Surplus water during floods as well as the low level of the soil surface determine the complicated ecological, economic and demographic conditions in the Nemunas delta. Subsequently, it leads to different types and intensity of use of farmland as well as different methods of water regime regulation: summer or winter type polders. The building of 17 summer polders in the 32 500 ha floodplain area was a compromise, seeking to improve farming conditions and maintain the stability of the flood regime and ecology in the delta. Field measurements and mathematical modelling were carried out. As study results have shown, grass yield losses due to flooding depend on the distribution of water levels and flood duration in the summer polders. Relative damage to agriculture is insignificant and makes up only 10-15% of the total amount of flood damage in the Nemunas lowland.The field measurements and hydraulic and mathematical modelling of flow and sediment regime as well as study of flooding and changed farming conditions also show that it is possible to reduce the height of floods and economic and ecological damage. Rearrangement of the system of protective dikes, regulation of the main river canal and water/sedimentation regime with the help of pumping stations, decrease of pollutant migration into the Curonian Lagoon and Baltic Sea, and recreation of natural meadows are important in that case. Copyright convient de réorganiser le système des digues de protection, de réguler le chenal principal du fleuve et le régime de sédimentation des eaux à l'aide de stations de pompage, de réduire les déversements polluants dans la lagune Curonian et la mer Baltique, et de régénérer des pâturages naturels.
Th e quality of water in rivers depends on numerous hydrological and anthropogenic factors. Th e Mūša catchment, belonging to the Lielupė river basin district (RBD) in the northern part of Lithuania, was taken for water quality investigation. In this catchment, 63% of the territory is under arable land. Th e conceptual FYRIS model was chosen to identify the impact of the sources of pollution with total nitrogen (N) in the Mūša river. Th e modelling encompasses the 1997-2006 period. Aft er calibration, the model effi ciency coeffi cient was E = 0.46, i. e. fairly good, and the correlation coeffi cient was r = 0.69. While modelling the variation of nitrogen concentrations during the study period (1997-2006), the results did not correspond to the monitored concentrations equally well in all subcatchments. One of the main reasons for the disagreement between the model and monitoring data can be a vast dispersal of the monitoring data. Th e model represents the monitoring results in the Mūša, Daugyvenė and Tatula rivers quite well. Th ere is a strong linear correlation between the model and the monitoring results. Th e determination coeffi cient of the regression equation R 2 varies from 0.48 to 0.56. Analysis of the total nitrogen load to the Mūša catchment from diff erent pollution sources has shown that about 87% of it comes from arable land, 10% enters from waste water treatment plants (WWTP), households and urban territories, and only 3% of all nitrogen within the catchment comes from wooded territories and pastures.
The dependence of runoff characteristics of the Lielupe River basin district rivers: the Mūša River, the Levuo River and the Tatula River on the wetland drainage is analysed in the article. The wetland covers 70–89% of the total area in the basins of these rivers. It was established, that drainage of 80–91% of wetland area had not changed the runoff characteristics. The excess of water is removed through the drainage systems most intensively during spring and summer rain floods. The analysis of the runoff of spring flood and summer rain floods revealed that the area of drained land had not changed the runoff characteristics. The tenuous positive or negative relations (coefficients of correlation from ‐0.52 to 0.19) between the area of drained wetland and the runoff characteristics were established. Santrauka Šlapių žemių sausinimo Lietuvoje įtaka upių nuotėkiui iki šiol vertinama nevienareikšmiškai, nes ji gana sudėtinga ir įvairiapusė. Straipsnyje analizuojama Lielupės baseino upių Mūšos, Lėvens ir Tatulos nuotėkio charakteristikų priklausomybė nuo periodiškai šlapių žemių sausinimo drenažo sistema. Šių upių baseinuose periodiškai šlapių mineralinių žemių plotai sudaro 70–89 % viso baseino ploto (pelkės užima tik 3,2–7,5 %). Analizuota sausinimo, nuotėkio bei jo pasiskirstymo per metus kaitos charakteristikos, nuotėkio netolygumo dėsningumai. Tiriant nustatyta, kad per pastaruosius 40 metų nagrinėjamų upių baseinuose nusausintų žemių plotai padidėjo nuo 4–7 % (šlapių žemių fondo) iki 80–91 %. Vandens perteklius iš sausinimo sistemos pasišalina per pavasario potvynius bei vasaros poplūdžius. Analizuojant pavasario potvynių ir vasaros poplūdžių duomenis akivaizdu, kad nusausintų žemių plotas neturi įtakos upių nuotėkio charakteristikų pokyčiui. Tarp nusausintų žemių ploto ir nuotėkio charakteristikų nustatytos labai silpnos teigiamos arba neigiamos sąsajos (koreliacijos koeficientai nuo –0,52 iki 0,19). Резюме Влияние осушения избыточно увлажненных земель на речной сток в Литве до сих пор оценивается неоднозначно, так как это влияние сложно и многообразно. В статье анализируются характеристики изменения речного стока и его внутригодового распределения, закономерности его неравномерности в бассейнах рек Муша, Левуо и Татула (эти реки составляют район речных бассейнов Лелупе), в которых периодически избыточно увлажняемые земли составляют 70−89% от площади бассейнов. Исследованиями установлено, что площади осушенных земель в исследуемых речных бассейнах за последние 40 лет увеличились с 4−7% (от фонда избыточно увлажненных земель) до 80−91%. Несмотря на то, что большая часть избыточно увлажненных земель уже осушена, это не оказывает существенного влияния на речной сток. Избыток влаги осушительными системами наиболее интенсивно удаляется в течение весенних половодий и летних паводков, поэтому изменения речного стока из-за осушения происходят в течение тех же сезонов. При анализе зависимости изменения слоя речного стока в течение весенних половодий и летних паводков (в долях от годового стока) от площади осушения установлено отсутствие корреляционной связи, либо эта связь является слабой (коэффициенты корреляции изменяются от −0,52 до 0,19).
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