Introduction
Measuring the amount of runoff and soil loss per unit area is very important to evaluate the condition of erosion in different areas. The runoff is caused by the condition of the upstream watersheds, which has a different response in different time and space ranges of a watershed. Some hydrological responses may be considerable in the upstream areas of a basin, but the same responses cannot be identified in the downstream. These different responses among different regions are referred to as the scale effect, including the mismatch among runoff responses at different scales. In this context, the role of spatial scale is considered as a factor that directly affects the occurrence of various processes of runoff production. Therefore, assuming the uniformity of discharge changes on different scales leads to the application of a single plan and management strategy for the entire watershed area, which ultimately results in the failure to achieve the desired outcomes. On the other hand, an accurate estimation of the discharge status of watersheds provides a correct understanding of the underlying hydrological processes. Comprehending the accurate influence of spatial scale on runoff is essential for precise streamflow estimation in watersheds and their zoning across various spatial extents. This forms the foundation for optimal water and soil resources planning and management, particularly in basins facing serious soil erosion challenges. Therefore, the objective of the present study was to examine the effect of spatial scale on the specific discharge and zoning of discharge values within the Karkheh basin, based on data of 28 hydrometric stations.
Materials and methods
In this research, the data used includes the average annual water discharge (Q) and the area of the basin and sub-basins (A). The data used was received from the Iranian Water Resources Management Organization. Subsequently, in order to identify the most suitable stations, any disturbances in the natural process of the basin processes were initially investigated. This process involved the identification and removal of stations that had the potential to alter the process. Finally, a total of 28 stations with an appropriate statistical period length and no upstream basin disturbances were selected. Also, in order to calculate the upstream area of each station, the Karkheh basin was divided into 28 sub-basins based on the location of existing hydrometric stations. This was achieved using the Arc Hydro extension and the digital elevation model. In addition, according to the topographical, Geomorphological and hydrological conditions, the Karkheh basin was divided into five main areas.
In the following, according to the values of area (A) and specific discharge (Qs) in the sub-basins, Qs-A relationships were drawn under log-log coordinates to calculate a linear or non-linear regression equation between Q and A, the effect of the scale on the amount the discharge in the main sub-basins should be specified. Finally, using the method provided by Church et al., the corrected specific discharge values were calculated for each of the main sub-basins and through interpolation, the specific discharge map of the basin was produced.
Results and discussion
The results of investigating the effect of scale on specific discharge indicated that there was no proper regression relationship between Qs and A for the entire Karkheh basin as a single basin (R2=0.3). However, the regression relationships with a coefficient of determination ranging from 0.7 to 0.98 were obtained for the sub-basins.
Other results showed that in the majority of sub-basins, there is a negative relationship between area and specific discharge. That is, as area increases, specific discharge decreases. However, in the Seimareh basin, the increase in area leads to an increase in specific discharge. The observed increase in discharge can be attributed to the reduction in the basin elevation and the broadness of the riverbed. In contrast to other sub-basins, the discharge in the Qarasu sub-basin exhibits a low initial value, followed by an increase and subsequent decrease. This phenomenon can be attributed to the location of the Qarasu sub-basin in the upstream region of the Karkheh basin, the topographical conditions, and the high slope of the area. In general, the results indicated that the specific discharge varies across sub-basins with different areas and spatial scales. So that, the highest specific discharge was observed in the vicinity of the Karkheh basin outlet. This can be attributed to a number of factors, including the high slope and rainfall intensity in the upstream areas, as well as the difference in climate conditions and vegetation cover between the downstream and upstream areas.
Conclusion
A comprehensive understanding of river discharge and the impact of spatial scale on it is of paramount importance in various fields. It provides a profound insight into the consequences associated with this phenomenon. To sum up, it is critical to understand river discharge patterns and how spatial scale affects them in a variety of contexts, including water resource management, flood risk reduction, sustainable development, and environmental preservation. It gives us a thorough awareness of the resulting effects. The findings of this study indicate that as the area increases, the specific discharge tends to decrease. Based on these results, it can be inferred that spatial scale and the upstream watershed area can exert an influence on discharge. A substantial body of research has demonstrated that spatial scale and the upstream watershed area play a pivotal role in determining river discharge. However, it is crucial to acknowledge the influence of a multitude of factors, including vegetation cover, climate, topography, slope, soil infiltration, and precipitation intensity, on specific discharge across diverse watersheds. Therefore, when evaluating the hydrological conditions of large-scale watersheds, it is essential to pay particular attention to the impact of spatial scale as a crucial factor. It is noteworthy that recognition this phenomenon necessitates more in-depth and comprehensive investigations across various spatial scale levels.
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