LAUSR

Abstract A study was conducted on a research farm at Urmia University under drip irrigation in order to schedule the irrigation of maize based on plant indices: crop water stress index (CWSI) and relative water content (RWC) and soil indices: soil water (SW) and soil penetration resistance (Q) in climatic conditions of Urmia region in Iran. This study was conducted in the form of randomized complete block design with four irrigation levels including two levels of deficit irrigation (I1) (Crop water requirement (CWR) of 0.5) and I2 (0.75CWR), a full-irrigation level of I3 (1.0CWR) and an over-irrigation level of I4 (1.25CWR) with three replications. I4 treatment for air-filled porosity was considered in the soil. For irrigation scheduling, lower and upper baseline and CWSI equations for I1, I2 and I3 treatments were calculated during plant growth period. Using the extracted baselines, the mean CWSI values during maize growth season for I1, I2 and I3 treatments were calculated to be 0.53, 0.44 and 0.28 respectively. The threshold limit of water stress index of I3 treatment was the basis of irrigation scheduling. Then, relationships were provided for determining the irrigation time using CWSI in Urmia climate for three maize growth stages. In addition, among soil and plant indices which were simultaneously measured at maximum stress hour were used as a complementary indicator to eliminate CWSI constraints. Using the regression relationships extracted between the indices, the water status of plant can be determined by only measuring the canopy temperature (Tc) without measuring the RWC, SW and Q. It should be noted that in the region of Urmia, the critical values of the difference between the Tc and the air temperature, Q and SW for maize were respectively 1.5 °C, 3.6 MPa and 0.27 cm3 cm−3 (matric potential of 8900 hPa).