Investigation of the Arjan Weir-bridge and Analysis of its Role in the Prosperity of Agricultural Economy Based on Historical Texts and Archaeological Studies

The Arjan Weir Bridge in southwest Iran from the Sassanid era to the era of Muslim Arabs, including irrigation and water supply structures that geographers and historians paid attention to the Islamic Middle Ages was the first. The upper Weir Bridge of the Arjan is a huge structure with different uses that some geographical and historical sources have described. After the destruction of the upper Weir bridge, a smaller sample was built downstream of the river, from which little archaeological evidence has been obtained. There are few traces of the structure of the Arjan double Weir bridges and the present study is based on such a necessity to document the effect of the upper Weir Bridge on the Arjan economy. The purpose of the forthcoming research; Analysis of the existence of an upper Weir bridge in the prosperity of the agricultural economy of the Arjan is from the Sassanid era and the early Islamic centuries. This article seeks to answer the question of what was the position of the upper Weir Bridge in the prosperity and prosperity of the agricultural economy of Arjan region during the Sassanid era and the first centuries of Islam. Historical texts provide a relatively confusing description of the Weir bridges, especially the upper Weir bridges. The answer to the research question has been done by examining historical texts, analyzing the flow rate and output of canals similar to Qanats. All research data are obtained from archaeological surveys and basic hydrological data of today’s Maroon River route. This work by calculating the amount of water in the Arjan Weir Bridge Dam basin in its active and prosperous conditions. The result was that the amount of irrigation of thr Arjan Weir Bridge caused relatively extensive artificial irrigation in the lands of the northern part of the Arjan plain and the case study of wheat as a crop that the texts have repeatedly spoken of; based on water needs and calculation of OPTIWAT software has been a significant success in this area.
Keywords: Weir-Bridge, Arjan, Agricultural Economics, Irrigation, Sassanid Era and the First Islamic Centuries.
 
Introduction
Arjan or Argan was one of the six states of Fars province during the Sassanid and then the Islamic era in southwestern Iran and on the border between Fars and Khuzestan provinces. This State is described in the texts of Islamic times due to its active economic capacities and the special strategic position of land and sea; it has been of considerable importance in the Sassanid era and the centuries of Muslim Arab rule over this region. One of the reasons for paying attention to this area has been the fertile plains along with permanent rivers, including the Khairabad (Zohreh / Shirin) and Tab (Maroon / Jarahi) rivers. Therefore, there was an urgent need to control water through the construction of a Weir bridge for traffic as well as agriculture. On the other hand, Arjan water structures, including Weir bridges, as well as covered canals similar to Qanates, have suffered serious damage due to natural and human factors, and few traces of these structures have been left. On the other hand, the study of Arjan agricultural economy in Sassanid times and also the first Islamic centuries of Arjan region is considered as one of the undeniable necessities of reconstruction of cultural development trends in this region as the boundary between two important regions of Khuzestan and Fars.
Research Questions: Historical texts provided confusing and relatively irregular data on the structure of the Weir Bridge and how it operated in the days of prosperity. The first part of the research examines Islamic texts in this field. However, the present article seeks to answer an important question that has been proposed as follows: What was the position of double Weir bridges and especially the upper Weir Bridge in the prosperity of the agricultural economy of the Arjan region during the Sassanid era and the first centuries of Islam?
Research Method: In the present study, by combining information from historical texts and archaeological data, an attempt was made to investigate the effectiveness of this water structure by analyzing the historical description. The data obtained from hydrological computational studies were used in favor of agricultural economics analysis with the focus on the Arjan upper Weir Bridge to be able to cultivate some agricultural products with the help of OPTIWAT software and the impact of the Arjan Weir Bridge on the agricultural economy in Sassanid and Islamic Eras Turned on.
Identified Traces
The share of the Arjan state in archaeological studies in the field of water engineering is very small. In this research, an attempt was made to eliminate the lack of such information with survey studies as well as data obtained from hydrological studies. Therefore, the upper Weir Bridge of the Arjan was considered as the main center of research. The amount of incoming water as well as calculating the discharge of outgoing water through the openings holes installed in the body of the bridge was considered as the basis of irrigated agriculture.
If the average width of the river up to a distance of 3500 meters from the dam is considered 30 meters (this is the case today), the following relationship is formed:
 
 
 
 
 
 In this regard, the desired A is the research area or the area of the basin triangle, b will be the base and h will be the height. Therefore, such a relationship can be defined as:
3500×30÷2=52500
m³52500×7=367500
Thus, the volume of water in Arjan Dam in normal and normal conditions was 367,500 cubic meters, which was directed to the agricultural lands and industrial uses in Arjan city and its suburbs through 9 Qanats entrances. Under normal conditions, the flow rate of Arjan Qanat entrances was about 35 liters per second, which in 9 valves of the Qanat in the dam body, a figure of about 315 liters per second, transferred water to agricultural lands. Due to the slope of the route and the arrangement of the entrances to the Qanats, more water flowed and a figure of about 90 liters per second was calculated. The total inflow of water was about 810 liters per second in 9 entrances of Qanats or canals. Out of this amount of water output; Based on the calculations of OPTIWAT software, the following relation is obtained:
810÷1000=0/81
M³0/81×3600×24=69984
Therefore, the volume of water that was discharged from Arjan Dam by Qanats and transferred to agricultural lands in 24 hours a day was about 69984 cubic meters. According to OPTIWAT software calculations, the maximum water requirement of the wheat plant in Behbahan plain is about 1.5 liters per second during the day. Therefore, despite the figure of 810 liters per second, approximately 540 hectares of the northern lands of Behbahan plain and around the city of the Arjan have been under irrigated cultivation, which is a significant figure in the Sassanid era and the first centuries of Islam.
 
Conclusion
Description of the Arjan Weir Bridge in geographical and historical texts of the first to middle Islamic centuries, although it can clarify some historical and structural features of this building; however, the confusion of the information obtained from these texts has necessitated an archaeological view of the functional importance of this irrigation and water supply structure. This Weir bridge with a relatively large pond as a source for feeding 9 canal openings was similar to the Qanat system, which included irrigation of farms around the city. As a result of the operation of this Weir Bridge, the water from the runoff of the Maroon River with a flow of 90 liters per second flows in each of the 9 canal openings and over 540 hectares of the Arjan plain lands are cultivated with artificial irrigation in 24 hours. Case study of a wheat plant as a product compatible with the region and also the product mentioned in most historical texts as the agricultural potential of Arjan from the early Islamic times based on calculations of OPTIWAT software is quite successful and the amount of water in the Arjan dam basin in one day’s ability to irrigate Shows significant areas under wheat cultivation.