# Presentation: 442588

## 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**

## 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)

## 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

## Study Area

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

- Data Sources
- Land use: 1995 LANDSAT TM images
- processed by SRWMD & SJRWMD
- Watershed boundary: SRWMD

**Land use**

## 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)

## 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

## 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 |

## NO3-N Trend Model

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

- Nitrate-Nitrogen
- μg.g-1 soil

- Jan. 2004

- 0.09
- 0.14
- 0.19
- 0.19
- 3.33
- 8.38
- 11.40
- 26.88

## 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

## Autocorrelation of Residuals

- Spherical model
- Nugget:15
- Sill: 75
- Range: 6,500 m

- January 2004

- The residuals were interpolated using
- the fitted semivariograms

- Residual(μg.g-1 soil)

## Spatial Distribution of NO3-N 

- Nitrate-Nitrogen
- μg.g-1 soil

- Trend model + Residual model               NO3-N Model

**Sept. 2003 **

## Spatial Distribution of NO3-N

- Nitrate-Nitrogen
- μg.g-1 soil

**Jan. 2004 **

## Spatial Distribution of NO3-N

**May 2004 **

- Nitrate-Nitrogen
- μg.g-1 soil

## 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 |

## 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

## Thank you

**Thank you**