Karkheh basin often experiences devastating river floods. The important factors that affect these events in the basin include the meteorological conditions, snowpack, dam storage levels, vegetation cover and soil moisture at the time of the event. The 21 March to 4 April 2019 flood events (herein called “the 2019 flood event”) was those in which several factors combined to create some of the most catastrophic flooding recorded in the basin. This event inundated five provinces in southwestern Iran and inflicted many casualties and much loss. It is clear that a hydrological evaluation of such an exceptional event is essential for flood risk management in order to implement necessary measures to mitigate the negative effects of such an event in the future.
In a hydrological evaluation, conceptual hydrological models are helpful. They make it possible to simulate and distinguish between the different factors that form the characteristics of flood events. To this end, reconstruction and simulation of flood events have been considered in different studies. For example, Stucki et al. (2018) applied the semi-distributed rainfall-runoff model PREVAH to reconstruct a historical extreme flood event in the Lago Maggiore catchment in 1868. Dasallas et al. (2019) simulated the 2002 Baeksan flood in Korea by implementing the combined 1D–2D hydraulic computation of the HEC-RAS model. The roles of the factors affecting flood events include the different climate and management scenarios that can be introduced to a hydrological model to investigate their effects on such flooding.
Hussein et al. (2019) studied the effect of land-use changes on flood events occurring on the east coast of the UAE using the HEC-HMS. In their study, the curve numbers (CN) of sub-watersheds were estimated using 1996, 2006 and 2016 remotely sensed images. They concluded that urbanization has increased the peak discharge, runoff volume and extent of the flooded area. However, their approach was less sensitive to extreme events. Similarly, Hu and Shrestha (2020) investigated the effect of land-use change on flood peaks using the same model a watershed in the Midwestern United States, but by applying different CNs. They predicted increases in the urban areas and dry land of 17.47% and 14.05%, respectively, by 2028. Accordingly, increases in flood peaks of 2.9–3.5% for small scale floods and 0.4–1.1% for a big scale flood were anticipated.
It was reported that the peaks and volumes of the 2019 Karkheh basin flood events were significant. However, it was emphasized by the Special Reporting Committee on Iran Floods 2019 (SRCIF) (SRCIF, 2019a) that the damage was mainly due to the flood volumes rather than the peaks. Having in mind these features as well as the scale of the basin and its vast agricultural activity, we decided to use the soil and water assessment tool (SWAT) conceptual model (Arnold et al., 1998). This model is continuous and capable of simulating different climate and management scenarios. It is also semi-distributed in that it divides each sub-basin into smaller hydrological response units (HRUs) based on soil type, crop patterns and management practices (Neitsch et al., 2011). The model incorporates many modules, including crop growth, groundwater, dams and river routing to accommodate the required simulations. Although the original time-scale of the model is daily, it has been applied for flood studies as well.
Schilling et al. (2014) applied the SWAT model to assess changes in downstream flood risks under different types of land use for a large rural basin in Iowa. Their study focused on increasing the cultivation of perennials on the landscape and found that this type of management could decrease the likelihood of flood events and reduce the frequency of severe floods, but that their durations would not be substantially affected. Lee et al. (2017) applied SWAT to evaluate the effects of two upstream dams on the frequency of downstream floods in the Han River basin in South Korea. They also applied this model to individually simulate regulated and unregulated daily streamflows entering the third dam, which is located at the outlet of the basin. The estimated daily flood peaks were used for frequency analysis and showed that the two upstream dams were able to reduce downstream floods by approximately 31%. The model also was applied by Rajib et al. (2020) in conjunction with LISFLOOD-FP for simulation of floods and their respective inundation maps along the Ohio River basin in the United States. The results showed 70–80% consistency in the areas inundated using what had been captured from remotely sensed images.
In addition to hydrological modeling, preparing a relevant set of rainfall data that can illustrate the spatial and temporal variation of a storm is crucial for flood simulation. In reality, meteorological stations are not usually evenly distributed in basins and some of them may even be inaccessible during flood events, as we experienced in 2019 (SRCIF, 2019b). Satellite-based grid precipitation products and reanalysis datasets can be a relevant solution to tackling this issue. For the area presently under study, databases such as the global land data assimilation system (GLDAS) (Delavar et al., 2019), TRMM (Xu et al., 2017), CMORPH (Sinta et al., 2022), CHIRPS (Gummadi et al., 2022) and PERSIANN-CDR (Zhang et al., 2022) were evaluated. Delavar et al. (2019) also evaluated a number of these databases and reported that GLDAS performed the best from among them. For this reason, it was selected for use in the present study.
The current study is in line with numerous efforts to enhance flood risk management downstream of Karkheh Dam, which features a large population and substantial agricultural activity. The research concentrated on inflows to the dam and addresses questions that specifically arose after the 2019 floods. It sought to identify the ability of the combined SWAT model and GLDAS data for flood simulation in the basin as well as for future flood forecasting, understand the role of the precipitation prior to the flood events in the basin on the specification of 2019 flood events and examined the role of upstream dams on mitigating the inflow to the Karkheh Dam.