Effect of inflow rate and initial soil moisture on Manning roughness coefficient in advance and storage phases in furrow irrigation

Manning roughness coefficient is a complex and effective parameter in furrow irrigation, and its exact determination is complicated due to spatial and temporal changes of soil characteristics and hydraulic parameters of the flow. Hence, this study was conducted to determine the Manning roughness coefficient in the advance and storage phases and in the first to third irrigation events. In this study, Manning roughness in furrow irrigation in the advance and storage phases was determined using the SIPAR_ID model and the Manning equation, respectively. For this purpose, the value of Manning roughness coefficient for two inflow treatments (an average of 0.27 and 0.53 L/S), two irrigation cycle treatments (5 and 10 days), and two different fields (E and F) was examined in the first to third irrigation events and in three replications. The results showed that the value of Manning roughness coefficient in the advance and storage phases of three irrigation events (first to third irrigation) was between 0.017 to 0.636 and 0.015 to 0.317, respectively. Also, it was found that the average roughness coefficient in the advance phase was more than the one in storage phase (0.083 and 0.054, respectively). In addition, the roughness coefficient in the two phases reduced by increasing irrigation events. In field F, due to the heavier soil texture, the difference in Manning roughness coefficient in both phases in the first and second irrigation was lower than the ones in field E. The results also indicated that the roughness coefficient had an inverse and poor relationship with the initial soil moisture and inflow. However, the inflow was more effective than the initial moisture.