FLOWS: a model to simulate water and solutes fluxes in agricultural and environmental systems: 2. Modules for specific applications in water and nutrient management

Abstract This paper is the second of two companion studies. The first paper focused on describing the model FLOWS, theory and the model benchmarking. This paper addresses FLOWS modules on specific applications: irrigation, temperature, organic carbon processes and nutrient transformation and transport in the soil. It also includes applications of FLOWS model in real scenarios. In the irrigation module, FLOWS allows for different scenarios: (1) no irrigation, i.e., only rainfall; (2) irrigation introduced by the user as a top-boundary, temporally variable, input flux; (3) irrigation computed as the daily potential evapotranspiration demand; and (4) irrigation computed by the model. In the latter, FLOWS optimizes irrigation fluxes and schedule according to a criterion based on the average pressure head along a user-specified depth, e.g., the root zone. Since soil temperature influences several soil processes, FLOWS simulates the temperature distribution in depth and time by using a solution to heat flow equations proposed by van Wijk and De Vries. FLOWS simulates fertilizer applications as organic matter (manure and crop residues) or mineral fertilizers. It can either simulate organic matter decomposition with coupled carbon, nitrogen, and phosphorus transformations governed by C:N and C:P ratios or simulate nitrogen transport alone using an empirical mineralization approach independent on organic matter dynamics. Two applications of the model are presented in this paper: (1) analysing the role of spatial variability in soil and vegetation for large-scale irrigation optimization, and (2) monitoring and modelling water and nutrient fluxes from irrigated fields to the surface drainage network. In the first application, FLOWS proved reliable for optimizing irrigation at large scales by accounting for the spatial variability of soil and vegetation properties and by enabling the analysis of the uncertainty associated with progressively overlooking this variability. In the second application, FLOWS was coupled with another physically based model (KWV) founded on kinematic wave theory. FLOWS was used to accurately simulate water, nitrate, and phosphorus fluxes to runoff, which were then routed through the KWV model to produce runoff hydrographs and nutrient concentrations at different sections of the drainage network and at the field outlet.