Determining the spatial correlation and pattern of changes in the height and volume of runoff in the sub-watersheds of Sharganj Birjand region

Statement of the Problem: Floods are a devastating natural disaster that can cause soil erosion, soil wastage, and damage to human infrastructure in flood-prone areas of watersheds. Therefore, it is crucial to predict and determine the amount of runoff production processes and its safe transfer to the outlet of the watershed. Various factors, including climatic and physiographic parameters, affect the conversion of precipitation into runoff. Climatic factors involve the intensity and duration of rainfall as well as the distribution of rainfall locations. Physiographic parameters include land use type, soil type, watershed area, basin shape, height, slope, direction, and type of drainage network. Therefore, several methods can be used to estimate the runoff generated by rainfall, and one of the commonly used methods in hydrology is the Soil Conservation Service (SCS) method. Furthermore, determining the spatial changes and correlation between the factors of runoff production and flooding can identify flood-prone areas. Therefore, this research utilized geographic information systems (GIS) to provide essential and fundamental information for obtaining direct runoff using the curve number (CN) method. The study aimed to estimate the height of excess rainfall runoff and analyze the spatial variations of runoff height and volume in the Sharganj watershed, Birjand. To achieve this goal, the SCS method was employed to estimate the amount of runoff generated by the maximum 24-hour rainfall in different return periods in the aforementioned watershed. Subsequently, the changes and spatial correlation of the values of the height and volume of the runoff were evaluated in the Sharganj watershed, Birjand.

This study estimated the amount of runoff produced from the maximum 24-hour rainfall in different return periods in the Sharganj watershed using the SCS method. To achieve this, the maximum 24-hour rainfall from surrounding stations was averaged using the IDW method. Next, the SCS method was utilized to estimate the runoff by combining land use maps and soil hydrological groups to prepare the curve number map. The basin runoff volume was then calculated for different return periods. Additionally, the spatial autocorrelation of the runoff height and volume in the 25-year return period was analyzed using the general Moran's index, and their clustering pattern was determined using Anselin Local Moran's Index.

The Sharganj watershed is primarily used for pastures and agricultural lands, with the former being located in hydrological group C. Despite this, rainfed and irrigated garden lands, mainly located in hydrological group B, have better permeability. Using ArcGIS software, land use maps and soil hydrological groups were combined to prepare a curve number map of the watershed. The results showed that the curve numbers of the sub-watersheds ranged from 68 to 79, with sub-watershed number 9 having the lowest curve number value (68) and sub-watersheds 22, 25, and 29 having the highest (79). The Global Moran's index values presented in Table 3 and Figure 10 showed that the spatial correlation of runoff height and volume had values of 0.1828 and -0.2694, respectively. The curve number decreased from west to east due to the presence of barberry gardens and irrigated agricultural lands. The spatial correlation map of runoff height values showed high-high (HL) clusters forming in the upstream parts of the basin, while low-low clusters (LL) were present in the downstream areas. These patterns were related to the amount of precipitation in the upstream part and the high slope of the upstream sub-watersheds. Meanwhile, the spatial correlation values of runoff volume indicated that the accumulation of runoff volume was higher in the sub-watersheds located downstream of the study area, with low formation of high-low (HL) clusters. This increase in flood volume occurred in the downstream sub-watersheds and adjacent to the outlet of the area. In general, the difference in the cluster pattern of runoff height and volume did not follow the same pattern, which depended on various factors affecting runoff production, such as the amount of precipitation in different parts and the difference in topography. It should be noted that the analysis was based on height and volume values in the 25-year return period, and the results can be expected to be similar for other return periods based on the curve number method.

The research findings suggest that the spatial changes in runoff height and volume components, which impact watershed response and flood producing procedures, are determined by various factors such as slope, land use, curve number, and soil hydrological group. Areas with high amounts of runoff are more susceptible to damages caused by floods and can be prioritized for management measures. Furthermore, the results indicate that the type of surface runoff control measures or flood volume control required will vary, providing valuable guidance for determining the appropriate damage reduction operations. Further studies analyzing the spatial correlation of height and runoff volume variables with other basin features and climatic factors could enhance the understanding of spatial changes. The formation of surface runoff is influenced by various factors related to rainfall and ground conditions, which could be considered in future research.