Journal of Irrigation and Drainage Engineering

The main producers of agricultural drainage water in Khuzestan province in Iran are the sugarcane agro-industries. As a result of the accumulation of drainage water in the region, environmental and social problems have arisen. In general, the salinity of this drainage water averages $6.3\mathrm{dS}/\mathrm{m}$ throughout the year. The cultivation of salinity-resistant varieties of paddy rice can be considered as a suitable substrate for the optimal use of drainage water. In this regard, a study was conducted in one of the sugarcane fields in southern Khuzestan based on randomized complete block design with three replications. The main factor was three intermittent irrigation regimes using drainage water of sugarcane fields, which included daily irrigation (I0), one-day irrigation interval (I1) and two-day irrigation interval (I2). In addition, the subplots included three salinity-resistant rice lines (${\mathrm{S}}_1$, ${\mathrm{S}}_2$, and ${\mathrm{S}}_5$), four drought-resistant rice lines (${\mathrm{M}}_2$, ${\mathrm{M}}_5$, ${\mathrm{M}}_7$, and ${\mathrm{M}}_{12}$), a control cultivar “Hashemi” and a local cultivar “Hoveyzeh.” The results showed that with a significant reduction in applied water from the I0 to I1 and I2, the yield in I1 increased by 6% over I0, but the yield in I2 decreased by 30%. The highest and lowest water productivity were related to I2 and I0 at the rates of 0.13 and $0.06\mathrm{kg}/{\mathrm{m}}^3$, respectively. In I1 irrigation regime, the local cultivar Hoveyzeh produced a yield of $\mathrm{2,444}\mathrm{kg}/\mathrm{ha}$, while the salinity-resistant lines ${\mathrm{S}}_1$ and ${\mathrm{S}}_5$ produced acceptable yields of 2,385 and $\mathrm{2,263}\mathrm{kg}/\mathrm{ha}$, respectively. According to the findings from monitoring the soil exchangeable sodium percentage (ESP), farm soils in both irrigation regimes I0 and I1 were nonsodic and saline. In I2, however, the soil profile status approached the sodic state as a result of the drying of the topsoil and the influx of salinity into the surface layer, resulting in a 17% increase in ESP. Consequently, when using agricultural drainage water for irrigation, leaching at the end of the growing season and drainage are necessary to remove salts. Based on the findings of this article, it is possible to maintain stability in rice production for a wide range of regions around the world with warm climates by reusing agricultural drainage water.

The existing equations for the discharge coefficient of Piano key weirs (PKWs) use a limited range of experimental data, which means that they are inappropriate for wide parametric ranges that might lead to significant errors. This study aimed to propose a reformed empirical equation using a wide range of data points gathered from previous experimental studies. Further, the appropriateness to use the existing equations for the collected data points, and the related errors, were investigated in detail using graphical and statistical analyses. The proposed equation predicted the discharge coefficients with $<5\%$ absolute errors for 83.5% data points and with $<10\%$ absolute errors for 100% data points, and the mean absolute error was 2.9%. Such variations may be attributed to the differences in experimental conditions that exist among the previous studies. The correlation indices were higher for the proposed equation as compared to the same for the existing equations, whereas the error indices were lowest for the proposed one. For some very specific parametric ranges, the existing equations still hold better accuracy. Overall, the proposed equation can precisely estimate the discharge coefficient of the basic geometry of Type-A PKWs for a wide parametric range and will be handy in the hydraulic design of such PKWs.

The labyrinth side weir structure is an optimal solution to regulate and divert excess flows across rivers, and irrigation and drainage channels when the opening length of the side weir is limited. Changing the geometry of the side weir is considered one way to improve its efficiency. The present study investigates the hydraulic effects of stepped labyrinth side weirs to increase their discharge capacity. To estimate the outflow over symmetrical stepped labyrinth side weir, the discharge coefficient needs to be determined. Normal triangular labyrinth side weir in one cycle mode has been modified by increasing the crest path using different steps. A total of 417 experiments were conducted on normal and stepped side weirs for various weir opening lengths, weir heights, internal head angles, and hydraulic conditions. The results were analyzed and compared. It was found that efficiency improved, and the discharge coefficient increased as the number of the steps decreased. Furthermore, the water surface profile becomes more uniform with decreasing number of steps. A lower number of steps, smaller head angle, and high weir length produce a higher discharge coefficient by approximately 15%–35% compared to a normal triangular labyrinth side weir; in addition, the Knope length decreases by approximately 40%–70% for the same effective length. Additionally, a reliable equation to estimate the discharge coefficient of the stepped labyrinth side weir was presented. The results of this study can be used to design side weirs with high hydraulic performance when the weir-included angle selected is small or there is a lack of space.