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The model MSM (Maize Simulation Model) was modified and validated, using data on furrow irrigation in Fars province in Iran during 2003 and 2004. The planted cultivar was SC704 (single cross) a late maturity hybrid. Three nitrogen levels (300, 150, and 0 kg N ha-1 as urea) and four irrigation treatments (applied water at 1.2, 1.0, 0.8, and 0.6 ratios of potential maize water requirements as I4, I3, I2, and I1 treatments, respectively) were considered. The potential water requirements were estimated based on the differences between soil field capacity and measured soil water contents at root depth in the I3 treatments before irrigation. The MSM model was modified based on field data in 2003 as: (i) Modification of root N uptake based on soil water pressure head and van Genuchten soil parameters for soil moisture retention curve α, and (ii) modification of grain yield prediction based on seasonal plant stover nitrogen uptake and grain N content. Model subroutines, i.e. leaf area index, dry matter production, prediction of soil water contents at different depths and at different days after planting did not require any modifications. Comparisons between measured data and predicted values by the modified model indicated favorable validation. [ABSTRACT FROM AUTHOR]

• Empirical CWSI calculation for irrigation scheduling of maize subjected to drip irrigation. •.Relations between plant indices (CWSI and RWC) and soil indices (soil water and soil penetration resistance). • Determination of the critical limits of the (T c - T a), soil water and soil penetration resistance. 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 (I 1) (Crop water requirement (CWR) of 0.5) and I 2 (0.75CWR), a full-irrigation level of I 3 (1.0CWR) and an over-irrigation level of I 4 (1.25CWR) with three replications. I 4 treatment for air-filled porosity was considered in the soil. For irrigation scheduling, lower and upper baseline and CWSI equations for I 1 , I 2 and I 3 treatments were calculated during plant growth period. Using the extracted baselines, the mean CWSI values during maize growth season for I 1 , I 2 and I 3 treatments were calculated to be 0.53, 0.44 and 0.28 respectively. The threshold limit of water stress index of I 3 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 (T c) 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 T c 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). [ABSTRACT FROM AUTHOR]