Weighing lysimeters are used to measure crop evapotranspiration (ETC) during the growing season. A ratio of crop evapotranspiration to reference evapotranspiration (ETo) determines a crop coefficient (Kc) value, which is related to a specific crop growth development stage. Determination of Kc is important for estimating crop irrigation requirements using meteorological data from weather stations. The research was conducted to determine growth-stage-specific Kc and compare them to existing FAO Kc values by investigating water use of maize (Zea mays L.) at the Water Technology Center Research Field in the Indian Agricultural Research Institute (IARI), New Delhi, India in 2010. Three lysimeters, weighing about 3.5 t, contained undisturbed 1.5 m deep soil monoliths. Accumulated seasonal crop water use was about 411 mm and the Kc values determined for maize during the growing season varied from 0.53 to 1.21. The calculated and measured evapotranspiration values were compared to assess the performance of the crop coefficient. The Nash-Sutcliffe efficiency (NSE), the ratio of the root mean square error to the standard deviation of measured data (RSR), the root mean square error (RMSE) itself, and the coefficient of determination (R2) values indicated that the Kc performed 'Good' in estimating the seasonal evapotranspiration of maize. However, with respect to particular growth stages, the agreement between the calculated and measured values varied from 'Satisfactory' to 'Very Good'. The Kc values for the initial, crop development, mid-season, and late stages were 0.40-0.60, 0.70-0.80, 1.1-1.21, and 0.50-0.65, respectively, while the values reported for maize by FAO are 0.3, 1.2, 0.3-0.6 for the initial, mid-season and late stage, respectively. The measured Kc values were different up to some extent from the FAO reported values; the cause might be that FAO Kc values are generalized ones and recommended for a wide range of climatic conditions. Other causes might be that different maize varieties have different crop water use and evapotranspiration patterns. So, determination of Kc for crops in different regions and climates is important to improve irrigation water management.
Crop Coefficient, Evapotranspiration, Maize, Weighing Lysimeter.