Modification and Development of the Tenant Method to Maintain Ecological Regime in Environmental Flow Management of Rivers
Authors: Khatar Behnaz , Shokoohi Alireza 
Published: January 2023

Abstract
The allocation of environmental flows should be done through legislation based on fundamental studies, while this does not seem to be the case in Iran, and the Tenant method for determining river water rights without assessment has been dictated to the responsible organizations. The main problem of the Tenant method is the allocation of a fixed amount to the environmental flow as a percentage of the average annual flow and therefore the inability to comply with the ecological regime of the river. The main reason for the attention of the country's water authorities to the Tenant method is that in this method as a hydrological method, the long-term average flow in the river is used, which is firstly easily available and secondly does not have to follow the allocation time pattern. In this regard, the responsible agencies can deliver the entire flow allocated to the environment in a volumetric manner over a period of time rather than actual environmental needs. Previous studies have suggested substitute methods concerning the problems of the Tenant method, but in the present study, the promotion of this method has been put on the agenda instead of offering a new method. The substitute methods, which are generally offered by prioritizing the environmental sector in developed countries of the Northern Hemisphere such as the United States, and the Southern Hemisphere such as Australia, are not welcomed in areas facing persistent water shortages and successive droughts. While everyone recognizes that sustainable development has three pillars of economic, social, and environmental justice, and without respect for environmental rights cannot achieve development goals to reach social welfare, the provision of environmental water rights is at the bottom of this circle and has the lowest priority in developing countries. In the present study, due to the importance of the subject and non-admittance of the ecological methods with environmental focus and priority in Iran, the Tenant method was accepted as the only applicable method for allocating environmental flow, then considering the ability and feasibility of Habitat simulation methods changes were made to enhance the Tenant method. In the enhanced Tenant method, instead of considering a fixed value for the whole or half of the year, first using the flow duration curve, the hydrological condition of the river is determined each month. Then, based on the normal, dry, or wet condition of the desired month, and employing the five Tenant scenarios for the management of the river ecosystem, the environmental flow is determined. Accordingly, the developed method, by allocating a percentage of the average monthly flow, practically follows the historical regime of the river without creating a conflict with the requirements of the existing habitats. The results showed that the algorithm used to develop the Tenant method, in addition to success in a proper simulation of -inter-seasonal changes in the river has been able to reduce the annual volume allocated to the environmental flow in the same range recommended in the conventional Tenant method, i.e. within the acceptable range in the opinion of the government.