Determination of maize crop coefficients in humid climate regime using the eddy covariance technique

•Five eddy covariance datasets are used to derive the Kc curve for maize in northern Italy.•Kc-ini are close to the FAO-56 adjusted values, for years of both abundant or scarce rainfall.•Kc-end range from 0.35 (grain maize, harvested dry) to 0.89 (silage maize).•Kc-mid are about 1.0, systematically lower than the corresponding FAO-56 adjusted values.•The low Kc-mid value is likely due to the humid climate and low wind speed of the study area Italy is one of the most important maize producers in the EU, and the Po Valley plain in northern Italy is the largest maize area in the country, with over 1millionha planted. Water consumption for maize is very high because the crop covers almost 30% of the agricultural land of the plain and is irrigated mainly by border irrigation. Conversion to more efficient methods is currently under discussion for many areas. Whatever the irrigation method adopted, an accurate knowledge of maize irrigation requirements is essential to improving irrigation scheduling and water use planning on a regional scale. The FAO-56 “single crop coefficient” procedure is currently the most widely used procedure for the estimation of crop irrigation requirements. However, in the last 15 years, many authors have shown that FAO-56 crop coefficients, even if adjusted to account for local conditions (climate, soil, irrigation and crop management), may differ by up to ±40% from experimentally determined values. In this study, Kc was derived as the ratio of the actual crop evapotranspiration in well-watered conditions (ETc), as measured by the eddy covariance technique at two maize fields over three years, to the reference crop evapotranspiration ETo. The crop cycle length ranged from 136 to 146 days, depending on the year and the harvesting date, and the accumulated growing degrees-days (GDD) varied from 1590 to 1730°C for silage and grain maize, respectively. The average locally developed Kc values for initial, middle and late growing seasons were 0.33, 0.99 and 0.89–0.35 (the Kc-end value depends on the harvesting time). The experimental Kc-mid was 15% lower than the FAO-56 value adjusted for local conditions. Average seasonal ETc values were 430 and 400mm for grain and silage second-crop maize, respectively