Humanity is at the point in its history where it must question its ability to live without irreversibly damaging the biophysical and environmental conditions it depends on (D’Odorico et al. 2018). Increasing anthropogenic pressure is causing ecological change to become abrupt, and sustainable global development seems like a dream. With rapid international development and accelerated population growth, Water, Food, and Energy (WFE (resources have been under tremendous pressure, which has led to the problems of water scarcity, food shortages, and energy insecurity (Mahlknecht et al. 2020; Afshar et al. 2021; Liu and Zhao 2022). Remarkably, the interconnectedness of WFE subsystems is fundamental to global sustainability (Fuso Nerini et al. 2018). These problems have also been exacerbated by extreme weather conditions and the COVID-19 epidemic (Udmale et al. 2020). Previous studies have emphasized the importance that the only solution ahead is a fundamental change in understanding the management of WFE resources, and integrated systems approaches should be replaced instead of traditional approaches. However, before realizing the importance of connecting these three resources (WFE resources), management strategies usually worked for one resource independently of the others (Smajgl et al. 2016; Soleimanian et al. 2022). As a result, these strategies did not consider the internal interactions between these three inseparable sectors, and subsequently, the results obtained from the strategies used were utterly contradictory. Also, due to not paying attention to the meaningful interactions between these three inseparable sectors and wrong management, the competition over these three resources increased. The WFE nexus concept refers to the interconnections between WFE subsystems to address the aforementioned problems and offers insight into how strategies in one subsystem will influence the other subsystems and vice versa (Huang et al. 2020; Molajou et al. 2021a).
The WFE nexus is a dynamic and complex system in which WFE resources are intimately intertwined (Ma et al. 2021; Qi et al. 2022). Throughout the WFE nexus system, energy is needed to pump, transport, distribute, and treat water, and likewise, water is consumed for energy generation and processing (Roidt and Avellán 2019; Vahabzadeh et al. 2022). Much water and energy are also consumed in agricultural planting and food production (Molajou et al. 2021a; Afshar et al. 2022). WFE's nexus system is complex because of its intricate interweaving relationships. The emergency resource shortage crisis must be alleviated by clarifying the interrelationships between WFE and comprehensively managing the WFE nexus system (Ma et al. 2021).
The food subsystem is one of the essential parts of the WFE nexus system. Throughout the food subsystem, people are linked to their food as they pursue production, distribution, and consumption activities, and these activities also affect society and the environment as a whole (Ingram 2011; Schipanski et al. 2016; D’Odorico et al. 2018). One of the most critical challenges of the Food and Agriculture Organization of the United Nations (FAO) is defining projects in line with planning to feed 9 billion people by 2050 (van Dijk et al. 2021). As a result, global food production should increase by 70%, and in developed countries, the amount of food production should be doubled. According to FAO reports, in the past decades, the number of people who cannot provide food has reached one billion, of which about 65% live in Asia (United Nations 2019).
Generally, the food subsystem in the WFE nexus can be divided into four main categories: 1- Livestock products 2- Poultry products 3- Agricultural products 4- Fisheries products. Natural resources and human labor are used in producing, processing, and transporting food and in individuals' food consumption decisions. The food subsystem is therefore shaped by agricultural, trade, and food policies and food consumers' cultural, economic, and educational dimensions (Ingram 2011; Zhang et al. 2018; Bhunnoo and Poppy 2020; Mahdavian et al. 2022). Agricultural products make up the most important and diverse part of the food chain. In other words, agriculture is the primary source of food supply because livestock and poultry products are also indirectly dependent on agricultural products. In addition, agriculture relies heavily on energy, so energy is consumed directly or indirectly in agricultural lands (Tyczewska et al. 2018; Grote et al. 2021; Vahabzadeh et al. 2022). Currently, the food subsystem consumes 30% of the world's total energy (Li et al. 2021). In modern agriculture, most of the activities are mechanized and agricultural operations such as tillage, planting, irrigation, and harvesting are done with high efficiency using agricultural equipment. As a result, all these activities require energy carriers. According to earlier research, 3400 million joules of energy are needed to grow winter wheat, and 800 and 650 million joules of energy are used to cultivate and harvest the crop, respectively (Chang et al. 2016; Taghizadeh-Hesary et al. 2019).
Considering the undisputed importance of agriculture in the food subsystem, from the nexus point of view, the simulation model of crops is fundamental in the food subsystem. The crop growth model (CGM) is a model that simulates the growth stages of leaves, branches, and roots gradually over time, like a natural plant. The CGMs can calculate crop growth, development, and yield by solving the governing equations of soil, plant, weather, and management measures (such as irrigation, fertilizer, pesticides, Etc.)(van Keulen et al. 1982; di Paola et al. 2016). Therefore, a crop simulation model predicts the results of a specific management or environmental condition. One of the applications of crop simulation models is to understand better the performance of different parts of the crop growth process. Also, by changing various parameters such as seed type, soil type, weather conditions, irrigation amount, type and amount of fertilizer, pesticides, Etc., the amount and manner of the effect of those parameters can be calculated. As a result, these models can perform thousands of test plans for different products during their growth period, which takes place in a minimal time (Steduto et al. 2009; Siad et al. 2019).
With the advancement of computers and the increase in the ability of calculations, the simulation of crops takes place in the shortest time. For this reason, the acceptance of CGMs has increased in recent years(O. Rauff and Bello 2015). However, this point should be noted that the agricultural products and food subsystem is highly dependent on energy and water for the production of agricultural food products(el Gafy et al. 2017); Therefore, the simulation models of the food subsystem should be developed in such a way that shows the interactions of the food subsystem with water and energy in the nexus system. With the development of such a model, water and energy consumption will be determined in the simulation model to define different cultivation patterns. As a result, macro-management decisions will be made with a broader vision. First of all, this review introduces two types of simulation models of the WFE nexus system. In the next step, the framework related to the CGMs, in which the mandatory nexus criteria are applied, is defined. Then, food subsystem simulation models for crop growth, which are used in the nexus system simulations, have been technically reviewed from the development perspective. It is worth mentioning that in some cases, the CGMs which have not been used in the WFE nexus system have been assessed. Finally, a checklist is provided based on the appropriate choice of CGMs that can be used in the nexus system.