Simulation of Nahand Reservoir water allocation and its performance evaluation under inflow increase and decrease scenarios using the Water Evaluation and Planning (WEAP) model

Introduction

The complex nature of water-related issues along with rapid population growth and limited water resources to meet needs require new methods in planning and managing the water resources in order to integrate technical, economic, environmental, social and logical perspectives in an integrated manner. The management of water resources is considered as the main possible way to solve the problems caused by the decrease in the quantity and quality of water. This is the concept of Integrated Water Resources Management (IWRM), which is one of the basic methods to achieve sustainable water resources at the national and international levels. This is generally believed that the traditional techniques have lost their capabilities in the management of water resources and water allocation problems. This requires the use of newly developed tools which may be beneficial in making optimal decisions in the exploitation of water resources. By the occurrence of phenomena such as climatic changes and numerous droughts along with the increase of population and agricultural cultivation area, the water demand has been significantly increased worldwide. In other words, the consecutive droughts accompanied by reduction of the average annual rainfall have decreased the amount of accessible water, while the population growth has worsened the situation. Effective approaches and policies including identifying priorities and optimal water allocation techniques especially in basins with different users are considered essential for sustainable development in each region. With 1100 cubic meters of renewable water per person per year, Iran is considered to be the most critical region in the world in terms of water resources. Unfortunately, most plans in the water sector of such countries are based on local economic growth, and no attention is paid to the amount of available water resources. Considering the issue of water crisis and the droughts of the last few years, the issue of water resources management has gained high importance. To overcome the mentioned problems, it is inevitably essential to make use of newly developed water management techniques based on advanced approaches. Although the optimization techniques are well-known tools in these issues, the simulation method utilizes as a helpful approach. To simulate of a water management in the basin, there are various available models. A RIBASIM, MIKE BASIN, WEAP and MODSIM models are famous and user-friendly ones of this collection. WEAP software is a comprehensive and advanced water resource system simulation tool that is widely used in watershed management and is able to consider both physical and hydrological processes. The scenarios that can be investigated with this software include population growth, economic development, changing the policy of operating reservoirs, extracting more from underground water resources, saving water, allocating ecosystem needs, integrated use of surface and underground water, reuse of water etc.

Materials and Methods

This study was conducted in the Nahand catchment area which is located in East Azerbaijan province. Nahand river is the main draining course of this catchment, on which a dam has been built to supply a part of Tabriz drinking water. In order to control the performance indicators of the reservoir, several management and exploitation scenarios were developed and evaluated in the WEAP model. The WEAP model was presented in 1990 by the Stockholm Environment Institute (SEI). It is a comprehensive and advanced model for simulating water resources systems, which is extensively used in the management of water resources in watersheds. This model has provided a practical tool for water resource planning and policy analysis in order to put all the issues related to water resources and uses in a single environment. The WEAP model is capable of simulating issues related to consumption such as water consumption patterns, water reuse strategies, costs and water allocation patterns, as well as issues related to resources such as river flow, groundwater resources, reservoirs and water transmission lines. Reservoir performance indicators are used to measure its performance under different operating circumstances.

Results and Discussion

The simulation results of the studied area indicated that the WEAP model with evaluation criteria including R2, RMSE and MAE in the calibration stage were 0.89, 1.16 and 1.01 MCM, respectively, and in the validation stage were 0.88, 6.22 and 6.01 MCM, respectively . The resultsalso showed that the amount of water demand for the near future period (2021-2040) will increase due to the increase in population, and therefore, the resources in the basin will not be able to meet all assumed needs. The findings showed that the studied system for the near future period (2021-2040) under the Reference (continuation of the status quo), SC1 (increase in flow by 10 percent) and SC2 (decrease in flow by 10 percent) scenarios from the drinking water supply point of view, will result in a shortage of 28.1, 7.3 and 44.3%, respectively, and from the supply of agricultural needs point of view will result in 31.4, 18.3 and 44.4%, respectively. Also, by evaluating the reservoir's performance indicators, it was found that under all assumed scenarios, the system will fail under the condition of supplying 100% and 80% of the needs, whereas the reservoir will be more sustainable by applying the SC1 scenario in comparison with the other two scenarios.

Conclusion

In order to choose the best management and exploitation scenarios, due to existing circumastanes and limitations such as time limitation, cost, possible risks to the environment, etc., it is not possible to apply all scenarios in the basins and, thence, it is logical to choose the most suitable scenario. Therefore, software tools can help experts to make decisions by condering all limitations. By examining the results of the reservoir performance indicators, it can be seen that the reservoir will encounter with failure in supplying 100 and 80 percent of the needs in the future period under all scenarios, and the sustainability index of the reservoir (remedial stability) in supplying 100 percent. The needs under the Reference, SC1 and SC2 scenarios will reach 31, 49 and 22%, respectively, and in meeting 80% of the needs, the sustainability index will be slightly higher.