Controlled drainage and sub-irrigation – an option to save water in crop production in Finland

In humid areas like Finland, the annual precipitation exceeds evaporation. Therefore, efficient land drainage is necessary for cultivation. However, during the dry growing period the water deficit may restrict crop growth, thereby occasionally necessitating irrigation. In our study we compared conventional subsurface pipe drainage (D), controlled drainage (CD) and sub-irrigation (CDI) in three fields in an 18.4 ha experimental area in Söderfjärden, Finland during 2010-2015. In that region the average long-term air temperature, precipitation and evaporation during the period of June-August were 14.9 °C, 180 mm and 349 mm, respectively. The runoff from CD and CDI fields was regulated with control wells, and CDI was sub-irrigated when groundwater dropped below the regulation level. The lateral flow of groundwater between the fields and seepage to the main drain was prevented by vertical plastic sheets reaching impermeable subsoil. Subsurface pipes were installed at a depth of 1.1 m, with spacing of 20 to 40 m. The fields were cultivated uniformly and all the farming operations were identical in all the fields excepting that the water management treatments were different in the three fields. The groundwater table, runoff and soil moisture were monitored continuously in the lowest part of the field, and groundwater was observed biweekly at two other places of the field. Grain yields of barley (Hordeum vulgare) and spring wheat (Triticum aestivum) were determined. Water deficit was calculated on the basis of the water balance of the fields during the water regulation period. In the sub-irrigated field, the water deficit was alleviated by sub-irrigation by an average of 50 mm in comparison to the conventional subsurface pipe drainage. During a summer (2015) when sub-irrigation was not possible due to lack of irrigation water, the controlled drainage alleviated the water deficit by 25 mm. Soil moisture at a depth of 70 cm was higher and the water table was markedly higher in the sub-irrigated field than in the other fields during the regulation period. Grain yield was higher in CDI compared to D in a summer during which evaporation roughly corresponded to the long-term average values. The summers were wet and supposedly factors other than water deficit had a stronger impact on yields in other summers. Controlled drainage together with sub-irrigation is an option for storing water for crop production. The technical solution used in this study may also be useful elsewhere in other circumstances.