Applying the Hargreaves model to Catalpa bignonioides water requirements under drip irrigation on light soil

This study evaluated the Droogers and Allen modification of the Hargreaves model for estimating potential evapotranspiration (ETp), determining crop coefficients (kc), and assessing the water requirements of Catalpa bignonioides grown in row plantings on light soils. A field experiment ran from 2019 to 2022 at the Białe Błota forest nursery on sandy soil with limited water retention. Subsurface drip irrigation was applied in two variants based on soil water potential: W1 (−40 kPa) and W2 (−20 kPa). Meteorological data came from the Mochełek weather station. Reference evapotranspiration (ETo) was computed with the Hargreaves approach and converted to ETp using kc and shading-dependent correction factors (kr). Seasonal ETp under W1 ranged from 240.30 to 422.83 mm, whereas W2 produced higher totals of 251.83–455.46 mm. The kc coefficients differed between irrigation variants, most clearly during the first three years after planting, and were consistently higher under W2; this pattern continued in subsequent years. Correlation analysis between field water consumption and Hargreaves-based ETp showed very strong, highly significant relationships (r > 0.94 for July and r > 0.93 for June and August), confirming the robustness of the methodology. The derived kc values provide the first climate-criterion-based estimate of C. bignonioides water needs in central Poland and support irrigation scheduling for row-planted nursery species on light soils. Field water consumption (S) was determined using a soil water-balance method for the 0–60 cm soil layer. These parameters offer a straightforward framework for planning irrigation dates and doses in nurseries.