A three-tier approach is recommended for estimating diffuse pollution loads,
similar to the one of the Intergovernmental Panel on Climate Change (IPCC)
for estimating greenhouse gas emissions (IPCC, 2006). From Tier 1 to 3, the
accuracy increases but the feasibility decreases.
• Tier 1 uses a fixed fraction to translate data on the amount of chemicals
applied to the soil to an estimate of the amount of chemicals that enter
the groundwater or surface water system. The fraction is to be derived
from existing literature and may depend on the chemical considered. This
Tier 1 estimate will suffice as a first rough estimate but obviously excludes
relevant factors such as soil type, agricultural practice, soil hydrology and
interaction between different chemicals in the soil.
• Tier 2 applies standardized and simplified model approaches, which can
be used based on widely available data (such as agricultural nutrient
balances, soil loss data, basic hydrologic, petrologic and hydromorphologic
information). These simple and standardized model approaches should be
derived from widely accepted and validated models.
• Tier 3 uses sophisticated modelling techniques given that the available
resources allow it and the chosen topic requires it. Whereas detailed
mechanistic models of contaminant flows through soil are available,
their complexity often renders them inappropriate for use in Tier 3
type modelling of the diffuse pollution load. However, validated empirical
models driven by information on farm practices, and which use simplified
soil and weather data are presently available for use in diffuse-load studies.
Tier 3 studies should be used to further refine Tier 2 approaches.loads