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E F F E C T S O F DRAINAGE AND WATER TABLE CONTROL ON GROUNDWATER AND SURFACE WATER Q U A L I T Y P a r t 1 - M e t h o d o l o g y and P r e l i m i n a r y R e s u l t s

C . ~ u n s t e r l , G . M. C h e s c h e i r l , R . W . Skaggsl, J . E . ~ a r s o n s l , T . J . s h e e t s 3 , J . W. ~ i l l i a m ~ , R . 0. ~ v a n s l ,

E . arms en^, and R . B . ~ e i d y ~

D e p a r t m e n t of B i o l o g i c a l and A g r i c u l t u r a l E n g i n e e r i n g 1 D e p a r t m e n t of S o i l s c i e n c e 2

D e p a r t m e n t of ~ o x i c o l o g y ~

N o r t h C a r o l i n a S t a t e U n i v e r s i t y R a l e i g h , NC 2 7 6 9 5 - 7 6 2 5

The research on which this publication is based was supported in part by funds provided by The University of North Carolina Water Resources Research Institute. Additional support was provided by the North Carolina Agricultural Research Service, and by the University of North Carolina Sea Grant College Program.

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ACKNOWLEDGMENTS

T h i s r e s e a r c h was conducted i n t h e Department of B i o l o g i c a l and Agr i c u l t u r a l E n g i n e e r i n g , t h e Department of S o i l S c i e n c e , and t h e P e s t i c i d e Residue Research L a b o r a t o r y a t North

C a r o l i n a S t a t e U n i v e r s i t y . The r e s e a r c h was s u p p o r t e d i n p a r t by f u n d s p r o v i d e d by The U n i v e r s i t y of North C a r o l i n a Water R e s o u r c e s Research I n s t i t u t e . A d d i t i o n a l s u p p o r t was p r o v i d e d by t h e North C a r o l i n a A g r i c u l t u r a l R e s e a r c h S e r v i c e , and by t h e U n i v e r s i t y of North C a r o l i n a Sea Grant C o l l e g e Program.

F i e l d r e s e a r c h f o r t h i s p r o j e c t was c o n d u c t e d a t t h e T i d e w a t e r Research S t a t i o n i n Plymouth, N C . We a r e i n d e b t e d t o John

S m i t h f o r h i s a d v i c e and c o n s t a n t s u p p o r t t h r o u g h o u t t h i s

p r o j e c t . We thank Dr. P a u l L i l l y and Don Davenport f o r t h e i r agronomic a d v i c e and a s s i s t a n c e . We a l s o a p p r e c i a t e t h e

v a l u a b l e e f f o r t s of C h a r l e s Luten and t h e farm s t a f f f o r t h e i r a s s i s t a n c e d u r i n g t h e i n s t a l l a t i o n of t h e p r o j e c t . We thank Eugene Boyce f o r h i s a s s i s t a n c e and f o r b e i n g t h e l o c a l man on t h e s c e n e when i n s p e c t i o n or r e p a i r was n e e d e d .

S p e c i a l t h a n k s a r e e x p r e s s e d t o Wilson H u n t l e y and Char1 i e W i l l i a m s f o r t h e i r t i r e l e s s e f f o r t i n p r o v i d i n g t h e t e c h n i c a l a s s i s t a n c e needed t o i n s t a l l and m a i n t a i n t h i s p r o j e c t . We acknowledge and a p p r e c i a t e t h e v e r y s u b s t a n t i a l e f f o r t of B e r t h a C r a b t r e e i n p r o v i d i n g c h e m i c a l a n a l y s e s on t h e

m u l t i t u d e of water and s o i l samples g e n e r a t e d i n t h e p r o j e c t . Our a p p r e c i a t i o n i s e x p r e s s e d t o W . L . J o n e s , Thad Tremaine, and Jamie Boyd f o r t h e c o l l e c t i o n and p r e p a r a t i o n of p e s t i c i d e s a m p l e s . We a l s o a p p r e c i a t e t h e e f f o r t and e x p e r t i s e of J i m Laws and J i m Y e a t t s f o r p e r f o r m i n g t h e p e s t i c i d e a n a l y s e s on s o i l and water s a m p l e s .

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ABSTRACT

A 13.8 ha drained agricultural field located on the North Carolina Coastal Plain was instrumented to monitor field

hydrology and movement of pollutants in the soil, groundwater, and surface water. Different water table management treatments

(conventional drainage, controlled drainage, and subirrigation) were implemented on experimental plots to study the effect of water management on field hydrology and pollutant movement. This report focuses on the description of the site, the installation and design of the data collection system, preliminary results from the field site, and initial modelling of the site hydrology and pollutant movement. The soil on the research site is a

Portsmouth sandy loam with a low conductivity sandy clay loam layer located in the soil profile approximately at the depth of the drain tubes (0.8 to 1.1 m)

.

This restrictive layer reduced the effectiveness of the drain tubes and hindered comparison of the different water management treatments. At the end of the project period, new drain tubing was installed at deeper depths

(1.2 to 1.4 m) to improve drainage and reveal differences between water management treatments. Concentrations of the pesticide metolachlor determined in soil and drainage water were generally low and decreased with time after application.

The water management model DRAINMOD was coupled with the erosion and chemistry submodels of CREAMS and used to study nutrient and pesticide losses in surface runoff for a range of drain spacings and water table management scenarios. Simulated results

indicated that surface drainage improvements can significantly increase chemical losses. The surface losses can be reduced with improved subsurface drainage. However, this may increase

subsurface losses of mobile, non-adsorbed contaminants.

Agronomic benefits can be achieved with controlled drainage and subirrigation but this will increase runoff and hence surface losses of nutrients and pesticides in some cases. A model for estimating variably saturated 2-dimensional nitrogen transport was developed from the USGS VS2DT model and used to study

subsurface movement of nitrogen for different water and

fertilizer management scenarios. Simulated results indicated that controlled drainage with a single application of N reduced total nitrate-N loss to the drain tile by 46% and increased N uptake by the plants when compared to the free drainage system. By using the controlled drainage system but splitting the

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TABLE OF CONTENTS

. . .

ACKNOWLEDGEMENTS iii

ABSTRACT

. . .

v

LIST FIGURES LIST OF TABLES

. . .

x v SUMMARYANDCONCLUSIONS

.

.

.

.

.

.

.

.

.

.

.

.

.

.

x v i i RECOMMENDATIONS

. . .

INTRODUCTION 1

. . .

FIELD STUDY EXPERIMENTAL METHODS 5

. . .

THERESEARCHSITE 5

. . .

WELL NESTS 10

. . .

Well Design 10

. . .

Well Installation 10 DATA COLLECTION

. . .

11

~nstrumentation

. . .

19

The personal Computers

. . .

20

. . . .

Field Layout of the Data Collection System 20 Computer Room Wiring Layout

. . .

20

Data Collection Program

.

.

.

.

.

.

.

.

.

23

DataReduction

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

24

. . . .

~etermination of Drainage Rates and Volumes 26 Data Acquisition Problems

. . .

28

Future Plans

. . .

31

MONITORING WELL WATER LEVELS

.

.

.

.

.

.

.

.

.

.

.

31

DETERMINATION OF SOIL PROPERTIES

.

.

.

.

.

.

.

.

.

32

Soilsampling

. . .

32

Hydraulic Conductivity

. . .

33

. . .

SoilTexture 35 BulkDensity

. . .

35

. . .

Porosity 35 Totalcarbon

. . .

35

Soilwatercontent

. . .

36

Soil pH

. . .

36

Water pH

. . .

36

Soil Water Characteristic

. . .

36

SOIL AND HYDROLOGY RESULTS FOR THE FIELD STUDY

.

37 SOILPROPERTIES

. . .

37

Saturated Hydraulic Conductivity

. . .

37

SoilTexture

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

42

BulkDensity

. . .

50

Porosity

. . .

50

Totalcarbon

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

50

Soilwatercontent

. . .

50

(8)

Water pH

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

50

. . .

Soil Water Characteristic 75

Soil Water Content at 15 Bar Pressure

.

75

. . .

WATER TABLE AND SOIL WATER PRESSURE HEAD RESPONSE 79 PESTICIDERESULTS FORFIELDSTUDY

.

.

.

.

.

.

.

.

.

.

.

.

91

PESTICIDE APPLICATION

. . .

91 SAMPLING

. . .

91

. . .

ANALYTICALMETHODS 93

Filter Paper for Confirmation of Application Rate . 93

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Soil 1989 and 1990 93

.

. . .

Soil 1991 94

The 1989 Procedure for Surface Runoff, Tile

Drainage, and Well Water

. . .

94 Filter Paper Soxhlet Extraction Procedure

.

95 The 1990 Step-Wise Procedure for Surface Runoff.

TileDrainage. andwellwater

. . .

96 The 1991 Procedure for Surface Runoff. Tile

. . .

Drainage. and Well Water 97

. . .

QualityControl Samples 97

RESULTS

. . .

97

. . .

First Year - 1 9 8 9 97

.

Secondyear 1990

.

.

.

.

.

.

.

.

.

.

.

.

.

.

98

.

. . .

Thirdyear 1991 100

PREDICTING THE FATE OF PHOSPHORUS AND PESTICIDES

WITH DRAINMOD/CREAMS

. . .

115 INTRODUCTION

. . .

115 THEMODELS

. . .

116 INPUTDATA

. . .

- 1 1 8 RESULTSANDDISCUSSION

. . .

120 SUMMARY

. . .

129

. . .

VARIABLY SATURATED 2-DIMENSIONAL NITROGEN TRANSPORT

INTRODUCTION

. . .

. . .

MODEL DESCRIPTION

. . .

Water Movement

. . .

Solute Movement

. . .

EXPERIMENTAL PROCEDURES

Field Site

. . .

. . .

Field and Laboratory Measurements

. . .

PARAMETER EVALUATION

. . .

Hydrologic Parameters

. . .

Transport Parameters

. . .

Transient Input Variables

. . .

Initialconditions

. . .

RESULTSANDDISCUSSION

Midpoint Water Table Elevation. Cumulative

. . .

Drainage and Soil Moisture

. . .

Soil Nitrogen Concentrations

Nitrogen Uptake by Plants

. . .

. . .

Nitrogen in Drainage Water

Nitrogen Mass Balance

. . .

EXAMPLE OF ESTIMATING WATER MANAGEMENT AND FERTILITY

(9)

TREATMENT DIFFERENCES

. . .

155

MODELLIMITATIONS

. . .

160

SUMMARY AND CONCLUSIONS . . . 161

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LIST OF FIGURES

Figure 1: Research site layout at the Tidewater Experiment

Station (6/90)

. . .

7

Figure 2: Drainage collection and control tanks (elevation view)

. . .

8

Figure 3: Drainage collection and control tanks (plan view)

. . .

9

Figure 4: Well nest locations at plots 1. 2 and 3

.

. . . . 12

Figure 5: Monitoring well with well head device

. . .

13

Figure 6: Well head data collection device (elevation view)

. . .

14

. . . .

Figure 7: Well head data collection device (plan view) 15 Figure 8: Monitoring well design

.

.

.

.

.

.

.

.

.

16

Figure 9: Well jet device used to install monitoring wells

. .

17

F i g u r e 1 0 . W e l l j e t d e t a i l s

. . .

18

Figure 11: Site data acquisition wiring layout

.

21 Figure 12: Computer room 2 wiring layout

.

.

.

.

.

.

.

.

22

Figure13: Flowmetercalibration

.

.

.

.

.

.

.

.

.

.

.

.

25

. . .

Figure 14: Control tank drainage, plot 3. 11-12-90 27 Figure 15: Drainage tank water level elevations, plot 3 , 11- 01 to 11-05-90

.

. . .

29

Figure16: Particle sizeanalysis

. . . 49

Figure 17: Soil classification, plot 1

.

.

.

.

.

.

.

.

.

.

54

Figure 18: Soil classification, plot 2

. . .

55

Figure 19: Soil classification, plot 3

.

.

.

.

.

.

.

.

.

.

56

Figure 20: Total organic carbon

. . .

57

Figure 21: volumetric soil moisture content, plot 1

.

58 Figure 22: Volumetric moisture content, plot 2

.

.

.

59

Figure 23: Volumetric moisture content, plot 3

.

60 Figure 24: pH analysis, plot 1, soil

. . .

61

Figure 25: pH analysis, plot 2, soil

. . .

62

Figure 26: pH analysis, plot 3, soil

. . .

63

Figure 27: pH analysis, plot 1, control point well nest

.

64 Figure 28: pH analysis, plot 1, midpoint well nest

.

65 Figure 29: pH analysis, plot 1, drains and surface runoff . . 66

Figure 30: pH analysis, plot 2, control point well nest

.

.

67

Figure 31: pH analysis, plot 2, quarter point well nest . 68 Figure 32: pH analysis, plot 2, midpoint well nest

.

69 Figure 33: pH analysis, plot 2, drains and surface runoff .

.

70

Figure 34: pH analysis, plot 3, control point well nest

.

71 Figure 35: pH analysis, plot 3, midpoint well nest

.

72 Figure 36: pH analysis, plot 3, drains and surface runoff .

.

73

Figure 37: pH analysis, Tidewater Research Station, irrigation wells and ditch

. . .

74

Figure 38: Soil water characteristic, 0.08 m depth

.

.

.

76

Figure 39: Soil water characteristic, 0.57 m depth

.

77 Figure 40: Soil water characteristic, 0.79 m depth

.

78 Figure 41: Water table depth, control point well nest, 8-22 to 8-24-90

. . . .

81

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Figure 43: Water table depth, 24-90.

. . .

Figure 44: Water table depth,

to 9-7-90.

. . .

Figure 45: Water table depth,

to 11-1-90.

. . .

Figure 46: Water table depth,

to 11-4-90.

. . .

Figure 47: Water table depth,

. . .

to 11-13-90.

Figure 48: Water table depth, to 8-7-90.

. . .

midpoint well nest, 8-22 to 8-

. . .

control point well nest, 8-27

. . .

control point well nest, 10-24

. . .

control point well nest, 11-1

. . .

control point well nest, 11-5

. . .

control point well nest, 8-1 Figure 49: Comparison of drain spacings on simulated runoff

losses of phosphorous with controlled drainage.

Results are based on simulations for a Portsmouth soil with a relatively steep slope of 2%.

. . .

Figure 50: Simulated annual drainage and runoff volumes for

controlled drainage. . . Figure 51: Simulated metolachlor losses in surface runoff

for conventional drainage with good and poor surface

. . .

drainage.

Figure 52: The effect of drain spacing on simulated annual drainage and runoff volumes for conventional drainage system design spacings.

. . .

Figure 53: Modelled cross-section of tile drained field. . Figure 54: Plan view of Plymouth study site.

. . .

Figure 55: Vertical cross-section of Plymouth experimental

site.

. . .

Figure 56: Percent sand, silt and clay with depth at

Plymouthsite. . . Figure 57: Total elemental nitrogen with depth at Plymouth

site.

. . .

Figure 58: Observed and calculated (VS2DNT) m i d - ~ o i n t -C water

table elevation and drainage volume. . . Figure 59: Vertical hydraulic gradient at restrictive

layer.

. . .

Figure 60: Observed and calculated (VS2DNT) volumetric

moisture content with time for three upper depths. Figure 61: Observed and calculated (VS2DNT) volumetric

. . .

moisture content with time for lower depths.

Figure 62: Observed and concentration with Figure 63: Observed and concentration with Figure 64: Observed and concentrati'on with Figure 65: Observed and concentration with Figure 66: Observed vs.

calculated (VS2DNT) nitrate-N time for three upper depths.

.

calculated (VS2DNT) nitrate-N time for lower three depths.

.

calculated (VS2DNT) ammonium-N time for three upper depths.

.

calculated (VS2DNT) ammonium-N time for lower three depths.

.

calculated (VS2DNT) normalized nitrogen use by wheat crop with time. Observed was winter wheat grown in the coastal plain region of

. . .

Virginia taken from Alley et al. (1990).

Figure 67: Observed and calculated (VS2DNT) drainage water

. . .

nitrate-N concentration with time.

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Figure 68: Soil vs. well nitrate-N concentration.

. . .

Figure 69: Simulated conditions used for the example

. . .

problem.

Figure 70: Drainage water nitrate-N concentrations with time

. . .

for the four treatments.

Figure 71: N i t r a t e 4 concentration with depth and time at the tile location for the controlled drainage-single N

. . .

application treatment.

Figure 72: Nitrate-N concentration at the midpoint and tile locations 80 cm below ground surface (i.e. tile depth) with time for the controlled drainage-single N

. . .

application treatment.

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LIST OF TABLES Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table Table

Table 1: Vertical saturated conductivity determined by lab

tests for plot 1.

. . . .

.

. . . .

.

. .

.

.

.

. . . .

37

Table 2: Vertical saturated conductivity determined by lab tests for plot 2. .

. . . .

.

. . . .

. 38

Table 3: Vertical saturated conductivity determined by lab tests for plot 3.

. . . . .

.

. . . . .

.

.

.

. . . .

. 39

Table 4: Horizontal saturated conductivity determined by s l u g t e s t s i n p l o t l .

. . . . .

.

.

.

.

.

. . . .

39

Table 5: Horizontal saturated conductivity determined by s l u g t e s t s i n p l o t 2 .

. . . .

.

. . . .

. .

.

. 40

Table 6: Horizontal saturated conductivity determined by s l u g t e s t s i n p l o t 3 .

. . . .

.

. . . .

. .

. .

. 40

7: Average Hydraulic Conductivity Values

. . . .

.

.

. 41

8: Hydraulic Conductivity, 0.00 to 0.10 M Depth

. . .

. 41

9: ~ydraulic Conductivity, 0.10 to 0.30 M Depth

. .

. . 42

10: Hydraulic Conductivity, 0.30 to 0.50 M Depth

. .

. 43

11: Hydraulic Conductivity, 0.50 to 0.70 M Depth

. .

. 43

12: Hydraulic Conductivity, 0.70 to 1.00 M Depth

. .

. 44

13: Hydraulic Conductivity, 1.00 to 1.50 M Depth . .

.

44

14: Hydraulic Conductivity, 1.50 to 2.40 M Depth .

.

. 45

15: Particle Size Analysis, 0.05 to 0.71 M Depth .

.

. 46

16: Particle Size Analysis, 0.76 to 1.42 M Depth .

.

. 47

17: Particle Size Analysis, 1.52 to 4.47 M Depth .

.

. 48

18: S o i l B u l k D e n s i t y . . . .

.

.

.

.

. . . . .

. .

.

. 51

19: SoilPorosity.

.

. .

.

. . . .

.

. .

.

. . . .

52

20: Soil Organic Carbon Content

. . . .

.

. .

.

. . . .

53

21: ResidualMoisture Content

.

. . . .

.

. . . .

79

22: Recoveries of known amounts of metolachlor added to untreated soila.

. . .

98

Table 23: Metolachlor in soil samples taken before and after application.

.

. .

.

.

.

.

.

.

.

.

.

. .

.

.

. . .

. . .

99

Table 24: Recoveries of known amounts of metolachlor added to non-treated water. . . 100

Table 25: Metolachlor in surface runoff in 1990". . . . I 0 1 Table 26: Metolachlor in tile drainage water in 1990".

. .

. 101

Table 27: Metolachlor in water from wells sampled in 1990".

.

102

Table 27 (continued): Metolachlor in water from wells for 1990a.

. .

.

.

.

. . . .

.

. .

.

. . .

. .

- 1 0 3 Table 27 (continued): Metolachlor in water from wells for 1990a.

. . . .

.

. . .

.

. .

.

.

.

. . . .

.

. . .

104

Table 27 (continued): Metolachlor in water from wells for 1990a.

. . . .

.

. . . . .

.

. .

.

. .

. . .

. . .

105

Table 27 (continued): Metolachlor in water from wells for 1990a.

. .

.

.

. . . .

.

. .

. .

. . .

.

.

106

Table 28: Metolachlor in surface runoff in 1991". . . . I 0 7 Table 29: Metolachlor in tile drainage water in 1991". .

. .

108

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Table 30 (continued): Metolachlor in water from wells for

1991a.

. . .

110

Table 30 (continued) : Metolachlor in water from wells for

1991a.

. . .

111

Table 30 (continued): Metolachlor in water from wells for

1991a.

. . .

112

Table 30 (continued) : Metolachlor in water from wells for

1991a.

. . .

113

Table 31: The Hydrologic Parameters Required By The Erosion

Submodel Of CREAMS (USDA-SCS, 1984).

. . .

119

Table 32: Summary Of Input Data For CREAMS Erosion

Submodel. . . . 1 2 0

Table 33: Summary Of Input Data For CREAMS Chemistry

Submodel (Nutrients) For Continuous Corn.

. . .

121

Table 34: Summary Of Updatable Input Data For CREAMS

Chemistry Submodel (NUTRIENTS) For Continuous Corn. . . 121

Table 35: Summary Of Input Data For CREAMS Chemistry

Submodel (PESTICIDES) For Continuous Corn. . . 122

Table 36: Summary Of Updatable Input Data For CREAMS

. .

Chemistry Submodel (PESTICIDES) For Continuous Corn. 123

Table 37: Summary Of The Average Annual Parameters For The DRAINMOD Simulations. Results are based on simulations for a Portsmouth soil with a relatively steep slope of

2%. . . . 1 2 4

Table 38: Summary Of The Average Annual Phosphorous Losses For The CREAMS Nutrient Simulations. Results are based on simulations for a Portsmouth soil with a relatively

steep slope of 2%. . . . I 2 5

Table 39: Summary Of The Average Annual Pesticide Surface Losses For The CREAMS Simulations. Results are based on simulations for a Portsmouth soil with a relatively

steep slope of 2%. . . . I 2 7

Table 40. Drainage Branch Of The Soil Water Characteristic For Soil Samples Obtained At Plymouth, NC. Values

Given In Table Are In Volumetric Water Content.

.

138

Table 41: Hydrologic Parameters Used In Plymouth Simulation

.

138

Table 42: Transport Parameters Used In Plymouth Simulation

.

140

Table 43: Input To VS2DNT Which Varies On A Daily Basis.

. .

142

Table 44: Initial Total Nitrate-N And Ammonium-N With Depth

In Micrograms Per Gram.

. . .

144

Table 45: VS2DNT Simulated Results VS. Observed Data.

.

146

Table 46: VS2DNT Simulated Components Of N Cycle For

Plymouthsite.

. . .

156

Table 47: Partial Water Budget From Hypothetical

Simulation.

. . .

157

Table 48: Partial Nitrogen Budget From Hypothetical

Simulation.

. . .

158

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SUMMARY AND CONCLUSIONS

An agricultural field located on the North Carolina coastal plain was instrumented to monitor field hydrology and movement of

nutrients and pesticides in the soil, groundwater, and surface water. Different water table management treatments (conventional drainage, controlled drainage, and subirrigation) were implement- ed on experimental plots to study the effect of water management on field hydrology and pollutant movement. The water management model DRAINMOD was coupled with the erosion and chemistry sub- models of CREAMS and used to study nutrient and pesticide losses in surface runoff for a range of drain spacings and water table management scenarios. A model for estimating variably saturated

2-dimensional nitrogen transport was developed from the USGS

VS2DT model and used to study subsurface movement of nitrogen for different water and fertilizer management scenarios. During work on this project, the overall objectives became part of a larger project funded by the USGS Matching Grants Program; consequently, this initial report focuses on the description of the site, the installation and design of the data collection system,

preliminary results from the field site, and initial modelling of the site hydrology and pollutant movement. Field results

comparing water management treatments and verifying simulation models will be published in the final report for the overall project entitled "Effects of drainage and water table control on groundwater and surface water qualityH.

The 13.8 ha field experiment is drained by subsurface drain tubes placed 23 m apart and 1.0 m deep. The field was instrumented to monitor field hydrology and nutrient and pesticide movement. Surface and subsurface drainage rates and water table elevations were continuously recorded by flowmeters and potentiometers

interfaced with a personal computer. Water quality samples were automatically collected from surface and subsurface drainage water and manually collected from groundwater sampling wells. Soil samples were collected throughout the field for determining soil texture, bulk density, porosity, carbon content, water

content, vertical hydraulic conductivity, and pH. Horizontal hydraulic conductivity was determined by slug tests.

The soil on the research site is classified as a Portsmouth sandy loam. Bulk density values ranged from a low of 1.09 g/cm3 at the surface to 1.61 g/cm3 at a depth of 0.46 m. Porosity values

ranged from 0.43 at the surface to 0.31 at a depth of 1.12 m. The total organic carbon content of the soil was approximately 4% from the surface to a depth of 0.25 m. From 0.25 m to 0.50 m, total organic carbon content declined to 0.25% and remained at this level to at least a depth of 2.5 m. The values for soil pH were the highest at the surface, approximately 5.5, and steadily declined to a value of approximately 4.0 at a depth of 0.76 m. Soil water pH ranged from 4.5 at depths near 0.6 m to 6.2 at depths greater than 1.0 m.

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some cases. The water solubility and chemical application

practices, as well as the water management system design were all important interacting factors affecting surface losses. Since neither DRAINMOD nor CREAMS consider nutrient or pesticide

transformations in the soil, routines for simulating these

processes are needed to improve the models for predicting losses of nutrients and pesticides in subsurface drainage water and their response to controlled drainage and subirrigation. Al- though the reliability of both DRAINMOD and CREAMS have been tested individually, rigorous tests of the combination model are essential if this tool is to be used for a wide range of water management, fertility and pesticide practices.

A model for estimating variably saturated 2-dimensional nitrogen transport was developed from the USGS VS2DT model. The new

model, VS2DNT, in general, performed quite well. However, both the nitrate-N and ammonium-N lost via subsurface drainage were overestimated. On a relative basis the overestimates were large, but on an absolute basis they were small (1-2 kg/ha).

A hypothetical simulation comparing two water management treat- ments and two fertility treatments for a corn season was present- ed. The purpose of the hypothetical simulation was to illustrate the potential usefulness of the model for evaluating management practices. The results indicated that by using controlled

drainage with a single application of N the total nitrate-N loss to the drain tile was reduce by 46% and that N uptake by the plants was increased 6%, relative to the free drainage system. By using the controlled drainage system but splitting the nitro- gen application the drainage water quality benefit was only

slightly reduced (46% to 43%) but nitrogen uptake was increased dramatically from 6% to 27%, relative to the free drainage-single N application treatment. The usefulness of the model may be

limited by the large computer time and input data requirement.

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RECOMMENDATIONS

This initial report focuses on the description of the site, the installation and design of the data collection system, and

preliminary results from the field site and from the initial modelling of hydrology and pollutant movement. Consequently these recommendations result from our experiences with the experimental methods and from our preliminary results.

Recommendations resulting from the final results of the overall project will be included in the final report for this project. The field collection system developed and installed in this study is producing a very detailed data set of the hydrology and

pollutant movement in poorly drained agricultural soils. While the system is automated, it has required a substantial effort to assure that it is functioning correctly. We recommend redundancy within the system to assure that a complete and continuous data set is collected even if part of the system fails. Most failures of the system have occurred from corroded or loose wiring

connections or from voltages surges. We recommend the use of soldered connections instead of the more convenient plug in connections. We also recommend the use of transorbs to protect the computer from voltage surges. For the Portsmouth soils at this site, undisturbed soil samples taken by the 90 cm long soil probe for hydraulic conductivity analyses were difficult to

handle and may have been disturbed due to soil compaction. We recommend that undisturbed soil core be taken in 15 cm increments rather than at 90 cm increments to reduce soil compaction and to simplify sample handling.

Preliminary results from simulations using the combination DRAINMOD/CREAMS model and the modified USGS VS2DT model has supported findings from previous research. These simulations showed that surface drainage improvements can significantly

increase chemical losses. The surface losses can be reduced with improved subsurface drainage. However, this may increase

subsurface losses of mobile, non-adsorbed contaminants.

Controlled drainage reduces total nitrate-N loss to the drain tile and can increase agronomic benefits, but controlled drainage will increase runoff and hence surface losses of nutrients and pesticides in some cases. We recommend that these effects of water table management be carefully considered when designing water table management systems.

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INTRODUCTION

Protection of the nation's ground and surface waters is of vital importance. Over the past 25 years, attention has been focused on point source pollution of surface and groundwater systems. More recently, concern has increased about the effects of non- point sources on both groundwater and surface water quality. The primary concern about nonpoint source groundwater pollution

involves pesticides and fertilizer nutrients, mostly nitrates. Holden (1986) and Ritter (1986) reported detection of widely used herbicides such as alachlor, atrazine, metolachlor, and cyanazine in the groundwater in several states. Other pesticides of

concern include fumigants such as DCP and DBCP (Cohen et al., 1984) and aldicarb (a carbonate insecticide) which has been found in groundwater in New York, Florida, North Carolina, and several other states fumigants such as DCP and DBCP (Cohen et al., 1984). The development of water management, fertility, and pesticide practices to reduce nonpoint source pollution of surface waters

is a high priority. Again, the primary pollutants of concern are pesticides, fertilizer nutrients (N and P), and sediment.

Generally, the goal of management practices used to control

surface runoff and nonpoint source pollution of surface waters is to increase retention of water on the land and increase infiltra- tion. In some cases, these practices may have a detrimental

effect on the quality of groundwater. For example, installation of subsurface drains in poorly drained North Carolina soils will reduce surface runoff, sediment, and P loadings to surface waters

(Deal et al., 1986), but will increase nitrates in the shallow groundwater and nitrate outflows in the subsurface drainage water

(Gambrel1 et al., 1975a). Thus it is important to understand the linkage between surface water management and groundwater impacts. This project addresses the effects of agricultural water manage- ment practices on pollutant movement from poorly drained soil to

shallow groundwater and to surface water via both surface and subsurface runoff. These are important soils from both agricul- tural and environmental perspectives. Over 25 percent (110

million acres) of the total cropland in the United States re- quires improved drainage for agricultural production. In North Carolina 2.2 million acres or about 40 percent of our cropland is on soils that are poorly drained under natural conditions and require drainage improvements. Most of these lands are in the Coastal Plains and Tidewater regions of Atlantic Coastal states, where they are close to environmentally sensitive waters.

Effects of water management, fertility, and pesticide practices on pollutant loading from these lands are magnified because of the short time required for the outflow water to appear in the receiving waters.

Aquatic biologists and fishermen generally believe that agricul- tural drainage has been detrimental to the productivity of

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quality in both inland and coastal rivers and streams. Previous research (Gilliam et al., 1978; Gambrel1 et al., 1975b; Skaggs and Gilliam, 1981; Gilliam and Skaggs, 1986) has focused on

determining the effects of various water management practices on the rate and quality of runoff from,the poorly drained soils. Field studies (Gilliam et al., 1979; Gilliam and Skaggs, 1986) have shown that nitrate loss to the environment can be greatly reduced by the use of controlled drainage during certain periods of the year. The effectiveness of this practice depends on the intensity of management. Because of the potential environmental benefits of controlled drainage, it has been accepted as a Best Management Practice by the regulatory agencies in North Carolina. Structures to achieve control have been cost-shared by the State of North Carolina in nutrient sensitive watersheds for the past three years. Farmers have also readily accepted controlled drainage because it conserves water and increases yields. Control structures have been placed in ditches draining over 200,000 acres in North Carolina. Based on results of field experiments on several soils, it is estimated that nitrate

nitrogen outflows from the controlled areas have been reduced by over one million pounds annually. This practice has expanded to other areas along the Atlantic Coast with new programs to cost share structures for water quality purposes in Virginia, Dela- ware, and Maryland. Frequent inquiries from both regulatory and agricultural agency personnel in other Atlantic Coast states

indicate that interest in this practice is widespread. A closely related water management practice involves pumping water into the controlled outlet to raise the water table in the field. This practice, called subirrigation, provides both irrigation and drainage in one system and is also being rapidly accepted by

farmers in the coastal area. Protasiewicz et al. (1989) reported that subirrigation decreased nitrogen loading, but increased

losses of both phosphorus and atrazine from clay soils in Michi- gan for one year, 1987. Both controlled drainage and subirriga- tion are growing in popularity as farmers learn of their advan- tages; thus they have the potential for dramatic increases in use over the next few years.

Methods for estimating nutrient losses as related to drainage system design and management have been developed (Deal et al. 1986). However, these methods are based on field experimental data.and can be used with confidence for only a few soils. They cannot be used to determine the best way to manage controlled drainage systems to minimize nutrient loading to receiving waters, nor can the effects of increased or decreased use of

fertilizer or changes in the timing of fertilizer application be predicted. Pollutant loading from agricultural lands is clearly affected by cultural practices such as crop rotation and tillage methods, as well as water management. The effects of these

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management and cultural practices on sediment and pesticide losses from agricultural lands.

USDA Agricultural Research Service Researchers have developed the models CREAMS (Knisel et al., 1983) and GLEAMS (Leonard et al.,

1987) for predicting the effects of cultural practices such as reduced tillage and contour farming on the movement of sediment, pesticides, and fertilizer nutrients from agricultural lands. However, both models were developed for upland conditions and are not directly applicable to poorly drained soils. They do not consider subsurface drainage and water table control processes, which are dominate in soils with high water tables. The water management model, DRAINMOD (Skaggs, 1978) was developed to quantify the hydrology of high water table soils and could be used to simulate the hydrology for the CREAMS and GLEAMS models. The development and testing of a model to predict the effects of water management, cultural and fertility practices on pollutant loading to surface waters and to shallow groundwater is the goal of our study. The objectives of this research project were to:

1. Experimentally determine the effects of drainage, controlled drainage, and subirrigation on losses of pesticides, nutri- ents, and sediments via surface and subsurface drainage.

2. Combine the simulation model DRAINMOD with CREAMS and GLEAMS to predict the movement of sediment, nutrients, and pesti- cides from poorly drained agricultural soils.

3. Use the results of the field experiments to test the validi- ty of the combination model for-conventional drainage,

controlled drainage, and subirrigation.

4. Document and package the models for application by scien- tists and engineers.

During work on this project, the above objectives became part of a larger project I1Effects of drainage and water table control on groundwater and surface water quality" funded by the USGS Match- ing Grants Program. Also, the effectivness of the the original drainage system was found to be reduced due to a restrictive

layer located in the soil profile at approximately the same depth as the drain tubes. Since the term of the overall project was lengthened and the original drainage system hindered comparison between different water management treatments, an effort to improve the drainage system design was made at the end of this WRRI Project. New drain tubing was installed at deeper depths to

improve drainage and reveal differences between water management treatments for the continuation of the overall project. Conse- quently, this report focuses on the description of the site, the installation and design of the data collection system, initial modelling of the site hydrology and pollutant movement, and

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FIELD STUDY EXPERIMENTAL METHODS

THE RESEARCH SITE

A 13.8 hectare (34.0 acre) agricultural field was instrumented to study the movement of nutrients and pesticides in the soil,

groundwater and surface runoff. The site is located on the

Tidewater Research Station, near Plymouth, in the North Carolina Coastal Plain approximately 150 miles east of Raleigh.

The research site was instrumented for water table management studies with initial installation beginning in 1988. The field is bounded on all four sides by drainage ditches approximately 2.0 m (6.6 ft) deep. Plastic drainage tubes, 101 mm (4.0 in.) in diameter, were installed in 1985. They were 22.9 m (75.0 ft) on center, and buried 0.8 to 1.1 rr~ (2.6 to 3.6 ft) below the surface

(Figure 1). Three water table management treatments: free or conventional drainage, controlled drainage and subirrigation can be implemented using these subsurface drains. All drainage and subirrigation through the subsurface drains is measured.

The site is divided into six, 1.7 hectare (4.3 acre), experimen- tal plots. These plots are delineated by the area drained by three adjacent subsurface drains. Each experimental plot has an underground vault and instrument house (Figure 2). Each under- ground vault intercepts the drainage outflow from the three

adjacent subsurface drains as well as the surface runoff from two field catch basins. The field catch basins collect surface

runoff from two in-field surface runoff plots approximately 6.1 rn (20.0 ft) wide by 30.5 m (100.0 ft) long.

Each vault contains three cylindrical PVC holding tanks that are 0.61 m (2.0 ft) in diameter and 1.83 m (6.0 ft) in length. These three holding tanks intercept water from the field as follows: the control tank receives the water from the middle subsurface drain (control drain), the surface runoff tank receives water

from the two surface runoff collectors, and the guard tank receives water from the two outside subsurface drains (guard drains). All three holding tanks are equipped with sump pumps and control floats that automatically pump water from the holding tanks to the drainage ditch outlet (Figure 3).

The function of the guard drains is to separate the experimental plots. These two outside drains function to hydraulically

isolate the area that is drained by the middle control drain from the influence of adjacent experimental plots.

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elevation, which is higher than the drain. The pumps come on when the water level exceeds the set point and go off when the tank water level falls to the set point. No water is pumped in to maintain the control water level elevation in controlled

drainage. In subirrigation, the water level in the holding tank is maintained at a set point above the field drain outlet.

Irrigation water from one of the two shallow irrigation wells is pumped in to replace water lost from the holding tanks via

subirrigation; when rainfall occurs, drainage water is pumped out of the tanks to maintain the subirrigation set point.

Drainage outflow and subirrigation data were collected and processed by personal computers located in climate controlled rooms in equipment houses 2 and 5. The computer in house 2

collects data from houses 1, 2 and 3 while the computer in house 5 collects data from houses 4 , 5 and 6.

Each equipment house has a refrigerator with a freezer compart- ment to preserve samples. In each refrigerator, there are two

large sample containers which receive a portion of drainage from the control drain and surface runoff plots. Flexible, 12 mm (0.5 in.) diameter tubing, is connected to the discharge pipes from the control drain tank pump and the surface runoff tank pump. This flexible tubing passes through the wall of the refrigerators and discharges into the sample containers. Every time the

control tank pump or the surface runoff tank pump comes on, a

portion of the discharge flows into the refrigerated sample containers.

Each experimental plot has two 101.6 mm (4 in.) water table wells equipped with automated water level monitoring equipment. These wells, which are located midway between the control drain and each guard drain approximately 76.2 m (250 ft) from the eastern edge of the field, are equipped with a float, potentiometer mounted on a Stevens Recordera platform and an OmniData pod@. The research site has a complete Campbell SR-lo0 weather station as well as two additional automated rain gages. A deep irriga- tion well, 91.4 M (300 ft) deep, and two shallow irrigation

wells, 22.9 M (75.0 ft) deep, are located on the eastern edge of the field.

The field, which was cleared for agriculture in 1975, is nearly flat. The soil is classified as a Portsmouth sandy loam (Typic Umbraquult; fine-loamy, siliceous, thermic)

.

These series are very poorly drained soils that formed in loamy fluvial and marine sediments (Tant, 1981). The Ap horizon is a black fine sandy loam 0.30 m (12 in.) thick with an organic content in the 3 to 5%

range. Various layers of fine sandy loam extend from the Ap horizon down to a sandy clay loam located at 0.50 to 0.90 m (23 to 35 in.). The sandy clay loam is followed by a sand loam and then at 0.97 to 1.22 m (38 to 48 in.) a grey sand. Coarse sand is found from 1.22 to 2.44 m (48 to 96 in.). The coarse sand is underlain by a marine clay deposit that is approximately 6.1 m

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-SURFACE RUNOFF COLLECTORS

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WELL NESTS

A series of six to seven wells, referred to as a well nest, were installed in lines parallel to the drainage tiles. Wells in each nest were spaced approximately 0.31 m (1.0 ft) horizontally and varied in depth from 0.40 m (1.3 ft) to 2.25 m (7.4 ft). Each well in a nest was screened in a distinct soil layer. A total of 45 wells were installed in seven well nests.

The number, location, and depth of the wells were designed to provide the minimum number of sampling points that would permit a detailed analysis of the movement of the pesticide in the flow domain. Two well nests were located 11.40 m (37.5 ft) and 0.30 m

(1.0 ft) from the control line in plots one, two and three

(Figure 4). These well nests are referred to as the midpoint well nest and the control point well nest respectively. An additional well nest was installed in plot two, 5.70 m (18.8 ft) from the control line. This well nest is named the quarter point well nest. One well in each well nest, referred to as a water table well, is screened the entire length.

Each monitoring well in plot two has approximately 0.91 meters (3.0 ft) of well casing extending above the ground surface and an associated weight well (Figure 5). These wells were equipped with a well head devices to continuously measure and record water levels (Figures 6 and 7).

Well Desiqn. The monitoring wells were designed to provide groundwater samples and hydraulic head measurements at discrete points within the soil profile. For this purpose, a 50.8 mm (2 in.) diameter polyvinyl chloride (PVC) well casing, with flush threaded joints, 152 mm (6 in.) slotted screen and 152 mm (6 in. )

reservoir was selected. These wells, which were designed to act as piezometers, included a granular filter pack around the screen and a bentonite clay seal around the well casing (Figure 8).

Well Installation. The 51 mm (2 in.) diameter PVC monitorina wells were installed by either augering or jetting. The we& less than 0.9 m (3.0 ft) were installed with an auger. Wells deeper than 0.9 m (3.0 ft) were installed by jetting.

The 102 mm (4 in. ) auger holes less than 0.9 m (3.0 ft) deep

remained open permitting the installation of the well casing, the filter pack and the clay seal. Auger holes deeper than 0.9 m

(3.0 ft) penetrated a sandy layer. This sand slumped into the auger hole and prevented the well from being installed as de- signed. To keep the bore hole open at depths greater than 0.9 m

(3.0 ft), a 102 mm (4 in.) diameter PVC well casing was jetted into the ground. The monitoring well was then installed properly within the 102 mm (4 in.) outer casing.

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depth (Figures 9 and 10). This device consisted of a jet head with a jetting pipe and a valve board. 1.5 m (5.0 ft) sections of 102 mm ( 4 in.) diameter well casing were jetted as follows. The jet head device was threaded onto a 1.5 m (5.0 ft) section of 102 mm (4 in.) well casing. Next, the 25 mm (I in.) diameter jet pipe was inserted into the jet head and attached to the water supply from the valve board. Then the irrigation pump was turned on to supply the valve board with jetting water. The valve to the jet head was opened and the discharge valve was closed. As the jet pipe was worked up and down to direct the pressurized water onto the soil surface, the 102 mm (4 in.) casing was pressed into the ground using the jet head handles. After the

102 mm (4 in. ) casing had been inserted, the jet head supply valve was closed and the discharge valve was opened. The jet pipe was removed and the jet head was unscrewed from the well casing. A new section of 102 mm (4 in.) well casing was threaded onto the first section of well casing. The jet head with jet pipe was reinstalled. This process was repeated until the 102 mm

(4 in.) well casing had been installed to the designed depth. The jet head assembly was then removed and the well case extrac- tor was screwed onto the 102 mm (4 in.) casing. Next, the 51 mm

(2 in.) well casing was centered within the 102 mm ( 4 in.) well casing. The granular filter pack was poured into the 102 mm (4

in.) casing to fill the annulus between the 51 mm (2 in.) well screen and the 102 mm (4 in.) outer casing. The filter pack was extended to at least 0.2 m (0.5 ft) above the well screen. Then bentonite clay pellets were poured between the 51 mm (2 in.) and 102 mm (4 in.) well casing to at least a 0.3 m (1.0 ft) above the filter pack. As the clay pellets were being poured the 102 mm (4 in.) casing was slowly extracted using the handles on the well case extractor. After the clay pellets had been installed, the extraction of the 102 mm (4 in.) well casing continues until the last section is removed.

After the last 102 mm (4 in.) outer well casing was removed, a bentonite clay slurry mixture was then poured into the open

cavity that remained around the 51 mm (2 in.) well casing. This slurry mixture generally extended from the ground surface down to the sandy layer at the 0.9 m (3.0 ft) depth. This seal prevented infiltration along the well casing from water on the surface. DATA COLLECTION

The collection of accurate data is a primary objective of this field research project. The validity of subsequent analyses is dependent on accuracy of the field data. Large scale water table management research projects require an automated data collection system to measure spatial and temporal response of dependent

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CANAL

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The holding tanks that intercept surface and subsurface drainage have been instrumented to quantify outflow. In addition, twenty- one monitoring wells, installed in three well nests in plot two, are equipped with well head devices to measure water levels. Data from monitoring devices at this site are recorded and

undergo preliminary analysis on two personal computers. Drainage and surface runoff volumes as well as water levels in the moni- toring wells are continuously collected, reduced and stored on diskettes with the corresponding time each event occurred. Instrumentation. Two transducers are used to measure drainage volumes: potentiometers and flowmeters. The Bourns 3540s-001-103 potentiometer is a simple and inexpensive device that can output

zero to two volts in ten shaft revolutions. The potentiometer output is wired to a Lawson Labs Model 17B Differential Multi- plexer (multiplexer). This multiplexer is an electronic switch- ing device that can accept up to 16 potentiometer outputs. The multiplexer is controlled by a Lawson Labs model 140 analog to digital converter card (A/D card) that resides in an expansion slot in the field computer. This A/D card has four input chan- nels that can accept analog voltages from -5.00 to 4.75 volts. The analog voltages are converted to digital equivalents with a resolution of 0.25 millivolts. Up to four multiplexers can be used with each A/D card.

The water level in each drainage tank is measured with a potenti- ometer that is mounted on a Stevens recorder platform. The

potentiometers are geared to the recorder pulley, enabling the recorder float and pulley assembly to rotate the potentiometer. For 0.305 meters (1.0 ft) of float travel (0.305 m of water level change) the potentiometer will rotate one complete revolution. Flow rates and volumes are determined from the potentiometer voltages as described in the "Data Collection Programl1 section. The other transducer used to measure flow rates and volumes is the Signet Industrial MK 515 Paddlewheel Flowsensor (flowmeter). This flowmeter induces a signal when the magnets in the

paddlewheel are moved past a coil in the main body of the flowme- ter. The flowmeters are wired to an op-amp circuit, which condi- tions the signals by converting AC voltage to frequency. The op- amp output is wired to a TC24 timer/counter card (counter card) by Real Time Devices, Inc. This counter card, which is located in an expansion slot of the computer, can maintain and output 16 bit counts from up to five counting devices simultaneously.

Flowmeters are installed in the 1 inch discharge pipes that are connected to the pump outlets in the control and surface runoff tanks. A flowmeter is also located in the 1 inch PVC line that supplies irrigation water to the control tank (Figure 3).

The Bourns potentiometers, which are also used to monitor well water levels, are mounted in a well head device that is bolted the well casing and the associated weight well. These

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The Personal Computers. Two personal computers, located in

instrument houses 2 and 5, are used to collect, process and store field data. The personal computers, which were assembled in the laboratory using primarily Jameco computer parts consists of the following components: flip top computer case, 150 watt power

supply, 8/12 Mhz 80286 AT compatible motherboard with 8 expansion slots, floppy disk drive controller, color/graphics adapter with parallel printer port, Mitsumi 1.44 MB diskette drive, 512 KB RAM memory, keyboard, and a color display.

Field Layout of the Data Collection System. Computer rooms have been built in instrument houses 2 and 5. These insulated rooms are environmentally controlled with heating and cooling and are sealed to prevent dust and rodent intrusion. Each computer has an emergency battery power source which provides backup power for approximately 15 minutes and transient voltage surge protection. All electronic data collection devices are wired to these two computer rooms.

The computer in instrument house 2 collects data from plots 1, 2, and 3. The 21 monitoring wells are also wired into the instru- ment house 2 computer room. The computer in instrument house 5 collects data from plots 4 , 5, and 6 (Figure 11).

There are three flowmeters and three potentiometers in each instrument house. Leads from these six devices are wired to control panels mounted on the walls of houses 1, 3 , 4 , and 6.

Underground signal wires connect the control panels with a master control panel located in the computer room. In instrument houses 2 and 5 the leads from the data collection devices are wired

directly to the master control panel.

Computer Room Wiring Layout. The master control panel in each computer room is wired to nine potentiometers and nine flow-

meters. From the master control panel, the nine flowmeter inputs are wired directly to the two counter cards using 40 pin connec- tors. The nine potentiometer inputs are wired from the master control panel to the multiplexer. The multiplexer is connected to the A / D card using a 17 pin connector (Figures 11 and 12). The leads from the 21 potentiometers, mounted on the monitoring wells in research plot 2, are connected to a well control panel in the computer room in instrument house 2 by a 25 conductor wire laid on the ground surface. This conductor can be disconnected from the well potentiometers and removed from the field during tillage, harvesting, and other field operations.

From the well control panel, 16 leads are wired to multiplexer number 2 in the computer room. The five remaining well potenti- ometer leads are connected to multiplexer number 1. Multiplexer number 1 also has the 9 potentiometers from instrument houses 1 , 2, and 3. Multiplexer number 2 is wired to multiplexer 1.

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25 CONDUCTOR CABLE

NEST IN FIELD TO WELL

Figure

rWELL NEST CONTROL PANEL

I 1

_L

FLOWMETER CARD #2

(F6-F9, TI) CARD

P = POTENTIOMETER F = FLOWMETER T = TIME COUNTER W = WELL

MULTIPLEXER #1

MASTER CONTROL

DATA CABLE TO INSTRUMENT

HOUSE 1

DATA CABLE TO INSTRUMENT

HOUSE 2

DATA CABLE TO INSTRUMENT

HOUSE 3

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For transient voltage suppression, each data line from both master control panels and the well control panel has an ICTE-5 TransZorb in line. The ICTE-5 can dissipate a maximum of 1500

watts of peak pulse or 5.0 watts of steady state power surge without shorting to ground.

Data Collection Proqram. The software used to collect data from the flowmeters and potentiometers is written in ~ u i c k ~ a s i c ? The program has four basic components: fir~t~initialization of

flowmeter, well nest and drainage tank variables; second, scan- ning the counter cards for flowmeter activity; third, monitoring the A/D card for well nest potentiometer changes; and fourth, monitoring the A/D card for drainage tank potentiometer changes. The initialization of variables is performed one time while the program loops through the second, third and fourth components on a continuous basis. The last three components are independent of each other. For example, the monitoring of the well nest

potentiometers can be suspended while measurements and samples are obtained and then resumed without interfering with the collection of data from flowmeters or drainage tank

potentiometers.

The program has multiple display options controlled by a menu screen that can be brought up at any time during the data collec- tion procedure. The display options include continuous scrolled listing of flowmeter counts and potentiometer voltages, or a bar chart plotting the volume pumped ( M ~ ) versus time for each tank flowmeter and potentiometer.

The program converts the drainage tank potentiometer output to volume of drainage water pumped by determining the rate that the drainage tank fills and multiplying this rate by the elapsed time between pumping events. The filling rate is determined by moni- toring each potentiometer voltage every 10 seconds to select the minimum and maximum potentiometer voltages with corresponding times. The difference between the maximum and minimum voltages is then related to the distance between the highest and lowest water elevations by multiplying the voltage difference by the drainage tank potentiometer calibration factor. This calibration factor was determined to be 1.486 m/v (4.88 ft/v) from laboratory tests. The change in volume is determined by multiplying the tank area times the distance between the highest and lowest water

levels in the drainage tank. The discharge rate is then calcu- lated by dividing the volume accumulated in the drainage tank between pump down events by the elapsed time.

To determine the volume of drainage water pumped using the

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(47)
(48)

The data stored in these files are sorted, plotted, and output to permanent storage files by data reduction programs in the labora- tory. The following SAS/GRAPH@ programs have been developed to reduce the field data: a program to plot control tank and guard tank outflow, a program to plot surface runoff tank outflow, a program to plot drainage tank water level elevations derived from the maximum and minimum potentiometer statistics, and a program to plot water elevations in each monitoring well. The average daily storage requirements for all sorted data are 80,800 bytes. Determination of Drainase Rates and Volumes. The data collected from this project are used to evaluate the effects of the free drainage, controlled drainage, and subirrigation on hydrology, water quality and crop response. Theoretically, all water that enters or exits the field plots is quantified by the field data collection system.

Since the site is underlain with a thick clay layer starting at a depth of approximately 2.13 meters (7.0 ft), all subsurface

drainage from the research plots should be through the drain tiles. Therefore determination of the volume of water moving through the control drainage tanks is a primary objective of the data collection system (Figure 14).

After the field data have been sorted and plotted by the data reduction programs, drainage volumes calculated by the redundant collection devices are compared. The paper charts on the Stevens recorders provide a reliable baseline to gauge the effectiveness of the electronic collection devices. However, these charts often cannot be read during wet periods. Drainage water quickly fills the tanks causing the pumps to cycle on and off rapidly. When this happens, individual pen lines can not be discerned and the number of pump cycles cannot be counted. A similar situation occurs on tank charts where subirrigation is maintained during dry periods. Irrigation water is introduced so rapidly that the volume of irrigation water cannot be determined from the chart. Therefore the recorder charts are used in the following manner. The volumes calculated by flowmeters and potentiometers are

compared to the volume calculated from the chart for at least one day during the data collection period. During periods of low flow, all three devices should provide essentially the same volume.

If there is an unusually large disparity between the volumes calculated from potentiometer and flowmeter data, the device produced the volume closest to the chart volume is selected. device that produced erroneous data is flagged to be checked the field.

that The in

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Figure

Figure 43: Water table depth,  24-90.  . . . . . . . . .   Figure 44: Water table depth,
Table 1:  Vertical  saturated conductivity determined by  lab tests  for plot  1.
Table  2:  Vertical  saturated conductivity determined by  lab tests  for  plot  2.  VERT  SAMPLE  DEPTH  (M)  0
Table 4: Horizontal saturated conductivity determined by slug tests  in plot  1.  SLUG TEST  HORIZONTAL CONDUCTIVITY  W D A Y )   CONTROL  MIDPOINT  11  1  'YETH  1  POINT
+7

References

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