In Lithuania, originally wet and swampy lands cover about 3.5 million hectares. By 2011 2.98 million hectares of wet and waterlogged lands, of which 2.58 million ha are agricultural lands, were drained in Lithuania. Lately, because of social, demographic and economic reasons, significant areas of drained lands were no longer used in agricultural production, they became neglected and drainage systems became unsupervised. Around 7% of drained areas in Lithuania are of poor reclamation condition because of defective drainage systems. The major investment is necessary for their reconstruction therefore it is so important to define the expediency of restoration. Areas, where it is appropriate / inappropriate to restore drainage systems (depending on the economic benefits derived from agricultural production and rural development strategies), are set out in the paper.

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The mechanical opening of drainage trenches cannot ensure constant drain line slopes. Therefore vertical bends in the pipelines are being formed. Air accumulates in these bends. Consequently the conductivity of collector drains reduces. The air is carried out by the water flow. However significant air volumes are not removed until the beginning of the dry season. Particularly unfavourable conditions for drainage operation are created when the slope of the collector drains is undulating. For the hydraulic calculation of drain parameters, an estimation of air volumes accumulated in drainage systems at the control time has to be performed. The calculation of the air moving process is necessary during the entire snow melting period. The special hydraulic calculation method is needed for this purpose. The creation and applying of this method is described in the article.