In hydrology, modeling is a widely used technique for estimating how a basin will respond to the different rainfall events. There are many factors contributing to the complexity of the modeling process, such as climate change, urbanization, and the expansion of agricultural activities. Therefore, scientists and researchers all over the world are trying to develop accurate tools for modeling hydrologic processes. With the advancement of computer technology and the availability of near-real-time precipitation data, rainfall-runoff models are expected to be able to predict runoff in a more accurate manner than they did in the past [1]. Watershed Modeling System (WMS) and Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) are two powerful hydrologic modeling tools. These two tools enable watershed modeling to be completed at all stages, including delineation of watersheds and subbasins, computation of geometric parameters, computation of hydrologic parameters (curve number, time of concentration, rainfall depth, losses, baseflow, runoff transformation, and routing), and visualization of the results [2].

HEC-HMS model has also been used for simulating rainfall-runoff processes [3] [4] [5] with geo-informatics and atmospheric models for flood forecasting in different regions of the world [6] [7] [8]. The model was found accurate in spatially and temporally predicting watershed responses in event based and continuous simulation, as well as simulating various scenarios in snow-melt [9], flood forecasting [10] [11], and early flood warnings [12]. The main objective of the study was to assess the increase of rainfall-runoff processes and impervious soil layers on hydrologic processes using the HEC-HMS model. This research analyzed the effects of three increased hypothetical rainfall events and urbanization/industrialization (in terms of impervious areas) scenarios in a rural basin of North Dakota using the HEC-HMS simulation model.

All required data were collected for different federal and state agencies. The Digital Elevation Model (DEM) of the watershed (Figure 3(a)) was obtained from the USGS Map Viewer². The land use and land cover data (Statsgo soil data) were obtained from the NRCS Geospatial soil data gateway, Web Soil Survey³. Afterward, DEM and shape files were imported, and different cartographic maps were created in ArcGIS Pro V2.9.1. The flow directions and flow accumulations were determined using Run Topaz function in WMS, and the watershed was divided into five sub-basins.

Figure 4 shows the layout of the HEC-HMS model. Tables 2(a)-(e) show the simulation outputs such as the global summary at the final outlet (junction 12C) of the watershed for different scenarios. Figure 5 illustrates the hydrograph at the final outlet for Scenario-5.

The simulation models responded well with increasing rainfall amount and impervious areas, as presented in Tables 2(a)-(e). In the scenario-1 with storm depth of 12.34 cm for 100-years storm and with zero impervious areas, the simulated peak discharge and runoff volume were 157 m³/s and 2.7 cm, respectively. In scenarios 2 and 3 with 10% (13.46 cm) and 20% (14.73 cm) increase in storm depths, the corresponding peak discharges and runoff volumes were 214 m³/s and 3.63 cm, and 237 m³/s and 4.0 cm, respectively. Overall, peak discharges increased to 36% and 51% when rainfall depths increased by 10% (Scenarios-4) and 20% (Scenarios-5), respectively (Figure 6). Similarly, the runoff volumes increased to 35%, and 49% when rainfall depths increased by 10% (Scenarios-4) and 20% (Scenarios-5), respectively (Figure 7). The findings of this study are in agreement with the results of some other previous studies [16]. Overall, runoff volume was found to increase with increasing storm depths and thus showed a positive correlation with storm depths.

Similarly, the Scenario-1 with storm depth of 12.34 cm for 100-year storm and zero impervious areas, was compared with scenarios 4 and 5, which had a 10% and 20% increase in impervious areas due to assumed urbanization and industrialization (Table 3). The peak discharges increased to 12% and 78% when impervious areas increased to 10% (Scenarios-4) and 20% (Scenarios-5), respectively (Figure 8). Similarly, the runoff volumes were increased to 12%, and 76% when impervious areas increased to 10% (Scenarios-4) and 20% (Scenarios-5), respectively (Figure 9). Like storm depths, the runoff volume was found to in

crease with increasing impervious layers, and runoff volume also showed a positive correlation with increasing impervious layers.

A simulation model implemented in the HEC-HMS platform allows for observing the responses due to three increasing rainfall and urbanization scenarios. As expected, the simulation models responded well to increasing rainfall depths and impervious areas. With the change in land use and land cover, the impervious area increases, causing the curve size of the different sub-basins in the area to increase, which will, in turn, cause the peak discharge and runoff volume to increase. When rainfall depths grow to 10% and 20% from the initial rainfall depth, respectively, the peak discharges increased to 36% and 51%. The runoff volumes were increased to 35% and 49%, respectively, for the same scenarios as peak discharges. Peak discharges and runoff volumes both showed similar rising trends as the impervious area increased. Overall, this study demonstrated a correlation between peak flow volume and increasing urbanization or land use pattern of the area.

The study also showed that the prime factor of flow increases from a watershed, land use changes due to urbanization, and industrialization (in terms of impervious areas), had an impact on sub-basin curve numbers. The curve numbers were rising in line with rising urbanization, and as a result, total maximum discharges were rising as well. In brief, the study discovered concrete evidence of an increase in watershed peak discharge with the trend toward urbanization/industrialization. However, this study’s limitation was that the current state of water quality was not considered. Since the study showed a connection between the change in land use class brought on by urbanization and industrialization (in terms of impervious areas) and maximum discharge, it is obvious that urbanization will also have an impact on water quality. Future researchers may have the opportunity to focus on this issue.

The corresponding author wants to acknowledge the National Agricultural Technology Program-Phase II Project (NATP-2), Bangladesh Agricultural Research Council Dhaka, Bangladesh for financial support of this research. Both authors acknowledge their gratitude to the anonymous reviewers for their efforts and valuable time.

The authors declare no conflicts of interest regarding the publication of this paper.

Khanaum, M.M. and Borhan, M.S. (2023) Effects of Increasing Rainfall Depths and Impervious Areas on the Hydrologic Responses. Open Journal of Modern Hydrology, 13, 114-128. https://doi.org/10.4236/ojmh.2023.132006