The increased input of fertilizers and animal wastes after 1950 has boosted agricultural crop production to a high level in many industrialized countries, but it has also contributed to increased nitrogen and phosphorus emissions from agriculture to groundwaters and surface waters. This paper summarizes the pathways and controls of nitrogen and phosphorus losses to surface waters, and it presents estimates and predictions of the losses from agricultural soils in The Netherlands into surface waters, before and after the implementation of policies and measures to reduce nutrient losses from agriculture. Implementation of the nutrient accounting system MINAS, aiming at a step-wise lowering of nitrogen and phosphorus surpluses at farm level, will decrease the total nitrogen and phosphorus surpluses between the years 1985 and 2008 by 58 and 82%, respectively. These large decreases are the result of a strong decrease in the input via fertilizers and animal wastes, combined with only a minor decrease in the output via harvested products. Nitrogen emissions from agricultural land to surface waters will decrease by 38% between 1985 and 2008. Phosphorus emissions from agricultural land to surface waters are expected not to decrease on the short term.

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Parameters of the distributed irrigation water management model FRAME are determined by an inverse method using evapotranspiration (ET) rates estimated from the SEBAL remote sensing procedure and in situ measurement of groundwater heads. The model simulates canal and on-farm water management as well as regional groundwater flow. The calibration is achieved in two phases. The data on ET were introduced with the primary intent of improving predictions of ET through better estimated soil hydraulic parameters. During the first phase, soil hydraulic parameters sensitive to ET were optimized. As per the canal running schedule in the study area, the daily values of ET data were synthesized into 16 time periods with 15 periods each of 24 days and one period of 5 days. Use of cumulative (annual basis) ET data results in better estimates of soil hydraulic parameters as compared to temporal (24-day period basis) ET data due to possible errors in other input data. During the second phase of calibration, aquifer drainable porosity and maximum allowable groundwater extraction were optimized against groundwater heads for five years. The calibration was very successful in about 70% of the study area with a coefficient of correlation between simulated and observed groundwater levels of more than 80%. Subsequently the model is validated against groundwater heads for nine years. RÉ SUMÉLes paramètres du modèle FRAME de gestion de l'eau d'irrigation distribuée sont déterminés par la méthode inverse en utilisant l'évapotranspiration (ET) estimée par la procédure SEBAL de télédétection et la mesure in situ des niveaux des eaux souterraines. Le modèle simule la gestion de l'eau dans le canal et sur l'exploitation, ainsi que le flux des eaux souterraines dans la région. L'étalonnage est réalisé en deux phases. La donnée ET a été introduite avec l'intention première d'améliorer les prévisions de ET grâce à une meilleure estimation des paramètres hydrauliques du sol. Au cours de la première phase, les paramètres hydrauliques des sols sensibles à ET ont été optimisés. Pour le tour d'eau dans le canal sur la zone d'étude, les valeurs quotidiennes de ET ont été synthétisées en 16 périodes, 15 périodes de 24 jours et une période de cinq jours. L'utilisation du cumul annuel de ET conduit à de meilleures estimations des paramètres hydrauliques du sol par rapport aux données ET sur périodes de 24 jours en raison d'éventuelles erreurs dans les autres données d'entrée. Au cours de la deuxième phase de l'étalonnage, la porosité de l'aquifère exploitable et l'allocation maximum d'eaux souterraines ont été optimisées sur cinq ans en fonction du niveau des eaux souterraines. L'étalonnage a été un grand succès dans environ 70% de la zone d'étude avec un coefficient de corrélation entre les niveaux simulés et observés des eaux souterraines de plus de 80%. Par la suite, le modèle est validé pour le niveau des eaux souterraines sur neuf ans.

In the past, environmental Phosphorus (P) parameters like soil P indices have been used to catogorize the potential risk of P losses from agricultural land. In order to assess the actual risk of P pollution of groundwater and surface waters, dynamic process oriented soil and water quality models have been frequently used. Recently, an approximating model for phosphorus, called SIMPLE, has been developed. This model approximates the output from a complex dynamic water quality model. The approximating model is called a metamodel. This simple P-model proves to be a powerful tool for quick assessment of the risk of P pollution from agricultural land to surface waters.