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In Lithuania, knowledge of subsurface drainage performance is important in order to use reclaimed land rationally and to apply the scarce available financial means to repair improperly functioning drains. Subsurface drainage is also instrumental in the improvement of non-productive soils and it can assist in avoiding unsuitable soil conditions during farming operations. The experimental site, situated at Pikeliai in the Kdainiai district of central-Lithuania, consists of seven plots with various combinations of drainage materials. To evaluate drainage performance, discharges and water heads on top of and midway between drains were measured during four consecutive seasons. Low rainfall during the second and third seasons resulted in small discharges and low groundwater tables compared to the first and fourth seasons.
A second degree polynomial, similar to Hooghoudt's equation, was used to express the relationship between hydraulic head loss and discharge for each of the plots during each of the measuring seasons. Comparison of the data for the four measuring seasons clearly shows completely different equations for the low rainfall second- and third-year observations. Although applicable within the measured range, extrapolation with the equations beyond the measuring range are less accurate. The results clearly illustrate that the original design criteria are not satisfied and that the drains may be too widely spaced in all the plots. The true cause of drain malfunctioning cannot be established with the existing measuring set-up, neither can differences in performance of the drainage materials used be evaluated, mainly due to differences in hydraulic conductivity of the respective plots. Only the evolution with time of the drainage performance of the different material combinations will permit the drawing of some conclusions on their behaviour. The data analyses clearly show that a more elaborate measuring set-up as well as long-term observations are required to ascertain the cause of drain malfunctioning and to compare the performance of the various drainage materials. Only then, more conclusive results will be obtained.
Author Keywords: Drainage performance; Field evaluation; Drain discharge; Total head loss; Second degree polynomial; Drainage materials; Observation period
