Pressures due to agricultural activities play a major role in the status of water resources
systems. The evaluation of the effects of agricultural policies is therefore a key problem in
water resource planning and management. The work presented in this paper is an attempt to
include the predictable effects of the Common Agricultural Policy (CAP) of the European
Union (EU) in the process of water resources planning at the basin scale prescribed by the EU
Water Framework Directive (WFD). The approach is based on the combined use of three
models: (i) an economic model used to predict the likely land use scenarios at the basin scale
following CAP reform; (ii) a spatially distributed hydrological model for the assessment of the
related irrigation water requirements; and (iii) a decision model supporting stakeholders and
decision makers in the process of water resources planning. The paper presents an application
to a pilot study area in Northern Italy. The results highlight how the CAP may produce
an adjustment in the agricultural sector, with a shift towards more extensive land use and to
a greater diversification of production. The predicted changes in land use type and distribution
are shown to have significant influence on water requirements, with a generalised
decrease of their amount, which may exceed 10% in some areas. In turn, the decrease of
irrigation demand can be exploited to achieve a higher degree of satisfaction of other water
users in the basin and can improve the attainment of the WFD objectives.