On-farm strategies for reducing water input inirrigated rice; case studies in the Philippines

D.F. Tabbala, B.A.M. Boumana,*, S.I. Bhuiyana,E.B. Sibayanb, M.A. Sattarc

aInternational Rice Research Institute (IRRI), Los Banos, PhilippinesbPhilippine Rice Research Institute (PhilRice), Maligaya, Munoz, Philippines

cBangladesh Rice Research Institute, Joydebpur, Bangladesh

Accepted 31 January 2002

Abstract

Traditional transplanted rice with continuous standing water in Asia has relatively high water

inputs. Because of increasing water scarcity, there is a need to develop alternative systems that require

less water. This paper reports results of on-farm experiments in the Philippines to reduce water input

by water-saving irrigation techniques and alternative crop establishment methods, such as wet and

dry seeding. With continuous standing water, direct wet-seeded rice yielded higher than traditional

transplanted rice by 3–17%, required 19% less water during the crop growth period and increased

water productivity by 25–48%. Direct dry-seeded rice yielded the same as transplanted and wet-

seeded rice, but can make more effective use of early season rainfall in the wet season and save

irrigation water for the subsequent dry season. Direct seeding can further reduce water input by

shortening the land preparation period. In transplanted and wet-seeded rice, keeping the soil

continuously around saturation reduced yields on average by 5% and water inputs by 35% and

increased water productivity by 45% compared with flooded conditions. Intermittent irrigation

further reduced water inputs but at the expense of increased yield loss. Under water-saving irrigation,

wet-seeded rice out-yielded transplanted rice by 6–36% and was a suitable establishment method to

save water and retain high yields. Groundwater depth greatly affected water use and the possibilities

of saving water. With shallow groundwater tables of 10–20 cm depth, irrigation water requirements

and potential water savings were low but yield reductions were relatively small. The introduction of

water-saving technologies at the field level can have implications for the hydrology and water use at

larger spatial scale levels. # 2002 Elsevier Science B.V. All rights reserved.

Keywords: Rice; Water saving; Water productivity; Direct seeding; Philippines

Agricultural Water Management 56 (2002) 93–112

* Corresponding author. Tel.: þ63-2-812-7686; fax: þ63-2-845-606.

E-mail address: b.bouman@cgiar.org (B.A.M. Bouman).

0378-3774/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 8 - 3 7 7 4 ( 0 2 ) 0 0 0 0 7 - 0

1. Introduction

Rice is the major staple food in Asia, where about 92% of the world’s rice is produced

and consumed (IRRI, 1997). More than 75% of the world’s rice supply comes from 79

million ha of irrigated land in Asia. Thus, the present and future food security of Asia

depends largely on the irrigated rice production system. This system is a major user of fresh

water. In Asia, irrigated agriculture accounts for 90% of total diverted fresh water, and

more than 50% of this is used to irrigate rice. The available amount of water for irrigation,

however, is increasingly getting scarce (Gleick, 1993; Guerra et al., 1998). The reasons are

diverse and location-specific, but include decreasing resources (e.g. falling groundwater

tables, silting of reservoirs), decreasing quality (e.g. chemical pollution, salinization) and

increased competition from other sectors such as urban and industrial users. Since demand

for rice is still rising because of continuing population growth in Asia, there is a need to

‘‘grow more rice with less water’’ (Guerra et al., 1998).

In the Philippines, some 61% of the 3.4 million ha of rice land is under irrigation, with the

majority of the production coming from the rice bowl in central Luzon (IRRI, 1997).

Irrigation is provided by gravity systems (both run-of-the-river as well as reservoir systems)

and by shallow and deep tubewells. However, the availability of water has been threatened

during the last decade(s): water from the Angat reservoir in Bulacan Province is increasingly

diverted toward Greater Manila at the expense of irrigation (Bhuiyan and Tabbal, as

referenced in Pingali et al., 1997, pp. 196–197); water in the Agno River in Pangasinan

Province is polluted with sediments and chemicals from mining activities upstream, which

make water quality less and less suitable for irrigation (Castaneda and Bhuiyan, 1993); many

irrigation systems were destroyed and clogged by the earthquakes of 1990 and the Mount

Pinatubo eruption in 1991 (NIA, 1996); and, lately, farmers using pumps for irrigation

confront increasing fuel prices. The government of the Philippines, through its National

Irrigation Administration (NIA), is dedicated to maintaining irrigation water availability by

the propagation of water-saving irrigation technologies. Though water can be saved at

different spatial scale levels, from field to irrigation system to watershed, a fundamental

approach is to start at the field level where water and rice interact and where individual

farmers can manage their water. In support of NIA’s efforts, the International Rice Research

Institute (IRRI) and the Philippine Rice Research Institute (PhilRice) experimented since the

late 80s with different farm-level technologies to save water and increase water productivity.

An important aspect of the study was the practical feasibility of the technologies under

farmers’ conditions in existing irrigation schemes. This paper reports on the results of this

study. First, we describe the traditional manner of rice production in central Luzon and the

major water flows in and out of the field. Next, strategies for reducing water input at the field

level are explained, followed by a description of the experiments. Results are presented and

discussed in terms of yield, water input and water productivity. At the end, conclusions are

drawn and the field-level results are placed in a (spatially) broader context.

2. Rice and water input

Most irrigated rice in central Luzon is, as in most of Asia (De Datta, 1981), raised in a

seedbed and then transplanted into a main field (Systems A and B in Fig. 1). Land

94 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

preparation of the main field consists of soaking, plowing and puddling (i.e. harrowing

under shallow submerged conditions). Puddling is mainly done for weed control, but also

increases water retention and reduces soil permeability, and eases field leveling and

transplanting (De Datta, 1981). After land preparation, the main crop growth period runs

from transplanting to harvest.

Water for transplanted rice is needed for land preparation and to match several outflows

during crop growth (Fig. 2). Soaking is a one-time operation and requires water to bring the

topsoil to saturation and to create a ponded water layer. Seepage (S) is the lateral subsurface

flow of water and percolation (P) is the down flow of water below the root zone. Both S and

P occur during land preparation and the crop growth period, and are governed by the water

head (depth of ponded water) on the field and the resistance to water movement in the soil.

Because they are difficult to separate in the field, S and P are often taken together as one

term, i.e. SP. Typical SP values for irrigated rice vary from 1–5 mm per day in heavy clay

soils to 25–30 mm per day in sandy and sandy loam soils (Bouman and Tuong, 2001).

Fig. 1. Schematic presentation of rice growth under four establishment systems: transplanting with seedbed in

main field (A), transplanting with separate seedbed (B), direct wet seeding (C) and direct dry seeding (D).

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 95

Fig. 2. Components of the water balance of a flooded, puddled rice field.

96

D.F

.Ta

bb

al

eta

l./Ag

ricultu

ral

Wa

terM

an

ag

emen

t5

6(2

00

2)

93

–1

12

Evaporation (E) occurs from the ponded water layer or from the surface of the soil and

transpiration (T) is the process by which plants take up water from the soil and release it

into the air as vapor. Only T is considered as productive water use since it leads directly to

crop growth and production. Again, since E and T are difficult to separate in the field, they

are mostly considered together as evapotranspiration (ET). However, only E takes place

during land preparation, whereas both E and Toccur during crop growth. Typical ET values

of rice in the tropics are 4–5 mm per day in the wet season and 6–7 mm per day in the dry

season (De Datta, 1981). Finally, over-bund flow (or surface runoff) is the spillover when

water depths rise above the paddy bunds.

Total water requirements of transplanted rice depend on the sizes of the outflows and on

the duration of land preparation and crop growth. For a 100-day crop growth duration of a

modern short-duration variety, total ET flows are about 400–500 mm in the wet season and

600–700 mm in the dry season, and total SP flows about 100–500 mm in heavy clayey soils

and 2500–3000 mm in sandy loam or loamy sand soils. With a short turnover time between

land soaking and transplanting, water requirements for land preparation can be as low as

100–150 mm (De Datta, 1981). In large-scale irrigation systems, however, turnover time

can be quite high. For instance, in the largest surface irrigation scheme in central Luzon,

called Upper Pampanga River integrated irrigation system (UPRIIS), it took up to 63 days

in a contiguous 145 ha block from the first day of water delivery for land preparation until

the whole area was completely transplanted (IRRI, 1978). The total amount of water input

during that time was some 940 mm, of which 110 mm was used for soaking, 225 mm

disappeared as surface runoff, 445 mm was lost by SP and 160 mm was lost by evaporation.

In UPRIIS, farmers raise seedlings in part of their main field. Because of a lack of tertiary

field channels, the whole main field is soaked when the seedbed is prepared and remains

flooded during the entire duration of the seedbed (System A in Fig. 1).

3. Strategies to reduce water input

At the field level, large reductions in water input can potentially be realized by reducing

the seepage and percolation flows and by minimizing land preparation time. Seepage and

percolation rates can be reduced by changed water management. Instead of keeping the rice

field continuously flooded with 5–10 cm of water, the floodwater depth is decreased to keep

the soil around saturation (called ‘‘saturated soil culture’’), irrigation is applied a few days

after water has disappeared from the surface (called ‘‘intermittent irrigation’’), or alternate

‘‘standing water/saturated soil/drying of soil’’ regimes are imposed (see Fig. 3 for

examples). With such water-saving irrigation techniques, the pressure head of water in

the field is reduced, which leads to reduced SP rates depending on the hydraulic properties

of the puddled layer and of the underlying subsoil (Bouman et al., 1994).

The duration of the land preparation period can be reduced by separating the water

supply for the seedbed from that of the main field (System B in Fig. 1). Land preparation

then only needs to take place shortly before the seedlings are ready for transplanting.

However, this requires the presence of field-level infrastructure (small channels and drains)

as well as a main infrastructure tuned to the delivery of small amounts of irrigation water

during the seedbed period (Tuong, 1999). In the absence of such, as is the case in many

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 97

large-scale irrigation systems such as UPRIIS, a different establishment technique may be

a suitable alternative. In direct seeding, seeds are directly broadcast after land preparation,

thus, reducing the combined duration of land preparation and crop growth, compared with

that of land preparation, seedbed duration and subsequent crop growth in transplanted rice,

Fig. 3. Graphical presentation of the water-saving irrigation treatments of experiment 1 (see text and Table 1 for

explanation of the eight water treatments).

98 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

which has an additional duration due to the so-called transplanting shock. In direct wet

seeding, pre-germinated seeds are broadcast onto puddled fields (System C in Fig. 1),

whereas, in direct dry seeding, ungerminated seeds are sown on non-puddled dry soil

(System D in Fig. 1). Since the soil is not puddled in direct dry seeding, the seepage and

percolation losses in this system are expected to be different than in transplanted or wet

seeded rice on the same soil. Whereas direct wet seeding is increasingly practised by some

farmers because it requires less labor (De Datta, 1986; Erguiza et al., 1990), direct dry

seeding is relatively unknown in irrigated rice culture in Asia.

Whereas saturated soil culture and intermittent irrigation in transplanted rice have

received some attention in literature (e.g. Sandhu et al., 1980; Tripathi et al., 1986; Mishra

et al., 1990), the use of these techniques in combination with different crop establishment

methods has hardly been addressed. In our study, we tested and compared the effect of

water management (continuously standing water, saturated soil culture, intermittent

irrigation) in different crop establishment techniques (transplanting, direct wet seeding,

direct dry seeding) on yield, water use and water productivity of rice.

4. Method and materials

A total of five experiments was carried out in farmers’ fields at Munoz (158450N;

1208560E), Talavera (15836014.0300N; 120857032.500E), San Jose City (15842056.200N;

120857054.300E) and Guimba (15838040.600N; 120845043.200E), all in the province of Nueva

Ecija, central Luzon, Philippines. Besides water management and crop establishment

treatments, some experiments included extra treatments, such as mid-season drainage in

experiments 1 and 4, drought stress in experiment 2 and weed control in experiment 5 (see

below). All management aspects that were not part of the treatments (e.g. fertilizer use,

seeding rate, pest control, etc.) followed local farmers’ practices. Mean fertilizer N use in the

area is 100 kg ha�1 in the dry season and 80 kg ha�1 in the wet season, both with a standard

deviation of 50 kg ha�1 (Bouman et al., 2002). In our experiments, we followed local farmer

recommendations, resulting in slightly different application rates per location (see below).

The sites at Munoz, Talavera and San Jose were within the UPRIIS, and the site in

Guimba bordered it on the west. In Guimba, shallow tubewells (i.e. down to about 15 m)

are privately owned and operated by farmers. UPRIIS covers about 102,000 ha and gets its

water from a combination of run-of-the-river flows and the Pantabangan reservoir with a

storage capacity of nearly 3 billion cubic meters. It is owned and operated by the National

Irrigation Administration (NIA). The elevation of the area varies from 20 to 80 m asl. The

climate is characterized by two pronounced seasons, dry from November to April and wet

for the rest of the year. The average annual rainfall, calculated over 1974–1994 using

rainfall data collected at the PAGASA (Philippine Atmospheric, Geophysical and Astro-

nomical Services Administration) in Munoz, is about 1900 mm, of which some 90% falls

in the wet season. Soils are Vertisols, Entisols and Inceptisols (FAO: Vertisols, Gleysols,

Regosols and Fluvisols), and have typically silty clay, silty clay loam, clay loam, and clay

textures. Seepage and percolation rates vary between 0.2 and 20 mm per day (Wickham

and Singh, 1978; Tabbal et al., 1992). The average groundwater table depth is about 0.5 m

in the rainy season and 1.5 m at the end of the dry season, although locally it can come up to

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 99

0.1 m and go down to deeper than 5 m. Double cropping of rice is the most common land

use but where water is scarce, upland crops such as onion, tomato and maize are grown in

the dry season (such as at the Guimba site). In the dry season, rice is fully irrigated, whereas

in the wet season irrigation is only applied supplementary to rainfall. Average irrigated rice

yield is 2–3.5 t ha�1 in the wet season and 4–6 t ha�1 in the dry season, while high yields

are around 5–6 t ha�1 in the wet season and 8–9 t ha�1 in the dry season.

Experiment 1: Transplanted rice was grown in two farmers’ fields at Guimba in four

consecutive dry seasons of 1988–1991. The farms were located at the high end of a 50 ha

command area serviced by a deep tubewell. However, since the water from this deep well

was not adequate, some farmers had installed shallow tubewells as well. In 1988, the

treatments consisted of eight water regimes (Fig. 3): (1) continuous standing water (2–5 cm

depth); (2) standing water until panicle initiation (PI), saturated soil thereafter; (3)

continuous saturated soil; (4) application of irrigation water (up to 5–7 cm depth) every

8 days; (5) saturated soil until PI, standing water during the reproductive stage, then

saturated soil during the ripening stage; (6) continuous standing water, but with a 1-week

drying period at maximum tillering called mid-season drainage; (7) same as treatment 4

until PI, saturated soil thereafter; (8) application of irrigation water (up to 5–7 cm depth)

every 10 days. The last two water regimes were discarded in 1989 and 1990, and in 1991

only the first four water regimes were included. Treatment 4, however, was modified to

intermittent irrigation: irrigation was applied one day after the disappearance of standing

water. Mid-season drainage (treatment 6) was included because this was observed to

increase yields in Japan and China (De Datta, 1981; Mao Zhi, personal communication,

1999). Reasons cited for increased yields are the control of unproductive tillers, the

prevention of root-rot disease, the removal of anaerobic toxins and CO2, reduced lodging

because of better root anchorage and insect control. In all plots, treatments were started

about five days after planting and drained 15 days before harvest. Water depth in the plots

was controlled by opening and closing holes in the bunds. All experiments were laid out in a

randomized complete block design with three replications. The size of the plots was about

10 m � 12:5 m (average farmers’ plot size in the area). The rice variety used was IR66 in

1988–1989 and IR72 in 1990–1991. The fields had silty clay loam soils. Polyethylene

plastic sheets were installed down to 60–70 cm depth in the center of the bunds (30 cm

wide) to avoid shallow seepage of water between plots. According to farmer’s recommen-

dations, all plots received 100 kg N ha�1, 50% as basal application and 50% topdressed at

panicle initiation. Amount of 30 kg P ha�1 and 30 kg K ha�1 were applied basally.

Experiment 2: In the 1991 dry season, transplanted and wet-seeded rice was grown in

farmers’ fields at Munoz. Besides treatments aimed at saving water, extra treatments were

included to test rice sensitivity to prolonged periods of no standing water. The six

treatments were as follows: (1) continuous 5–7 cm standing water; (2) continuous saturated

soil; (3) standing water but withholding water for 10 days in the vegetative period, starting

with saturated soil at 30 days after sowing (DAS) in the wet-seeded plot and 9 days after

transplanting (DAT) in the transplanted plot; (4) the same as treatment 3, but withholding

water for 20 days; (5) withholding water for 10 days in the reproductive period, starting

with saturated soil at 50 DAS in the wet-seeded plot and 29 DAT in the transplanted plot;

(6) the same as treatment 5, but withholding water for 20 days. The experiment was laid out

in a split plot in a randomized complete block design with four replicates, with water

100 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

regime in the main plot and crop establishment methods in the split plot. The average size

of each split plot was 8 m � 10 m. The soil type was clayey. Plastic lining in the bunds was

45 cm deep to prevent lateral flow of water between the plots. The variety used was IR64.

The rate of fertilizer application was 120 kg N ha�1, 40 kg P ha�1, and 40 kg K ha�1. One-

third of the total N plus all the P and K were applied 30 DAS in the direct-seeded plots and

10 DAT in the transplanted plots. The remaining amount of N was applied as 50% at

maximum tillering and 50% at panicle initiation.

Experiment 3: In the 1993 dry season, the four water-saving irrigation treatments of

experiment 1 in 1991 were further evaluated in both transplanted and wet-seeded rice in

farmers’ fields at Talavera. The experiment was laid out as a split plot in a randomized

complete block design in three replicates, with the irrigation treatments in the main plot

and the crop establishment methods in the split plot (15 m � 10 m). Like in experiment 1,

plastic sheets were installed to a depth of 60–70 cm at the center of the bunds between the

split plots. In addition, drainage ditches were constructed to facilitate the removal of excess

water from each of the split plots and to prevent surface and shallow subsurface inflow of

water from neighboring fields surrounding the experiment. The fields had a clay loam soil.

The rice variety used was PSBRc10. All plots received 120 kg N ha�1, of which 33% was

applied 10 days after seeding or transplanting, 33% topdressed at maximum tillering and

33% applied at panicle initiation. Also, 40 kg P ha�1 and 40 kg K ha�1 were applied with

the basal N. The water treatments were started about 10 days after seeding and 5 DAT in the

direct wet-seeded and transplanted rice, respectively.

Experiment 4: In the 1997 dry season, direct wet-seeded was grown in farmers’ fields at

San Jose. Three treatments were laid out in a randomized complete block design in four

replicates: 1: continuous standing water (2–5 cm); (2) continuous saturated soil with mid-

season drainage; (3) continuous saturated soil. The average plot size was 8 m � 12 m and

the upper 30 cm of soil had textures from silty clay to clay loam. No plastic sheets were

installed but bunds were constructed and sealed between the plots. Perimeter drainage

ditches were constructed to remove excess water from each plot and to intercept water from

adjacent fields. Fertilizer input was relatively high with 170 kg N ha�1 and 90 kg ha�1 of

both P and K. The N was applied at 40 kg at 10 days after emergence (DAE), 45 kg at 25

DAE, 55 kg at 35 DAE (which was about 10 days before panicle initiation) and 30 kg at

booting. One-half of the P and K was applied basally and the other half at 25 DAE for P and

35 DAE for K. The mid-season drainage entailed draining the field at maximum tillering

and withholding water until hairline cracks developed in the soil.

Experiment 5: During two consecutive wet seasons of 1996–1997, direct dry-seeded rice

was grown in farmers’ fields at San Jose. Besides two irrigation regimes, two weed control

treatments were established in a randomized complete block design: (1a) continuous

standing water (2–5 cm depth) with chemical weed control; (1b) continuous standing water

(2–5 cm) with chemical weed control plus hand weeding; (2a) continuous saturated soil

with chemical weed control; (2b) continuous saturated soil with chemical weed control and

hand weeding. In 1996, the experiments were conducted on three farms with four replicates

each, and in 1997 on two farms with four replicates each. The average plot size was

8 m � 12 m. Since direct dry seeding was completely new to the area, no farmer practices

existed for fertilizer input. To ensure that nutrients were not limiting crop growth, high

fertilizer N rates were applied of 160 kg ha�1 in four splits in 1996, and 170 kg ha�1 in four

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 101

splits in treatments 1a/b and 2a, and 250 kg ha�1 in five splits in treatment 2b in 1997. The

high N dose in treatment 2b was meant to test whether N was really not a growth-limiting

factor. The P and K were applied basally at 90 kg ha�1 each.

4.1. Measurements and data processing

Rainfall and evaporation were measured at all sites using rain gauges and class A pans,

respectively. Sloping gauges were installed in the plots to measure (daily) standing water

depth. The irrigation water input was measured using 908 V-notch weirs starting from

transplanting or direct seeding. In experiment 2, irrigation water input was also measured

during the period of land preparation. Yields were determined by crop-cut sampling in two

diagonally opposite corners of each plot using a 1 m � 1 m sampling frame in experiment 2

and a 2 m � 2:5 m sampling frame in the other experiments. Ground water depths (perched

water tables) were measured through plastic tubes installed at the sites. In 1997 in

experiment 5, crop cut samples were collected at harvest from 10 farms surrounding

the experiment (five practising wet-seeding and five transplanting). Reported grain yield

refers to weight of rough rice corrected to 14% moisture content. Water input was

calculated as the sum of irrigation water and rainfall from transplanting to harvest in

transplanted rice and from sowing to harvest in direct-seeded rice. In all dry-season

experiments (nos. 1–4), rainfall was low and was completely intercepted and stored in

the fields (no runoff). In the wet seasons of experiment 5, rainfall was relatively high. In the

standing water treatments, rainfall was effectively stored in the bunded fields. In the

saturated soil treatments, however, spillways in the bunds were at ground level and rainfall

in excess of the soil storage capacity was lost as surface runoff. Because runoff was not

separately measured, it is included in the calculation of total water input. Water produc-

tivity was calculated as the ratio of grain yield (in g) over total water input (in kg). Seepage

and percolation rates were calculated from the daily observed standing water depths in the

plots with continuous standing water, corrected for evapotranspiration and any rainfall. The

evapotranspiration was calculated from the measured pan evaporation using the equations

for lowland rice derived by Kampen (1970).

5. Results

Grain yield, water input and water productivity are given in Tables 1–5. In experiment 5,

no differences between the weed control methods were observed and therefore the data are

lumped per water treatment.

5.1. Conventional water management

5.1.1. Yield

Transplanted and direct wet-seeded rice yields with conventional standing water

management (control treatments) were relatively high with 7–8.4 t ha�1 in the dry seasons

of experiments 2–4 but relatively low with 4.9–5.8 t ha�1 in the dry seasons of experiment

1. The relatively low yield in experiment 1 can be partly explained by the lower level of

102 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

Table 1

Yield, water input and water productivity in transplanted rice, experiment 1, Guimba, 1988–91

Treatmenta Yieldb

(kg ha�1)

Water

inputb,c (mm)

Water productivity

(g grain kg�1 water)

1988 dry season

1 5010 a 2197 a 0.23

2 4860 ab 1059 bc 0.46

3 4590 ab 914 bc 0.50

4 4040 b 880 c 0.46

5 5200 a 1693 ab 0.31

6 5190 a 2187 a 0.24

7 4540 ab 874 c 0.52

8 4000 b 870 c 0.46

Mean 4679 1334 0.40

1989 dry season

1 5840 a 1679 a 0.35

2 5650 a 1266 ab 0.45

3 5630 a 1164 ab 0.48

4 4300 b 700 b 0.61

5 5630 a 1485 a 0.38

6 5500 a 1713 a 0.32

Mean 5425 1335 0.43

1990 dry season

1 5325 a 2028 bc 0.26

2 5100 a 1352 abc 0.38

3 4800 ab 1227 ab 0.39

4 4200 b 912 a 0.46

5 4800 ab 2195 c 0.22

6 5300 a 1802 ac 0.29

Mean 4917 1586 0.33

1991 dry season

1 4903 a 3504 a 0.14

2 4587 a 2252 b 0.20

3 3597 b 2053 b 0.18

4 3313 b 1126 c 0.29

Mean 4100 2165 0.20

a 1: continuous standing water (2–5 cm depth); 2: standing water until panicle initiation (PI), saturated soil

thereafter; 3: continuous saturated soil; 4: application of irrigation water (up to 5–7 cm depth) every 8 days; 5:

saturated soil until PI, standing water during the reproductive stage, then saturated soil during the ripening stage;

6: continuous standing water, but with a 1-week drying period at maximum tillering called mid-season drainage;

7: same as treatment 4 until PI, saturated soil thereafter; 8: application of irrigation water (up to 5–7 cm depth)

every 10 days.b In a column, means followed by a common letter are not significantly different at 0.05 level.c The seasonal rainfall was 0 in 1988, 94 mm in 1989, 140 mm in 1990 and 99 mm in 1991.

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 103

fertilizer application and by a smaller number of splits in the N application. However, these

low yields in experiment 1 are comparable to the yields (4.3–5.0 t ha�1) obtained from

previous (1984–1985) experiments within the same area (Gines et al., 1988).

The average yields of direct dry-seeded rice with continuous standing water in the wet

season was relatively high with 4.3 in 1996 and 4.7 t ha�1 in 1997. For comparison, the

Table 2

Yield, water input and water productivity in transplanted and wet-seeded rice, experiment 2, Munoz, 1991 dry

season

Treatmenta Transplanted Wet-seeded

Yieldb

(kg ha�1)

Water

inputb,c

(mm)

Water productivity

(g grain kg�1 water)

Yieldb

(kg ha�1)

Water

inputb,c

(mm)

Water productivity

(g grain kg�1 water)

1 7357 a 694 a 1.06 7585 a 631 a 1.20

2 6743 b 373 c 1.81 7338 a 324 c 2.27

3 6297 c 673 a 0.94 7007 b 602 a 1.16

4 5303 d 637 ab 0.83 6075 d 589 a 1.03

5 6038 c 571 b 1.06 6397 c 492 b 1.30

6 4204 e 568 b 0.74 5210 e 476 b 1.09

Mean 5990 586 1.07 6602 519 1.34

a 1: continuous 5–7 cm depth standing water; 2: continuous saturated soil; 3: standing water but withholding

water for 10 days in the vegetative period, starting with saturated soil conditions at 30 DAS in the wet-seeded

plot and 9 DAT in the transplanted plot; 4: the same as treatment 3, but withholding water for 20 days; 5:

withholding water for 10 days in the reproductive period, starting with saturated soil conditions at 50 DAS in

the wet-seeded plot and 29 DAT in the transplanted plot; 6: the same as treatment 5, but withholding water for

20 days.b In a column, means followed by a common letter are not significantly different at 0.05 level.c Rainfall during the experiment was 13 mm.

Table 3

Yield, water input and water productivity in transplanted and wet-seeded rice, experiment 3, Talavera, 1993 dry

season

Treatmenta Transplanted Wet-seeded

Yieldb

(kg ha�1)

Water

inputb,c

(mm)

Water productivity

(g grain kg�1 water)

Yieldb

(kg ha�1)

Water

inputb,c

(mm)

Water productivity

(g grain kg�1 water)

1 6987 a 728 a 0.96 8173 a 577 a 1.42

2 6200 a 491 b 1.26 8441 a 456 a 1.85

3 6626 a 477 b 1.39 7410 a 391 a 1.90

4 6135 a 600 ab 1.02 8199 a 456 a 1.80

Mean 6487 574 1.16 8056 470 1.74

a 1: continuous standing water (2–5 cm depth); 2: standing water until panicle initiation, saturated soil

thereafter; 3: continuous saturated soil; 4: application of irrigation water (up to 5–7 cm depth) one day after

standing water has disappeared.b In a column, means followed by a common letter are not significantly different at 0.05 level.c Rainfall during the experiment was 20 mm.

104 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

average yield of the five wet-seeded fields of surrounding farms in the 1997 wet season was

3.8 t ha�1 and that of the five transplanted fields 4.1 t ha�1. Despite the high level of N, P

and K fertilizer and the large number of splits in the N application, yields were

considerably lower than in the dry season experiments. This is because yield potentials

are lower in the wet season than in the dry season because of less solar radiation in the wet

season (more clouds). Moreover, more rains and typhoons occur in the wet than in the dry

season, particularly during the pre and post flowering periods.

5.1.2. Water use

Total water input in the puddled transplanted and direct wet-seeded control treatments

varied much among experiments: 1679–3504 mm in experiment 1, 631–694 mm in

experiment 2, 577–728 mm in experiment 3, and 2875 mm in experiment 4. Differences

among experiments are due to differences in soil hydrological properties, groundwater

Table 4

Yield, water input and water productivity in wet-seeded rice, experiment 4, San Jose City, 1997 dry season

Treatmenta Yieldb

(kg ha�1)

Water

inputb,c (mm)

Water productivity

(g grain kg�1 water)

1 8432 a 2874 a 0.29

2 7811 a 1305 b 0.60

3 7843 a 1516 b 0.52

Mean 8029 1898 0.47

a 1: continuous standing water (2–5 cm); 2: continuous saturated soil with mid-season drainage; 3:

continuous saturated soil.b In a column, means followed by a common letter are not significantly different at 0.05 level.c Rainfall during the experiment was 0 mm.

Table 5

Yield, water input and water productivity in dry-seeded rice, experiment 5, San Jose City, 1996–1997

Treatmenta Yieldb

(kg ha�1)

Water inputb,c (mm) Water productivity

(g grain kg�1 water)

Irrigation þrainfall

Irrigation Irrigation þrainfall

Irrigation

1996 wet season

1 4338 a 1417 a (38%) 531 0.31 8.16

2 4172 a 1330 a (33%) 432 0.32 9.65

Mean 4255 1373 482 0.32 8.91

1997 wet season

1 4696 a 1920 a (49%) 941 0.25 4.99

2 4546 a 1269 a (28%) 355 0.36 12.81

Mean 4621 1594 648 0.30 8.90

a 1: continuous standing water; 2: continuous saturated soil.b In a column, means followed by a common letter are not significantly different at 0.05 level.c Values in parentheses indicate the percentage of irrigation water of total water input.

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 105

table depths, drainage and hydraulic isolation of the experiment fields. Compared with the

other sites, the fields of experiment 1 had a relatively high elevation, were mostly

surrounded by fallow fields or non-rice crops, and had a lighter soil texture and deeper

groundwater depth. All these factors favored high percolation rates (Table 6). Although

shallow subsurface seepage was blocked by the plastic sheets, the high percolation rates

caused high water inputs. In 1991, the deep well in the area stopped operating and no rice

was planted in the area surrounding our field experiment. As a consequence, there was no

groundwater recharge by percolation from paddy fields and the groundwater table fell

quickly from its shallow level of about 0.7 cm at the start of the dry season to the 2 m at the

end of the season. Therefore, the water use and seasonal-mean percolation rate were higher

in 1991 than in the other years. The fields of experiments 2 and 3 were in the midst of a

fully irrigated rice area. The continuous percolation of water from the paddy fields raised

the groundwater table to shallow levels of about 10–20 cm depth on average during the

growing season. With such shallow (perched) groundwater tables, percolation rates are

very low and the roots of rice extract water for transpiration from the groundwater, thus

lessening the demand for irrigation water. In addition, the plastic sheets prevented most of

the shallow seepage flow. Thus, total water inputs were relatively low. In experiment 4,

water inputs were relatively high again although the surrounding conditions and ground-

water table depths were comparable with those of experiments 2–3. In experiment 4, there

were no plastic sheets to block seepage flow and the peripheral ditch drained water quickly

from the plots, resulting in relatively large seepage losses (Table 6).

The total water input in the continuous standing water treatment in non-puddled soil in

experiment 5 was at an intermediate-high level with 1417–1919 mm (Table 5). Because of

high rainfall in the rainy season, the irrigation water input was only 531 mm in 1996 and

941 mm in 1997. The texture of the soil was heavy enough to keep seepage and percolation

rates low and to allow the ponding of water at the surface.

5.1.3. Water productivity

In the dry seasons, water productivity of the transplanted and direct wet-seeded control

treatments was low with 0.14–0.35 g grain kg�1 water in experiment 1 and 0.29 in

experiment 4, and high with 1.06–1.20 in experiment 2 and 0.96–1.42 in experiment 3.

The relatively high water productivities in experiments 2 and 3 were caused by the

very low water inputs and the high yields. In the wet season, the water productivity of

direct dry-seeded rice with continuous standing water in experiment 5 was low with

0.25–0.31 g grain kg�1 water, caused by low yield levels and relatively high water inputs.

The productivity of the irrigation water alone, however, was high with 5–8 g kg�1.

5.2. Water-saving technologies

5.2.1. Saturated soil culture

Water-saving irrigation techniques that kept the soil around saturation during part of or

the entire growing season generally had lower yields than the control treatments with

2–7 cm of standing water, though in most cases the differences were not statistically

significant. This trend is the same in transplanted as well as in direct wet-seeded and direct

dry-seeded rice. Keeping the soil continuously around saturation reduced yield from the

106 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

Table 6

Characterization and comparison of water inputs in the continuous standing water (CSW) and the continuous saturated soil (CSS) treatments in experiments 1–4a

Experiment

no.

Site Year Methods of crop

establishmentbWater inputc (mm) Water savedd in

CSS over CSW

SPe in CSW

treatments

per plots

Seasonal perched

water table depth

in the CSS plots

Soil type of the upper

30 cm depthf

CSW CSS mm % mm d�1 m %C Class

1 Guimba-1 1988 TPR 2197 914 1283 58 18 0.5–2.5 34 SiCL

1 Guimba-1 1989 TPR 1679 1164 515 31 16 0.5–2.5 34 SiCL

1 Guimba-2 1990 TPR 2028 1227 801 40 18 0.7–2.0 33 SiCL

1 Guimba-2 1991 TPR 3504 2053 1451 41 24 0.7–2.0 33 SiCL

2 Munoz 1991 TPR 694 373 321 46 2 0.0–0.8 64 C

2 Munoz 1991 WSR 631 324 307 49 2 0.0–0.8 64 C

3 Talavera 1993 TPR 728 477 251 35 3 0.0–0.5 38 CL

3 Talavera 1993 WSR 577 391 186 32 3 0.0–0.5 38 CL

4 San Jose 1997 WSR 2875 1516 1358 47 21 0.0–0.6 41 SiCL

a Central Luzon, Philippines. 1988–1997 dry seasons.b TPR: transplanted rice; WSR: direct wet seeded rice.c Water inputs includes rainfall and irrigation water applied during the crop growth period only, does not include water input for land preparation.d Water saved ð%Þ ¼ ½ðwater input in CSW treatment � water input in CSS treatmentÞ/water input in CSW treatment � 100.e SP: seepage and percolation rate.f %C: percentage clay; SiCL: silty clay loam; C: clay; CL: clay loam.

D.F

.Ta

bb

al

eta

l./Ag

ricultu

ral

Wa

terM

an

ag

emen

t5

6(2

00

2)

93

–1

12

10

7

standing water control on average by 7% for the dry-season transplanted and wet-seeded

rice and by 3% for the wet-season dry-seeded rice. Where yield reductions were relatively

small and statistically mostly not significant, reductions in water input were relatively large

and statistically significant. On average, saturated soil treatments reduced water inputs by

31–58% in transplanted and 32–49% in direct wet-seeded rice (Table 6), and by 6–34% in

direct dry-seeded rice (Table 5). Variation in water reduction among experiments was

caused not only by differences in hydraulic properties of the soil and the terrain but also by

rainfall distribution. In the wet seasons of experiment 5, more than half of the total water

input was by rainfall over which we had no control. Although not measured, we observed

that considerable amounts of rainfall were lost as surface runoff. Computed for irriga-

tion water input only, saturated soil treatments reduced water inputs by 19–62% from

the continuous flooded treatments. The relatively large reductions in water input

combined with the slight reductions in grain yield led to large increases of 29–118%

in water productivities in the saturated-soil treatments over the standing-water controls

(calculated over all experiments). Only in the direct dry-seeded crop in the wet season of

1996 was the water productivity the same under both water treatments. This was caused

by the relatively large contribution (about 60–70%) of uncontrollable rainfall to total

water input.

5.2.2. Intermittent irrigation

In experiment 1, with the relatively deep groundwater table, intermittent irrigation every

8 or 10 days (treatments 4 and 8; Table 1) reduced the yield on average by 25% of that of the

control value and the average water input by 60% of that of the control. Water productivity,

therefore, nearly doubled the control value. In experiment 3, with the high perched

groundwater table, yield in the intermittent irrigation treatment (treatment 4; Table 3)

declined only 12% from that of the control in the transplanted plots, whereas no yield

reduction occurred in the wet-seeded plots (see ‘‘Section 5.3’’). Even without ponded

water, the rice crop could extract water from the shallow perched water table to satisfy most

of its transpiration demands. Since percolation rates were low to start with, water input

decreased by only 19% from the control value in both establishment methods.

The imposition of drought periods by withholding irrigation water for 10 or 20

consecutive days in experiment 2 (treatments 3–6; Table 2) reduced yields severely.

However, with only 10 days of drought, water input also decreased to such an extent that

water productivity was nearly the same as with continuous flooding (treatments 3 and 5).

With 20 days of continuous drought, yields declined so much that even water productivities

declined to 82% of the value under continuous flooding (treatments 5 and 6). Mid-season

drainage by withholding water for 1 week at mid-tillering in otherwise flooded (treatment 6

in experiment 1) or saturated-soil (treatment 2 of experiment 4) conditions had hardly any

effect on either yield or water productivity.

5.3. Crop establishment

5.3.1. Direct wet seeding

Wet-seeded rice outyielded transplanted rice in the continuous standing-water control

treatments by 3% in experiment 2 and by 17% in experiment 3. Moreover, the highest

108 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

recorded yield was 8.4 t ha�1 for wet-seeded rice with continuous standing water in

experiment 4 (Table 4). The differences in yield between direct wet-seeded and trans-

planted rice became larger with reduced water input: the direct wet-seeded rice outyielded

its transplanted counterpart by 6–24% in treatments 2–6 in experiment 2 (Table 2) and by

12–36% in treatments 2–4 in experiment 3 (Table 3). Over all treatments, total water input

was on average lower in direct wet-seeded rice than in transplanted rice by 11% in

experiment 2 and by 18% in experiment 3. The relatively low water input in direct wet-

seeded rice in experiment 3 was because (1) no irrigation was applied for the first 10–15

DAS, (2) standing water levels were kept low in the early part of vegetative growth so as not

to submerge the upcoming seedlings (thereby reducing seepage and percolation flows), and

(3) the crop was harvested some 10 days earlier than the transplanted crop. Thus, despite

the longer duration between sowing and harvest in the direct wet-seeded crop, compared

with the duration between transplanting and harvest in the transplanted crop, total water

input was some 100 mm less (Table 3). In experiment 2, the water used during land

preparation was 544 mm for transplanted rice but only 388 mm for direct wet-seeded rice.

The reason for this lower water use was that the duration of land preparation under farmers’

conditions in UPRIIS was longer in transplanted rice (26 days) than in direct wet-seeded

rice (8 days). In both experiments 2 and 3, the higher yields combined with the lower water

inputs of direct wet-seeded rice resulted in higher water productivities: on average, water

productivity was 38% and 50% higher in direct wet seeding than in transplanting in

experiments 2 and 3, respectively.

5.3.2. Direct dry seeding

Though there are only limited yield data for direct dry-seeded and transplanted rice in

experiment 5 to compare, direct dry seeding of rice was feasible and resulted in high wet

season yields with intermediate values of water input (Table 5). The average direct dry-

seeded yield in 1997 was 4.6 t ha�1 compared with 3.9 t ha�1 for 10 surrounding farms

practising direct wet seeding and transplanting. Water productivities of direct dry-seeded

rice under continuous standing water conditions in the wet season were comparable with

values realized for transplanted or wet-seeded rice in the dry season of experiments 1 and 4.

6. Conclusions and discussion

Water-saving technologies could successfully be implemented in farmers’ fields under

shallow pump irrigation and in the large-scale surface irrigation scheme UPRIIS under the

prevailing water delivery schedules. Although, in most experiments care was taken to try to

hydraulically separate the experimental plots from their surroundings (by using plastic

sheets), lateral subsurface flow could not be fully prevented when groundwater tables were

shallow, and this may have affected the results of the water-saving irrigation treatments.

However, these are typically the conditions farmers actually experience in irrigated

lowlands and our results are representative for such situations. Saturated soil culture

reduced water inputs substantially from traditional continuous flooding without signifi-

cantly decreasing yield. Keeping the soil continuously around saturation resulted on

average in a 5% yield reduction, 35% water input reduction and 45% increase in water

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 109

productivity. Intermittent irrigation can further reduce water inputs but mostly at the

expense of increased yield loss. The depth of the (perched) groundwater table determined

to a large extent the amount of total water input and the magnitude of water reductions and

yield losses under water-saving irrigation practices. With shallow water tables (10–20 cm

below the surface), percolation rates were low and water inputs small (600–700 mm).

Potential water savings were small as well, but the risk of yield loss was low since the crop

could extract water for transpiration from the groundwater. With deeper groundwater

tables, percolation rates and water inputs were larger (up to 3500 mm), and potential water

savings and yield losses higher. In our whole study area, groundwater tables generally

varied from 0.1 to more than 2 m below the surface, depending on topography, nearness to

drains, creeks and irrigation canals, and the surrounding land use. The opportunities and

optimum water management practices for water saving are, therefore, very site-specific.

Combined crop growth and water balance modeling can help to extrapolate experimental

results and establish recommendations based on soil hydrological and environmental

properties (e.g. Wopereis et al., 1994; Bouman et al., 1994).

Direct dry or wet seeding can be a means to save water during land preparation in

irrigation schemes with a lack of tertiary field structures (channels) to separate the seedbed

from the main field, as in UPRIIS. In our study, wet-seeded rice yields were 3–17% higher

than transplanted rice yields under continuously flooded conditions while having 11–18%

less water input during the crop growth period. Our higher yields of direct wet-seeded rice

contrast with those of Garcia et al. (1995) and Dingkuhn et al. (1991), who concluded that,

with adequate N and water supply, yield potentials of transplanted and wet-seeded rice

were comparable. Under water-saving irrigation and with prolonged periods with no

standing water, wet-seeded rice outyielded transplanted rice in our experiments by 6–36%.

Sanchez (1973) and Bhuiyan et al. (1995) reported that with decreasing water input, direct

wet-seeded rice can extract more water from the soil profile because of a better and more

deeply developed root system than transplanted rice. Direct wet-seeded rice is a suitable

establishment method under water-saving irrigation techniques or when water delivery is

erratic to maintain relatively high yield levels.

Direct dry seeded rice yielded the same as transplanted and wet-seeded rice in the wet

season with comparable water input during the crop growth period. In general, direct dry

seeding can reduce water input in the rainy season by advancing the growth period and

taking better advantage of the early rains. It eliminates the need for irrigation water during

land preparation because the field is prepared under dry soil conditions. Instead of waiting

for reservoirs and canals to be filled (for transplanting or direct wet seeding), direct dry

seeding can be done at the onset of the rains that can be used to support germination and

early crop growth (Lantican et al., 1999; Tuong, 1999). Moreover, water saved in a

reservoir during the wet season can be used for irrigation in the subsequent dry season.

However, since the soil is not puddled, percolation rates may be higher than in puddled

conditions, depending on the permeability of the top- and subsoil and the depth of the

groundwater table. Saturated soil culture in direct dry-seeded rice in the wet season does

not save water since large amounts of rainfall can be lost via surface runoff.

The adoption of water-saving technologies at the farm level will have consequences for

the hydrology and water use at larger spatial scale levels. First, water saved at the farm level

does not always mean that water is saved in the whole irrigation system. Water lost from

110 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112

individual fields by seepage and percolation will enter the surface flow system through

creeks and drains and the subsurface system through (shallow) groundwater. Both surface

and subsurface systems can be exploited downstream by water re-use. If such is the case,

field-level water savings upstream do not lead to water savings at the system level. Second,

water-saving irrigation practices at the farm level may affect the groundwater table depth.

As found in our study area, groundwater tables in extensive irrigated rice areas can be

shallow because of continuous percolation from the paddy fields. Shallow groundwater

tables reduce percolation rates. When farmers massively adopt water-saving technologies,

the groundwater recharge through percolation will be less which may lead to a drop in the

groundwater table. This can then increase the percolation rates, offsetting the gains in

water saving introduced by the water-saving irrigation technologies. More study is needed

into the relation between field-level and system hydrology to predict the large-scale and

long-term effects of the introduction of water-saving irrigation technologies at the field

level.

The experimental conditions used in our study are typical for irrigated rice farming in

Asia, and our results and conclusions can be extrapolated to other areas outside the

Philippines.

Acknowledgements

The authors acknowledge the contributions of Mr. Lucio N. Caramihan and Mr. Roberto.

Soriano in field work and data collection and Ms. Lizzel P. Llorca in data encoding,

computation and the preparation of tables and figures

References

Bhuiyan, S.I., Sattar, M.A., Khan, M.A.K., 1995. Improving water use efficiency in rice through wet seeding.

Irrig. Sci. 16, 1–8.

Bouman, B.A.M., Tuong, T.P., 2001. Field water management to save water and increase its productivity in

irrigated lowland rice. Agric. Water Manage. 49, 11–30.

Bouman, B.A.M., Castaneda, A., Bhuiyan, S.I., 2002. Nitrate and pesticide contamination of groundwater under

rice-based cropping systems: evidence from the Philippines. Agriculture, Ecosystems and Environment

X-ref:S0167-8809(01)00297-3 (in press).

Bouman, B.A.M., Wopereis, M.C.S., Kropff, M.J., ten Berge, H.F.M., Tuong, T.P., 1994. Understanding the

water use efficiency of flooded rice fields. Part II: percolation and seepage losses. Agric. Water Manage. 26,

291–304.

Castaneda, A.R., Bhuiyan, S.I., 1993. Sediment pollution in a gravity irrigation system and its effects on rice

production. Agric. Ecosystems Environ. 45, 195–202.

De Datta, S.K., 1981. Principles and Practices of Rice Production. IRRI, Los Banos, Philippines. 618 p.

De Datta, S.K., 1986. Technology development and the spread of direct-seeded flooded rice in Southeast Asia.

Exp. Agric. 22, 417–426.

Dingkuhn, M., Penning de Vries, F.W.T., De Datta, S.K., van Laar, H.H., 1991. Concepts for a new plant type for

direct seeded flooded tropical rice. In: Direct seeded flooded rice in the tropics, IRRI, Los Banos,

Philippines, pp. 17–38.

Erguiza, A., Duff, B., Khan, C., 1990. Choice of rice crop establishment technique: transplanting versus wet

seeding. IRRI Research Paper Series 139. IRRI, Los Banos, Philippines, 10 p.

D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112 111

Garcia, F.V., Peng, S., Cassman, K.G., 1995. Yield potential of transplanted and wet-seeded rice in high-yield

environments in the Philippines. In: Moody, K. (Ed.), Constraints, Opportunities, and Innovations for Wet-

seeded Rice, IRRI, Los Banos, Philippines, pp. 246–256.

Gines, H.C., Moya, T.B., Pandy, R.K., Carangal, V.R., 1988. Crop diversification: problems and prospects in

partially irrigated rice-based farming systems. In: Valera, A. (Ed.) Proceedings of a National Workshop on

Crop Diversification in Irrigated Agriculture in the Philippines. International Irrigation Management

Institute, Sri Lanka, pp. 147–166.

Gleick, P.H. (Ed.), 1993. Water crisis: a guide to the world’s fresh water resources. Pacific Institute for Studies in

Development, Environment, and Security. Stockholm Environment Institute. Oxford University Press, New

York, 473 p.

Guerra, L.C., Bhuiyan, S.I., Tuong, T.P., Barker, R., 1998. Producing more rice with less water from irrigated

systems. SWIM Paper 5. IWMI/IRRI, Colombo, Sri Lanka, 24 p.

IRRI (International Rice Research Institute), 1978. Annual report for 1977. IRRI, Los Banos, Philippines. 548 p.

IRRI (International Rice Research Institute), 1997. Rice Almanac, 2nd Edition. IRRI, Los Banos, Philippines.

181 p.

Kampen, J., 1970. Water losses and water balance studies in lowland rice irrigation. PhD Thesis, Cornell

University, Ithaca, New York, 416 p.

Lantican, M.A., Lampayan, R.M., Bhuiyan, S.I., Yadav, M.K., 1999. Determinants of improving productivity of

dry-seeded rice in rainfed lowlands. Exp. Agric. 35, 127–140.

Mishra, H.S., Rathore, T.R., Pant, R.C., 1990. Effect of intermittent irrigation on groundwater table contribution,

irrigation requirements and yield of rice in mollisols of Tarai region. Agric. Water Manage. 18, 231–241.

NIA (National Irrigation Administration), 1996. Annual Report 1996. NIA, Manila, Philippines. 46 p.

Pingali, P.L., Hossain, M., Gerpacio, R.V., 1997. Asian rice bowls; the returning crisis? CAB International,

Oxon, UK (in association with IRRI, Los Banos, Philippines). 341 p.

Sanchez, P.A., 1973. Puddling tropical rice soils. Part 2: effects of water loss. Soil Sci. 115, 303–308.

Sandhu, B.S., Khera, K.L., Prihar, S.S., Singh, Baldev, 1980. Irrigation needs and yield of rice on a sandy–loam

soil as affected by continuous and intermittent submergence. Indian J. Agric. Sci. 50, 492–496.

Tabbal, D.F., Lampayan, R.M., Bhuiyan, S.I., 1992. Water-efficient irrigation technique for rice. In: Soil and

water engineering for paddy field management. AIT, Bangkok, Thailand, pp. 146–159.

Tripathi, R.P., Kushawa, H.S., Mishra, R.K., 1986. Irrigation requirements of rice under shallow water table

conditions. Agric. Water Manage. 12, 127–136.

Tuong, T.P., 1999. Productive water use in rice production: opportunities and limitations. J. Crop Production 2

(2), 241–264.

Wickham, T.H., Singh, V.P., 1978. Water movement through wet soils. In: Soils and rice. IRRI, Los Banos,

Philippines, pp. 337–358.

Wopereis, M.C.S., Bouman, B.A.M., Kropff, M.J., ten Berge, H.F.M., Maligaya, A.R., 1994. Water use

efficiency of flooded rice fields. I. validation of the soil-water balance model SAWAH. Agric. Water

Manage. 26, 277–289.

112 D.F. Tabbal et al. / Agricultural Water Management 56 (2002) 93–112