The aim of this paper is to evaluate the green, blue and grey water footprint (WF) of crops in the Duero river basin. For this purpose CWUModel was developed. CWUModel is able to estimate the green and blue water consumed by crops and the water needed to assimilate the nitrogen leaching resulting from fertilizer application. The total WF of crops in the Spanish Duero river basin was simulated as 9473 Mm³/year (59% green, 20% blue and 21% grey). Cultivation of crops in rain-fed lands is responsible for 5548 Mm³/year of the WF (86% green and 14% grey), whereas the irrigated WF accounts for 3924 Mm³/year (20% green, 47% blue and 33% grey). Barley is the crop with the highest WF, with almost 37% of the total WF for the crops simulated for the basin, followed by wheat (17%). Although maize makes up 16% of the total WF of the basin, the blue and grey components comprise the 36% of the total blue and grey WF in the basin. The relevance of green water goes beyond the rain-fed production, to the extent that in long-cycle irrigated cereals it accounts for over 40% of the total water consumed. Nonetheless, blue water is a key component in agriculture, both for production and economically. The sustainability assessment shows that the current blue water consumption of crops causes a significant or severe water stress level in 2–5 months of the year. The anticipated expansion of irrigation in the coming years could hamper water management, despite the Duero being a relatively humid basin.

Virtual water is defined as the water needed to produce a product. We can use virtual water flow calculations to estimate the water efficiency of a country, as well as its economic dependence on water resources. Former studies on this area have focused on quantifying the virtual water flows between countries, in an international context. In this study we reduce the action framework to regions within a country, determining the virtual water balance between two Spanish regions: Castilla-La Mancha and Murcia. In 2004, Castilla-La Mancha exported to Murcia 2,453,442 tons of commercial products, from which 1,191,628 tons were agricultural goods. In terms of virtual water, it means 1,365 hm3, including food-processing, and industrial products. It is necessary to add 350 hm3 to the result, because of the water transfer (Tajo-Segura transfer) between the rivers basins of these regions, so the final virtual water number, in 2004, was 1,715 hm3. The other way round, Murcia exported in 2004 2,069,000 tons of products, from which 490,351 tons were agricultural goods. That supposes 712 hm3 of virtual water. Virtual water flow is unbalanced and displaced towards Murcia with a difference of 1,003 hm3.