The

Aims. As part of the WISH (Water In Star-forming regions with Herschel) key project, systematic observations of H2O transitions in young outflows are being carried out with the aim of understanding the role of water in shock chemistry and its physical and dynamical properties. We report on the observations of several ortho- and para-H2O lines performed with the HIFI instrument toward two bright shock spots (R4 and B2) along the outflow driven by the L1448 low-mass proto-stellar system, located in the Perseus cloud. These data are used to identify the physical conditions giving rise to the H2O emission and to infer any dependence on velocity. Methods. We used a large velocity gradient (LVG) analysis to derive the main physical parameters of the emitting regions, namely n(H2), Tkin, N(H2O) and emitting-region size. We compared these with other main shock tracers, such as CO, SiO and H2 and with shock models available in the literature. Results. These observations provide evidence that the observed water lines probe a warm (Tkin ~ 400−600 K) and very dense (n ~ 106−107 cm-3) gas that is not traced by other molecules, such as low-J CO and SiO, but is traced by mid-IR H2 emission. In particular, H2O shows strong differences with SiO in the excitation conditions and in the line profiles in the two observed shocked positions, pointing to chemical variations across the various velocity regimes and chemical evolution in the different shock spots. Physical and kinematical differences can be seen at the two shocked positions. At the R4 position, two velocity components with different excitation can be distinguished, of which the component at higher velocity (R4-HV) is less extended and less dense than the low velocity component (R4-LV). H2O column densities of about 2 × 1013 and 4 × 1014 cm-2 were derived for the R4-LV and the R4-HV components, respectively. The conditions inferred for the B2 position are similar to those of the R4-HV component, with H2O column density in the range 1014−5 × 1014 cm-2, corresponding to H2O/H2 abundances in the range 0.5−1 × 10-5. The observed line ratios and the derived physical conditions seem to be more consistent with excitation in a low-velocity J-type shock with strong compression rather than in a stationary C-shock, although none of these stationary models seems able to reproduce the whole characteristics of the observed emission