Understanding the seasonal causes of floods is important for safe flood management (Ye et al. 2017). Floods can be occurred in periodic and seasonal manner and may occur infrequently during different months and seasons. It may also be severe or short-lived and even have spatial effects (Yang et al. 2017; Marhaento et al. 2018; Pokhrel et al. 2018).
Considering the complex nature of flood and its spatial changes, mapping its intensity and frequency at the watershed scale can be vital for making correct decisions in flood management, as well as understanding changes and predicting its occurrence (Avand et al. 2021). The spatial mapping of flood events and its seasonal analysis of flood occurrence is useful and optimal tool for investigating the hydrological aspects of flooding (Ruiz et al. 2014; Fischer et al. 2016). Distinguishing floods in different seasons can help planners and managers determine the different source of floods.
According to the results (Table 3 and Fig. 6), it can be said that in the stations studied in Ardabil province, out of 32 stations studied in 21 stations, the monthly flood frequency was high in April, which showed that about 63.63% of the floods occurred in April, which was the same as the results of (Sivapalan et al. 2005; Shankman et al. 2006; Hall and Blöschl 2018). Meanwhile, (Diakakis 2017) in Greece analyzed the seasonal floods and according to their results, the autumn season had the largest share of floods. The highest percentage of flood frequency was related to Naneh-Karan and Samian stations, about 93.33 and 77.55%, respectively. Based on the results, it can be said that the maximum floods were related to spring and in all return periods studied (Jain and Lall 2000; Reihan et al. 2012; Nolin et al. 2021; Sadeghi et al. 2021), the maximum amount and percentage of floods occurred in this season, which was different from the results of (Hall and Blöschl 2018) that floods are spring.
The highest share of spring floods was related to the 100-year return period with 87% and the lowest was related to the 25-year return period with 73%. However, in most of the return periods after spring, it was winter, which, with a slight difference compared to autumn and summer, accounted for a high percentage of floods, and the highest amount (14%) was related to the 25-year return period (Fig. 3.). Also in some stations such as Yamchi, Nouran, Naneh-Karan, Mashiran, Amouqin, Droud and Pol-e-Almas, nearly 100% of the floods were related to spring (Therrell and Bialecki 2015; Nolin et al. 2022). The reason for the occurrence of most floods in the spring season can be related to the mountainous and high area of the above-mentioned stations. In this regard, due to the snowfall in the winter season and the melting of snowpacks in the spring season and the simultaneous occurrence of precipitation, the maximum flow rate of the rivers increases. The seasons of winter, summer and autumn, with a slight difference, have allocated a part of these floods in each river gauge station. The significant trend of decreasing the average of the studied components showed that in the long term, the average values of the components had undergone many changes, and unfortunately, these changes have been in the form of a decrease, which has had a negative effect on the water resources of the province (Mostafazadeh et al. 2023).
Based on the results of the diagrams in Fig. 5, which show the I-F curves in the return periods of 2-, 5-, 10-, 25, 50 and 100-year in the studied river in Ardabil province, it can be said that in 2-year return period, floods in the southern and upstream regions of the province compared to the northern and downstream regions had a high frequency. Also, during the 5-year return period, a decreasing trend was observed in the floods of Ardabil province from the southern regions to the central and northern regions. According to a study by (Diakakis 2017) in Greece, the eastern parts had the most floods in the fall. The results of (Mostafazadeh et al. 2023) showed that the reliability of climatic events has been disturbed in recent decades and the intense flow pulses had increased over time, which confirmed the occurrence of destructive Also, the evaluation of average values of the seasonal index showed that Pol-e-Soltani station had the lowest and Nair station had the highest amount of seasonal flow in the study area. The analysis of flow seasonality and its relationship with flood occurrences has been confirmed in different studies (Radecki-Pawlik et al. 2020; Mwinuka et al. 2021).
On the other hand, the decrease in the frequency of spring floods in the eastern part of Ardabil Province can be related to the precipitation system from the Caspian Sea. Also, based on the seasonal patterns of flood frequency, the west of the province had the most spring floods among other studied river gauge stations (Fig. 3.).
These changes in the seasonal pattern can be attributed to changes in climatic parameters (Nastos and Zerefos 2008; Llasat et al. 2010; Philandras et al. 2010). For example, in the 2-year return period at Bafrajerd RGS with an altitude of 2020 meters above sea level, the frequency of floods was 33; while in the same return period at Bouran RGS with an average altitude of 244 meters above sea level, the frequency of floods was 4. This can be due to the increase in altitude and consequently the increase in rainfall, which shows the impact of climatic components and was similar to the results of (Hall and Blöschl 2018), based on the impact of climatic components and altitude on the flood occurrence. This difference in flood frequency was also observed based on 10-, 25-, 50- and 100-year return periods diagrams, which based on the results in the upstream and highlands of the province, an increase in flood frequency was evident.