The problem of data assimilation for a nonlinear evolution model is considered with the aim to identify the initial condition. The equation for the error of the optimal solution through the errors of the input data is derived, based on the Hessian of the misfit functional. The solvability of the error equation is studied. The fundamental control functions are used for error analysis. The error sensitivity coefficients are obtained using the singular vectors of the specific response operators in the error equation. An application to the data assimilation problem in hydrology is given. Numerical results are presented.