Linking Crop Water Stress Index (CWSI) and water stress coefficient (Ks) to support irrigation scheduling of wheat grown in Mediterranean environments

Wheat is among the crops that are affected by increasing drought and water scarcity, and deficit irrigation (DI) is considered to be an effective strategy to stabilize yields under limited water availability. To support on-farm DI management, methods based on soil water balance and canopy temperature measurements have been suggested to be reliable for field application. A preliminary experiment was set at CIHEAM-IAMB to evaluate the effect of different water regimes on two durum wheat varieties. In our experiment, under full irrigation (FI) the crop was kept quite constantly under unstressed conditions, while under DI (supported with 50% of corresponding FI supplies) and rainfed regimes, moderate water stress started respectively during the 'early grain filling' and the 'booting to heading' stages, subsequently increasing until the end of the cropping season. Grain yield reached 4.4 t/ha under FI, while it was reduced by only 14% in the case of DI treatment (3.8 t/ha) but with a corresponding potential water saving of about 44% (from 281 mm to 159 mm of total irrigation supplies). On the other side, DI increased yield of about 43% with respect to rainfed production (2.6 t/ha). The 'biomass water use efficiency' (WUEb) decreased moving from rainfed (3.51) to FI treatment (2.42), and similarly the 'yield water use efficiency' (WUEy) increased moving from FI (1.19) to DI (1.32) and to rainfed treatment (1.47). Midday canopy temperature (Tc) was remotely measured at plot scale by means of a thermal camera and the corresponding 'empirical' CWSI was computed. The CWSI followed the same daily trend for the two varieties and it appeared to be a good indicator of plant water stress as it was ranging between 0-0.1 under non-stressed conditions, while it increased steadily after the imposition of water stress, first for rainfed (at booting stage) and after for DI (at early filling stage), reaching values of 0.7-0.9 at the end of the season. A comparison of the 'measured' CWSI with the 'simulated' Ks has been done under the different water regimes, and the modification of some selected crop parameters resulted in the improvement of model simulations.