Abstract In the Latvian S.S.R. experiments on hydrological drainage action have been carried out for some 16–17 years on 200 drainage fields. It is found that the average annual removal of excess water is 150–250 mm and, in particularly moist years, 400–500 mm and more. Drainage results in a considerable reduction of the duration of over-wetting and over-flooding of the active soil layer. The importance of drainage in the control of the soil water regime is the greatest in the winter and spring periods. The most effective draining and the best economic indices have been achieved by applying deep systematic drainage. For sandy and loamy soils, a 1.3–1.5 m deep drainage installation and use of larger diameter drain pipes are recommended. It is expedient to determine the drain spacing by combining the hydromechanical and the empirical methods, making the most of the data of many years of observations of the action of drainage systems.