Applied optimal control problems are usually formulated with parameters. Sensitivity analysis is then of foremost importance to understand how optimal solutions are affected by uncertainties of the parameter(s) under consideration. Here, we focus on an irrigation problem to minimize the amount of water used to guarantee the production of a healthy crop. The parameter of interest is the percentage of water loss due to run-off and deep infiltration in the field which is hard to estimate. We apply sensitivity analysis to our parametric optimal control problem. Noteworthy, we determine a neighborhood for a nominal value of the parameter where the approximations yield by sensitivity analysis are valid. We illustrate our research through numerical simulations.