Field research on the performance of various drainage materials in Lithuania
Abstract Field evaluation of the performance of a drainage system or of various drainage materials is a long-lasting research, very often with disappointing results. These disappointing results stem either from a too wide a spread of the field data, or one or more necessary parameters which were not measured, or an insufficient knowledge of the flow occurrence in the vicinity of the drain resulting in a wrong interpretation and evaluation of the research findings. Yet, a correct evaluation of the performance of a drainage system is of primary importance to ascertain the real value of a drainage system or to compare various drainage materials. As a result of the wide spread of field data within a measuring season, a large amount of data from several seasons is required to obtain reliable results. From discharge and total head loss measurements a second-degree polynomial results which only permits a determination of whether the design criteria are met. The similarity with Hooghoudt’s equation allows the determination of soil hydraulic conductivity and the equivalent depth of the impervious layer. With this data the drain spacing can be checked. Too wide a spacing can be the cause of malfunctioning of a drainage system, but the drainage materials can also affect its performance. To evaluate or compare drainage materials it is necessary to measure the so-called approach-flow head loss in the vicinity of the drain. The relationship between discharge and approach-flow head loss should theoretically be linear and the regression coefficient gives the approach-flow resistance. However, the relationship depends to a large extent on the approach-flow occurrence in the vicinity of the drain. For non-linear relationships, the approach-flow resistance can be estimated from the slope of the tangent line, which tends to a constant value at greater discharges for flow patterns covering the whole drain circumference. The approach-flow resistance depends on the hydraulic conductivity of the soil and does not permit a direct comparison of various drainage materials. Therefore, the approach-flow constant or the dimensionless approach-flow resistance is calculated. This parameter is related to the material itself and to the installation conditions reflected in the hydraulic conductivity of the trench backfill. Consequently the comparison of the various drainage materials includes the trench backfill. The approach-flow head loss fraction is that part of the total head loss which is dissipated by the approach-flow. The relationship between approach-flow head loss and total head loss should theoretically be and is, indeed, linear so that the slope of the regression line gives the approach-flow head loss fraction. This parameter combines the influence of drainage material and flow pattern so that real flow situations in the vicinity of the drain, which are not necessarily the same for all materials, can be compared.
Evaluation of subsurface drainage performance in Lithuania
Abstract 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 K≐dainiai 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.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.