Evaluation of the performance of sediment connectivity and sediment transport capacity indicators in the spatial analysis of the sediment flux pattern in the Neyriz watershed, Fars province, Iran

Introduction

The amount of sediment production, the manner and time of sedimentation, the size and composition of sediment grains, its transport among the waterways network are important features of the sedimentation regime of watersheds; Because changes in each of these factors cause changes in watershed performance. Therefore, sediment production is a reflection of the importance and amount of erosion processes and sediment sources in the upstream parts of the watershed and how sediment is transported and deposited from the moment of movement of erosion materials from the point of separation to the exit of it. Sediment connectivity indicators indicate the spatial changes of connectivity patterns in different parts of the watershed and provide a suitable estimate of the contribution of sediment sources as well as sediment transport routes. For this purpose, investigating the spatial pattern of sediment flux at the scale of the watershed is of particular importance in developing watershed comprehensive management plans and measures to control erosion and sedimentation. Although various indicators and models have been developed in this field; But so far, their performance has not been evaluated based on observational data and statistical methods. The purpose of this research is to analyze the sediment flux pattern of the Neyriz watershed located in the east of Fars province, based on the indicators of sediment connectivity and sediment transfer capacity and compare their performance based on field sediment evidence.

Materials and Methods

At first, a digital model of the ground elevation of Neyriz watershed with a spatial resolution of 12.5 meters was prepared and its drainage network was extracted. Then, the sediment connectivity index was calculated by considering the upstream and downstream features of the watershed and by considering the roughness factor as the sediment movement resistance factor, and the sediment connectivity map of the watershed was made. Then, the sediment transport capacity index map was also prepared based on the concept of topography and using the digital model layer of land height. For this purpose, sediment transport capacity index was calculated for each pixel of Neyriz watershed in SAGA-GIS software and a sediment transport map was prepared. Based on field visits to different parts of the watershed, 30 positions that had evidence of sediment transfer were recorded as observation points of the sediment transfer event. Also, 30 other positions that did not have signs and evidence of sediment transfer were also added to the validation database as observation points of no sediment transfer event. The corresponding values of each index were also extracted in the geographic information system and based on the available information, using evaluation methods based on the error matrix including: true skill statistic (TSS), efficiency (E) and F score (F-score), the validation of the mentioned indicators was done quantitatively.

Results and Discussion

The value of the sediment connectivity index of the Neyriz watershed in Fars province varied from -7.24 to 2.43 and its median value was -4.36. The spatial pattern of sediment connectivity index in this watershed is such that the middle and western parts have a low amount and the northern, southern and eastern parts have more amounts. Since in this research, the drainage network of the watershed was introduced as the target of receiving the sediment; Parts of the slopes of the watershed level, which had the conditions of sediment production and transfer and were connected to the drainage network in terms of the transfer path, have shown a higher value of connectivity index. This index provides valuable information for land management by considering the upstream characteristics of each point as well as the characteristics of the transfer path to the sediment receiving target. The value of the sediment transfer capacity index varied from 7.2 to 23.16 and the average value was 10.19. The value of this index is high in the marginal parts of the watershed where there are sloping lands, and the middle parts of the watershed have a small value. Based on the findings, the index of sediment connectivity with the true skill statistic (TSS) of 0.833, the efficiency value (E) of 0.916 and the F score of 0.915 is a better performance than the sediment transport capacity index (TSS= 0.633, E=0.816, F-score=0.825). In addition, based on the values of the false positive component in the error matrix, the sediment transport capacity index predicts high sediment flux potential in many situations; While in the field observations, it was not true.

Conclusion

Based on statistical evaluation criteria, sediment connectivity index has been able to better describe the state of sediment flux and is more consistent with field realities. So that the sediment connectivity index, in addition to considering the characteristics of the upstream area of each point of the watershed, it is possible to consider the path of the sediment particle to the target location of the receiver (such as the nearest branch of the network drainage) has made it possible. Based on the findings obtained in this research, although the sediment transfer capacity index in some parts of the Neyriz watershed of Fars province was in line with the sediment connectivity index; However, due to the energy-based nature of the flow, this index only considers the local conditions of the points and ignores the features of the upstream area as well as the process of transporting sediment particles to the downstream. Therefore, it is suggested to use the sediment connectivity index in the erosion and sedimentation studies of watersheds. Because when information about the amount of sediment production is not available; Sediment connectivity index can provide useful information about the sediment transfer process in the watershed.