wAbstract
@Typically, regional assessment of the spatial variability and
distribution of environmental properties are constrained by sparse
field observations that are costly and labor intensive. We adopted
a hybrid geospatial modeling approach that combined sparsely measured
soil NO3-N observations collected in three
seasons (Sept. 2003, Jan. and May 2004) with dense auxiliary environmental
datasets to predict NO3-N within the Santa
Fe River Watershed (3585 km2) in north-east Florida.
Elevated nitrate-nitrogen concentrations have been found in this
watershed in spring, surface and ground water. We collected soil
samples at four depths (0-30, 30-60, 60-120, 120-180 cm) based
on a random-stratified sampling design. Classification and regression
trees were used to develop trend models for soil NO3-N
predictions based on environmental correlation and predict values
at the watershed scale. Residuals were spatially autocorrelated
only for the Jan. 2004 sampling and regression kriging was used
to combine the kriged residuals with tree-based trend estimates
for this event. At each step of the upscaling process, error assessment
provided important information about the uncertainty of predictions,
which was lowest for the Jan. sampling event. Sites that showed
consistently high NO3-N values throughout
the cropping season (Jan-May 2004) with values
5Κg g-1
covered 95.7 (3363.9 km2) of the watershed. Values
in the 5-10Κg g-1 range covered 4.3 (150.7 km2))
while values exceeding 10Κg g-1 covered only 0.59
(20.7 km2) of the watershed. Elevated soil NO3-N on karst, unconfined areas with sand-rich
soils, or in close proximity to streams and water bodies pose
the greatest risk for accelerated nitrate leaching contributing
to elevated nitrogen found in spring, surface and ground water
in the watershed. This approach is transferable to other land
resource problems that require the upscaling of sparse site-specific
data to large watersheds.
Keywords: Nitrate-nitrogen; Geospatial; Classification and regression
trees; Regression krigingx
1. Introduction
2. Materials and methods
@2.1. Study area
@2.2. Field data collection
@2.3. Spatial datasets
@2.4. Upscalling methods
3. Results and discussion
@3.1. The NO3-N dataset - exploratory
data analysis
@3.2. Regional predictions of soil NO3-N
@@3.2.1. Classification tree model - Sept. 2003 sampling event
@@3.2.2. Regression tree model - Jan. and May 2004 sampling events
@@3.2.3. CART model error
@@3.2.4. Upscaling of the tree model predictions
4. Summary and conclusions
Acknowledgements
References