AgroNews

Introduction

As urban activities expand and use more water, Texas water policy makers are

encouraging farmers to use irrigation methods that reduce water use and increase yield

in order to allow water to flow to other economic sectors. The LCRA, a water district that

manages the Lower Colorado River, is the owner and operator of three large irrigation

districts that have developed conservative water use technologies for rice irrigation. The

LCRA has sought to increase the reliability of irrigation water for rice farming because

irrigation is the largest single water user in the river basin today.

The LCRA water conservation policies and programs reflect the guidelines adopted by

the Texas Water Commission in 1988 for water conservation. Under the Texas Water

Code, holders of water rights permits may only apply water for beneficial uses. The

Texas state law defines “beneficial use” as use of the amount of water which is

economically necessary for a purpose authorized by law, when reasonable intelligence

and reasonable diligence are used in applying the water to that purpose.2

Any use of water in excess of “a reasonable duty of water for rice irrigation”3 can be

defined as a “non-beneficial” use of water. Any non-beneficial use of water at some point

could affect the water rights of the irrigation districts. In 1988, the Texas Commission on

Environmental Quality (TCEQ) adopted a standard of 5.25 acre-foot of water per acre of

rice cultivation as an expected volume.4 Also, the Texas Water Code states that

persons supplying state water for irrigation purposes shall charge the purchaser on a

volumetric basis. The TCEQ may direct suppliers of state water to implement

appropriate procedures for determining the volume of water delivered.5

Promoting water conservation in irrigation is a high priority for the LCRA as described in

the LCRA’s water conservation policy outlined in its document: “Board Policy Statement

WFC 505.00.”6 That policy is applied in the Water Management Plan7 and is supported

by Board policy statements.8 The 2000 LCRA agricultural water conservation policy

stated that the LCRA shall:

… support and assist public and private-sector initiatives to develop,

demonstrate, and apply cultivation and irrigation practices to improve on-farm

water use efficiency.

…assist with the transfer of information and technology for improving on-farm

water use efficiency from research to the producer.

… undertake maintenance, rehabilitation and management practices to minimize

water losses from LCRA irrigation water delivery systems.9

The LCRA has conducted rice water conservation studies for over 15 years. In 1987, the

LCRA initiated the “Less Water-More Rice” project to identify ways that farmers could

conserve water without reducing yields. This research determined that yields could be

increased by as much as 17 percent and water use reduced between 25 and 30 percent

by maintaining a shallow level of water in the fields and improving the methods of water

delivery into and through fields. A conversion to a volumetric pricing system started in

1990. The system went through trial years before it was adopted at the end of 1993.

In 2001, the LCRA began three experimental seasons testing subsurface drip irrigation

for rice farming. The section below describes that effort.

The Drip Irrigation Experiment

LCRA water conservation measures adopted for rice farming during the 1980’s and

1990’s allowed rice farmers to increase yield while reducing the average per acre water

use by around 25 percent. Further water use efficiencies might enhance the reliability of

irrigation water. In 2001 the LCRA began to examine the possibility of using subsurface

drip irrigation for rice. This research was conducted in collaboration with Netafim, an

Israeli drip irrigation manufacturing company. It involved experiments conducted at the

Texas A&M University Agricultural Research and Extension Center in Beaumont, Texas.

The Texas Department of Agriculture’s Texas-Israel Exchange program partially funded

this research. The research compared water uses and yields for conventionally flooded

and subsurface drip irrigation treatments.

Flood irrigation is the conventional method for rice irrigation and generally demands

more water than other irrigation practices. For example, in one recent study of the LCRA

rice irrigation districts, farmers used around 5.2 acre-feet of water per year per acre for

flood irrigation.11 One of the reasons farmers have used flood irrigation is that it

accomplishes multiple tasks. Flooding provides water for plant growth. It controls weeds

which can thus substitute for chemical herbicide and mechanical weeding. Flooding can

substitute for mechanical land leveling.

One of the rationales for considering subsurface drip irrigation is that such a system can

save water lost in direct evaporation, runoff and seepage. A well-designed drip system

will lose little water to runoff, deep percolation or evaporation. For such a drip system,

irrigation scheduling can be precisely managed to meet crop demands. In other words,

subsurface drip irrigation holds the promise of increased crop yields and quality while

reducing water use significantly. Also, by eliminating irrigation return flow all together;

sub-surface drip irrigation may help water quality of receiving water bodies.

Flood irrigation also produces methane, a greenhouse gas, as organic material in the

soil decomposes anaerobically. Methane gas produced from rice field contributes to the

global warming as shown in Table 2. Subsurface drip irrigation provides water to the

roots of the rice plant without cutting the oxygen supply from the atmosphere to the soil,

thus reducing anaerobic decomposition of soil organic matter. As in a natural wetland,

flooding a rice field cuts off the oxygen supply from the atmosphere to the soil, which

results in anaerobic decomposition of soil organic matter. Methane is a major end

product of anaerobic decomposition. It is released from submerged soils to the

atmosphere by diffusion and through roots and stems of rice plants.12

Subsurface drip irrigation has the potential to increase the rice crop yield while reducing

evaporation, runoff and seepage water losses, methane production, improved water

quality of receiving water bodies and reducing energy consumption by reducing the

amount of water pumped. Table 3 lists potential advantages of subsurface drip irrigation.

The section below describes three years of subsurface drip irrigation experiments during

the farming seasons of 2001, 2002 and 2003. These experiments were conducted byLCRA at the Texas A&M University Agricultural Research and Extension Center at

Beaumont, Texas. For the three years of the experiment, the approach was to establish

replicates for rice farming, use different irrigation methods in these replicates, and

compare yields of rice and water use among the different replicates. Table 4

summarizes the results from the experimental years. The experiments seek to isolate

the effect of factors other than drip irrigation such as planting time, presence of weeds,

use of pesticides, etc. so the only difference between the compared plots is the irrigation

method. Two categories of irrigation methods are used: conventional flooding, which

consists of flooding the field many times per season, versus subsurface drip irrigation,

which consists of supplying water to the plant near its roots and based on the plant need

for water. The experiments considered spacing options between drip lines for the

subsurface drip irrigation method.

During the first experimental season in 2001, two plots were established, each

consisting of three different fields: one conventionally flooded field, and two other fields

irrigated with subsurface drip lines at 16 and 32-inch spacing, respectively. When

subsurface drip lines are used, they were installed at a depth of seven inches. To

eliminate water stress on the rice plant, the treatment plots were monitored daily for

dryness. Approximately half inch of water is applied to the plot when the soil is dry for

two inches below the surface. The average volume of water used in 2001 in the

conventionally flooded treatment was 2.13 acre-feet of water per acre of rice cultivation.

The calculated amounts of water used were 0.35 and 0.39 acre-feet of water per acre of

rice cultivation for the 32-inch and 16-inch spacing treatments, respectively,

corresponding to about 16 percent and 18 percent of the water used in conventional

system, respectively.13 Mean yields were 2,895, 2,535, and 2,157 lb of rice per acre of

land for the 32-inch spacing, 16-inch spacing, and conventionally flooded treatment,

respectively. This indicates a higher yield of rice for the treatments where subsurface

drip irrigation is used for the experiment in 2001. Figure 2 illustrates the distribution of

the plots among the different irrigation treatment methods for 2001, 2002 and 2003

growing seasons. The plots established at year 2001 include the 16-, 32-inches and

flooding treatment methods. The same applies to the plots established in 2002. Some of

the plots established in 2003 have a 48-inches treatment method. The plots established

in prior years of the experiment were used subsequent years until the end of the

experiment.

For the 2002 experiment, the experimental area was doubled, resulting in four

replications of each treatment as shown in Figure 2. The area included the 2001 two

replicates, referred to as the “old replicates”, and the two “new replicates.” A chemical

injector was added in year 2002 to fertilize the drip treatments. The amount of water

applied for the 16 and 32-inch drip treatments during 2002 were 1.45 and 1.56 acre-feet

of water per plot of rice cultivation, respectively. In 2002, Cypress, a medium season rice

variety required 2.72 acre-feet of water per acre of rice cultivation for the crop in

conventionally flooded plots.14 The flooded control showed the highest yields for 2002,

6,056 pounds of rice per acre of land, followed by 5,032 lb of rice per acre of land for the

32-inch treatment and 4,865 lb of rice per acre of land for the 16-inch treatment. The two

newer replications that were established for the 2002 season showed higher yields than

the replications which were established during the 2001 season. Rice farmers do not

usually farm the same plot for three years. The usual pattern is to farm a piece of land

one year and let it rest for two years. The reason is that rice farming tends to strip

nutrients from land, so that yields in subsequent years drop if lands are not left fallow.

One of the reasons for this yield drop is observed in the 2002-2003 experiments, couldbe such loss of nutrients from the land from the previous year’s experiment. In other

words, it is likely that the 2002 and 2003 yields would have been larger if plots were

farmed according to current fallowing practice.

Six replications were planted in 2003. Four of these, replications 1, 2, 3, and 4, were

planted on April 2, while remaining two were planted on April 16. Replication 6 consisted

of a 32-inch and a 48-inch drip treatment in addition to the conventionally flooded control

plot. Replications 1, 2, 3, 4 and 5 each consisted of a 16- and a 32-inch drip treatment in

addition to the conventionally flooded control plot.

This analysis considered replications 1, 2, 3 and 4 and replications 5 and 6 as two

separate sets because of the difference in planting dates. The mean water use in

replications 1, 2, 3 and 4 was 1.34 and 1.22 acre-feet per acre for the 16- and 32-inch

treatments, respectively. The mean water use per plot in replications 5 and 6 was 1.21,

1.31 and 1.38 acre-feet per acre for the 16-, 32-, and 48-inch treatments, respectively.

Conventionally flooded plots required 2.72 acre-feet of water per acre to bring rice to

maturity. The corresponding water savings exceed 50 percent for both the 16 and 32-

inch drip treatment plots.

Analysis of the 2003 yield data showed a slight difference among 16- and 32-inch

treatments and the control plot for replications 1, 2, 3 and 4. For these replications, the

16-inch treatment showed the highest yields of 3,721 lb of rice per acre of land followed

by the 32-inch treatment of 3,684 lb of rice per acre of land and the least yields

correspond to the flooded control 3,639 lb of rice per acre of land. A much larger

difference was observed between the 16-, 32- and 48-inch treatments and the flooded

control plot in replications 5 and 6. The flooded control showed the highest yields 4,785

lb of rice per acre of land, followed by the 16-inch of 4,293 lb of rice per acre of land, the

32-inch treatment of 4,066 lb of rice per acre of land, and the 48-inch of 3,794 lb of rice per acre of land.Experimental Conclusions

Preliminary results suggest that drip irrigation has the potential for large water savings

compared with conventionally flooded rice production. The results from the three years

suggest that at least a 50 percent water saving can be achieved while maintaining a

comparable rice yield.

During the first year of the study, both drip treatments produced more rice per acre than

the conventionally flooded rice treatment. Water use amounted to approximately 16

percent and 18 percent of the water used in conventional systems for the 32 and 16-inch

treatments, respectively. In other words, drip saved between 82 and 84 percent of the

water even as yield increased by 10 percent.

Lower yields were produced in the subsurface drip treatment plots in the 2002 season.

Water use amounted to approximately 47 percent and 43 percent of the water used in

conventional systems for the 32 and 16-inch treatments, respectively. Both the 16- and

32-inch treatments out-yielded conventional flooded rice production during the 2003

season in replications 1, 2, 3 and 4 while the reverse was observed for replications 5

and 6. In replications 5 and 6, the 48-inch treatment had the lowest yield. Water use

varied between 49 and 56 percent of the conventional use for the different treatmentsRecent experience is that a subsurface drip irrigation system costs in the order of $500

per acre to install. Netafim’s experience is that a subsurface drip irrigation system is

likely to last for 15 years. At these costs, with water savings either 50 or 80 percent, and

with yield either flat or increasing at 10 percent, economical analysis result in a cost

benefit-ratio could range from 0.88 to 4.78. Water price is assumed to be $33 per acrefeet

for rice irrigation and $105 per acre-feet for municipal and industrial use. The lower

benefit-cost ratio of 0.88 reflects a 50 percent water saving, a stable crop yield and

conserved water is marketed for rice irrigation. Increasing the assumed water savings to

80 percent, rice yield increase by 10 percent, rice price is assumed to be $0.07 per

pound and conserved water marketed to municipal and industrial customers may result

in benefit-cost ratio of as much as 4.78. Table 6 shows benefit cost ratios of various

scenarios of water conservation, yield impact and water markets. The analysis did not

take into account problems in implementing subsurface drip irrigation, such as many of

the farmers are tenant farmers and have little incentive to finance the system installation.

These figures do not incorporate the practice of leaving a field fallow for up to two years

due to nutrient depletion. The economic value of the use of subsurface drip irrigation

would be affected by the increase demand for water from many sectors of the economy.The following section discusses projected savings in cost of water for rice farming inview of the change in rice acreage due to changes in water availability and the rice

market.

Conclusions and Future Work

Evidence from the experiment shows that subsurface drip irrigation can save water for

rice farming, while producing at least stable yield. Water savings of at least 50 percent

versus conventional method might loosen the pressure exerted now on rice farmers

because of water scarcity. The future of rice farming in Texas depends on the wise and

economical use of water. If the subsurface drip irrigation experiments continue, they can

be designed to provide more definitive answers to questions about cost effectiveness,

water efficiency and crop yields. Future work can address the potential reduction in

methane production and also can isolate the factors that affect yield. A larger-scale

experiment and the separation of factors, such as the effect of weeds, late planting,

pesticides, and farming the same plot year after year, should give better answers and

improve the prospect of rice farming in Texas. Although the LCRA’s three years of

experiment demonstrated economic feasibility of drip technology, it unanswered the

question of whether farmers would implement drip systems. Efforts should be made to

move from the “laboratory” scale of the current experiment to a full field trial of rice

farmed by drip irrigation. Such experiments would go far in resolving farmer’s

uncertainties regarding the value of investment in sub-surface drip irrigation for rice. 

Advantages of Drip Irrigation

1 Subsurface drip irrigation may require less water since only a portion of

the soil surface is wetted. Thus evaporation losses can be reduced and

water use efficiency can be improved.

2 Subsurface drip irrigation may lead to increased crop yields which are

attributed to reduced fluctuations in soil water content and well aerated

plan root zone. Crops subjected to fewer water deficit conditions may also

increase yields.

3 Subsurface drip irrigation may reduce agrochemical application because

drip irrigation can deliver agrochemicals precisely in the wetted area

where active roots are distributed. Fertigation and chemigation via drip

irrigation facilitate the relatively uniform distribution of fertilizer and other

chemicals, thus reducing the amounts applied.

4 Subsurface drip irrigation may eliminate anaerobic decomposition of plant

materials and thus substantially reduce methane gas production. Methane

gas is considered one of the most potent global warming gases.

5 Since irrigation return flow will be totally eliminated, water quality of the

receiving water bodies could improve.