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Actualisering van model NLOAD voor de nitraatuitspoeling van landbouwgronden; beschrijving van model en GIS-omgeving(1998-01-31)Model NLOAD employs a simple, straightforward and empirical approach, describing the steady state nitrate_N leaching of agricultural soils on annual basis. NLOAD is used every year to assess the effects of environmental policy on nitrate leaching and exceedance of the standard for nitrate_N concentrations in shallow groundwater in the Netherlands. Over a period of five years the model was changed at every occasion. For the State of the Environment of 1995/96 it was necessary to update the model, compare it with the measured nitrate leaching of experimental fields and make a new documentation. Some highlights in this report are conceptual improvements, validation of the model and implementation in a GIS environment. Improvement of the modelconcept: Nitrate_N leaching is calculated as the sum of background leaching, leaching due to N-fertilization, additional leaching from animal manure and leaching of dung_N and urine_N supplied by grazing cattle on grassland. In the new model background leaching is decreased. The leaching due to atmospheric_N deposition is now explicitly taken into account (added to the fertilizer_N). Efficiency coefficients for plant available N in manure were increased. The leaching of arable land was a constant fraction of the total applied plant available N. Now this fraction is an increasing function of total applied available N (just like for grassland). The urine_N production of grazing cattle is now a function of N-fertilization. Old vs. new: The model gives an improved prediction of the leaching of experimental fields. However the new model underestimates the leaching at very high fertilization levels. The model is judged to be suitable for grassland on sandy soils, but still is not reliable for other combinations of agricultural land use and soil types.