**Models Are Essential for Water Resource Management**

**Dr. Ali Mohtashami**

**Postdoc Researcher**

**UNESCO Chair on Aflaj Studies (Archaeohydrology)**

A model is an informative representation of an object, phenomena, person, or system, and modelling is the use of these models as a basis for simulations of the related object, phenomena, person, or system for making decisions. Modeling enables us to predict the future behavior of proposed system and present suitable management policies. We have made major advances in the past 40 years in our ability to model engineering, economic, ecological, hydrologic, and sometimes even institutional or political aspects of large complex multi-objective systems and phenomena. By applying and verifying models to real systems, we have gained a better understanding of them, which has often resulted in improved system design, management, and operation. There are many types of models such as physical, conceptual, mathematical and so on. Physical model is a smaller or larger physical copy of an object. Conceptual model is a representation of entities and their relationships and mathematical model is a description of a system using mathematical relations and concepts.

In this article we go through to mathematical ones. To enhance the proficiency of the models, computer-based modeling is used. The use of these quantitative mathematical models is often considered a requirement for carrying out the prediction of that system and also investigate its impacts.

In the three recent decades, due to the importance of water in the whole world, humans’ attention to using mathematical models for these resources has increased. Planners, managers and decision makers in water fields start to use these models in many ways. Because they can predict the behavior of any proposed water resources design or behavior or management policy by using mathematical simulation and optimization models within interactive computer programs and can also give them an access to accurate and effective information. The variety purposes of water decision makers (WDMs) include of studying how water flows in both aquifers and rivers, evaluation the quality changes related to both groundwater and surface water of basins and watersheds, identifying regions at high risk of not having adequate groundwater in both quantity and quality aspect, determining flood risk areas, analyzing the economic and social impacts of water, design of hydraulic structures such as dams, weirs and spillways, design of water distribution networks, sedimentation movement from mountains, optimal consumption in industrial, agricultural, and drinking and trading in water.

A remarkable help of models is that they allow WDMs to make estimates of future conditions by considering historical data and asking "what if" questions about complex water systems. In order to sustainably manage these resources, WDMs rely on the information achieved by mathematical models because they provide information that is necessary to understand the impact of changes to water resources on users, our environment, and our country and finally our planet.

It is possible to determine how actions or choices in one area will affect others by using models. As a result, they make it possible to predict what might happen without it actually happening. Our models have the capability of running under different scenarios, so we can test hypotheses without the expense or impact of a physical pilot project, which is one of the most interesting things about them. For example, modelling can show what will happen in different parts of the watersheds because of high rainfall or drought as follows:

For rainfall:

- How much rainfall infiltrates and recharges the aquifer?
- How much does this recharge affect the flowrate of the other near wells and aflaj?
- Does this recharge lead to wider farming?
- Can this recharge be used to extend water distribution networks in urban areas?
- How much rainfall converts to runoff?
- What is the amount of runoff? What happens if this value becomes flood? If yes, how much is it? And what are the high-risk regions?
- Can this runoff transport sediment? If yes, where is the deposit location?
- And so on.

Therefore, the advantages of the models especially in water resources management can be summarized as follows:

**Less expense**

You can test an external element on the real system without changing the real system. For example, you want to allocate 75 percent of the water resources to agriculture, but you don’t know what will happen for other consumers, In this matter, you do not change the consumption rate in a real system, but you change it in the model and see what happens next.

**More safety**

Suppose poisonous chemicals enter the water resource. With the help of the model, you can find out whether it is suitable for drinking even though you can't directly touch it to measure the pollution concentration.

**Saving time****speed things up**

Therefore, water resource modeling is a valuable tool for gaining a comprehensive understanding of the present situation of that resource and its outlook.

Groundwater Flow Simulation for a sample Aquifer (adopted from Mohtashami et al. 2022) |

Flood Risk Map for a sample Basin (adopted from Mohammadi et al. 2021) |