MODELING THE IMPACT OF IRRIGATION ON NUTRIENT EXPORT FROM AGRICULTURAL FIELDS IN THE SOUTHEASTERN UNITED STATES
W. Lee EllenburgGraduate Research AssistantEarth System Science CenterUAHuntsviile
2013 Alabama Water Resources Conference
Introduction Agricultural runoff has the potential to
contribute to a litany of water quality problems
Excessive Nutrients
Storm Runoff
Inter-storm Periods
Does Irrigation Impact Nutrient Runoff? Irrigation increases soil moisture and thus
runoff and leaching
Irrigated crops are more efficient than rain-fed crops in up-taking nutrients/sequestering biomass.
Droughts are known to substantially increase the nitrogen left in the soil [Nafziger, 2013; USDA NRCS, 2012]
In The News
Iowa copes with nitrate surge in drinking water
Summer 2012 - Drought plagues the Midwest
Fertilizer in the fields goes unused
Spring, 2013 - Wettest spring in 141 years
Sharp increase in nutrient runoff and leaching
Does Irrigation Impact Nutrient Runoff? Nutrient export and cycling kinetics is
closely related to soil moisture via “hydraulic flushing” [Hornberger et al. (1994)]
Soil moisture content, temperature and organic carbon content are the dominant factors in the nutrient cycling process
This project explored the differences of the nitrogen cycle dynamics (i.e. nitrogen movement) in irrigated and rain-fed fields using a physically based distributed hydrologic model coupled with a sophisticated nutrient cycling and plant uptake model (DSSAT).
All that… to say this
Background Many models have been developed to
simulate agricultural runoff ARS Models
CREAMS, EPIC, SWAT, etc. Hydrologic-Ecosystem Models
PnET-BGC, SPARROW, LASCAM, etc.
More commonly, field studies have been employed
Agricultural Model DSSAT
Decision Support System For Agro-technology Transfer
Experimental inputs: Planting dates Irrigation thresholds Fertilization schedules Climate data Soil Data
Used to simulate: nitrogen cycle plant uptake
Nutrient Transport Kinematic wave approximation:
Surface
Subsurface
Nitrogen Concentrations
𝜕𝑄𝜕 𝑥 +
𝜕 𝐴𝜕𝑡 =𝑞
weNO
ConZ
z
SATw
ZMN
)1()
)1((
,3,
Study Area Corn (Zea mays L.) Silty Clay Loam
(NRCS) Bulk Density
1.29-1.2 Porosity (Sat.)
.521-.485 Lower Limit
.172-.332 Drained Upper Limit
.346-.483
Experimental Set-up Split Plot Design
4 Replications 2010; 5 Replications 2011; All Irrigated
Treatment Number
Nitrogen Application
Treatment Number
Nitrogen Application
1 100-0 9 60-402 50-50 10 250-03 30-70 11 125-1254 150-0 12 80-1705 75-75 13 0-06 50-100 14 50-07 200-0 15 25-258 100-100 16 0-50
Irrigation Applications 2010
0 100 200 300 400 500 6000
20
40
60
80
100
Irrigation Applications and Precipitation
Precipitation Irrigation
Day of Simulation
Amou
nt, m
m
DescriptionAmount (inches)
Yearly Precipitation 71.2
Seasonal Precipitation 27.5
Irrigation 8.4
Irrigation Applications 2011
0 100 200 300 400 500 6000
20
40
60
80
100
120
Irrigation Applications and Precipitation
Precipitation Irrigation
Day of Simulation
Amou
nt, m
m
DescriptionAmount (inches)
Yearly Precipitation 68.2
Seasonal Precipitation 35.5
Irrigation 5.5
Model Calibration - 2010
4500 6500 8500 10500 125004500
6500
8500
10500
12500
f(x) = 0.930733316558872 x + 1339.36205599654R² = 0.626799060860674
Observed Grain Weight, kg/ha
Sim
ulat
ed G
rain
Wei
ght,
kg/
ha
Observed Mean = 146.5Simulated Mean = 157.7 P-value = .105
Assuming Unequal Variances:
Model Calibration - 2010
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 165500
6500
7500
8500
9500
10500
11500
12500
Simulated Observed
Treatment Number
Gra
in W
eigh
t, k
g/ha
Variance in the observed replications: 18195 kg/ha (289.3)
Model Validation - 2009
Observed Mean = 8375 kg/ha (133.2)Simulated Mean = 8288 kg/ha (131.8) P-value = .460Assuming Unequal
Variances:
500 2500 4500 6500 8500 10500 12500500
2500
4500
6500
8500
10500
12500f(x) = 1.3233047100773 x − 2827.50099198876R² = 0.951759030442325
Observed Grain Weight, kg/ha
Sim
ulat
ed G
rain
Wei
ght,
kg/
ha
Model Validation - 2011
0 100 200 300 400 500 6000
50100150200250
SimulatedObserved
Day of Simulation
Nitr
ate,
kg/
ha Observed Mean = 4.38 mg/lSimulated Mean = 4.34 mg/l
Observed Variance = 7.57 mg/lSimulated Variance = 50.89 mg/l
RMSE = 164%
In terms of the processes being modeled, the overwhelming source of nitrogen loss from the field is the plant uptake which accounts for about 90% of the losses in this study (compared to de-nitrification and volitization).
**
Results
Surface Runoff
2010 20110
20
40
60
80
100
120
140
160
180
IrrigatedRainfed
Year
Dep
th,
mm
Surface Nitrate Export
0102030405060708090
100
IrrigatedRain-fed
Nitr
ate,
kg/
ha
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160
102030405060708090
100
Treatment Number
Nitr
ate
kg/h
a
2010
2011
Results and Discussion – Treatment 10 (280-0)
Comparing 2011 with 2010
0 50 100 150 200 250 300 350 400 450 5000
102030405060708090
Surface Nitrogen TransportJan. 2010 - Mar 2011
Nitr
ate
kg/h
a
0102030405060708090
Total Nitrate RunoffJan 2010 - Mar 2011
Nitr
ate
kg/h
a
0 50 100 150 200 250 300 350 400 450 5000
102030405060708090
Jan. 2011 - Mar 2012
Day of Simulation
Nitr
ate
kg/h
a
N
0102030405060708090
100
Jan 2011 - Mar 2012
Nitr
ate
kg/h
a
N
Surface Nitrate Export
2010 20110
100
200
300
400
500
600
700Irrigated Rain-fed
Year
Nitr
ate,
kg/
ha
Surface Nitrate Export
Trt 1
Trt 11
Results and Discussion – Subsurface Treatment 6-2011 (56-
112)
154157
160163
166169
172175
178181
184187
190193
196199
202205
2080
10
20
30
40
50
60
70
80Rainfall and Irrigation
Precipitation IrrigationDay of Simulation
H2O
, m
m
154 159 164 169 174 179 184 189 194 199 204 2090
10
20
30
40
50
60Soil Nitrate Concentrations
Irrigated Rainfed
Day of Simulation
Nitr
ate
kg/h
a
154157
160163
166169
172175
178181
184187
190193
196199
202205
2080
50
100
150
200
250
300N Uptake
Irrigated RainfedDay of Simulation
Nitr
ogen
, kg/
ha
154157
160163
166169
172175
178181
184187
190193
196199
202205
2080
0.050.1
0.150.2
0.250.3
0.350.4
0.450.5
Soil Moisture
Irrigated RainfedDay of Simulation
Soil
Moi
stur
e, %
Lateral Subsurface Nitrate Export
2010
2011
0.00
0.50
1.00
1.50
2.00
2.50
3.00
IrrigatedRainfed
Nitr
ate,
kg/
ha
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160.00
0.50
1.00
1.50
2.00
2.50
3.00
Treatment Number
Nitr
ate,
kg/
ha
Growing Season
Lateral Subsurface Nitrate Export
2010
20110.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50IrrigatedRainfed
Nitr
ate,
kg/
ha
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
Treatment Number
Nitr
ate,
kg/
ha
Fallow Season
Total Lateral Nitrate Export
2010 20110.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
IrrigatedRain-fed
Year
Nitr
ate
kg/h
a
Conclusions Surface Runoff
Early season frontal systems have major impact on surface nitrate runoff. Irrigation benefits by allowing the nitrate to move beyond the surface layer.
Runoff can be mitigated with flexible application schedules.
Conclusions Subsurface Lateral Leaching
Irrigation has minimal effect on lateral leaching of N during the growing season.
Irrigation decreases the residual (fallow/post season) nitrate in the soil column decreasing the lateral and vertical leaching
Irrigation provides the vertical movement and aerobic conditions for nitrogen to be consumed by the plant.
