Identification of flood atmospheric patterns and simulation of rainfall to runoff conversion in Shiraz Watershed

Flood is caused by a sudden increase in the water level of a river due to heavy rainfall, in which excess water flows out of the basin and the riverbed and enters adjacent areas, causing extensive damage to the region's ecosystems and infrastructure. In order to determine and implement a strategic plan in the direction of crisis management, understanding the mechanism and development of the flood phenomenon and the amount of subsequent runoff is a priority. In recent years, flood events have occurred in the catchment area of ​​the dry river of Shiraz; The most terrible flood event occurred on the 25th and 26th of March 2019 in the basin and caused severe economic, social and human damage to the people of the city. By recognizing the mechanisms and developing atmospheric patterns of flood generation, the first step can be taken in flood control; Also, by predicting the amount of runoff produced, how to deal with it can be identified. For this reason, in order to manage the environment of the dry river basin of Shiraz, the present study tries to approach flood management based on recognizing flood generating patterns and simulating the amount of runoff produced.

In order to study rainfall in the dry river of Shiraz, the daily rainfall was obtained from 12 rain gauge stations in the area in the period 2001 to 2019. The rainfalls were then sorted on an annual and monthly scale, and the rainfall was zoned using the Kriging algorithm in ArcGIS. Also, rainfalls of more than 40 mm were extracted from 12 rain gauge stations and were considered as flood events. For each rainfall station, the number of flood events was determined and flood events were zoned by the kriging algorithm in ArcGIS. Next, three events of flood on 16 February 2017, 26 March 2019 and 23 January 2020, when the average rainfall in the Shiraz watershed was 100 mm every three days, were selected and interacted with a synoptic approach. Atmospheric conditions and flood patterns were identified. Next, using the HEC-HMS hydrological model, the amount of rainfall to runoff conversion in each of the sub-basins in the Shiraz watershed was simulated.

The results showed that the maximum precipitation occurs in February and the minimum in July. The maximum annual rainfall is 627 mm in the mountainous areas of Qalat and Golestan and the minimum is 245 mm in Maharloo Lake. Synoptic results showed that the low altitude and cold-water trough at the level of 500 hPa in the eastern Mediterranean along with the dynamic low-pressure system in Iran caused surface air instability in Fars Province and by injecting moisture from the Persian Gulf to the massif. The weather has caused floods in Shiraz. The results of rainfall-runoff modeling showed that the highest runoff in Sadra and Qalat sub-basins were 5773 and 5076 thousand m3, respectively, and the maximum peak discharge in Sadra and Qalat was 666 and 389 m3/sec, respectively. It happened at 17:00. The highest volume of rainfall penetration in Qalat and Chenar Rahdar was 5423 and 2546 thousand m3, respectively, and the lowest level of penetration in the Quran Gate was 247 thousand m3, the main reason being the high density of residential-commercial use in this sub-basin.

Therefore, it is suggested that in order to manage the flood crisis in Shiraz, controlling runoff produced in northern sub-basins such as Qalat, Golestan and Sadra and increasing the level of permeability can be effective in controlling floods.