This research project aims to include solute transport (of nitrate) to a model based on partial differential equations. This is closely linked to previous research work on integrated surface and groundwater model, see Figure 1. Von Gunten and colleagues (2014) calibrate the water balance in the Lerma catchment with a physically-based approach. Using this model, von Gunten and co-workers (2015) investigate the impact varying climate scenarios on this catchment. The Mediterranean small-scale Lerma catchment in northern Spain is an ideal study area. The main land-use in the catchment is agriculture. In the recent 10 years, it had undergo the transformation from a rain-fed to irrigated agriculture. The associated evolution of the water basin is well monitored (Merchán, Causapé & Abrahão 2013, Merchán et al. 2014). A comprehensive data base for water quality and quantity is the result. An ideal situation to calibrate and validate a physically-based model on nitrate transport. Key parameter in nitrate transport, computational costs, and data-requirements need to be determined. This model would then be useful to answer questions on the impact of varying land-use (different crops, their respective water and fertilization demand) on the water quality of the catchment.