توسعه شبکه‌های آبیاری فرعی کم‌فشار از دیدگاه اقتصادی، مبتنی بر سناریوهای فشار-انرژی

Background and Objectives
Replacing the pressurized irrigation system with surface irrigation is one of the proposed solutions to increase irrigation efficiency. The low-pressure network consists of pipes that receives water from main and/or secondary canals or pipes and usually delivers it to farm units or hydro-flume pipes for surface irrigation with a pressure of lower than 10m of water. Various parameters affect the feasibility and design of low-pressure irrigation, which determine the economic justification of the plan, energy consumption, and its social desirability. In recent years, there has been more interest in implementing low-pressure irrigation instead of open tertiary canals. The reason for this is the reduction of water losses in the distribution and delivery process, the possibility of better management of water distribution, the possibility of volume delivery of water to agricultural plots, and the possibility of irrigating more lands. But there are still ambiguities in its economic and technical issues, and no significant research has been done on its design issues and parameters affecting its economic aspect. In this research, the design of a low-pressure distribution system instead of the open canals and the simultaneous consideration of social and executive issues in an irrigation area are considered. In addition, network design based on different design options (pressure-energy consumption), evaluation and comparison of two types of sub-networks from an economic and operational view have been proposed as research objectives. Focusing on the correct selection of the pump (with a lower energy consumption approach) and estimating for the economic comparison of the two methods of irrigation have been done.

Methodology
The land of Keyzarin village (with an area of 144 hectares) is located on the right bank of the Tangab river in Fars province. The lands of the right bank use the springs of the region, but the low efficiency of water distribution in the traditional canals has caused a significant reduction in the delivered water to the farms. The characteristics of the irrigation network in the current situation are as follows: a canal with a length of 610meters and a design flow of 360 lit/s, leads a part of the water of the "Qomp Atashkade" spring to the dividing point and divides it equally into two canals. In order to design low-pressure instead of the open canal, the layout of water conveyance and distribution lines is determined. The initial pipes’ diameter is determined according to the optimal speed, and then, the appropriate pump is selected based on the design flow rate and the topography of the area. In this research, the pressure control system is selected according to the topography of the region and the slope of the land, the type of closed system.
Socially, the available option for the layout of pipes is to follow the arrangement of the traditional canals. From a technical point of view, there are several options that, the main difference between them is in the hydraulic design and the type of pump used; So that, accordingly, the number of pumps, the commercial diameter of the pipes and the amount of energy consumed in each option are determined. Therefore, these options need to be compared technically and economically. In this research, 14 scenarios are defined based on 7 types of pump designs and two output pressures. The energy consumption of each scenario has been calculated according to the flow rate and cropping pattern in each irrigation season. Then, the comparison of costs in low-pressure and open canals network is calculated and compared.

Findings
The cost of constructing an open tertiary network per hectare of land is equal to 990 million-Rials. The costs of earthworks with machines, which are related to the volume of cut and fill, and then the cost of in-situ concrete, constitute the largest percentage of the total costs of the canal networks. Based on the results of the quantity surveying and estimating of the low-pressure sub-network, the cost of constructing for each hectare is equal to 1654 million-Rials. However, if the operation and maintenance costs are not met and if the maintenance and operation of the low-pressure pipes is neglected, the resulting problems will disable the network completely. The construction of the low-pressure network in the studied area can make 3.6 hectares of land available to farmers due to the less space occupied by this network and also increase the cultivated area by 21.6hectares (A total of 17.5% is added to the cultivated land in the irrigated area). Considering the quality of land in the region, the price of each hectare of irrigated land and rainfed land in the region is estimated to be 1 billion-Rials and 300 million-Rials respectively. Therefore, as a result of the implementation of the low-pressure network, the profit from the increase in the price of agricultural land (conversion of rainfed to irrigated) will be equal to 18 billion-Rials, which is equal to 83% of the total cost of the construction of the low-pressure network. Proper layout of lands in low pressure irrigation can reduce the costs of construction of the network. Also, the amount of more land that can be cultivated after the implementation of low pressure irrigation and the amount of reduction of land waste compared to open canals can be effective in the economic justification of low pressure irrigation. In low-pressure networks, repairs and maintenance, unlike open canals, are somehow mandatory.

Conclusion
At the end of this plan, low-pressure networks were chosen as the superior option over the tertiary canals due to various technical advantages and also to compensate for the initial implementation costs by increasing the water transmission efficiency and increasing the cultivated area. Based on the results obtained, it can be said that despite the acceptability of low-pressure irrigation, comprehensive studies should be done in each region for its implementation, and the specific economic-social conditions in a region may not be consistent with its implementation. Also, special attention should be paid to the final output pressure, of the pumps based on different options and the type of low-pressure irrigation network.