『Abstract
Dairy farms comprise a complex landscape of groundwater pollution
sources. The objective of our work is to develop a method to quantify
nitrate leaching to shallow groundwater from different management
units at dairy farms. Total nitrate loads are determined by the
sequential calibration of a sub-regional scale and a farm-scale
three-dimensional groundwater flow and transport model using observations
at different spatial scales. These observations include local
measurements of groundwater heads and nitrate concentrations in
an extensive monitoring well network, providing data at a scale
of a few meters and measurements of discharge rates and nitrate
concentrations in a tile-drain network, providing data integrated
across multiple farms. The various measurement scales are different
from the spatial scales of the calibration parameters, which are
the recharge and nitrogen leaching rates from individual management
units. The calibration procedure offers a conceptual framework
for using field measurements at different spatial scales to estimate
recharge N concentrations at the management unit scale. It provides
a map of spatially varying dairy farming impact on groundwater
nitrogen. The method is applied to a dairy farm located in a relatively
vulnerable hydrogeologic region in California. Potential sources
within the dairy farm are divided into three categories, representing
different manure management units: animal exercise yards and feeding
areas (corrals), liquid manure holding ponds, and manure irrigated
forage fields. Estimated average nitrogen leaching is 872 kg/ha/year,
807 kg/ha/year and 486 kg/ha/year for corrals, ponds and fields
respectively. Results are applied to evaluate the accuracy of
nitrogen mass balances often used by regulatory agencies to assess
groundwater impacts. Calibrated leaching rates compare favorably
to field and farm scale nitrogen mass balances. These data and
interpretations provide a basis for developing improved management
strategies.
Keywords: Nitrate leaching; Dairy farm; Groundwater modeling』
1. Introduction
2. Site description
2.1. Manure management and nitrogen sources
2.2. Climate, soils and hydrogeology
2.3. Groundwater monitoring and tile drainage network
2.4. Groundwater quality
3. Groundwater flow and transport model
3.1. Modeling approach
3.2. Sub-regional scale flow model - model-1a
3.3. Dairy farm scale model - model-2
3.4. Parameter estimation
4. Results and discussion
4.1. Parameter optimization
4.2. Model fit and uncertainty
4.3. Validation
4.4. Spatial and temporal dynamics of N-leaching
4.4.1. Fields
4.4.2. Corrals
4.4.3. Ponds
4.5. Comparison with nitrogen mass balances at the field and
farm scale
4.5.1. Case specific field N-balance
4.5.2. Whole farm N-balance by regulatory guidelines
4.5.3. Case-specific whole farm N-balance
5. Conclusion
Acknowledgments
References