Comparison of water footprint in various irrigated and rainfed wheat agro-ecosystems from Iran

A proper understanding of crop yield and corresponding water consumption is important to reach the sustainable agriculture. Water footprint (WF) is one of the indicators for describing water consumption and water use efficiency so that its accurate estimation in different climates and cropping systems (i.e. rainfed and irrigated) could be effective in managing water resources. WF consists of four components included green water, blue water, gray water, and white water. The magnitude of WF and its components is notably affected by deferent factors such as soil type, climate, and water management practices. Accordingly, the current study aimed to assess water footprint and its components in various irrigated and rainfed wheat agro-ecosystems of Iran. In the current study, water footprints and its components (blue, green, gray, and white waters) were simulated for irrigated and rainfed wheat agro-ecosystems in six locations in Iran (Ardebil, Hamedan, Sanandaj, Tabriz, Urmia, and Zanjan) during the 37-year period (1980-2016) using APSIM (Agricultural Production Systems sIMulator) crop model. To do this, four inputs for the crop model including crop characteristics (i.e. genetic coefficients), climatic data (i.e. maximum and minimum temperatures, rainfall, and solar radiation), soil characteristics (i.e. soil water content at wilting point and field capacity, saturation water content and bulk density), and management practices (i.e. sowing date, nitrogen fertilizer, irrigation, and etc.) were gathered to run the crop model. Outputs from the model were used to estimate WF components. The outputs included grain yield, cumulative net irrigation, and evapotranspiration in irrigated and rainfed systems, respectively. The results showed that grain yield and total WF were 2.4 t ha-1 and 1498 m3 t-1 for rainfed, and 5.7 t ha-1 and 1393 m3 t-1 for irrigated systems. The magnitude of WF was not only associated with type of cropping system (i.e. irrigated and rainfed) but also strongly region-dependent. Shifting from rainfed wheat cultivation to irrigated one increased WF of +15.2, +14.6, +28.1 % for Tabriz, Urmia, and Zanjan and decreased WF of -5.94, -9.03, and -9.12 % for Ardebil, Hamedan, and Sanandaj, respectively. In irrigated wheat systems, the shrare of green, blue, gray, and white waters in total WF simulated by 32.3, 24.2, 20.9, and 22.6 %, respectively. However, in rainfed wheat systems, the shrare of green and gray waters in total WF estimated by 90.3 and 9.60 %, respectively. The findings of the current research showed that shifting from irrigated to rainfed cultivation of wheat in some areas of northwestern Iran (i.e. Ardebil, Hamedan, and Sanandaj) could lead to a significant reduction in blue water consumption (less irrigation) and subsequently increase sustainability in these areas.