ArcNEMO, a new spatially distributed nutrient emission model to quantify N and P losses from agriculture to surface waters

Pollution of surface water bodies with N and P from agricultural sources is a major problem in Flanders and other areas with intensive agriculture in Europe. The problem is attributed to excess-use of animal manure and mineral fertilizers, hampering the achievement of the environmental targets set out in the EU Water Framework Directive and Nitrate Directive. In order to provide information to the Flemish Environmental Agency on how spatially explicit policy measures on manure and fertilizer use, and changes in land use and soil management affect the N and P concentrations in the surface waters in the region of Flanders in Belgium, a new spatially distributed mechanistic nutrient emission model was developed in ArcGIS. The model is called ArcNEMO (Nutrient Emission Model) and operates on a 50m x 50m grid scale. The high spatial resolution allows to take full benefit of the detailed information that is available in Flanders for almost every agricultural parcel (yearly information on crops grown), and farms (parcels managed by the farm and manure and fertilizer used in that year). A fertilizer allocation model was developed to calculate from these data, yearly fertilizer and manure input per grid cell for further use in the ArcNemo-model. In ArcNemo, vertical water flow and nutrient transport in the unsaturated zone are described per grid cell using a cascading bucket-type model with daily time steps. Groundwater flow is described by solving the 2D-groundwater flow equation using an explicit numerical solution with daily time steps. The unsaturated and saturated zone water flow sub-models are tightly coupled: predicted groundwater levels are used to continuously adjust the lower boundary of the unsaturated zone model while the storage coefficient in the groundwater model is constantly adjusted using information from the unsaturated zone model to ensure the water balance is closed in every grid cell. Solute transport is described using a mixing cell concept in the unsaturated zone, and bu numerically solving the 2D solute transport equation in the groundwater. Denitrification in soil and groundwater is described as a first order process. Mineralisation of organic N and P in the top soil of every grid cell is modeled according to the principles of the ROTHC model and by assigning C:N and C:P ratios to organic matter pools. Opposed to water flow and nutrient transport, denitrification and mineralization are considered to be slow processes and are modeled with monthly rather than daily time steps. Soil erosion and N and P transport with sediment flow is modeled in line with the WaTEM/SEDEM spatially distributed soil erosion and sediment delivery model, also with monthly time steps. Discharge measurements and surface water quality measurements are used to calibrate and validate the model parameters that influence modeled discharges and concentrations of N and P in surface waters in a number of subcatchments.