The climate in Latvia is moderately warm and humid. To provide optimal groundwater heads (levels) for plant and tree growth, agricultural and forest melioration systems (AMS and FMS) have been set up in many places. Reconstruction of the FMS “Zamelu-Tisu mezs” will be carried out. To assess the reconstruction impact, hydrogeological and hydrological conditions in and around the FMS territory were studied. To accomplish this task, specialists of Riga Technical University for the study area developed and used the local hydrogeological model (HM). HM included natural hydrographic network elements (4 rivers and 2 brooks) and drainage ditches (11 AM and FM ditches and 306 FMS ditches). The aeration zone thickness maps, which characterize FMS performance, were obtained. FMS territory was divided into areas, where the drainage runoff before and after the system reconstruction was calculated. It was found out in which areas reconstruction will provide the best effect. Changes caused by FMS in hydrographical network flows were estimated. The modeling methodology and results explained in this paper may be useful for professionals designing large melioration systems.