In the present study, the effect of different irrigation water treatments on black grams was investigated. The experimental design was a randomized complete block design with four aqueous treatments in three replications (Fig. 1). Water treatments included 50% (I1), 75% (I2), 100% (I3) or the control treatment, and 125% (I4) of crop water requirement. To determine the water requirement, meteorological parameters were obtained from the meteorological station of the Faculty of Agriculture of Urmia University, and the reference evapotranspiration (ETo) was calculated by V3.1 ETo Calculator24. The equation used in this software to calculate ETo is the FAO-modified Penman–Monteith equation25. Using the following equation, the ETo calculated by multiplying the crop coefficient (Kc)26 was generalized to the potential evapotranspiration (ETc) values of black gram (Table (Table33).

Plot layout of field experiment.

Black gram irrigation scheduling for control treatment (I3), during 2017 growing seasons.

Immediately after transferring the seedling, an irrigation step was carried out to plant the seedlings in the field. The black gram is used as a second crop in the area while water stress was applied in mid-July (tenth irrigation) for optimal plant establishment. The irrigation water amounts for each treatment, ETc and ETo, during the crop growth period are presented in Table Table4.4. Plant irrigation was performed during the growing season using a 16 mm dripper pipe located next to each row21. The 16 mm dripper pipe had constant pressure and the flow rate of emitters was 4 L h-1. At the beginning of each 16 mm pipe, a 16 in 16 mm valve was used to control water stress over time21. According to soil analysis, four fertilizer treatments were used to prevent nutrient deficiency27 and these four treatments received equal amounts of fertilizers28. Fertilizer was used as a spray and soil fertilizers (Table (Table11).

Irrigation amounts and ETo (mm), during 2017 growing seasons.

ETo: reference evapotranspiration.