Assessment of long-term changes of Gully Eroiosn Growth in Kondouran Catchment, Hormozgan Province

Gully erosion is the most obvious form of soil erosion, which leads to reduced production capacity, significant soil losses, and the production of large amounts of sediment and restricting land use. It can be a severe risk to agricultural areas and villageschr('39') roads and structures. Due to the lack of information on soil erosion and sediment production in many watersheds of the country, the use of new technologies such as remote sensing and GIS is necessary so that the use of this technology can be a valuable source of information to understand the trend of Gully changes. The study area is located in the west of Bandar-e-Lengeh city in the Hormozgan provinc. The approximate geographic coordinat is 54° 19¢ 11² to  54° 30¢ 40 longitudes and 26° 40¢ 57² to 26° 56¢ 04² latitude. The study area is mainly formed by sedimentary rocks belonging to the Mesozoic period (Fars group formations) with low permeability. This factor, along with the low rainfall and very high intensity, has destructive effects on natural resource ecosystems.

This research is focused on gully erosion mapping and monitoring at multiple spatial scales using multi-source Arial photo and remote sensing data for gullies extend in the past decades. Aerial photos of 1957, Landsat satellite images of 1994, 1984, 2004, and GeoEye satellite images for 2014 and 2016 have been used to study the long-term changes in the growth of Gully erosion. Image visual interpretation with field verification was employed to map the geometric gully features and evaluate the growth of the gully in the last 60 years.

The results showed the gully areas increased from 112 hectares to 206 hectares from 1984 to 2004. The results also show that during the period 2004 to 2014, the gully areas increased from 206 hectares to 316 hectares and 370 hectares in 2014 to 2016.50 gullies were studied to investigate the longitudinal growth of ditches in the study area. The results show the annual rate of gullieschr('39') longitudinal growth is 4.9 meters for1984-2004 and 6.8 and 2.95 meters for 2004 to 2012 and 2012 to 2016 periods, respectively.

The long-term growth rate of gullies in the study area (1975-2016) is an average of 5.2 meters per year, which is very high comparing to researches conducted in the world and Iran. For example, in their study, Ghezelsofloo et al. (2018) estimated the annual erosion rate of 0.99 m for the Shurdreh catchment, and Ahmadi et al. (2007) estimated the annual growth rate of 0.206 m for the Hablehroud basin.In other studies, conducted by Rostamizad et al. (2015), the annual growth rate of 0.85 m for the city of Darhshahr, Tavakoli Rad et al. (2014) 1.29 m in the Samal Basin of Bushehr, Nazari Samani et al. (2014), growth 1.3 meters for the Kore Dareh catchment.In other countries, Malik (2007) estimated the average erosion rate of the small gully at 0.63 m per year. Millington et al. (2013) also estimated the average rate of gully erosion on slopes from 0.21 to 0.51 m per year on the southern plateau of Poland.In another study, the growth rate of the ditch was calculated by Pellikka et al. (2005) for the Tiata hills of Kenya as 0.1 to 0.7 m per year.In addition, the research results show that the High sensitivity of Fars Group formations to erosion, arid climate, weakness and degradation of vegetation and very high intensity of 24-hour rainfall in the study area, lack of flood management are main reasons for the high longitudinal growth of gully in the study area. Other reasons are human manipulation in flood diversion and concentration, a very high percentage of soil silt, release of rainfed lands and the existence of a wide network of local access roads. The resuls are consistent with the results of Karimi et al. (2007), Mohammad Ebrahimi et al. (2017), Asghari Sarskanrood (2017), Besharati et al. (2018).