Estimation of crop coefficient of irrigated transplanted puddled rice by field scale water balance in the semi-arid Indo-Gangetic Plains, India

Estimation of specific crop coefficient (kc) based on local climate, genotype and management conditions for irrigated rice will help in improving irrigation scheduling and water productivity. We estimated kc values of continuous flooded transplanted puddled rice (CFTPR) (cultivar Pusa-44) from field scale water balance measured actual crop evapotranspiration (AETc), Penman-Monteith reference evapotranspiration (kc₁) and pan evaporation (kc₂) losses in the Indo Gangetic Plains (IGP), New Delhi, India. Measured kc₁ values ranged from 1.15–1.58, 1.44–1.75, 1.90–1.96, 1.59–1.82 and 1.0–1.41 at tillering (14–18 DAT), panicle initiation (27–34 DAT), flowering (62–65 DAT), physiological maturity (97–101 DAT) and harvesting (112–113 DAT), respectively. Based on FAO-56 defined stages, the measured kc₁ values were 1.06–1.120 for initial (kc-ini), 1.73–1.88 at mid- (kc-mid) and 1.36–1.45 at end-season (kc-end) growth. Due to higher pan evaporation (8–11%) losses over estimated reference evapotranspiration of grass surface, the measured kc₂ values at initial and end- seasons were 2–5% less while at mid-season, it was 7–8% less than kc₁. The FAO-56 climate adjusted model improved kc-ini by 9.5% but improved kc-mid and kc-end values only marginally (<2.5%) over FAO-56 climate unadjusted model. As a result, FAO-56 climate adjusted model considerably under-estimated kc-mid by 42–53% and kc-end by 50–58% over measured kc values (kc₁ &kc₂) while kc-ini value (1.15) was comparable to measured kc values during both the seasons. Measured kc values suggest adjustment of available FAO-56 model derived kc values by 1.30–1.53 times at mid-and 1.40–1.60 times at -end seasons of CFTPR ecosystems for improving irrigation scheduling and water productivity in semi-arid and other similar agro-ecological regions.