Effects of land use changes on water and nitrogen flows at the scale of West African inland valleys: an explorative model.

Land use and cover, as influenced by agricultural practices, and the changes in these with increasing pressure on land, are among the factors determining water flows in inland valleys. Changing water flows affect nitrogen flows both at the plot level and at levels higher than plots. We present a conceptual model to show the potential influence of changes in land use on water and nitrogen flows. The model first calculates the three-dimensional redistribution of water at the inland valley level using topographic characteristics as input. For that purpose, the inland valley is divided in columns. Subsequently, the annual one-dimensional water balance is computed for each column using transfer functions. These functions use soil characteristics as input variables. Finally, the annual nitrogen balance is calculated with an equation which uses the calculated infiltration as input variable and differentiates between nitrogen uptake efficiency of annual and perennial vegetation. Both this equation and the transfer functions were tested for West Africa in an earlier study. The model-runs simulating different land-use scenarios illustrate that effects of changes in land use on water and nitrogen flows are not additive, when spatial effects of water redistribution are explicitly modelled. The addition of an explicit time dimension will make the model more dynamic in order to analyse the effects of patterns of land use throughout the year on water and nitrogen flows. The model can be used to target further research on the processes underlying water and nitrogen flows in inland valleys: in particular, the scale dependency of run-off in relation to agricultural practices merits attention in field experiments.