A : IC, internal catchment [domain]; LGM, National Groundwater Model for the Netherlands; MHW, mean high water; MLW, mean low water; SWAP, Soil Water Atmosphere Plant [model]; WP, water productivity.S S : V Z M The Soil Water Atmosphere Plant (SWAP) model simulates transport of water, solutes, and heat in the vadose zone in interac on with vegeta on development. Special features of the model are generic crop growth, versa le top boundary condi ons, macroporous fl ow, and interac on of soil water with groundwater and surface water. We discuss typical model applica ons that have appeared in recent scien fi c literature. New model developments are explained with respect to the numerical solu on of Richards' equa on, macroporous fl ow, evapotranspira on, and interac ons with groundwater and surface water.We describe case studies on agricultural water produc vity, regional nutrient management, and groundwater conserva on by surface water management. Finally we envision model developments with respect to SWAP for the coming 5 to 10 yr.

The contribution of nonpoint‐source pollution of P leaching and P runoff from agricultural soils is very important, especially in those areas with intensive livestock production systems. Prediction of the impacts of fertilization reduction measures on P loads from agricultural soils to surface water requires a sound description of the accumulation of P. In this article an overview is given of the Dutch approach. The presented methodology has been developed for noncalcareous sandy soils in flat areas where P loss by subsurface runoff is the dominating pathway. Based on the kinetics of P sorption and desorption of these soil types, P accumulation in soils, and inorganic P concentrations in soil solution can be modeled. Furthermore, methods have been deduced to translate the description of soil chemical P processes into commonly used soil P tests. As a result of this interaction, soil P tests can be used as an initial approach to estimate the potential risk of P leaching and runoff from agricultural soils in noncalcareous sandy areas. The description of the kinetics of inorganic P reactions and the modeling of soil P tests have been implemented in the ANIMO model in addition to an organic P cycle that is related to organic C and N dynamics. With this model, the fixation and immobilization of P by soll components can also be assessed. As a result of annual losses by fixation, immobilization, and subsurface runoff/leaching, changes in soil P test values with time can be estimated.

Nitrogen and phosphorus retention estimates in streams and standing water bodies were compared for four European catchments by a series of catchment-scale modelling tools of different complexity, ranging from a simple, equilibrium input-output type to dynamic, physical-based models: source apportionment, MONERIS, EveNFlow, TRK, SWAT, and NL-CAT. The four catchments represent diverse climate, hydrology, and nutrient loads from diffuse and point sources in Norway, the UK, Italy, and the Czech Republic. The models' retention values varied largely, with tendencies towards higher scatters for phosphorus than for nitrogen, and for catchments with lakes (Vansjø-Hobøl, Zelivka) compared to mostly or entirely lakeless catchments (Ouse or Enza, respectively). A comparison of retention values with the size of nutrient sources showed that the modelled nutrient export from diffuse sources was directly proportional to retention estimates, hence implying that the uncertainty in quantification of diffuse catchment sources of nutrients was also related to the uncertainty in nutrient retention determination. This study demonstrates that realistic modelling of nutrient export from large catchments is very difficult without a certain level of measured data. In particular, even complex process oriented models require information on the retention capabilities of water bodies within the receiving surface water system and on the nutrient export from micro-catchments representing the major types of diffuse sources to surface waters.

In EUROHARP, an EC Framework V project, which started in 2002 with 21 partners in 17 countries across Europe, a detailed intercomparison of contemporary catchment-scale modelling approaches was undertaken to characterise the relative importance of point and diffuse pollution of nutrients in surface freshwater systems. The study focused on the scientific evaluation of different modelling approaches, which were validated on three core catchments (the Ouse, UK; the Vansjo-Hobøl, Norway; and the Enza, Italy), and the application of each tool to three additional, randomly chosen catchments across Europe. The tools involved differ profoundly in their complexity, level of process representation and data requirements. The tools include simple loading models, statistical, conceptual and empirical model approaches, and physics-based (mechanistic) models. The results of a scientific intercomparison of the characteristics of these different model approaches are described. This includes an analysis of potential strengths and weaknesses of the nutrient models.