NDVI‐based estimates of evapotranspiration of winter wheat indicate positive effects of N fertilizer application on agronomic water‐use efficiency

Abstract Yield increases by fertilizer application impose higher crop water use, as biomass production is positively correlated with transpiration. To quantify effects of N fertilizer supply on evapotranspiration (ET) of winter wheat, a field experiment with three nitrogen rates (N0, N120 and N230) under non‐water‐limited conditions was performed during 2014 and 2015. Normalized difference vegetation index (NDVI) was used to derive crop coefficients which were used to calculate N effects on bare soil evaporation (E), transpiration, evapotranspiration (ET), grain yield and harvest index (HI). E during the early post‐winter growth period was measured with micro‐lysimeters and compared with model estimates. N fertilizer supply resulted in lower cumulative E, and increases in grain yield were higher than increases in ET, resulting in a higher agronomic water‐use efficiency (WUE Y ). HI of treatment N120 was higher than that of treatment N230 indicating that HI was not the main reason of higher WUE Y of treatment N230. It is concluded that estimates of ET under variable N supply require consideration of N‐induced effects on canopy development which could be successfully monitored by NDVI measurements. N supply increases ET and WUE Y potentially imposing a trade‐off between water conservation and efficiency of water use for crop production.