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Upscaling of Site-Specific Nitrate-Nitrogen Measurements to the Watershed Scale in the Santa Fe

River Watershed

S. Lamsal, S. Grunwald, C. M. Bliss, I. Lopez-Zamora, N. B. Comerford and M.W. Clark

University of Florida

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Introduction

• The Santa Fe River watershed (SFRW) (3,585 km2) is part of the Suwannee Basin (≈ 25,900 km2)

• The Santa Fe River (SFR) Reach 2 covers 5.7 % of the basin

• In 1998, SFR Reach 2 (west) accounted for 15.9 % (1,130 tons) and SFR Reach 1 (east) accounted for 0.9 % (65.6 tons) of the total nitrate–nitrogen delivered to the Gulf of Mexico (SRWMD, 1998)

• In 2002, the contribution of SFR Reach 2 increased to 19.6 % (SRWMD, 2003)

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Objective

• Overall Goal

To gain a better understanding of the geo-temporal distribution and variability of nitrate –nitrogen (NO3-N) in the SFRW

• Objective

To upscale site-specific NO3-N measurements to the watershed-scale using field observations and ancillary environmental datasets

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Study Area• Santa Fe River Watershed in north-east Florida• Major soils: Ultisols (37 %), Spodosols (26 %), Entisols (15%), Histosols (2 %) (NRCS, SSURGO)

Data SourcesLand use: 1995 LANDSAT TM imagesprocessed by SRWMD & SJRWMDWatershed boundary: SRWMD

±Land use category

UrbanRangelandPine plantationsUpland forestWetlandImproved pastureForest regenerationCrops

0 12,500 25,000 37,500 50,000Meters

Land use categories in the SFRW

±Land use category

UrbanRangelandPine plantationsUpland forestWetlandImproved pastureForest regenerationCrops

0 12,500 25,000 37,500 50,000Meters

Land use categories in the SFRW

N

Land use

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Sampling Design

• Stratified random sampling: land use–soil combinations

• Composite sampling at 4 depth increments

0-30, 30-60, 60-120, 120-180 cm• Comprehensive space-time sampling, 2 years with 3

samplings per year (Sept. 2003, Jan. 2004, May 2004, and 3 future sampling events)

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Nitrate-Nitrogen Measurements

• NO3-N measurements at each site were profile averaged

• Distribution highly skewed to the right

Statistics Sept. 2003 Jan. 2004 May 2004

n 101 123 128

Mean 0.74 3.72 1.16

Min 0.01 0.06 0.10

Max 9.88 193.83 90.12

SD 0.11 1.84 0.68

Statistics of NO3-N (μg.g-1 soil) distribution

High values in Jan. followed by May and Sept sampling events

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NO3-N by Land Use

Land uses Sept 2003 Jan 2004 May 2004

Mean Max Mean Max Mean Max

Pine plantation 0.22 0.79 0.14 0.60 0.23 0.86

Crops 1.95 5.02 26.88 103.12 5.36 13.96

Forest regeneration 0.83 10.1 0.19 0.70 0.30 0.50

Improved pasture 1.17 6.54 11.40 103.70 3.01 19.92

Rangeland 1.58 4.30 8.38 41.80 1.49 6.50

Tree grove 3.07 6.06 2.79 5.36 4.22 8.86

Upland forest 0.78 4.58 3.66 67.42 0.22 0.39

Urban 0.14 0.39 0.19 0.61 0.38 1.12

Wetland 0.24 1.60 0.09 0.28 0.49 1.79

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NO3-N Trend Model

• Trend model based on mean NO3-N under land use Pixel based upscaling

0 10,000 20,000 30,000 40,0005,000Meters

±

OK prediction of nitrate-nitrogen residuals across the SFRW

Nitrate-Nitrogen(ug/g soil)

0.0990.1450.1900.1993.6608.38511.40626.880

Nitrate-Nitrogenμg.g-1 soil

Jan. 20040 10,000 20,000 30,000 40,0005,000

Meters

±

OK prediction of nitrate-nitrogen residuals across the SFRW

Nitrate-Nitrogen(ug/g soil)

0.0990.1450.1900.1993.6608.38511.40626.880

0.090.140.190.193.338.3811.4026.88 N

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Residual Modeling

• Residuals were computed for each sampling location • Spatial autocorrelation of residuals was modeled

using semivariograms• Residuals were interpolated using Ordinary Kriging• Residuals compensated for under- and over-

predictions made by the trend model

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Autocorrelation of Residuals

Spherical modelNugget:15Sill: 75Range: 6,500 m

D1

M1

0. 10. 20. 30.

Lag (km)

0.

25.

50.

75.

100.

125.

Semivaraince

January 2004

The residuals were interpolated using the fitted semivariograms

>=70.72

46.46

28.27

10.07

-8.12

<-26.31

Text stringResidual(ug/g)

Residual(μg.g-1 soil)

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Spatial Distribution of NO3-N

±0 11,000 22,000 33,000 44,000Meters

Distribution of nitrate-nitrogen across theSFRW

Nitrate-Nitrogen(ug/g soil)

0 - 0.20.2 - 0.50.5 - 1.01.0 - 1.51.5 - 2.02.0 - 3.2

±0 11,000 22,000 33,000 44,000Meters

Distribution of nitrate-nitrogen across theSFRW

Nitrate-Nitrogen(ug/g soil)

0 - 0.20.2 - 0.50.5 - 1.01.0 - 1.51.5 - 2.02.0 - 3.2

Nitrate-Nitrogenμg.g-1 soil

Trend model + Residual model NO3-N Model

Sept. 2003

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Spatial Distribution of NO3-N

± 0 12,500 25,000 37,500 50,000Meters

0 - 11 - 22 - 55 - 1010 - 2020 - 4040 - 87 ± 0 12,500 25,000 37,500 50,000

Meters

0 - 11 - 22 - 55 - 1010 - 2020 - 4040 - 87

Nitrate-Nitrogenμg.g-1 soil

Jan. 2004

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Spatial Distribution of NO3-N

May 2004

Nitrate-nitrogen distribution in the SFRW in May 2004

Nitrate-Nitrogen(ug/g soil)

0 - 0.50.5 - 1.01.0 - 2.02.0 - 5.05.0 - 10.010.0 - 15.015.0 - 20.5

±0 12,500 25,000 37,500 50,000Meters

Nitrate-nitrogen distribution in the SFRW in May 2004

Nitrate-Nitrogen(ug/g soil)

0 - 0.50.5 - 1.01.0 - 2.02.0 - 5.05.0 - 10.010.0 - 15.015.0 - 20.5

±0 12,500 25,000 37,500 50,000Meters

Nitrate-Nitrogenμg.g-1 soil

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Prediction Success

Prediction error is a composite of the error from the trend model and the residual model

Error statistics Sept. 2003 Jan 2004 May 2004

Mean -0.12 -0.37 -0.10

Variance 1.69 247.96 8.28

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Conclusions

• Soil NO3-N was variable in space and through time

• NO3-N was highest in Jan. 2004 and lowest in Sept. 2003

• High NO3-N was found in crops, improved pasture and rangeland

• Future research will focus on more advanced modeling of space-time distributions of NO3-N across the SFRW

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Thank you