The Impact of Tajan Irrigation and Drainage Network on Meeting the Environmental Flow Requirements of Water Resources

In recent years, due to the repeated emphasis of the country's development programs on food security and promotion of self-sufficiency in the production of basic agricultural crops, many efforts have been made for agricultural development. Ignoring the sustainability of the environment, as one of the fundamental pillars of the sustainability of agricultural production systems, has put a lot of pressure on the fragile environment and especially on the aquatic ecosystems. One of the important aspects for assessing the sustainability of regional water resources is how to determine the long-term satisfaction of the environmental flow requirements (EFR) of the resources. This research investigates the satisfaction trend of quantitative and qualitative EFR of surface and groundwater resources in the area of Tajan Irrigation and Drainage Network (TIDN) in Mazandaran province.

Hydrological and qualitative EFR of surface water (S.EFR) including Tajan River (T.EFR) and Ab-bandans (A.EFR) and groundwater (G.EFR) was determined for pre (1997- 1984) and post (1998-2019) TIDN periods. Hydrological EFR of Tajan River is calculated using four hydrological methods variable monthly flow (VMF), Tennant, Tasman and Smakhtin. The qualitative EFR of this river is determined based on the three main surface water pollutants in the region, including nitrogen, phosphorus and salinity. The minimum volume of water required to sustainably maintain Ab-bandan ecosystems is considered as A.EFR. Due to the fact that until now no specific method has been provided to determine the EFR of groundwater resources; in this research, by combining hydrologic and qualitative parameters such as the depth of groundwater, salinity and, nitrogen concentrations, the EFR of these resources is determined.

The average river flow before and after the operation of TIDN was 14.53 and 8.36 m3 s-1, respectively. Before TIDN, based on MVF, Smakhtin, Tasman, and Tennant methods, the hydrological EFR of the river satisfied in 79.1, 59.2, 69.1, and 90.1% of cases, respectively. The satisfaction was 53.4, 27.1, 41.4, and 73.3% of cases, respectively, during the TIDN operation. From nitrogen and salinity perspectives, the violation rates of the qualitative EFRs of this river during TIDN operation increased by 11.1 and 9.9%, respectively, compared to the pre-TIDN period. EC has the main role in the deficit of qualitative EFR of the river, followed by nitrogen and phosphorus. The operation of TIDN caused an increase in the depth and nitrogen concentration of the groundwater, resulting in an increase in the unstable area regarding these two parameters during the operation period. Before the construction of TIDN, no part of the region had a hydrological EFR deficiency of more than 353 m3, but after that, about 40.6% of the region experienced higher deficiencies than this value. Also, the area with nitrogen related qualitative EFR deficiency rises from 13.4% in pre-TIDN to 35.6% in post-TIDN.

The development of TIDN increases the violation of quantitative and qualitative EFR of surface and groundwater resources in the region. Considering the limitation of EFR satisfaction after TIDN, especially in low rainfall seasons, it seems necessary to revise the cropping pattern and irrigation method. Otherwise, the continuation of the current trend, through completely disrupting the ecological balance, will make agriculture unsustainable.