The paper looks at the impact of imperfect assessment of water discharge on the level of surface water pollution and allocation of environmental budget in a framework of one river basin region. Analysis is based on the partial equilibrium model with unidirectional spatially accumulated external effect in the presence of partial natural absorbtion. The effect of water discharge assessment imperfection on the level of water pollution at every location and social welfare is investigated. It is shown that decentralization accompanied by the improvement in water discharge quality assessment may reduce the welfare loss. However, even under complete elimination of assessment imperfections resource allocation under decentralization is still inefficient.