The Effect of Controlled Drainage on Crop Yields and Nitrate Nitrogen Losses on Drained Lands in Eastern North Carolina.

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ABSTRACT

POOLE, CHAD ASHLEY. The Effect of Controlled Drainage on Crop Yields and Nitrate Nitrogen Losses on Drained Lands in Eastern North Carolina. (Under the direction of co-chairs R.W. Skaggs and M.A.Youssef.)

Research studies on a wide range of soils, crops, locations, and climates have shown

that Controlled Drainage (CD) has the potential to substantially reduce the loss of nitrogen

(N), from drained agricultural lands to surface waters. Controlled drainage is an integral part

of Drainage Water Management (DWM), of which the adoption and widespread application

depends on its impact on crop yields and overall performance on reducing nitrogen loading.

This dissertation presents results from a long term field study on the effect of CD on crop

yields and nitrate-N losses in a 3 crops in 2 years corn/wheat-soybean rotation.

Crop yields were measured on replicated field scale plots under CD and conventional

or Free Drainage (FD) treatments for a total of 18 crops on two experimental sites during the

period 1990 to 2011. Data were collected on 7 corn crops, 5 wheat crops and 6 soybean

crops. Controlled drainage had no significant effect on yields of winter wheat, which in

North Carolina is grown in the wettest, coolest part of the year. Controlled drainage

significantly increased corn yields compared to FD in all 7 years. The average yield increase

for corn was 11% with a range between 4 and 23%. Controlled drainage also significantly

increased soybean yield in all years with an average increase of 10% compared to FD with a

range between 2 and 20%. The data shows that small amounts of drainage water conserved

by CD during critical periods of the growing season can have a substantial impact on crop

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1992 and 2012. Annual nitrate-N export was reduced by an average of 6.3 kg/ha or 30%

compared with FD. Annual reductions ranged from 0.5 to 14.1 kg/ha. The observed

reductions were a function of significant increases in NO3-N concentrations and significant

decreases in drainage volumes with CD. Controlled drainage reduced annual drainage

volumes by 33% on average. The reduction in drainage volume and increases in nitrogen

removed by the crop under CD were the primary factors responsible for reducing NO3-N

export. The reduction in the loss of NO3-N with CD is about the same as the increase in N

taken up and harvested by the crop. The successful reduction in NO3-N is directly linked to

continuous management and control settings. The results indicate that CD is an effective

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The Effect of Controlled Drainage on Crop Yields and Nitrate Nitrogen Losses on Drained Lands in Eastern North Carolina

by

Chad Ashley Poole

A dissertation submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the Degree of

Doctor of Philosophy

Biological and Agricultural Engineering

Raleigh, North Carolina

2015

APPROVED BY:

_______________________________ _______________________________

Dr. R. Wayne Skaggs Dr. M. Youssef Co-Chair, Minor Representative Co-Chair

_______________________________ _______________________________

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DEDICATION

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BIOGRAPHY

Chad A. Poole was born on January 24, 1981 to Steve and Lucinda Poole. He was

born and raised on a small family farm in Belhaven, N.C., in Beaufort County. His parents

and grandparents instilled in him the value of hard work and stressed the importance of

dedication to everything one undertakes.

He graduated Valedictorian of his class at Northside High School in 1999. That fall,

Chad enrolled in North Carolina State University to pursue a degree in Biological and

Agricultural Engineering. He received a B.S. in Biological and Agricultural Engineering in

2003 with dual concentrations in Agriculture and Environmental Engineering while also

receiving a minor in Agribusiness Management. He also graduated Valedictorian of his class

at NCSU in 2003.

In May 2006, he graduate from the Department of Biological and Agricultural

Engineering (BAE) with and M.S. degree under the direction of Dr. R.W. Skaggs and Dr.

G.M. Chescheir. His thesis focused on the effects of shallow subsurface drains on nitrate-N

and orthophosphorus losses from drained lands.

In January of 2007, while working as a ¾ time Research Associate in BAE, he began

his efforts to pursue a PhD degree under the direction of Dr. R.W. Skaggs and Dr. Mohamed

Youssef. His dissertation work was focused on determining the effects of drainage water

management on crop yields and nitrate-N losses in North Carolina.

While studying at NCSU, Chad has remained active in the day-to-day operations of

the family farm. He has continued working as a Research Associate and received his

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water control structure for open ditch drainage systems. He has planned/or participated in

over 20 extension related Drainage Water Management field days, workshops, or meetings

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ACKNOWLEDGMENTS

We can do all things through the support of God and family. Without their help, no

matter how dedicated or passionate one is about the work he undertakes, success will not

come. First and most importantly, I want to thank the Lord for standing beside me and

offering support throughout my life. This dissertation would not have been possible without

you in my life. Thank you to my family for your support.

Secondly, I’d like to thank my committee for the opportunity to undertake this

project, and for the countless hours of support and encouragement given to me along the

way. Dr. Skaggs has not only been a professional role-model but a caring friend during some

difficult times. His patience and understanding will always be appreciated. In his own way,

he could always point out the areas where improvement was necessary while at the same

time offering encouragement to get the job done. It has truly been an honor to work with Dr.

Skaggs.

Dr. Youssef has supported me throughout the project. He has offered valuable

encouragement and insight during this process. A special thanks is offered to him for his

support and assistance with the completion of this work.

Dr. Chescheir and Dr. Crozier offered valuable support for field work, equipment,

and data analysis. Without their extensive knowledge and assistance in these areas, it would

have been difficult to complete this project. A special thank you is given to them for their

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My family has served not only in a supportive role, but also in a cooperative role.

Assistance was offered numerous times with site setup, downloading data, and collecting

water samples. Their help will always be greatly appreciated.

I worked closely with Wilson Huntley and Dr. Tim Appelboom in the maintenance

and installation of equipment. Tim served in electronic troubleshooting of the site, and

Wilson always assisted when maintenance work had to be performed. Jewell Tetterton

provide support by periodically checking the site for problems and coordinating the planting

and harvesting operations with Dr. Crozier and myself.

The Biological and Agricultural Environmental Analysis Laboratory and the Soil

Science Analysis Laboratory at North Carolina State University deserve special thanks for

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TABLE OF CONTENTS

LIST OF TABLES ... ix

LIST OF FIGURES ... xi

CHAPTER 1. INTRODUCTION ... 1

INTRODUCTION ... 1

Conventional Systems ... 1

Controlled Drainage Systems or Drainage Water Management Systems. 2 Management of DWM Systems for Crop Yield ... 4

Stresses Due to Excessively Wet Soil Water Conditions (Wet Stresses). 5 Stresses Caused by Deficit Soil Water Conditions (Dry Stresses) ... 6

Corn... 7

Soybeans ... 10

Wheat ... 12

Management of CD systems for Nitrate-N Reduction ... 13

OBJECTIVES ... 16

REFERENCES ... 18

CHAPTER 2. THE EFFECTS OF DRAINAGE WATER MANAGEMENT ON CROP YIELDS IN NORTH CAROLINA ... 23

ABSTRACT ... 23

INTRODUCTION ... 24

MATERIAL AND METHODS ... 28

Site 1- TRS ... 29

Site 2- BATH ... 31

RESULTS AND DISCUSSION ... 36

SUMMARY AND CONCLUSIONS ... 44

ACKNOWLEDGEMENTS ... 45

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CHAPTER 3. THE EFFECT OF DRAINAGE WATER MANAGEMENT ON

NITRATE NITROGEN LOSS TO TILE DRAINS IN NORTH CAROLINA ... 49

INTRODUCTION ... 49

MATERIALS AND METHODS ... 54

Site and Drainage System ... 54

Water Table Management ... 63

Statistical Analysis ... 65

RESULTS AND DISCUSSION ... 65

Hydrology: Effect of CD on Drainage and Water Table Depth ... 71

Effect of CD on Losses of Nitrate-N in Drainage Water ... 76

CONCLUSIONS... 83

REFERENCES ... 84

APPENDIX ... 89

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LIST OF TABLES CHAPTER 1

Table 1.1. Crop growth stages of corn plants ... 7

Table 1.2. Crop susceptibility factors for corn... 9

Table 1.3. Crop growth stages of soybean plants ... 10

Table 1.4. Crop susceptibility factors for soybeans ... 11

Table 1.5. Guidelines for CD management in NC ... 15

CHAPTER 2 Table 2.1. Measured crop yields for conventional free drainage (FD) and controlled drainage (CD) for three crops on two sites in North Carolina ... 37

CHAPTER 3 Table 3.1. Field effective hydraulic conductivity and drainable porosity for Portsmouth sandy loam on field plots 2-5 on the Tidewater Research Station, Roper, NC ... 56

Table 3.2. Cropping sequence, tillage practices, liming, and fertilizer rates and timings for 1992-1994, 2007-2012 ... 61

Table 3.3. Monthly and annual precipitation (mm) from the research site compared to 50 year average ... 66

Table 3.4. Measured rainfall minus potential evapotranspiration (mm) for April-October ... 66

Table 3.5. Average annual drainage outflow for conventional (free) drainage and for controlled drainage at the Tidewater Research Station, Plymouth, NC ... 74

Table 3.6. Water table elevations during the study ... 76

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the study period ... 76

Table 3.8. Losses of NO3-N measured in subsurface drainage waters by

treatment and replication at the Tidewater Research Station,

Plymouth, NC ... 79

Table 3.9. Nitrogen removed in the grain harvest and total measured export

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LIST OF FIGURES CHAPTER 1

Figure 1.1. Nitrogen Cycle ... 15

CHAPTER 2

Figure 2.1. Flashboard riser control structure (left). Weir for measuring

drainage rates (right) ... 26

Figure 2.2. General layout of the Tidewater Research Station

experimental site near Plymouth, NC and site locations in NC ... 29

Figure 2.3. General layout of the Bath open-ditch experimental site near

Bath, NC ... 34

Figure 2.4. Controlled and conventional DWM for corn in 1993 ... 39

Figure 2.5. Controlled and conventional DWM for soybeans in 1994 ... 39

Figure 2.6. Monthly precipitation values for 1993, 1994, and 50 year

average, (1957-2006) at TRS ... 40

Figure 2.7. Spatial distribution of corn yield in Bath, 2008 ... 44

CHAPTER 3

Figure 3.1. General layout of the drainage system at the Tidewater

Research Station site ... 57

Figure 3.2. Detailed layout of an experimental field plot at the Tidewater

Research Station site ... 59

Figure 3.3. Effect of controlled drainage on hydrology and NO3-N losses in

drainage water during 1994. Crops were wheat followed by soybean. (A) Daily drainage and NO3-N concentration and

sampling dates for CD and FD; (B) Daily water table (WT) depth, precipitation, and control setting for CD and FD; (C) Cumulative drainage and precipitation for CD and FD; (D) Fertilizer applications and flow weighted NO3-N measured

concentrations for CD and FD; (E) Cumulative NO3-N Export

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drainage water during 2008. Crops were wheat followed by soybean. (A) Daily drainage and NO3-N concentration and

for CD and FD ... 69

drainage water during 2009. Crops was corn. (A) Daily drainage and NO3-N concentration and sampling dates for CD

and FD; (B) Daily water table (WT) depth, precipitation, and control setting for CD and FD; (C) Cumulative drainage and precipitation for CD and FD; (D) Fertilizer applications and flow weighted NO3-N measured concentrations for CD and FD;

(E) Cumulative NO3-N Export for CD and FD ... 70 Figure 3.6. Linear regression analysis for annual CD NO3-N export vs FD

NO3-N export ... 79 Figure 3.7. Total nitrogen removed from field plots under FD and CD

drainage treatments during the 9 year study ... 82

APPENDIX

Figure A.1. Effect of controlled drainage on hydrology and NO3-N losses in

drainage water during 1992. Crop was wheat/soybean. (A) Daily drainage and NO3-N concentration and sampling

dates for CD and FD; (B) Daily water table (WT) depth, precipitation, and control setting for CD and FD;

(C) Cumulative drainage and precipitation for CD and FD; (D) Fertilizer applications and flow weighted NO3-N measured

Figure A.2. Effect of controlled drainage on hydrology and NO3-N losses

in drainage water during 1993. Crop was corn. (A) Daily drainage and NO3-N concentration and sampling dates for

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and flow weighted NO3-N measured concentrations for CD

and FD; (E) Cumulative NO3-N Export for CD and FD ... 92 Figure A.3. Effect of controlled drainage on hydrology and NO3-N losses in

drainage water during 2007. Crop was corn. (A) Daily drainage and NO3-N concentration and sampling dates for CD and FD;

(B) Daily water table (WT) depth, precipitation, and control setting for CD and FD; (C) Cumulative drainage and

precipitation for CD and FD; (D) Fertilizer applications and flow weighted NO3-N measured concentrations for CD and FD;

(E) Cumulative NO3-N Export for CD and FD ... 93

Figure A.4. Effect of controlled drainage on hydrology and NO3-N losses in

drainage water during 2010. Crop was wheat/soybean. (A)Daily drainage and NO3-N concentration and sampling

dates for CD and FD;(B) Daily water table (WT) depth, precipitation, and control setting for CD and FD;

(C) Cumulative drainage and precipitation for CD and FD; (D) Fertilizer applications and flow weighted NO3-N measured

Figure A.5. Effect of controlled drainage on hydrology and NO3-N losses

in drainage water during 2011. Crop was corn. (A) Daily drainage and NO3-N concentration and sampling dates for CD

and FD; (B) Daily water table (WT) depth, precipitation, and control setting for CD and FD; (C) Cumulative drainage and precipitation for CD and FD; (D) Fertilizer applications and flow weighted NO3-N measured concentrations for CD and FD;

(E) Cumulative NO3-N Export for CD and FD ... 95 Figure A.6. Effect of controlled drainage on hydrology and NO3-N losses

in drainage water during 2012. Crop was wheat/soybean. (A) Daily drainage and NO3-N concentration and

concentrations for CD and FD; (E) Cumulative NO3-N

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CHAPTER 1. INTRODUCTION Conventional Systems

Draining surplus water out of the soil profile has been practiced as long as production

agriculture has existed. Van Schilfgaarde (1974) cited writings of the Roman Cato in the

second century B.C. who “referred to the need for removing water from wet fields, stating

that in low–lying areas, it is necessary to have many drains”.

Sustained profitable crop production in poorly drained areas is not possible without

improving drainage. Consequently, crop producers have built drainage systems for centuries

to enable them to utilize poorly drained lands. The purpose of all drainage systems is to

increase the efficiency and reliability of production. The goal is to have high crop yields

with a minimum investment of energy, water and other resources. Skaggs (1982) stated that

drainage systems in humid regions historically have been installed for trafficability and to

remove excess water from the root zone, which prevents crop yields from being reduced by

oxygen deficiencies or other stresses associated with wet soils. In humid regions, the

prevailing drainage practice has been to install drain tubes or ditches as deep as possible and

as wide as possible to meet the primary drainage objectives (Doering, 1982). Though the

deep and wide principle reduces the length and costs of drains required for an area, excessive

drain depths may lead to over-drainage and increased drought stresses in some cases (Skaggs

and Chescheir, 2003).

Ditches have been used along the eastern seaboard of the United States to drain

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drainage ditches were installed by individuals or plantation owners. This later moved to

groups of individuals that organized drainage management districts and worked in

cooperation with government agencies (Haynes, 1966). Typical ditch depths in eastern North

Carolina varied from 1 to 1.5 m.

While conventional drainage enables and improves crop production on poorly drained

soils, it can significantly change the quality of water draining from the soils (Gambrell et al,

1975). Since subsurface drainage is increased, a greater amount of soluble nitrogen (mostly

in the form of nitrate nitrogen) can be leached from the soil and exported in the drainage

water (Gilliam et al., 1999). With the increase of subsurface drainage intensity, surface

runoff is decreased which reduces the amount of sediment and sediment bound contaminants

being exported. Nitrogen export from drained land; however, has received the most attention

and has been considered a major source of nutrients leading to algal blooms and hypoxia in

nutrient sensitive receiving waters. Since the late 1970s, a large percentage of drainage

research has focused on methods to reduce nitrogen export from drained lands without

compromising crop production.

Controlled Drainage Systems or Drainage Water Management Systems

Controlled drainage (CD) systems limit the amount of water draining from artificially

drained fields by managing the elevation of the water surface at the drainage outlet. The

water level at the outlet is controlled by a water control structure which is typically a

flashboard riser. When flashboards are added to the riser, the water level at the outlet, during

a drainage event, rises to the level of the boards and drainage rates decrease compared to the

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given system by reducing the drainage depth through control of the water level in the drains

and at the outlet.

Controlled drainage was used in the northern everglades of Florida as early as the

1930s (Clayton, 1941). During the 1970s, studies completed by Skaggs et al., 1972; Skaggs,

1973; and Doty et al., 1975 showed that water table control practices could be applied to a

wide variety of soils if design parameters were fitted to soil and site conditions. There are

seven basic design goals for controlled drainage systems as presented by Fouss et al., (1999).

1. Provide trafficability

2. Reduce plant stress caused by excessive soil water

3. Control soil salinity and alkalinity

4. Reduce plant stress caused by deficit soil water conditions

5. Minimize harmful offsite environmental impacts

6. Conserve and efficiently utilize water supplies

7. Maintain a soil-water environment so that other practices, such as conservation

tillage, are more effective and beneficial.

Research has shown that CD can reduce contaminant loads to surface waters from

drained agricultural lands (Gilliam et al., 1979, 1999). More recent research conducted on

various soils and climates in Sweden, Canada, and the Midwest, USA, has shown that CD

reduced nitrogen loads by 18 to 80%. Results from these recent studies along with those from

earlier studies have been summarized and discussed by Skaggs et al. (2010, 2012). Most

studies show that nitrogen reductions due to CD result from the reductions in flow volume

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Management of DWM Systems for Crop Yield

Proper management of CD systems is required to achieve maximum crop yield

benefits. Failure to lower the control level during excessively wet periods may increase

excess water stress and damage the growing crop. Not raising the control level after planting

will result in loss of drainage water that could be used by the crop during dry periods. Poor

management of CD can, therefore, lead to lower yields than those occurring with

conventional drainage. Since crop yields play a major role in the adoption of the practice by

producers, yield reductions resulting from poor management can be perceived as a

disadvantage of CD. This perception may be one reason that CD has not been readily

accepted, despite the fact that controlled drainage is an accepted BMP in North Carolina and

is cost shared by programs such as the Environmental Quality Incentives Program (EQIP)

and the North Carolina Agricultural Cost Share Program. Another reason for less than

enthusiastic acceptance and use of CD is the effort needed to manage the system. In order

for CD systems to be enthusiastically accepted and properly managed, producers need to see

an economic incentive to install and manage the system.

There have been a few studies that have reported yield increases under a controlled

drainage system; however, the data are limited. Jaynes (2012) reported an 8% yield increase

of soybean in central Iowa due to CD, but corn yields were not significantly affected by

controlled drainage. In northern Ohio, Fausey et al (2004) reported no significant effect of

CD on corn and soybean yields, while Helmers et al (2012) in southeast Iowa, reported no

increase in soybean yields, but a significant decrease in corn yields. Consistent yield

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added to the controlled drainage system as subirrigation (Fisher et al, 1999, Tan et al., 2004,

Allred et al., 2003, and others). Documenting the long term yield benefits of the practice has

been limited due to the long time required to complete such studies. As a result, much of the

predicted benefits have come from short term studies followed by long term modeling

(Skaggs and Gilliam, 1981; Ale et al, 2009). More data are needed on yield benefits of this

drainage management system to ensure the continued support of the BMP by producers and

to develop management strategies that increase crop yields. Specific issues relating to

managing a drainage system to reduce water related stresses and increase crop yields are

briefly discussed the following sections

Stresses Due to Excessively Wet Soil Water Conditions (Wet Stresses). Excess water in the root zone leads to oxygen depletion which is the main cause of stress in plants.

Processes affected by excessive water in the root zone include soil oxygen and carbon

dioxide exchange with the atmosphere, oxidation-reduction reactions in the soil and

subsequent diffusion of gaseous metabolites, nutrient availability and uptake, and

accumulation of CO2, ethylene, and other toxic substances and byproducts of reduced

conditions such as nitrite, highly soluble Fe and Mn, and hydrogen sulphide (Evans, 1999).

These processes can both directly and indirectly affect crop growth by suppressing seed

germination, restricting root growth and nutrient uptake, shoot wilting, reduction in available

nutrients such as nitrate-N, epinastic curvature of main stems, chlorosis, and desiccation.

Comprehensive reviews of the effects of excessive water stresses in growing crops is given

by Clement (1921), Russell (1952), Wesseling and van Wijk (1957), Grable (1966),

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(1985), Heritage (1985), Kowalik (1985), Patwardhan et al. (1988), Evans (1999), and

multiple others.

Stresses Caused by Deficit Soil Water Conditions (Dry Stresses). Inadequate access to soil water during any part of a plant’s physiological growth can have significant

impacts on yields. Drought stress decreases nutrient transport in soil by diffusion and mass

flow to the root surfaces thus decreasing nutrient absorption by roots. Under dry stress, roots

are unable to take up nutrients from the soil because of lack of activity of fine roots, water

movement, and ionic diffusion of nutrients (Prasad, 2008). Several research studies have

reported reduced N response by plants under low soil moisture contents (Larson 1950,

Singleton, et al 1950).

Inadequate moisture when crops are planted can slow, and even prevent, germination.

When inadequate soil moisture exists, conductance is decreased through stomata which limits

the process of transpiration (Cornic, 2000). Plant temperature increases as a result of

decreased water available for transpiration leading to damage to respiratory and

photosynthesis mechanisms. Consequently, biological functions are limited and plant growth

and yield reductions occur.

Photosynthesis processes are significantly reduced under deficit soil water conditions.

At late stages of drought stress, tissue dehydration can occur, leading to metabolic

impairment. Plant respiration is also affected.

The effects of drought stress on whole-plant processes are large and can ultimately

influence germination, emergence, leaf, root, tiller, and stem development and growth, dry

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seed yield, and seed quality (Prasad, 2008). Drought stress can not only influence the

transition of one development stage to another, but also the duration of the development

stage. For example, drought stress during a critical period such as pollination for some plants

can lead to failure of fertilization because of decreased pollen or sterility thereby shortening

the pollination period. In general, crops are more sensitive to drought stresses during

reproductive stages of development which influences seed yield.

Corn plants have multiple stages of growth. The stages are broken down into two categories including vegetative (V) and reproductive (R) stages, as given in Table 1.1 below.

Table 1.1. Crop growth stages of corn plants.

Subdivisions of the vegetative stages represent the number of emerged leaves before

the tassel emerges. This can vary from one variety of corn to another and is denoted with

Vn, where n represents the last emerged leaf before tasseling. The R stages are straight

forward and the same for all varieties of corn, though the number of days necessary for each

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Water related stresses vary throughout the growth stages of corn. Excessive wet

stresses can cause severe yield losses at any plant stage, but is especially true at the early

vegetative stages. Evans (1999) stated that most crops are more susceptible to moisture

stresses, both excessive and deficient, at some physiological growth stage(s) than others. He

used the stress-day-index (SDI) concept developed by Hiler (1969) to mathematically

describe the response of yield to stresses caused by planting delays, excessive soil water, and

deficit soil water. Hiler used a crop susceptibility factor (CS) to describe a unit of stress

which is a function of crop species and its stage of development. The relationship is given

by:

CSi= (X-Xi)/X, (1.1)

where:

Xi: is the harvested crop yield when subject to the critical stress at growth stage i

X: is the crop yield when no stress is applied

Higher CS values imply that the crop is more susceptible to stress at a particular

stage. Multiple authors have studied this relationship and developed CS values for both corn

and soybeans (Hardjoamidjojo et al., 1982, Desmond et al, 1985, Scott et al. 1989, Mukhar et

al., 1990a, Evans et al 1990, and Shaw, 1974) which are given in table 1.2 below (Evans,

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Table 1.2. Crop susceptibility factors for corn.

The CS values imply that the emergence period through silking and pollination are

the more important periods to avoid excessive wet stresses in corn. Kanwar (1988) showed

that excessive water early in the growing season lowered corn yield significantly. The most

critical period for avoiding wet stress is during the late vegetative period when

evapotranspiration and nutrient uptake are maximum.

The late vegetative periods is also important for avoiding dry stress. This period is

2nd only to pollination for drought stress issues. It is therefore important to maximize water conservation to prevent deficit soil conditions at tasseling and silking. These observations

are consistence with studies reported by Denmead (1960), and Robins (1953). Denmead’s

studies showed that drought stress prior to silking (VT-R1) reduced grain yield by 25%,

deficit water at silking (VT-R1) by 50% and moisture stress just after silking (R2) by 21%.

Robin’s study showed that deficit water during one or two days during VT-R1 reduced yield

by 22% and six to 8 days during this period reduced yields by 50%.

A CD water management system for corn must provide adequate drainage from

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during this period to prevent saturated conditions in the root zone for a period greater than 24

hours. However, some water must be conserved during this period to ensure a viable water

supply in the subsoil for late vegetation (Vx-Vn) (typically X would represent about the 6th leaf stage). This will ensure that ET demands are met for tasseling, silking, and pollination

(VT-R1) to promote viable pollen for reproduction. Denmead (1960) observed that the

critical period for deficit water stress does not extend longer than 3 weeks after 75% silking.

This would be in line with the R4 period. This implies that water conservation would not be

necessary in most cases 10-14 days after silk emergence, but CD mode should continue

through the growing season for water quality purposes. Operation of irrigation pumps are

also not necessary 10-14 days after silk emergence.

Soybeans plants also have multiple stages of growth. The stages are also broken down into two categories including vegetative (V) and reproductive (R) stages. These stages

are given in Table 1.3 below.

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Subdivisions of the vegetative stages are denoted with Vn, where n represents the last

emerged unrolled trifoliolate leaf before bloom. Indeterminate and determinate varieties

grow virtually the same until the R1 stage at which time indeterminate varieties continue to

develop leaves on the main stem and branches throughout the flowering period. Determinate

varieties differ because growth ceases on the main stem after the R1 stage, but leaves

continue to develop on the branches. The R stages are the same for all varieties of soybean,

though the number of days and timing of when each stage starts is dependent on the maturity

group (i.e Group 00 to IX).

Water related susceptibility factors (CS) vary throughout the growth stages of

soybeans (Table 1.4). Excessive wet stresses can cause severe yield losses at any plant stage,

but CS values are higher before ripening. The most critical period to prevent saturated

conditions is during pod development and fill.

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The CS values in Table 1.4 are consistent with studies reported by Sionit (1977),

Hobbs (1983), Sugimoto (2000), and Doss (1974). Doss (1974) showed that irrigation after

flowering and just before pod fill was the critical time for maximum yields. Sionit (1977)

showed that drought stress during flower induction and flowering produced fewer flowers,

pods and seeds, whereas stress during earlier pod formation caused the greatest reduction in

the number of pods and seeds at harvest. Hobbs (1983) demonstrated soybean water use to

be low during early season growth with a peak use during late flowering and pod

development in southern Alberta. Sugimoto (2000) noted that seed yield was notably

depressed with excessive soil moisture conditions at the flowering and bud stages. He also

noted that soybeans were more tolerant to wet stresses at the ripening stage than the

flowering or pod fill stages.

It appears that it is essential in a water management system for soybeans to provide

adequate drainage from emergence through pod fill. A CD system should be closely

monitored during this period to prevent saturated conditions in the root zone longer than 24

hours. However, water must be conserved during this period to ensure a viable water supply

in the subsoil for the critical periods of flowering and pod development and fill. As noted,

from the CS values and the reported studies herein, water stress or deficit water stress during

the ripening period is of minimal importance. This implies that water conservation or

irrigation would not be necessary in the majority of cases after pod fill.

Wheat is typically planted in the fall in NC and tillering slows or stops when winter weather turns cold which coincides with the normal high water tables present in the poorly

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theory should have little impact on wheat yield. Wheat production may or may not occur in a

typical crop rotation in NC and is highly driven by commodity prices. The two crops that are

most critical to producers in the area where CD can be utilized are corn and soybeans.

Management of CD systems for Nitrate-N Reduction

Drainage and water management practices have a profound influence on the soil N

cycle, Figure 1.1. Drainage promotes aerobic conditions which significantly influences the

rate of mineralization of the soil organic N pool. Good drainage also influences the rate of

nitrification. Both processes are influenced by aerobic bacteria. The enhanced

mineralization and nitrification resulting from improved drainage along with N fertilizer

leads to high amounts of nitrate in the soil profile. Excess nitrate is available for leaching

which is enhanced by improved subsurface drainage. Nitrate leached from the soil through

drainage pipes or ditches can eventually reach nutrient sensitive receiving waters and cause

environmental problems. Nitrate can also be transported through deep seepage to

groundwater and eventually to streams.

Drainage water management can be utilized to control the rate of loss of nitrate by

three processes. The first is to artificially increase the water table elevation in the soil profile

which will remove oxygen from the soil system. This increases the volume of the anaerobic

zone where denitrification can convert nitrate to N2 gas, which is released into the

atmosphere. The second mechanism that drainage water management uses is to control the

drainage outlet elevations reducing the drainage intensity and the subsurface drainage rate.

This reduces the volume of water moving through the soil and leaching NO3-N to surface

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enhance crop growth, and ultimately yield, by providing favorable soil water conditions for

crop growth. Excess soil water conditions are prevented by drainage. Deficit water

condition are avoided by not overdrawing the soil profile and conserving water that can later

be utilized by the crop. If soil water is properly managed, crop yields will be improved and

more N will be utilized by the crop and removed from the system at harvest. The key is to

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The following guidelines, provided by the Division of Soil and Water Conservation in

North Carolina (Kelly Hedgepeth, personal communication, July 2015), are given to

producers who participate in the NC Cost share program for CD structure management

(Table 1.5). These guidelines where developed based on the information given above and the

typical cropping schedule in NC to protect crop yields and maximize nitrate-N reductions.

This is the typical starting point for CD management with most systems, but can be modified

based on field conditions determined by crop stage, soil and water conditions in the field, and

drainage system capacities.

Table 1.5. Guidelines for CD management in NC

OBJECTIVES

The main goal of this research is to quantify the effects of Drainage Water

Management (DWM) through Controlled Drainage (CD) on crop yields effects and water

quality over a wide range of weather conditions and cropping practices in North Carolina.

Specific objectives are:

1. Quantify the crop yield effects of CD in long-term field experiments for the

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2. Quantify the effects of CD on drainage volumes, nitrate-N concentrations and

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Gambrell, R.P., J.W. Gilliam, and S.B. Weed. (1975). Denitrification in subsoils of the North Carolina Coastal Plain as affected by soil drainage. J. Environ Qual. 4:311-316

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Glinski, J., and W. Stepniewski. (1985). Soil aeration and its role for plants. CRC Press, Inc., Boca Raton, Fl.

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Hardjoamidjojo, S., R.W. Skaggs, G.O. Schwab. (1982). Corn Yield response to excessive soil water conditions. Trans. ASAE. 25:922-927, 934.

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Hiler, E.A. (1969). Quantitative evaluation of crop drainage requirements. Trans. ASAE

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Hobbs, E.H., and H.H. Muendel. (1983). Water requirements of irrigated soybeans in southern Alberta. Can. J. Plant. Sci. 63: 855-860.

Jaynes, D.B. (2012). Changes in yield and nitrate losses from using drainage water management in central Iowa, United States. J. Soil Water Conserv. 67, 485–494.

Kanwar, R.S., J.L. Baker, S. Mukhtar. (1988) Excessive Soil Water Effects at Various Stages of Development on the Growth and Yield of Corn, Trans ASABE 31 (1):0133-0141.

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16:988-993

Skaggs, R.W. and G.M. Chescheir III. (2003). Effects of subsurface drain depth on nitrogen losses from drained lands. Transactions of the ASAE 46(2): 237-244.

Skaggs, R.W., and J.W. Gilliam. (1981). Effect of drainage system design and operation on nitrate transport. Transactions of the ASAE 24(4):929-934.

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Skaggs, R.W., M.A. Youssef, R.O. Evans, and J.W. Gilliam. (2010). Effect of controlled drainage on water and nitrogen balances in drained lands. Trans. ASABE 53(6): 1843-1850.

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CHAPTER 2.

THE EFFECTS OF DRAINAGE WATER MANAGEMENT ON CROP YIELDS IN NORTH CAROLINA

Published:

Poole, C.A. R.W. Skaggs, G.M. Cheschier, M.A. Youssef, and C.R. Crozier. Effects of

drainage water management on crop yields in North Carolina. Journal of Soil and Water

Conservation. Nov/Dec 2013 vol 68, no 6, p. 429-437

ABSTRACT

Research studies on a wide range of soils, crops, locations, and climates have shown

that Drainage Water Management (DWM) (or Controlled Drainage, CD) can be used to

substantially reduce the loss of nitrogen (N), and in some cases, phosphorus (P) from drained

agricultural lands to surface waters. The adoption and widespread application of DWM

depends on a variety of factors including its impact on crop yields. This paper presents

results from a long term field study on the effect of DWM or CD on crop yields in a 3 crops

in 2 years corn/wheat-soybean rotation. Yields were measured on replicated field scale plots

under CD and conventional or Free Drainage (FD) treatments for a total of 18 crops on two

experimental sites during the period 1990 to 2011. Data were collected on 7 corn crops, 5

wheat crops and 6 soybean crops. Controlled drainage had no significant effect on yields of

winter wheat, which in North Carolina is grown in the wettest, coolest part of the year.

Controlled drainage increased corn yields compared to FD in all 7 years. The average yield

increase for corn was 11%. Controlled drainage also increased soybean yield in all years

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application of drainage water management which will result in both economic and

environmental benefits.

Key words: drainage water management (DWM)—controlled drainage (CD)—yield— corn—soybean—wheat

INTRODUCTION

The primary purpose of agricultural drainage has historically been to increase crop

yields, profits, and the reliability of production. Improved or artificial drainage systems have been used for centuries (Luthin, 1957) to provide trafficable conditions for field

operations, remove excess water from the root zone, prevent crop yield reductions due to

oxygen deficiencies and associated stresses, and control soil salinity. These systems may

include improvements to surface drainage by land smoothing to fill potholes and provide an

outlet for surface runoff, and subsurface drainage by the installation of ditches or drain

tubes (clay, concrete, or plastic tubes or tile). Research since the 1970s has shown that

improved subsurface drainage has increased the loss of some potential contaminants,

particularly nitrogen (N), to surface waters while reducing the loss of others (Gilliam et al.,

1999). This has resulted in negative environmental impacts at both local and basin scales.

The most prominent example is in the Midwest, USA where N losses from subsurface

drained land is a primary source of N linked to the development of the hypoxic zone in the

Gulf of Mexico (Rablais et al., 2002; Burkhart and James, 1999; EPA-SAB, 2007). By the

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of drainage design and management was well recognized by drainage scientists and

engineers (Skaggs, 1992).

Improved drainage continues to be needed to provide increased sustainable yields

to satisfy increasing global demand for food production. Federal conservation programs

have been established to reduce the loss of nutrients and other contaminants to surface and

ground waters. Drainage water management (DWM) has emerged as an effective method

of conserving water, increasing yields and reducing N and phosphorus (P) losses to the

environment. However the adoption of the practice is highly dependent on production

economics.

Drainage water management or controlled drainage (CD) is not a new practice. It

has been used to control soil subsidence and conserve drainage water in drained organic

soils in the Florida Everglades and other locations for many years (Clayton and Jones,

1941; Stevens, 1955). Studies in North Carolina during the 1970s (Skaggs et al., 1972;

Skaggs, 1973; Doty et al., 1975) showed that water table control practices could be applied

to a wide variety of soils if design parameters were fitted to soil and site conditions.

Gilliam et al. (1999) reviewed research on the effect of DWM on drainage water quality.

More recently Skaggs et al. (2010) reviewed results of studies on the effects of N losses to

surface waters. Early research in NC (Gilliam et al., 1979; Evans et al., 1995) showed that

CD reduced nitrate-N losses by 40% to 50%, and P losses by 25% to 35% compared to

conventional free drainage. Similar findings have been confirmed at other locations

(Lalonde et al., 1996; Tan et al., 1998; Fausey, 2005; Wesstrom and Messing, 2007; Drury

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that DWM had been reported to reduce N losses in drainage waters by 17 to over 80% and

discussed the mechanisms controlling the effectiveness of the practice and how soil

properties, drainage intensity, and site conditions affect its performance.

Because DWM has been referred to as CD in North Carolina since the 1970s, we

will use that term in this paper. CD is implemented by using a water control structure to

control the outlet water level elevation. A typical water control structure used for CD in an

open ditch is shown in Figure 2.1. A similar in-line system is used in drain tube

applications (not shown). When boards are added, the water level in the drain rises during

wet periods and the subsurface drainage rate is decreased. Conversely, when boards are

removed, the subsurface drainage rate increases to that of conventional or free drainage

(FD). Proper management of the system is required to achieve maximum water quality and

crop yield benefits.

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CD reduces N losses in drainage waters by three fundamental mechanisms: (1) it

reduces subsurface drainage volumes, which is the principal pathway of N losses from

drained soils; (2) it increases denitrification by increasing the anaerobic zone in the soil

profile; and (3) it conserves water, reduces deficit soil water stresses, and increases yields

resulting in more N removed from the field with the harvested crop and less available for

loss in the drainage water.

Drained agricultural lands make up about 40% of North Carolina's cropland. The

lands are primarily located in the coastal plain in the eastern and southeastern parts of the

state. Controlled drainage has been employed in these areas to reduce losses of N and P to

surface waters (Gilliam et al., 1999; Evans et al., 1995). Controlled drainage was accepted

in the mid-1980s as a Best Management Practice (BMP) for reducing nutrient losses in

drainage waters with the control structures cost shared by the state of North Carolina at the

rate of 75% (state) to 25% (farmer). The practice is now eligible for cost share under the

USDA EQIP program, as well as from North Carolina through the N.C. Division of Soil

and Water Conservation (DSWC). Since the first cost share contracts were issued in 1984,

over 4,000 water control structures affecting about 160,000 ha (400,000 acres) have been

installed in the state (Evans and Skaggs, 2004). Controlled drainage is one of the most

effective BMPs for reducing N and P losses from drained agricultural lands in North

Carolina.

The driving force behind the adoption of the practice in NC has been the state and

federal cost-share incentives for installation of the practice with the primary goal of reducing

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research concentrated on the effectiveness of the practice in reducing N and P loads to

receiving waters. Effects on yields were observed in some studies, but few replicated field

experiments were conducted. Evans and Skaggs (1985) found that water conserved by CD

substantially increased yields in some years but had only minor impacts in others. General

guidelines to producers indicated that, properly managed, CD could be expected to increase

yields by an average of about 5%, but the emphasis was on management for reduction of N

and P losses to surface waters. Tan et al. (2004) showed that a combination controlled

drainage-subirrigation system reduced nitrate loss by 38% and increased corn yields by 64%

in a sandy loam soil in Ontario, Canada. However, subirrigation requires a water supply and

associated pumps and controls. The most common mode of DWM is CD which conserves

drainage water but does not provide irrigation during long dry periods. Controlled drainage

conserves water that would otherwise drain out of the soil profile. This may be expected to

increase yields, but its effectiveness depends on when rainfall and drainage events occur,

plant growth stage, and weather conditions following those events. Crop yield effects of

water conservation resulting from CD in North Carolina have not been fully studied. This

paper presents data on the effect of controlled drainage on yields of corn (Zea mays L.), soft

red winter wheat (Triticum aestivum L.), and soybean (Glycine max L.) collected from two

experimental research sites in North Carolina during the period 1990 through 2011.

MATERIAL AND METHODS

Data were collected from two study sites to determine effects of CD on crop yields

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drained with 10 cm (4-in) diameter corrugated subsurface drain tubing, and Site 2 (BATH),

located near Bath, NC, is drained with parallel open ditches. Locations of the sites are shown

in Figure 2.2.

Figure 2.2. General layout of the Tidewater Research Station experimental site near Plymouth, NC and site locations in NC.

Site 1- TRS

The response of crop yields to CD was evaluated using data collected between (1990

and September 2011) from TRS. The site is a 13.8-ha (34.1 ac) agricultural field divided into

eight experimental plots, four of which were used in this study (Figure 2.2).

Soil on the site is classified as Portsmouth sandy loam (Typic Umbraquult;

fine-loamy, siliceous, thermic), which is very poorly drained under natural conditions. The site

has nearly flat topography and is bounded on all four sides by drainage ditches,

approximately 2.0 m (6.6 ft) deep. There are two different drainage systems installed at the

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and 1.0 m (3.3 ft) deep. The original system was closed at the end of 1990 and a second

system was installed and used in subsequent years. That system consisted of parallel drains

23 m (75 ft) apart and 1.2 m (4.0 ft) deep. Center drain lines were used for experimental

purposes. The function of the guard lines on each side is to hydraulically isolate the area

drained by the center drain line from the influence of adjacent experimental plots. The

subsurface drains of each experimental plot discharge to receiving tanks installed in an

instrument house. Each plot has two 100 mm (4 in) diameter water table monitoring wells

equipped with automatic recorders and data loggers programmed to measure and record

water table depth at a one hour interval. Tipping bucket and manual rain gauges were

installed at the site to continuously measure and record precipitation.

Plots 2-5, which were used in this study, were managed in either FD or CD. In FD,

pump controls were set to keep the water level in receiving tanks below the elevation of the

subsurface drains. In CD, water was pumped out of the receiving tank when the outlet water

level in the tank exceeded a preset control elevation which, except for periods of seedbed

preparation, planting and harvest, was higher than the elevation of the subsurface drains. The

control elevation represents the weir elevation in a conventional drainage water management

structure. No water was pumped into the system.

Crop yields were determined for each plot at the end of each growing season. Grain

yield was determined by harvesting 2 subplots located 6 m (20 ft) on either side of the center

drain line. Generally, the corn harvest areas were 1.8 m (6 ft) wide by 23 m (75 ft) long.

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were collected from plots 2-5 on this site from 1990 through September 2011. The impact of

CD on yields was determined on 14 crops during this period at TRS.

Wheat was not harvested in 2010 due to wet planting conditions and poor emergence

on all plots in the fall of 2009. A power outage during tropical storm Nicole in October 2010

resulted in complete submergence of some of the plots for a prolonged period and other plots

for a shorter period. It confounded results for soybean that year which were omitted from the

analysis.

Site 2-BATH

The (BATH) site consists of 24 ha (60 ac) in northeastern Beaufort county near Bath,

North Carolina. The soil is also Portsmouth sandy loam, (Typic Umbraquult; fine-loamy,

siliceous, thermic). The site originally had irregular spaced lateral ditches with relatively

poor surface drainage. In December 2007, all interior ditches were filled and replaced with

parallel ditches 60 m (197 ft) apart. Surface leveling was required to facilitate surface

drainage to the new ditches and permit efficient production.

The drainage water management model DRAINMOD 6.0 (Skaggs 1978; Youssef et

al. 2005; Skaggs et al. 2012), was utilized to predict the effects of surface grading, ditch

depth, and spacing on crop yields and N losses for a typical Portsmouth soil. Decision

parameters used to determine proposed ditch spacing on the site included crop yield,

trafficability, and machine efficiency. Machinery for farming practices at the site was 7.3 m

(24 ft) wide. In order to have an even number of passes in each plot, ditch spacing needed to

be a multiple of 14.6 m (48 ft). Simulated crop yields for ditch depths varying from 0.61 to

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m (197 ft). A spacing of 60 m (197 ft), which allows for a 1.5 m (5 ft) wide ditch, was

selected because it was both efficient for the producer and sufficient for draining the site.

The field site (Figure 2.3) was divided into 2 separate blocks consisting of

approximately 12 ha (30 ac) each. The blocks were isolated from one another by leaving a

2.4 ha (5.9 ac) plot between the treatments. Each block is drained by five lateral ditches with

the exception of the FD treatment, which is drained with 4 lateral ditches and a subsurface

drainage line. The subsurface line was used in place of an open ditch at the western edge of

the field to provide more efficient operating conditions and convenient access to the field.

The west block was operated in conventional FD mode with 1.07 m (3.5 ft) deep lateral

ditches. The eastern block was operated in CD mode with 1.07 m deep (3.5 ft) lateral ditches

managed using flashboard risers with the exception of the wheat crop in 2009 which was

managed in free drainage for ditch bank stabilization purposes. The land drained by each of

the three inner ditches in each block was treated as a field scale experimental plot, giving

three replications for each treatment. The two outer ditches in each block served as guard

drains to hydraulically isolate the area drained by the three inner drains from the influence of

the adjacent block.

The FD and CD blocks were surface leveled (field crowning and land smoothing) by

practices that are standard in the area. Box-blades and land planes are typically used in the

NC coastal plain to do this type of surface grading and are typically not laser controlled.

This results in approximately a 7.6 cm-15 cm (3 to 6 in) surface crown with some shallow

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Water table elevations at two locations per block were continuously monitored in

wells equipped with automatic recorders and data loggers. Eight flashboard risers were

installed at the site. Three risers were used on the FD system and five on the CD system.

Risers on the center-lines of each plot were used for drain flow monitoring. Weirs were

installed and used to measure flow after March 2009. Precipitation data were collected with

automatic tipping bucket and manual rain gauges at the site.

Crop yield was determined for each plot by harvesting and mapping the yield on the

whole plot with a calibrated yield monitoring system. Crop samples were collected to

determine grain moisture and test weights. Yield data from 4 crops were collected from April

2008 through June 2011 from the BATH site. Controlled drainage was not applied to the

wheat crop of 2009/2010; the site was managed in free drainage mode in all plots during that

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Figure 2.3. General layout of the Bath open-ditch experimental site near Bath, NC.

The crop rotation on both study sites consisted of three crops in two years: corn, year

1; wheat, year 1-2; and soybean, year 2. This cropping sequence is common on farmlands of

the North Carolina lower coastal plain (Chescheir et al., 1996). Corn, the first crop in the

rotation, was planted in April and harvested in late August to early September. Wheat was

then planted in mid-November and harvested in early June of the following year. Soybean

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fallow after soybean harvest until April of the following year when corn was planted and the

rotation repeated.

Both conventional tillage and no-till practices were implemented on the sites at

different times during the observation period. The tillage treatment remained consistent

across the plots in any given year. Corn and wheat were grown under both practices.

Soybean was grown using no-till only.

Nitrogen fertilization followed common practices in the region with one exception for

experimental reasons. Two N fertilization rates were used during 1998 as part of an

experiment to study the effect of N fertilization on crop yield and N leaching losses at (TRS).

The two rates were used for both drainage treatments which allowed the use of the yield data

in this study. Lime application rates followed soil test report recommendations (Chescheir et

al., 1996).

Controlled drainage settings from the surface varied slightly during the study. During

1990, 1991, and 1994 the control elevations were set to 30 cm (1 ft) from the surface. In all

other seasons, the CD control settings were either 45 or 50 cm (1.5 ft) from the surface (table

2.1). Adjustments to the control levels were made based on site conditions, crops to be

grown, and scheduling of necessary field operations. Controlled drainage control settings

were adjusted to FD occasionally to facilitate planting and harvesting operations, or during

wet periods.

The crop yield data were paired by year and location, and analyzed with a paired

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variability in climate, soils, tillage, varieties, and the drainage systems that existed at the TRS

and BATH sites.

RESULTS AND DISCUSSION

Annual crop yields and associated data are summarized in Table 2.1 for all crops,

years and locations. Yield data were collected on field plots under CD and FD on a total of

18 crops over the period 1990-2011. Results from each treatment were grouped by crop and

paired by year. Controlled drainage significantly increased corn yield (p= 0.01). Yields in

the CD plots were higher than in the plots under FD in every year during the study with

increases of greater than 10% in 3 out of the 7 years of observation. During 1993 and 2010

corn yields on CD plots were increased by more than 20% compared to FD. On average

across all years at both locations, CD increased corn yield by 11%.

Analysis of data for wheat yields indicated that CD did not significantly affect yield

for this crop (p=0.22). Wheat yields were higher on the CD treatment for only two of five

crop years, 1.6% in 1990 and a 0.7% increase in 1992. Wheat yields were lower on the CD

plots in 1998, 2008, and 2011 by 11.1%, 1.9%, and 3.1%, respectively. The lack of response

for wheat is not unexpected as it is grown in the coolest, wettest part of the year (Nov.-June)

when gains due to water conservation in some periods are potentially offset with losses due

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Table 2.1. Measured crop yields for conventional free drainage (FD) and controlled drainage (CD) for three crops on two sites in North Carolina.

Analysis of the yield data for soybean indicated that CD significantly increased yields

for that crop (p=0.008). The soybean yields in CD plots were greater than FD in all six years

observed with the highest increase of 20% in 1990. On average, CD increased soybean yield

by 10%. Water table data were available for 15 of the 18 crop/years. Analysis of the data by

production year using a paired two sample t-test for the means showed that the water table

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plots than in FD plots) (p=0.006), over all crop seasons. When the data were grouped by

crop and paired by year, results showed that the water table depth was significantly less in

the CD plots for wheat (p=0.1), and for soybean (p=0.08), compared to the respective FD

plots. Controlled drainage reduced the average water table depth by 8.5 cm (3.3 in) for

wheat and by 8.5 cm (3.3 in) for soybean. Average water table depths under CD were 6.5 cm

(2.6 in) less than FD for corn, but the difference was not statistically significant (p=0.25).

Depending on the crop and year, outlet elevations in the CD treatments were set 30 to 50 cm

below the soil surface (Table 2.1). The drains were 107 to 120 cm deep, so CD resulted in a

substantial decrease in the effective drain depth during the treatment period. Thus the

relatively small effect of CD on water table depth may at first seem unusual. Measured water

table depth is plotted for the 1993 corn growing season in Figure 2.4 and for the 1994

growing season in Figure 2.5. Results plotted in Figures 2.4 show that the water tables in

both CD and FD plots were below the drain depth, and hence unaffected by drainage

treatment for relatively long dry periods in 1993. Similar results were obtained in most other

years (not shown). Controlled drainage does have an effect on water table depth during

wetter periods (e.g., days 140-160 in 1993) as expected. However, the nearly negligible

impact of CD during dry periods causes its effect on average water table depth to be

relatively small in most years. The effect was larger during wet growing seasons such as

1994 (Figure 2.5). Rainfall during the 1994 growing season was above average (Table 2.1

and Figure 2.6) and drainage from the FD plots occurred during most of the time. Controlled

drainage, with outlet weirs set at a depth of 30 cm, decreased subsurface drainage by 14.3