Context : Molecular outflows powered by young protostars strongly affect the kinematics and chemistry of the natal molecular cloud through strong shocks resulting in substantial modifications of the abundance of several species .
In particular , water is a powerful tracer of shocked material due its sensitivity to both physical conditions and chemical processes .

Aims : As part of the ‘ ‘ Chemical Herschel Surveys of Star forming regions ’ ’ ( CHESS ) guaranteed time key program , we aim at investigating the physical and chemical conditions of H _ { 2 } O in the brightest shock region B1 of the L1157 molecular outflow .

Methods : We observed several ortho- and para-H _ { 2 } O transitions using HIFI and PACS instruments on board Herschel toward L1157-B1 , providing a detailed picture of the kinematics and spatial distribution of the gas .
We performed a Large Velocity Gradient ( LVG ) analysis to derive the physical conditions of H _ { 2 } O shocked material , and ultimately obtain its abundance .

Results : We detected 13 H _ { 2 } O lines with both instruments probing a wide range of excitation conditions .
This is the largest data set of water lines observed in a protostellar shock that provide both the kinematics and the spatial information of the emitting gas .
PACS maps reveal that H _ { 2 } O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H _ { 2 } O line at 556.9 GHz , and a compact ( \sim 10 ^ { \prime \prime } ) bright , higher-excitation region .
The LVG analysis of H _ { 2 } O lines in the bow-shock show the presence of two gas components with different excitation conditions : a warm ( T _ { \mathrm { kin } } \simeq 200-300 K ) and dense ( n ( H _ { 2 } ) \simeq ( 1–3 ) \times 10 ^ { 6 } cm ^ { -3 } ) component with an assumed extent of 10 ^ { \prime \prime } and a compact ( \sim 2 ^ { \prime \prime } -5 ^ { \prime \prime } ) and hot , tenuous ( T _ { \mathrm { kin } } \simeq 900-1400 K , n ( H _ { 2 } ) \simeq 10 ^ { 3 - 4 } cm ^ { -3 } ) gas component , which is needed to account for the line fluxes of high E _ { u } transitions .
The fractional abundance of the warm and hot H _ { 2 } O gas components is estimated to be ( 0.7–2 ) \times 10 ^ { -6 } and ( 1–3 ) \times 10 ^ { -4 } , respectively .
Finally , we identified an additional component in absorption in the HIFI spectra of H _ { 2 } O lines connecting with the ground state level .
This absorption probably arises from the photodesorption of icy mantles of a water-enriched layer at the edges of the cloud , driven by the external UV illumination of the interstellar radiation field .

Conclusions :