Agroecological zoning of agricultural lands in Golestan province for Faba bean (Vicia faba L.) cultivation by ordered weighted average (OWA)

Faba bean is one of the main winter crops in Golestan province (northeast Iran), with an estimated production of about 106,284 tons of green pods and 7256 tons of dry grain while the harvesting land area  is about 1838 ha. Topographic characteristics, climatic conditions and the soil quality of an area are the most important parameters to evaluate land suitability. To develop successful cropping systems, it is necessary to understand how a crop such as faba bean responds to biological, chemical, physical, and climatic variables, and how this response can be influenced by management. Accordingly, farmers can improve their production in this district, and at the end of the study, the distribution and areas of land suitable for faba bean cultivation in Aq Qala were determined. In addition, this research provides information at a local level that could be used by farmers to select cropping patterns in accordance with suitability results.

The study areas of the research include agricultural lands and rangelands of Golestan province  Using 1: 50,000 maps of the national cartographic center of Iran, we created a 20 m digital elevation model (DEM) with a topo-to-raster function. In this study, the first agro-ecological requirements of faba bean were determined according to scientific resources. Several parameters were considered in this study, including the annual average, minimum and maximum temperatures, annual precipitation, slope, elevation, and some soil properties such as organic matter, pH, EC, texture, phosphorus, potassium and calcium. The last 12-year climatic data were gathered from 4 meteorological stations and 21 rain gauge stations in the Golestan province. The mean temperatures were calculated from daily mean minimum and maximum temperature data. The rainfall and temperature data of the whole province were used to interpolate and draw map of annual rainfall, and annual average, maximum and minimum temperature by geostatistical and interpolation methods. The slope and elevation information was obtained from the Digital Elevation Model (DEM) using GIS software. The standardization of data was used from fuzzy method and Analytic Hierarchy Process (AHP) was used for weighting the criteria. Finally, using ordered weighted average (OWA) in the  IDRISI software, faba bean cultivation potential map was prepared.

The results indicated that the most important variables, according to their specific weighting, were annual precipitation during the growing season (0.2876), slope (0.5396) and EC (0.3913). The digital environmental layers overlaid and integration in GIS media, then zoning of lands were carried out in 4 classes (highly suitable, suitable, less suitable and non-suitable). The study area includes different classes with diverse capacities for faba been cultivation comprising: a highly suitable potential, which constitutes 55.78% of the total area; a suitable class, 27.82%; a less suitable, 12.23%; and non-suitable, 4.17%. The final maps showed that more than 80% of agricultural lands of township are suitable for faba bean production. This research provides information at a local level that could be used by farmers and decision makers to select cropping patterns in accordance with their suitability. The topography, soil characteristics and climatic conditions were the most important determinant parameters of this evaluation. Our results revealed that 86.60% of the land area is suitable for faba bean cultivation. From the land suitability analysis, it was concluded that, in general, some nutrient contents of the soil are low or high for the study region. In addition, the research results indicated that the main limiting factors are high EC and low rainfall.

In conclusion, a combination of GIS, AHP and OWA is a practical and applicable method for determining land suitability for faba bean crops. In this study, the time-saving during the development of the land suitability map for faba bean was considerable. This method is almost a new application in Iran for land suitability analysis. However, decision makers should consider the side effects of applying these methods in the determination of suitable areas for crop growing. The result in of this study indicated that the OWA is a valuable approach in land suitability assessment, therefore, we suggest that the OWA should be used with AHP in future land suitability evaluation. We recommend that similar studies should use other parameters such as relative humidity, wind speed, solar radiation and heavy soil metals.