نوع مقاله : مقاله پژوهشی
نویسندگان
گروه مهندسی آبیاری و آبادانی، دانشکدگان کشاورزی و منابع طبیعی، دانشگاه تهران، کرج، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
This study aimed to estimation of the Manning roughness coefficient (n) in different phases and events of irrigation using empirical relations. For this purpose, six inflow rates in two flow categories, low and high, three consecutive irrigation events, advance and storage phases, two irrigation intervals and two types of soil texture were investigated. Next, the correlation between roughness and these parameters was investigated using Pearson and Kendall statistical tests. Then, using its results, regression equations were developed to estimate Manning’s n in different irrigation phases. The results indicated that the advance time and the size of clods before irrigation had a high correlation and the slope, initial soil moisture and the size of clods after irrigation had a low correlation with the Manning’s n data in the whole irrigation event. The roughness coefficient of the advance phase also had the highest correlation with the advance time. The highest and lowest correlation coefficients between the parameters and roughness coefficient of the storage phase were related to advance time and inflow rate with values of 0.65 and -0.31, respectively, which shows high correlation and direct relationship between advance time and roughness and weak correlation and inverse relationship between flow rate and roughness. The average values of R2, RMSE, and NRMSE indices in the provided relationships were 0.87, 0.014, and 26.97%, respectively, which indicated the appropriate accuracy of these relationships. Finally, it was suggested to conduct similar studies in different field and hydraulic conditions so that the presented relations are more comprehensive and can be recommended in other fields since the development of such relations can increase the speed of roughness estimation in different phases and the ease of using it.
کلیدواژهها [English]
Presenting empirical equations for estimating Manning roughness coefficient in furrow irrigation in different irrigation phases
EXTENDED ABSTRACT
Manning equation is an empirical equation that is commonly used to calculate water flow in open channels. However, its application in investigating the flow hydraulics in surface irrigation, especially in irrigation furrows, faces limitations due to the assumptions made for it. These limitations have led the researchers to consider Manning roughness coefficient as a fixed number during each irrigation event by accepting possible errors in the estimation and to pay less attention to its changes during an irrigation event. This is despite the fact that if the value of Manning roughness coefficient is estimated more (less) than the actual value, considering that the roughness is a force resisting the flow, the flow rate is estimated less (more) than the actual value and leads to many errors in the simulation and hence, it is necessary to pay more attention to the time changes of the roughness coefficient by using various methods and assumptions that lead to the simplification of roughness estimation complexities.
This research aimed to developing regression equations to estimate roughness in different irrigation phases, using different hydraulic and non-hydraulic parameters.
To investigate the values of Manning roughness coefficient in different phases and events in furrow irrigation, different treatments were considered in such a way as to cover most of the factors influencing roughness. For this purpose, six inflow rates in two flow categories, low (with an average of 0.27 liters per second) and high (with an average of 0.54 liters per second), three consecutive irrigation events, advance and storage phases, two irrigation intervals (5 and 10 days) and two types of soil texture were investigated. Secondly, Manning roughness coefficient was determined in whole irrigation event, advance and storage phases and respectively by using SIPAR_ID model, Manning equation and WinSRFR. Finally, the mutual effect of various hydraulic and non-hydraulic parameters on Manning roughness coefficient was investigated and regression equations based on the influential parameters were developed using SPSS software to estimate the roughness coefficient in different phases.
The results showed that Manning roughness coefficient in the advance, storage phases and whole irrigation event in the first to third irrigations ranged between 0.017 and 0.636, 0.015 and 0.317, and 0.015 and 0.34, respectively. The average was 0.083, 0.054 and 0.055. The results also showed that the advance time and the size of clods before irrigation had a high correlation and the slope, moisture and the size of clods after irrigation had a low correlation with the Manning roughness coefficient data in the entire irrigation event. The roughness coefficient of the advance phase also had the highest correlation with the advance time. The highest and lowest correlation coefficients between the parameters and roughness coefficient of the storage phase were related to the advance time and inflow rate with values of 0.65 and -0.31, which shows high correlation and direct relationship between advance time and weak correlation and inverse relationship between flow rate and roughness.
Results indicate the suitable accuracy of the methods used to estimate the roughness coefficient even in the advance phase where the application of Manning equation can be considered more than before.
The sensitivity analysis of Manning roughness coefficient in the equations developed to estimate roughness in different phases also showed that the cross-sectional area and flow rate (inflow, outflow and average) had the greatest influence on Manning roughness coefficient in all irrigation phases, and the recession and advance time were other parameters influencing the roughness coefficient. The importance and mutual influence of flow and cross-section is clear and has been given much attention.
As results indicate, it is very important to take into account the mutual relationship between roughness and advance and recession time, since as an example, the increase of roughness coefficient in the furrow is due to various reasons such as the presence of clods, Obstruction in the furrow path, irregularity in the path (irregular plowing) etc. can lead to an increase in the advance time and as a result an excessive increase in depth infiltration and consequently water loss or an imbalance in the distribution of water along the furrow, which may have received less attention so far and might require more precision.