**1. Introduction**

The inclusion of the social component in hydrologic modeling is necessary for considering the implications of water managemen<sup>t</sup> decisions in coupled socio-hydrologic systems. Sivapalan et al. [1] presented the importance of socio-hydrology to illustrate the evolution of coupled socio-hydrologic systems, potential co-evolution tracks, and emerging or even unexpected patterns that depart from the previous framework setting for scenario analyses. System dynamics modeling is a particularly useful method to simulate the dynamic mechanisms within a coupled system [2]. By properly modeling the interactions within a coupled socio-hydrologic system, proposed changes such as policy implementation may be included to model future impacts on varying aspects of both systems. In this way, simulations can offer insights as to which components of the system may benefit or be negatively impacted, which may contribute to water policy decision-making processes.

**Citation:** Bai, Y.; Langarudi, S.P.; Fernald, A.G. System Dynamics Modeling for Evaluating Regional Hydrologic and Economic Effects of Irrigation Efficiency Policy. *Hydrology* **2021**, *8*, 61. https://doi.org/10.3390/ hydrology8020061

Academic Editor: Tamim Younos

Received: 4 March 2021 Accepted: 30 March 2021 Published: 2 April 2021

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### *1.1. System Dynamics and Its Application for Sociohydrology*

Communicating a coupled socio-hydrologic system's complexity is difficult when considering the dynamics of the system, varied perspectives of water stakeholders, and potential conflicts among water users. Local changes to a system result in unexpected changes to the broader system and interconnected systems such as economies [3]. A change in the system can yield nonlinear and indirect effects [4–6]. The causes for an outcome with a managemen<sup>t</sup> strategy are varied due to delays between actions and effects, making it challenging to identify policy options.

System dynamics modeling is one approach to represent the key feedback structures of a coupled system with a social component. Key feedback structures in socio-hydrologic modeling track systematic impacts that arise due to anthropogenic factors. Liu et al. [7] used one exploratory and simplified conceptual model to depict the socio-hydrologic system's co-evolution processes. Gober et al. [8] stated that system dynamics view human activities as internal driving factors and include the interaction between social and hydrologic processes that threatens current water systems' viability through feedbacks and unintended consequences.

System dynamics modeling has long been recognized as an approach to realize sociohydrology research since Forrester developed the concept [9,10]. System dynamics modeling can long-term reflecting the impacts of social components. Then influence of social components may not have been obtained by modeling of the individual parts of the system separately. In this method, systems are represented through feedback loops, stocks, and flows. Feedback loops dictate the behavior of the system based upon physical processes and thresholds. Stocks depict the state of the system and maintain stepwise trends. Flows affect the stocks as inflow and outflow and interlink the stocks within a system [11].

System dynamics modeling is categorized as either qualitative and quantitative or conceptual and numerical [12,13]. Feedback loops improve the understanding of a system qualitatively [14]. Stocks and flows visualize the effects of system behavior through simulation quantitatively. Meanwhile, system dynamics modeling aggregates a wide range of input parameters as key factors in a meaningful way [15]. In particular, it can incorporate different forms of human decision-making processes and behavioral rules [16]. These characteristics can guide water managemen<sup>t</sup> strategies responding to the crucial changes in an adaptive way [17,18]. System dynamics modeling can dynamically simulate the consequences of evolutionary systems as a decision support tool for strategic policy testing [19,20].
