Modeling Subsurface Phosphorus Transport in Icecreamdb.

The ICECREAMDB calculation tool is since 2005 used to estimate the diffuse phosphorus (P) losses from Swedish agricultural soils for both national and international environmental reporting, and as a tool to evaluate the effects of different measures to reduce P losses. The major pathway for P losses from Swedish agricultural soils is through sub-surface flow and especially through macropore transport, while surface runoff and erosion is a more limited problem. Hence, it has been crucial to develop model processes for macropore transport of P to reproduce the pattern for P losses observed in the field. Recently, the hydrology parts, and especially transport in macropores have been further developed in ICECREAMDB. It is now built on the two models SOIL and ICECREAM. ICECREAM is a management oriented phosphorus loss model, which simulates field-scale erosion and losses of dissolved and particulate P. SOIL is a soil water and heat model,  and simulates water discharge through surface runoff, and matrix and macropore flow as input to the phosphorus simulations. 

The SOIL model includes a full water balance. Richards’ equation is used to calculate infiltration and water transport through the soil matrix to drainage pipes. If the precipitation rate exceeds the infiltration rate a surface water pool is generated. The surface pool is then divided between surface runoff and macropore flow, depending on the soil surface conditions, i.e. the fraction of soil surface where macropores are sealed due to soil compaction or freezing. Crop cover protects the soil from being sealed due to the impact of raindrops.  Simulated daily results of plant water uptake, soil water content, matrix and macropore  flow, surface water runoff, snow content, and soil temperatures are exported from the SOIL model and used as driving variables in the ICECREAM model.

Particulate P is lost either through surface runoff with erosion or through macropore transport. Erosion is calculated from the modified USLE equation and P-concentration in the sediments is accounted for by an enrichment factor. For macropore transport, particles are generated from a surface pool of readily available particles. The pool is replenished by processes rearranging the soil, such as soil tillage and freezing-thawing, but also by a time factor. Sediment is lost from the pool when macropore flow occurs. Losses through macropores are immediately channelled to the drainage pipes. Losses of dissolved P, assumed to be in equilibrium with the solid phases of soil P, are accounted for in matrix flow, macropore flow and surface runoff.

The model performance is currently being evaluated against measured data on drainage and P-losses from agricultural soils in Sweden.