Hydrodynamic model of the formation of horizontal drainage runoff on drainless and slightly drained irrigated lands in the dry steppe zone of Ukraine

The article presents the results of the research, which are the basis of making a hydrodynamic model of the formation of drainage runoff of closed horizontal drainage on drainless and slightly drained irrigated lands in the dry steppe zone of Ukraine. The relevance of the research is in their need when designing the systems of horizontal drainage, determining drain spacing, modes of operation and the evaluation of drainage efficiency during its operation when irrigating with the use of modern sprinklers from a closed farm network. The objective of the research is to develop a generalized hydrodynamic model of formation of closed horizontal drainage runoff when irrigating from a closed farm network on drainless and slightly drained lands, which are typical for the watershed plains and coastal lowlands of the dry steppe zone of Ukraine. The task of the research to determine the basic conditions and factors of formation of  horizontal drainage runoff, to specify the structure of groundwater inflow to the drains when having optimal drain spacing in the closed farm network and to define the prospects and areas of further research. Research methods and techniques: long-term (1975-2020) field experiments on drained areas with different drain spacing in typical hydrogeological conditions for watersheds and coastal lowlands; water balance studies; theoretical research methods (analysis and synthesis, comparison, generalization, zoning); to determine the structure of groundwater inflow to the drains, the method of electrohydrodynamic analogies when using the laboratory integrator EGDA 9/60 was applied. As a result of the research it was determined that in the conditions of a closed water farm network it is possible to increase drain spacing from 120-220 m to 240-400 m. When studying all the conditions of drainless and slightly drained watershed plains and coastal lowlands, the main sources forming the regime of groundwater and drainage runoff are the precipitation of 420 mm or 55.0% of water input, irrigation water - 340 mm or 45.0%, including 266 mm or 35,0% from irrigation and 75 mm or 10,0% from filtration from the canals. The analysis of the hydrodynamic model of drainage runoff formation shows that when having drain spacing as 240-400 m, the inflow from the zone located above the bottom of the drain is 2.6-4.8% of the total inflow to the drain. The ascending flow under the bottom of the drain enters it at an average angle of 600 and in all variants of drain spacing is 95.2-97.2% of the total inflow. When drain spacing increases from 240 to 300-400 m, the horizontal inflow from the area located below the bottom of the drain decreases with a corresponding increase in the ascending flow under the bottom of the drain. The average width of the ascending flow to the drain at the edge of the active zone of groundwater (9.0-10.0 m below the drain) is 13.0-20.0 m. The resulting model complements the existing theoretical and methodological knowledge base for designing horizontal drainage and is necessary in perspective researches on the formation of expert systems for optimization of the parameters and modes of irrigation and drainage functioning when applying modern broadcast sprinkler equipment irrigating from the closed farm network.