The mathematical model Daisy was originally developed in 1986 by the Royal Veterinarian and Agricultural University, now University of Copenhagen, in Denmark, as a response to the need to study nitrogen (N) transport from agricultural land to groundwater amid increasing crisis in the Baltic region of soil, ground- and surface-water pollution with N, especially nitrate (Hansen et al. 1993). As such and with knowledge on linked biogeochemical cycles, the model is able to mechanistically simulate water, heat, N, and carbon (C) flows, including leaching and gaseous fluxes (CO2, N2/N2O), crop production and fate of agro-chemicals (e.g. pesticides) in agroecosystems at daily time scale subjected to various field management strategies and driven by daily weather data (Fig. 1). Actually, since the development of the model, it has been a priority for operational reasons to keep the number of necessary driving variables at a minimum and to limit the requirement of temporal resolution of the driving variables to one day. In Europe, N and water are the two most common limiting factors in crop production to obtain high yields. Therefore, the Daisy model simulates plant growth as a function of only water and N availability, although interspecies plant competition as a limiting factor can also be simulated. This however means, according to the law of the minimum (the Liebig's Law), the model cannot simulate plant growth limited by other nutrients on, for instance, the phosphorus-deprived soils in much of tropical Africa or the potassium-limited soils in Southeast Asia. In the early 1990s, the theory was disseminated to a wider international audience for the description of the model simulation of crop production, water dynamics, and N dynamics in crop production at various agricultural management practices and strategies, integrating at that time a substantial knowledge from various disciplines including soil science and crop science (Hansen et al. 1993). In the late 1990s, Daisy has been re-written from the original Fortran to C++ object-oriented structure code in about 170.000 code lines. A major rewrite in year 2000 allowed a number of new models within Daisy to be implemented, including the simulation of intercropping systems and multiple soil columns (Abrahamsen and Hansen 2000). The theory behind the model is documented with a comprehensive set of user’s manuals available at the Daisy homepage ( www.daisy.ku.dk ).