irrigation in many developing countries are
rooted in the technology of the colonial era and the lack
of adaptation to the new socio-economic environment of
the post-colonial period. During the colonial times,
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Faulty Design Parameters and Criteria of Canal Irrigation Systems
design and operation resided in one ministry that gave
feedback from operation to planning and design possible.
Consequently, technology developed in balance with the
management capability. In the post-colonial era,
however, design has been carried out mainly by foreign
consultants while government agencies have been
responsible for operation. They argued that this
separation of design and operation has led to
discrepancies between design assumptions and
operational realities. Together with shortage of technical
personnel and expansion of areas under irrigation,
irrigation in developing countries has been haunted for
decades by a multitude of problems such as low
performance, low water use efficiencies and deteriorating
physical structures.
Meijer (1992) argued that a major pitfall in irrigation
design is too much adherence to high water use
efficiency per se which results in water distribution
schedules that tend to be much too complicated and far
too rigid for everyday practice. He contended that apart
from crop water requirements, additional water is needed
to facilitate a fair and simple water distribution.
The design shortcomings of an irrigation system
manifest themselves in the engineering performance of
the system such as actual area served in relation to its
design irrigation service area and the ease of its operation
and maintenance. The persistently very low irrigation
intensity of existing NIS, CIS and SWIP in spite of the
massive efforts exerted during the past two decades
toward their rehabilitation is both an indication of
inappropriate or erroneous criteria used in their design
and the inability to carry out necessary adjustments in
design and operation during rehabilitation (David 2003,
2008).
Conventional rehabilitation works have focused on
restoring the original physical structure. The design
philosophy and design criteria based on water balance
parameters and hydraulic design assumptions of the past
continue to be used in incumbent system rehabilitation
without review.
There are several ongoing rehabilitation projects for
irrigation systems. These include Phase 1 of the
International Bank for Reconstruction and Development
(IBRD) Supported Participatory Irrigation Development
Project for NIS (World Bank 2009), Phase 2 of the
IBRD-supported Mindanao Rural Development Project
for CIS in Mindanao (World Bank 2007) and a pipeline
of large irrigation projects such as the Balog-Balog
Multipurpose Project (NIA 2010). There is a pressing
need to evaluate the soundness of some of the design
criteria used in the planning and development of
irrigation projects in order to avoid repeating previous
mistakes and incorporate corresponding design
adjustments in ongoing and future irrigation systems
rehabilitation and modernization efforts.
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Wilfredo P. David et al.
A study to assess the soundness of design criteria
used in the planning of canal irrigation systems in the
Philippines was carried out during 2006–2007. The first
part of the study investigated the validity of the water
balance parameters used in estimating the farm water
requirements. The results of the second part, which
looked at some design issues concerning the headwork
and water distribution and control facilities, were
reported by David et al. (2012).
This paper reports the findings of the first part of the
study which measured some of the on-farm water balance
parameters in sample canal irrigation systems. The
measured parameters were used to compute crop and
farm water requirements. The measured and computed
water balance parameters were compared with those
assumed during project preparation. The discrepancies
between the assumed and the measured design criteria
were then related to the ratios of the actual area served
over the design service area of the sample irrigation
systems.
MATERIALS AND METHODS
In the selection of irrigation systems, a sample region
(cluster of provinces) was drawn at random from the 14
regions of the country. From the sample region, a
province was also drawn at random. From this province,
the sample irrigation systems were then drawn at random.
Ilocos Norte, which belongs to Region 1, was drawn
as the sample province. Two (2) NIS, 6 CIS and 2 SWIP
were then selected at random as the sample irrigation
systems for the study.
The engineering performance of the sample NIS,
SWIP and 4 out of the 6 CIS were evaluated. The actual
areas irrigated were estimated based on the records of the
agencies or associations managing the irrigation systems
and information obtained from farmer-irrigators and
irrigators associations through interviews.
Measurements of percolation and seepage and farm
ditch losses were carried out in the 10 sample irrigation
systems from May 2006 to April 2007. The locations
(latitude and longitude) of these measurement sites were
determined using a Global Positioning System (GPS)
receiver unit (Fig. 1). For each site, the measurements for
percolation and seepage losses were carried out until at
least three replicates showed comparable results.
Measurement of Percolation and Seepage
For each of the sample irrigation systems, a tertiary canal
(main farm ditch) and a representative farm ditch of this
canal were selected. A sample field paddy for each of
these farm ditches was selected as measurement site for
percolation and seepage losses. The criteria for the
selection of field paddy included proximity to a water
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Faulty Design Parameters and Criteria of Canal Irrigation Systems
Fig. 1. Site locations of measurements for percolation
and farm ditch losses in Ilocos Norte. (CIS –
communal irrigation system, SWIP – small
water-impounding project, RIS – river irrigation
system, PIS – pump irrigation system)
source for better control of the water level within the
field paddy. The field measurement activities were
coordinated with the respective farmers of the selected
field paddies to minimize the unwanted effects of water
management and other regular farming activities on
percolation and seepage measurements.
The percolation and seepage were measured by using
the tank method. One set of instruments consisting of
three 40-cm diameter cylindrical tanks, rain gage and
halo rings were installed in each measurement site. The
tanks were fabricated from a gauge 16 galvanized iron
(GI) sheet. The first tank (labeled “E”) was 40 cm high
and with a bottom. Water in this tank escaped only
through evaporation or evapotranspiration (if there was a
standing crop). The second tank was 50 cm high and
without a bottom. One end of this tank was embedded in
the hard or plow pan of the rice paddy. Water in this tank
was lost through combined evapotranspiration and
percolation. For a given time interval, the measured
evapotranspiration loss in the "E" tank should be the
same as that in this second tank. The difference in the
readings of the two tanks was taken as the percolation
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Wilfredo P. David et al.
loss rate. Thus, the second tank was labeled as the “P”
tank.
The third tank was 30-cm high and without a bottom.
The bottom was not abutting the hard pan. When its
reading was corrected for percolation and evaporation, it
gave an estimate of seepage losses, hence, it was labeled
herewith as the “S” tank.
The water level measurements were taken inside a
small stilling basin installed in each tank. The water level
in each tank was measured using a hook gage. A rain
gage was also installed at each measurement site to
correct the rainfall readings in the 3 tanks. A schematic
diagram of the 3 tanks and rain gage and the water
balance parameters measured is shown in Figure 2.
A distance of about 1 m between the edge of each
tank and the levee and between tanks was observed for
ease of measurement and to minimize errors as a result of
possible shading from the sun’s rays and shielding of the
tanks from the wind by the levee. The “P” tank was
placed far from the banks or near the center of the paddy.
The “S” tank was placed nearer the bank of the paddy.
The “P” and “S” tanks without bottoms were driven into
the soil. The “S” tank was driven into the soil such that
its lower end did not abut the hard pan. The “E” tank,
which had a watertight bottom, was lowered in a dug
hole in the paddy. The excavated mud was then placed
inside the tank. Figure 3 shows a set of installed tanks
including a rain gage (“R”).
The reams of the tanks were leveled using a
carpenter’s level. The same clearance between the rim of
each cylinder and the standing water in the paddy was
maintained in all tanks in order to minimize the
difference in the thermal mismatch errors.
To determine the direction and gradient of seepage
flow, halo rings were installed in and around the paddies
where percolation and seepage measurements were made.
The halo rings were 7.5 cm in diameter and their stems
were 46 cm long. They were fabricated from 10-mm
round bars. The halo rings labeled “H0” were maintained
inside the sample paddy for the duration of the field
measurement. The relative elevations of the rims of the
tanks and rings of the halo rings with respect to that of
the “P” tank were measured using an engineer’s transit.
This measurement enabled comparisons of the elevations
of the water levels in the tank and in the surrounding
paddies.
All the water level measurements inside the tanks
and halo rings and the time of reading were recorded in
duplicate. At least two measurements were made each
day, one in the morning and one in the afternoon.
Additional measurements were made in the events of
high evapotranspiration, seepage and percolation rates or
during periods of heavy rainfall to minimize
measurement problems due to the depletion or
overflowing of water from the tanks. Water level was
also measured before and after water was removed or
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Faulty Design Parameters and Criteria of Canal Irrigation Systems
Wilfredo P. David et al.
Fig. 2. Schematic diagram showing the tanks and rain gage for measuring the different water balance parameters in a
rice paddy. (“P” tank measures percolation and evapotranspiration/evaporation; “E” tank measures
evapotranspiration/evaporation; “S” tank measures seepage, percolation and evapotranspiration/evaporation).
“P”
“R”
“S”
“E”
Fig. 3. Installed cylinders and rain gage within a newly
planted rice paddy. (“P” tank measures
percolation
and
evapotranspiration
/
evaporation;
“E”
tank
measures
evapotranspiration /evaporation; “S” tank
measures
seepage,
percolation
and
evapotranspiration /evaporation; “R” rain gage
measures rainfall).
added into a tank. Rainfall was measured soon enough as
small amounts of rainfall collected in the rain gage (e.g.,
less than 4 mm) could evaporate within a few hours
during a bright sunny day. For a given site, the
measurements were continued until the readings had
stabilized, or when three comparable readings had been
obtained. The series of measurement for a site usually
lasted 1–3 wk.
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Measurement of Farm Ditch Losses
The ponding method was used to measure farm ditch
losses. A section of each farm ditch serving the sample
paddy (where percolation and seepage measurements
were carried out) were selected as site. Each chosen
section was more or less straight, with a uniform crosssection and at least 10 m in length. Measurement
activities for farm ditch losses were coordinated with the
concerned farmers and farmer-leaders to avoid conflicts
with their irrigation and other farm activities.
Each canal section selected as the measurement site
was cleared of debris and of grasses growing along its
canal banks to simulate conditions under recommended
canal maintenance. Stakes were used to mark the ends of
the selected span of the farm ditch and to support the
dikes or mud embankments (Fig. 4). The dikes were
sealed with plastic sheets. An additional dike at about 1.5
m on each end of the pond was constructed to create a
buffer pond. The canal section for farm ditch loss
measurement and its buffer pond were filled with water
up to a depth of the designed full supply level of the farm
ditch. A 20-cm diameter, 40-cm high stilling basin was
installed in the middle of the pond for measuring the
water level. It was made up of gauge 16 GI sheet and was
without a bottom. The side of the tank was drilled with
small holes to allow water to freely move in and out. The
surface of the stilling basin was leveled. A rain gage was
placed beside the pond for rainfall corrections. Figure 4
shows a typical setup of the ponding method for
measurement of farm ditch losses.
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Faulty Design Parameters and Criteria of Canal Irrigation Systems
Fig. 4. Setup for measuring the losses in a farm ditch
section.
Sample points along the span of the pond were
selected for measuring wetted top widths. The length of
pond was also measured. The water level in the stilling
tank was measured using a hook gage. Water level
readings were initially taken at 15-min intervals. After
the first hour, the time interval between measurements
was increased to 30 min. The maximum drop in water
surface elevation that could be measured from an initial
reading by a hook gage was about 8 cm. When the water
level dropped below 8 cm from the initial reading, the
reading was reset by adding water to the pond.
Computation of Farm Water Requirements
The design of tertiary and lower level irrigation canals
and their related water control structures was meant to
accommodate the peak water requirements of the crops.
Their discharge capacities were determined based on the
crop water requirement (CWR) and farm water
requirement (FWR). For lowland rice, CWR refers to the
amount of water needed to be delivered by the turnouts to
the rice paddies to insure a certain dependability level of
irrigation water. In equation form, CWR can be
expressed as
CWR = ET + P + S + Tro – ER
where ET is the actual evapotranspiration, P is
percolation, S is seepage, Tro is tail water runoff or
spillage over paddy boundaries and ER is effective
rainfall.
In the Philippines, the canal irrigation systems
irrigate mainly lowland rice monocultures. The design
dependability level of their irrigation water supply is
usually 80%. The CWR is, therefore, the water
requirement at the rice paddy level that is expected to be
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Wilfredo P. David et al.
exceeded only 20% of the time. For lowland rice
monocultures, the period of high water demand is usually
during land soaking following a dry season rice crop or
during peak vegetative growth stage during the dry
season. By manipulations of cropping systems and
cropping calendars, the water requirement for land
preparation can be spread over a longer period of time.
The appropriate CWR value for design purposes was the
maximum water requirement of the dry season lowland
rice crop.
Effective rainfall (ER) is the amount of rainfall that
contributes to meeting the CWR. Ilocos Norte is
characterized by very distinct wet and dry seasons.
Hence, ER during the dry season was assumed to be
negligible.
In estimating CWR, ET was estimated using the 30yr (1976–2005) open pan data of the weather station at
the Mariano Marcos State University (MMSU) agrometeorological station in Batac, Ilocos Norte. The
readings from Standard Class A pan were adjusted for the
pan and crop coefficients established from studies at the
University of the Philippines Los Baños in Laguna by
using large, drainage-type lysimeters.
For a given field paddy, seepage may be positive or
negative depending on the level of the standing water
within the paddy with respect to those of neighboring
paddies. During or immediately following irrigation
water delivery, the level of standing water in the paddies
nearer the turnouts was higher than those of other
paddies. Water seeped from paddies with higher water
level and were recovered in the downstream paddies. So
for a given turnout service area, the net seepage is that
water lost in the levees bounding the turnout’s service
area. Among others, this loss is influenced by the shape
or configuration of the turnout service area, the seepage
flow gradient from the levees bounding the turnout
service area into field drains, the soil physical properties
and the quality of water management.
In the study, the seepage loss in the sample field
paddy (where percolation and seepage losses were
monitored) exhibited considerable fluctuations that were
associated with irrigation water delivery into neighboring
paddies. Due to the enormity of the task required, it was
not possible to monitor the seepage flow from paddy to
paddy down to the paddies bounding the turnout service
area.
Similarly, the Tro is influenced by many factors that
include the service area per turnout ratio, the quality of
water management by farmers and the control of spillage
of water in the levees bounding the service areas of the
offtakes. Because of the difficulty in measuring Tro and
seepage, it was necessary to resort to the common
practice of design engineers to assume the total of these
two parameters to be a percentage of the combined ET
and P losses. In the computation of the crop water
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Faulty Design Parameters and Criteria of Canal Irrigation Systems
requirement, the combined seepage and Tro were
assumed to be 10% of the measured ET and P losses. The
bases for this low estimate are the higher percolation loss
rate obtained through field measurement than that used
during the design stage and, consequently, the lower
design CWR. The higher percolation and the
underestimation of CWR resulted in limited irrigation
water reaching the downstream paddies bounding the
service areas of turnouts, hence, a lower Tro.
Farm water requirement (FWR) is a measure of the
amount of water needed at the offtake for the tertiary
canal. It is expressed as
FWR = CWR + FDL + OPL
where farm ditch losses (FDL) are the losses due to
evapotranspiration, seepage and percolation in the farm
ditches and the tertiary canal and OPL are operational
losses. Operational losses result from spillage over canal
banks and inefficiency in the distribution of water where
some farm ditches get more water than what was needed.
Among others, operational losses are influenced by the
quality of on-farm water management, the degree of head
and discharge control from the tertiary canal down to the
farm ditches and the density of on-farm water
management structures.
In the sample irrigation systems, the adequacy of the
facilities for head, velocity and discharge control was
assessed through field surveys. In all the systems studied,
a good degree of head, velocity and discharge control
was not possible due to the lack of cross regulators,
wrong placements and combinations of flow control and
intake structures, improper alignment of tertiary and farm
ditches, ungated intake structures, direct offtaking of
farm ditches from the main canal and very high service
area to farm ditch and turnout ratios. From these
observations, the application efficiency in all systems
studied was estimated to be about 80%. As such, the farm
water requirement equation may be rewritten as follows:
FWR = 1.2 x (CWR + FDL)
RESULTS AND DISCUSSION
Table 1 shows the design irrigation service area and the
actual area served during the 2004–2005 crop year of
some of the sample irrigation systems. The actual area
served over the design irrigation service area ranged 20–
90%, averaging a low 27%. This large range of
fluctuation may be due to the following reasons: (1)
severe flood damage to the headworks of the Madongan
River Irrigation System (RIS) (David et al. 2012); (2)
presence of shallow tubewells (STW) and low-lift pump
(LLP) irrigation to supplement the irrigation water
supply in many of the sample irrigation systems; and (3)
the excising or cutting out of considerable portions of the
original service areas of some of the sample irrigation
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Wilfredo P. David et al.
systems [e.g., Bonga #1 Pump Irrigation System (PIS)
and Nagsurot Tanap Avis CIS)] due to irrigation water
supply limitations. Nevertheless, the very low
percentages of actual area served over design irrigation
service area, despite the last two reasons mentioned,
pointed to optimistic estimates of crop areas to be served
by irrigation projects during their planning stages.
Table 2 shows the percolation loss rates measured in
farmers’ fields in the irrigation service areas of the 10
sample irrigation systems. For most of the systems
studied, the measured percolation rates were much higher
than those assumed during the design stage. For the
Madongan RIS, the actual percolation rates were order of
magnitudes higher than the assumed rates. For the 6 CIS
and 2 SWIP studied, only Nagsurot Tanap Avis came
close to the assumed rate during the planning stage. The
rest had actual percolation loss rates that ranged 2.1–6.1
times the rates assumed in the design of the systems.
These findings also pointed out to a serious design flaw
that resulted in gross underestimation of irrigation water
requirements and overly optimistic assumptions
regarding water application efficiency.
The information shown in Table 2 helps explain the
low ratios of actual area irrigated to design service area
shown in Table 1. The erroneous estimates of the
percolation rates used during the design stage were
subsequently carried over in the formulation of other
design criteria such as crop water, farm water and
diversion water requirements.
Table 3 summarizes the measured water losses in
L s-1 km-1 of canal length in selected farm ditches in the
sample irrigation systems. There were considerable
variations in farm ditch losses from system to system and
within individual systems. These variations reflect the
type of soil and the state of the physical condition and
maintenance of the farm ditches. In some systems, some
of the farm ditches are lined with concrete.
Together with the measured percolation rates and
evapotranspiration estimates, the measured farm ditch
losses were used in the estimation of farm water
requirements. The estimated CWR and FWR in selected
irrigation systems using the measured percolation and
farm ditch losses are summarized in Table 4. Hydrologic
frequency analysis showed that the measured daily ET
fitted a normal density function. A design ET rate of 7.6
mm d-1, which has an associated exceedance probability
of 20%, was assumed in all the sample systems.
From the above assumptions and based on the
commonly used definition of field application efficiency
[(ET + P) x 100 / FWR] for lowland rice irrigation, the
resulting application efficiency was about 67%. It should,
however, be pointed out that this definition of irrigation
efficiency is used only in lowland rice where percolation
is considered an unavoidable loss. In reality, the actual
efficiency was much less because percolation should be
considered a water loss.
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Wilfredo P. David et al.
Faulty Design Parameters and Criteria of Canal Irrigation Systems
Table 1. Design irrigation service area and actual area served of selected irrigation systems in Ilocos Norte,
Philippines during crop year 2004–2005.
Design Irrigation
Reported Actual
Actual to Design
Irrigation System
Type
Service Area (ha)
Area Served (ha)
Service Area (%)
Madongan RIS
Bonga Pump # 1 PIS
Nagsurot Tanap Avis CIS
Pila CIS
Estancia CIS
Nagpatayan CIS
Bingao SWIP
San Cristobal SWIP
Total/Average
NIS, gravity, ROTR
NIS, pump
CIS, gravity, ROTR
CIS, gravity, ROTR
CIS, gravity, ROTR
CIS, gravity, ROTR
SWIP, gravity
SWIP, gravity
3,621
298
308
80
125
56
70
100
4,658
740
165
100
53
113
32
32
39
1,274
20.4
55.4
32.5
66.3
90.4
57.1
45.7
39.0
27.4
CIS – communal irrigation system, NIS – national irrigation system, PIS – pump irrigation system, ROTR – run-of-the-river, SWIP – small waterimpounding project
Table 2. Measured and design percolation loss rates in selected irrigation systems in Ilocos Norte, Philippines.
Service Area
Design Percolation Measured Percolation
Ratio of Measured
Irrigation System
Location
Loss Rate (mm d-1)
Rate (mm d-1)
over Design Rate
National Irrigation Systems (NIS)
Lateral MC-MR, San
Jose, San Marcelino
Madongan RIS
Average of 2.0
15.4
7.7
Lateral MC-MR
San Miguel, San
Marcelino
Average of 2.0
77.0
38.5
Lateral MR2,
San Raquinto, Baresbes
Average of 2.0
32.0
16.0
No available data
No available data
No available data
3.0
3.0
6.0
----
1.5
1.6
1.1
1.5
1.5
1.5
1.5
1.5
6.7
4.5
5.4
9.2
5.0
4.5
3.0
3.6
6.1
3.3
1.0
1.0
1.0
4.9
6.1
2.1
4.9
6.1
2.1
1.0
5.0
5.0
Main Canal (Zone 1)
Main Canal (Zone 2)
Main Canal (Zone 3)
Communal Irrigation Systems (CIS)
Nagsurot Tanap
Lateral A
Avis CIS
Pila CIS
Main Canal
Estancia CIS
Lateral A
Nagpatayan CIS
Main Canal
Tambidao CIS
Main Canal
Sallaguid CIS
Lateral 1
Small Water Impounding Projects (SWIP)
Lateral 1 (Zone 1)
Bingao SWIP
Lateral 2 (Zone 2)
Lateral 3 (Zone 3)
San Cristobal
Lateral 1
SWIP
Bonga #1 PIS
PIS – pump irrigation system, RIS – river irrigation system
Table 3. Measured farm ditch losses (L s-1 km-1) of canal length in sample irrigation systems.
Measurement Location
Irrigation System
Site No.
Canal Name
Farm Ditch
1. Madongan RIS
2. Bonga #1 PIS
3. Nagsurot Tanap Avis CIS
4. Estancia CIS
5. Sallaguid CIS
6. Nagpatayan CIS
7. Pila CIS
8. Tambidao CIS
9. Bingao SWIP
10. San Cristobal SWIP
1
2
3
1
2
3
1
1
2
3
1
2
1
1
1
1
1
MC-MR
MC-MR
MR 2
Main Canal
Main Canal
Main Canal
Lateral A
Main Canal
Lateral A
Lateral A
Lateral 1
Lateral 3
Main Canal
Main Canal
Main Canal
Lateral 1
Main Canal
CIS – communal irrigation system, NIS – national irrigation system, PIS – pump
small water-impounding project
206
Padila # 3
Padila # 12
Padila # 11
Zone 1
Zone 2
Zone 3
TO # 4
TO # 4
TO # 5-A
TO # 5-B
TO # 1
TO # 4
Zone 1
Farm Ditch Losses
(L s-1 km-1)
2.16
0.75
3.10
1.05
0.96
22.76
0.21
5.42
1.26
1.9
6.17
0.67
0.37
2.28
0.69
1.35
0.14
irrigation system, RIS – river irrigation system, SWIP –
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
Wilfredo P. David et al.
Faulty Design Parameters and Criteria of Canal Irrigation Systems
Table 4. Comparison of design and computed crop water requirements (CWR in mm d-1) and farm water requirements
(FWR in mm d-1) in sample irrigation systems.
Design Values
Computed Values
Actual to
Design over Design over
Site
Design
Irrigation System
Computed
Computed
No.
Service
CWR
FWR
CWR
FWR
CWR (%)
FWR (%)
Area (%)
1. Madongan RIS
2. Bonga #1 PIS
3. Nagsurot Tanap Avis CIS
4. Estancia CIS
5. Sallaguid CIS
6. Nagpatayan CIS
7. Pila CIS
8. Tambidao CIS
9. Bingao SWIP
10. San Cristobal SWIP
1
2
3
1
3
1
1
1
1
9.43
9.43
9.43
6.06
9.05
6.06
6.06
6.0
6.7
14.5
14.5
14.5
7.62
6.5
25.56
94.0
44.0
10.22
13.44
14.0
14.44
15.89
18.67
13.89
14.0
34.1
118.6
57.1
11.4
30.34
18.25
18.49
21.88
24.36
19.16
17.60
22.8*
26.7*
20.4
59.3
67.3
42.0
38.1
43.2
47.9
39.8
36.9
Average
55.4
32.5
90.4
57.1
66.3
45.7
39.0
27.4
*Average for 3 measurement sites.
CIS – communal irrigation system, NIS – national irrigation system, SWIP – small water-impounding project, RIS – river
irrigation system, PIS – pump irrigation system
Table 4 also compares the water requirements
estimated in the study with those assumed in the design
of the sample systems. The design CWR and FWR were
taken from available feasibility and detailed design
documents of the sample systems (NIA data, Provincial
and National Offices). Three of the sample systems did
not have documentation or available information on their
design. It is possible that these systems were constructed
by adopting design parameters for neighboring systems.
The results of the study indicated gross underestimation
of the crop and farm water requirements at the design
stage of the irrigation systems. Except for Nagsurot
Tanap Avis CIS, none of the design CWR was within
100% of the actual values. Nagsurot Tanap Avis had the
lowest CWR and FWR. The reasons for its low water
requirements were the lowest percolation rate and the
recent rehabilitation of the system where the main farm
and secondary farm ditches were lined with concrete.
Table 4 compares the proportions in percent of the
design over measured CWR, the design over measured
FWR and the actual area served over design service area.
To a large extent, the very low actual area served over
design service area can be attributed to the gross
underestimation of the design CWR and FWR.
In most of the sample systems, farmers pumped
water from shallow aquifers and from creeks and
drainage ways to supplement the irrigation water coming
from the sample canal irrigation systems. This situation
explains the higher actual area served over design area
than the design over measured CWR ratios in some of the
sample systems. For example, considerable portions of
the service areas of the Bingao, Bonga, Pila, Estancia and
Nagpatayan irrigation systems were served by STW and
LLP irrigation systems during the dry season. Most of
these systems were owned and operated by individual
farmers. As estimated by David (2003, 2009), the
The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)
combined area served by pump irrigation systems was
larger than that served by all the other modes of irrigation
not only in Ilocos Norte but in the entire country.
The very low actual area served over design area for
the Madongan RIS is partly due to the damage to the
Madongan dam by a flood in 2004. The flood destroyed
one of the two main canals of the irrigation system. The
results of a study of David et al. (2012) indicated that one
cause of dam failure is the underestimation of the design
flood.
Noteworthy is the very limited interaction or
coordination between irrigation systems design and
operation and maintenance. In fact, design engineers are
not required to test run the system they designed before
they turn it over to the operation and maintenance staff.
As a result, design mistakes are repeated without having
been corrected.
The development of irrigation in the Philippines also
suffers from inadequate baseline information and
institutional capacity for project planning. Government
agencies such as the National Irrigation Administration
and the Bureau of Soils and Water Management do not
carry out field measurements to generate essential
baseline information for formulating sound design
criteria. They rely on age-old design criteria based on
assumptions that had never been field validated.
During the past 10 yr, the expenditures for NIS and
CIS rehabilitation and new systems construction
averaged well over US$ 100 million per yr. Yet,
irrigation infrastructures continue to deteriorate rapidly.
The reason was that the design flaws are not being
rectified during rehabilitation. In response to the present
rice crisis, the government increased its allocation for
irrigation substantially. Based on experience, this move
would have minimal impact unless the design mistakes
are identified and rectified.
207
Faulty Design Parameters and Criteria of Canal Irrigation Systems
CONCLUSION AND RECOMMENDATIONS
The sample canal irrigation systems in Ilocos Norte,
Philippines are performing poorly. On the average, less
than a third of their total design service area is actually
irrigated during the dry season. This poor performance
can largely be traced to the gross underestimation of onfarm water losses during the planning stage of the
irrigation systems. In fact, none of the design farm water
requirements studied compared favorably with the actual
values computed using measured percolation and farm
ditch losses.
Considering that the designs of canal irrigation
systems in the Philippines are carried out by the same
government agencies, the same design faults may be
expected in the design and development of most of the
NIS, CIS and SWIP. Moreover, such mistakes are usually
repeated before they are corrected because of the very
limited interaction among design engineers and operation
and maintenance staff and the inadequate baseline
information and institutional capacity for project
planning. Hence, the massive efforts and expenditures
aimed at canal irrigation systems rehabilitation during
1992–2005 did not significantly improve the performance
in terms of cropping intensity of canal irrigation systems
because their design shortcomings were not properly
identified and corrected. It is, therefore, high time to give
more emphasis on the formulation of appropriate design
criteria. The design assumptions on percolation rate,
application
and
conveyance
efficiencies
and
evapotranspiration should be reviewed. Likewise, the
design of canal irrigation systems should allow for the
cultivation of non-rice crops during the dry season in
light-textured and shallow soils in order to minimize the
excessive loss of water from percolation. Greater
attention should also be given to minimizing operational
losses through appropriate and adequate water control
structure for on-farm water management.
In most of the canal irrigation systems studied,
farmers resorted to the conjunctive use of surface and
groundwater sources. In fact, the privately owned and
developed STW and LLP irrigation systems now serve a
much larger area than the NIS and CIS. This practice of
supplementing the available water from canal irrigation
system with groundwater should be encouraged not only
to improve the performance of irrigated agriculture in the
province of Ilocos Norte but also to diversify the irrigated
cropping systems.
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The Philippine Agricultural Scientist Vol. 95 No. 2 (June 2012)