عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Introduction Considering the vital importance of water in arid and semi-arid regions like Iran, conservation and optimal use of water resources is necessary. In this regard, increasing conveyance efficiency and reducing water losses in distribution canals are very imperative. Low water conveyance efficiency in distribution canals is mainly due to seepage losses in channels. However, since infiltration and seepage cause groundwater recharge, it is sometimes beneficial. Reviewing the literature revealed that some specialists believe lining channels is not justified, technically and economically. Other experts do not reject lining at all, however, suggested that since seepage could help groundwater recharge indirectly, in regions which there is no ponding problem, channel lining should justify from the economic point of view. Additionally, another group of researchers found that although construction costs of lining seems very high, considering its long term benefits for countries with vital need to water in future, such as Pakistan and Iran, is economic. Meanwhile, according to some studies, concrete lining is not economically justified. Based on numerous different and sometimes opposite opinions regarding economic justification of lining, it seems necessary to investigate this issue on regional scale. Thus, the current study aimed at economic analysis of infiltrating water from unlined canal to groundwater and pumping again through wells to surface, in comparison with lining canals and not allowing to seep.
Method In order to do this research, the concrete canal of modern Irrigation and Drainage network of Gandoman and Boldaji, Borujen city, Chaharmahal and Bakhtiari province, with ?.? m width, ?.? m depth and ?.? m top width, was chosen as prototype. In order to analyze the lining, this channel was compared with an earth canal, having the same dimensions. Soil texture was determined using hydrometer method, which was obtained as loam that was constant to the depth of water table (?.?? m from soil surface). In order to estimate the amount of seepage in the earth channel, using physical modelling and dimensional similitude relations and considering ?.?? as scale ratio, dimensions and discharges of lined canal have converted to laboratory dimensions at soil mechanics lab of Shahrekord University. Lining lifetime was selected??, ?? and ?? years, and the interest rates were ??, ?? and ?? percent.
Results Based on the results, the measured seepage loss in experimental flume of laboratory for the unlined and lined canal was ?? and ?? %, respectively. According to calculations, the price of groundwater in the study region is nearly ?.?? cents per cubic meter. Bearing in mind the low depth of groundwater in the study area (?.?? m), this small value in comparison with the price of surface water (?.?? cents per cubic meter), will justify. According to ‘base unit price index for irrigation and drainage works’, the costs of earth and lined canal construction with length ?????, top width ?.? and bottom width ?.? m, was calculated and obtained as ????? and ????? M.Rials respectively. It has been noticeable that for construction of both type of canals, half of cutting practices have considered in stony lands and the remaining in earth lands. Furthermore, renovation and maintenance costs for unlined channel were taken into consideration, which have been applied from year ? to ??, ??, and ?? years. These costs have increased by ??% increase per year (according to the inflation rate). Results showed that increasing the interest rate from ?? to ??% has been resulted in decreasing both NPV and B/C values (except for B/C in concrete-lined canal, ?? years). On the other hand, increasing the lifetime of lining from ?? to ?? (constant interest rate), the NPV and B/C values were increased at first and then were decreased. Investigating the calculations showed that after ?? years, with maximizing the maintenance costs of lined canal, its costs exceed its benefits. Therefore, lining canal with the mentioned properties will not be economical after ?? years. Consequently, with all lifetime values, the earth canal is more economic than the lined one, considering both economic indices and for all interest rates. Therefore, it could be concluded that when the water table depth is ?/?? m and it is possible to pumpage the infiltrated water from well (while the canal is unlined with a loam soil), which are the properties of Borujen region in the current study, lining with concrete is not justified economically.