Estimation of crop coefficients of dry-seeded irrigated rice–wheat rotation on raised beds by field water balance method in the Indo-Gangetic plains, India

• Stage wise crop coefficient (kc) values of irrigated direct seeded rice-wheat on raised bed and conventional flat planting were estimated from field water balance crop evapotranspiration (ETc) and Penman–Monteith evapotranspiration (ET0).
• Bed geometry led variation in plant density, leaf area and soil surface exposed to evaporation loss resulted in lesser ETc loss as well as kc values in later stages of rice–wheat compared to flat planting.
• Estimated kc values of rice–wheat on beds and flat planting differ considerably from FAO-estimated values.

Lack of information on crop coefficient (kc) values of bed planted rice–wheat system in the Indo Gangetic Plains (IGP) of India has become a constraint for irrigation planning to improve the crop water productivity. In this paper, we estimated kc values from field water balance measured crop evapotranspiration (ETc) and Penman–Monteith estimated reference ET0 for dry-seeded irrigated bed planted rice–wheat rotation and also compared with conventional dry-seeded flat system of planting. The experiment was conducted in 2001–2003 at New Delhi, India. Estimated kc values at initial stage (4–6th weeks of sowing) on raised beds were comparable with flat conventional planting for rice but significantly higher for wheat. However, in later stages of growth, kc values for both rice and wheat were lesser on beds than flat land. The kc values of rice during initial, crop development, mid-season and late-season stages on beds were 0.62, 0.75, 1.16 and 0.67, respectively while in conventional flat land, corresponding kc values were 0.61, 0.97, 1.42 and 0.91, respectively. The kc values for wheat at four crop growth stages (initial, crop development, mid- and late-season) on raised beds were 0.98–1.06, 1.10–1.14, 1.25–1.26 and 0.46–0.47, respectively while on flat land with conventional row (20 cm) spacing, corresponding kc values were 0.87, 1.12, 1.45 and 0.55, respectively. Flat beds similar spacing (20–47 cm) with raised beds had comparable kc values. Bed geometry led variation in plant population density influenced strongly both crop ETc losses as well as kc values. The results provide estimates of ET0, ETc and kc for use in irrigation scheduling in bed planted rice–wheat system in the IGP of India and elsewhere with similar environmental conditions.