FEATURES OF WATER BALANCE FORMATION ON DRAINED PADDY FIELDS IN THE DANUBE DELTA RICE IRRIGATION SYSTEMS

Abstract

The article is devoted to a comprehensive study of the processes of water balance formation on flooded paddy check plots equipped with drainage networks of various parameters within the rice irrigation systems of the Danube Delta. The relevance of the research is determined by the need to increase water-saving efficiency in Ukrainian rice-growing systems under conditions of limited water resources, increasing hydrological variability, and the necessity to rationalize irrigation water use when cultivating a water-demanding crop such as rice. The purpose of the study is to identify the regularities of the formation of the main components of the water balance of paddy fields depending on the design features of the drainage-discharge network and the structural parameters of the check plots, as well as to determine the influence of drain spacing on the intensity of filtration losses. The article examines both natural factors of the water balance (precipitation, total water consumption, hydrogeological conditions) and technical ones (design of the irrigation units, drainage spacing, flooding regime). The research was conducted on typical plots of the Kiliya rice irrigation system covering 4832 ha, where the soil cover is represented by alluvial loams and sandy loams typical for deltaic formations. Field measurements were carried out using vegetative evaporation vessels to determine evapotranspiration components and infiltrometers to obtain spatial patterns of filtration at check plots with different drain spacing. Each component of the water balance was determined independently, ensuring high accuracy of the obtained results. Water balances were compiled for check plots with drain spacing of 200, 250 and 500 m. It was found that water consumption for soil saturation averaged 1560…1803 m³/ha (up to 8.6% of total water expenditures) and strongly correlated with the depth of the aeration zone (r = 0.77). Based on statistical analysis, an equation was derived that allows predicting water consumption for saturation depending on groundwater depth, which can be used in the design of rice systems in the Danube Delta. Total water consumption (evapotranspiration) during the study period ranged from 7240 to 7840 m³/ha, which accounted for up to 37% of the expenditure part of the water balance. Seasonal dynamics of water evaporation from the flooded surface and rice plant transpiration were analyzed; maximum water consumption was observed in late June and early July during the most active growth period and the highest air temperatures. Special attention is paid to filtration losses, which under the studied conditions occurred mainly as drainage outflow, since the shallow groundwater level excluded significant vertical filtration to deeper layers. Filtration was found to vary spatially: in drain-adjacent zones, filtration rates reached 4-20 mm/day, whereas in the central parts of the fields they were only 1-2 mm/day. The average seasonal filtration rate at drain spacing of 250 and 500 m was lower than at 200 m by 24.8% and 42.5%, respectively, demonstrating the water-saving advantage of increased drain spacing. Regression equations describing the relationship between filtration rate and distance from the drainage canal were developed. A practically significant finding is the possibility of reducing filtration losses by 10-15% through the creation of hydraulic backwater in the drainage network during the rice growing season. This measure allows saving 1000-1500 m³/ha of water without changing the structural parameters of the check plots. The obtained results are valuable for improving water regime management technologies in rice irrigation systems, optimizing their hydromeliorative performance, and planning the reconstruction of irrigation networks under delta conditions.