ABSTRACT A significant area of Hungary is cultivated with maize. To date, a limited number of direct field-based evapotranspiration studies have been performed. A Thornthwaite-Mather-type compensation evapotranspirometer was used to investigate both weather variables and biochar effects on maize evapotranspiration in Keszthely, Hungary, in the 2023 growing season. This research revealed that two meteorological variables daily mean temperature (Ta) and solar radiation (Rn) exhibited significant positive correlation coefficients in both daily evapotranspiration (ETa) applied treatments (0 and 400 kg ha−1 biochar), whereas precipitation (P) and daily relative humidity (RH) had negative impacts on both ETa treatments. Biochar application to a clay loam soil increased cumulative evapotranspiration (ETc), reaching 408.21 mm, while WUE (water use efficiency) stood at 3.46 kg m−3. Similarly, growth parameters (leaf area cm2, stem diameter cm, plant height cm, and chlorophyllSPAD) showed a positive effect after biochar amendment during the growing season intervals. This research outlined the most effective weather variables on ETa and how ETa could be computed depending on two climatic variables (Ta and RH). Furthermore, it concluded that biochar addition at low dose to clay loam soil is a powerful tool to improve ETc, WUE, and maize growth and productive features.