Vol.:(0123456789) 1 3 https://doi.org/10.1007/s00704-021-03710-0 ORIGINAL PAPER Actual evapotranspiration and crop coefcients for tropical lowland rice (Oryza sativa L.) in eastern India Sumanta Chatterjee 1,2  · Paul C. Stoy 3,4  · Manish Debnath 1  · Amaresh Kumar Nayak 1  · Chinmaya Kumar Swain 1  · Rahul Tripathi 1  · Dibyendu Chatterjee 1  · Smruthi Sagarika Mahapatra 1,5  · Ammara Talib 4  · Himanshu Pathak 6 Received: 21 October 2020 / Accepted: 26 June 2021 © The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature 2021 Abstract Accurate measurements of actual evapotranspiration (ET a ) and crop coefcients (K c ) are essential to know crop water require- ments and to improve irrigation scheduling. The eddy covariance (EC) technique is increasingly being used to do so. Precise information on K c for lowland rice is essential for local- and regional-scale irrigation planning but it is lacking for tropical humid climates such as those found in eastern India. We used the EC technique to measure ET a and K c —the ratio of ET a to reference potential evapotranspiration (ET 0 )—of tropical lowland rice in eastern India over 2 years. ET 0 was estimated by four diferent approaches—the Food and Agriculture Organization-Penman–Monteith (FAO-PM) method, the Hargreaves and Samani (HS) method, the Mahringer (MG) method, and pan evaporation (E pan ) measurements. Measurements were taken when rice was grown in the dry season (January–May) and wet season (July–November) and in between growing seasons when the feld was kept fallow. The magnitude of average ET a during dry seasons (2.86 and 3.32 mm d −1 in 2015 and 2016, respectively) was higher than that of the wet seasons (2.3 and 2.2 mm d −1 ) in both the study years. Of the four methods tested for ET 0 estimation, the FAO-PM method best-represented ET 0 in this region of India. The energy balance was found to be more closed in the dry seasons (75–84%) and dry fallow periods (73–81%) as compared to the wet season (42–48%) and wet fallow (33–69%) periods of both years of study, suggesting that lateral heat transport was an important term in the energy bal- ance. The estimated K c values for lowland rice in dry seasons by the FAO-PM method at the four crop growth stages, namely, initial, crop development, reproductive, and late-season, were 0.23, 0.42, 0.64, and 0.90, respectively, in 2015 and 0.32, 0.52, 0.76, and 0.88, respectively, in 2016. The FAO-PM, HS, and MG methods produced reliable estimates of K c values in the dry seasons, whereas E pan performed better in wet seasons. The actual K c values derived for tropical lowland rice in eastern India are diferent from those suggested by the FAO implying revision of K c values for regional-scale irrigation scheduling. Keyword Eddy covariance · Lowland rice · Crop evapotranspiration · Crop coefcient · Energy balance closure 1 Introduction Rice, a major global staple food crop, occupies about 44 mil- lion ha (Mha) of cropped land in India meeting food require- ments for about 65% of the population of India (Mohanty and Yamano 2017; Chatterjee et al. 2020a). In India, rice is cultivated in uplands and lowlands under both rainfed and irrigated water inputs which consume about 42% of the available water supply. It has been projected that 20–60 Mha of irrigated rice in Asia may sufer from water scarcity by 2025 (Bouman 2007; Elliott et al. 2014). Worldwide, irriga- tion utilizes between 90–94% of global water consumption and it has the maximum share (96–98%) in South Asia in part due to large-scale rice cultivation (Siebert et al. 2015). In India, the withdrawal of groundwater for agricultural Highlights • Actual evapotranspiration (ET a ) and crop coefcients (K c ) for tropical lowland rice in Eastern India were calculated using eddy covariance approach. • The magnitude of ET a during dry seasons were higher than the wet seasons in both the study years. • The FAO-PM, Hargreaves-Samani and Mahringer methods produced reliable estimates of K c values in dry seasons. • The K c values derived in this study are diferent from those suggested by the FAO for rice. * Amaresh Kumar Nayak aknayak20@yahoo.com Extended author information available on the last page of the article / Published online: 23 July 2021 Theoretical and Applied Climatology (2021) 146:155–171