عنوان مقاله [English]
Background and Objectives
In arid and semi-arid regions, meeting the environmental need of rivers and natural lakes is drastically diminished due to the climate changes, drought, and mismanagement of the water supply and distribution in agricultural activities. The closed basins, like Lake Urmia, are more sensitive to reducing the environmental need, climate change and drought. In Lake Urmia basin with arid and semi-arid climate, water shortage and high demand for water consumption in agricultural activities have caused to increase the free accessibility of the extraction of the surface water and groundwater resources. Therefore, in arid and semi-arid regions, it is necessary to implement an integrated dynamic system (IDS) for water consumption management in all sectors of water consumption, including drinking, industrial, and agriculture needs to stabilize the groundwater resources; hence, the optimal use of water resources to increase production capacity per unit of water consumption is a fundamental solution to meet the nutritional needs of humans and improve the economic conditions of operators. The multiplication of greenhouse agricultural products compared to the open-field cultivation for the same water consumption is one of the crucial solutions in the optimal use of water resources.
The Ajichay basin with an area of 12600 km2 is one of the greatest sub-basins of Lake Urmia. The average annual temperature of this basin is 11.3oC and the average annual precipitation is 320 mm. The soil and water assessment tool (SWAT) model was used to evaluate the impacts of the implementation of development scenarios of greenhouse towns. The Soil and Water Assessment Tool (SWAT) is considered as a comprehensive hydrological model to evaluate the impacts of different factors such as climate changes, land use variations, changes in irrigation methods, agricultural management, and greenhouse development on runoff volume and groundwater level fluctuations. In this study, the SWAT model has been calibrated and verified using a 30 m digital elevation model (DEM), a soil map with 217 homogeneous zones, 5 land use maps from 1987 to 2016, data gathered from 10 hydrometric stations and more than 50 meteorological stations, hydrological studies and field measurements. In the present study, two scenarios were adopted according to the current policies of greenhouse towns development in Iran and an ideal scenario (third scenario) was also considered for the development of greenhouse towns. The first scenario for the development of greenhouse settlements was adjusted using new water resources with the aim of
increasing the production of agricultural products in the region. The second scenario is based on the removal of part of the arable lands of each aquifer and the use of harvested water from the removed lands in the development of greenhouse settlements. The third scenario was based on the average production statistics in open and closed environment in Ajichay catchment, in order to maintain the current production per hectare. In modelling the Ajichay basin, the time periods of 1987-1991, 1992-2009, and 2010-2016 have been assigned to the model adaptation to the basin conditions, calibration, and verification, respectively. The statistical criteria comprising correlation coefficient (CC), Nash-Sutcliffe efficiency (NSE), and root mean square error (RMSE) have been utilized to evaluate model performance for stream flow simulation in hydrometric stations.
Statistical criteria indicated the high accuracy of simulation for all ten hydrometric stations. At Akhula station (watershed outlet), the statistics including correlation coefficient (R), Nash-Sutcliffe efficiency (NSE), and root mean square error (RMSE) were 0.92, 0.83 and 6.48 m3 s-1, respectively, during calibration, and 0.86, 0.73 and 3.23 m3 s-1, respectively, during verification periods. Development of 1875 hectares of greenhouse towns in the Ajichay watershed caused an average depletion of 11.68 m and 4.41 m within the first and second scenarios, respectively; in contrast, for the third scenario, an increase of 3.86 m for the average groundwater level of the aquifers of the Ajichay watershed was acquired rather than the initial condition. At the end of the simulation period of the third scenario, the flow volume increase of 28.5 MCM y-1 was observed at the Akhula hydrometric station. The results revealed that the development of greenhouse towns using new water resources can increase agricultural crop production and also intensify the downward trend in groundwater levels. The second scenario has caused a negative balance in the aquifers due to the eliminating the water withdrawal under the traditional irrigation system from the hydrological cycle. Only in the third scenario, the removal of nearly ten hectares of farmlands for the development of one hectare of greenhouse has caused the aquifers of Ajichay basin to have an increasing trend of groundwater level. The results of the third scenario revealed that 24% of the total reduced volume of groundwater abstraction, was discharged into Lake Urmia through rivers, and 38% was related to the water withdrawal of the traditional irrigation, and the other part was spent on increasing the water abstraction from groundwater and rivers for the agricultural purposes.
The results of the implementation of the scenarios to develop greenhouse towns revealed that the type of policies and how to implement the development of greenhouse towns can enhance agricultural production capacity as well as decrease/increase the flow of rivers in the Ajichay basin. The adverse effect on the groundwater resources occurs when the administrative institutions' approach is focused only on the increasing production capacity. Furthermore, when the ratio of replacing traditional farmlands with greenhouse towns is not commensurate with the corresponding irrigation efficiencies, it applies more stress to the groundwater resources to meet the evapotranspiration need of the greenhouse plants. The positive effect on aquifers and increased river flow occurs when the actual evapotranspiration of the plants cultivated in greenhouse towns is less than the eliminated traditional farmland.