Spatio-Temporal Analysis of Morphometric Changes of Gully Shape in Alborz Province (Ismailabad and Najmabad Regions)

Gully erosion is one of the linear and progressive forms of water erosion that has an expanding role in land degradation. A gully is a channel with a steep side slope and head cut that is formed by surface or subsurface soil erosion processes though heavy rains, soil disturbance or landuse changes. Upon once the gully initiation head cut retreating is the main effective erosion process influencing land degradation and soil loess. The development of the head cuts have caused the removal of fertile soil, the aggravation of flooding by wild land and production of sediment from agricultural lands. Therefore, it is necessary to monitor the growth and expansion of gullies in order to identify the factors involved in soil loss and effectiv implimentation of soil conservation projects. In the same regard, besides paying attention to the special climatic conditions and the high sensitivity of the soil in the study area, the main aim of this research is to compare the growth of the gullies by aerial photographs and field operations to determaine the most important factors on gully head development over the short and long time spans. Also, other geometric of gullies including length, volume and surface growth were estimated and modelled by regression analysis procedure to find out the most important variables.

The present research was conducted by selecting 30 permanent gullies using a survey based on field sampling, and conducting experiments to extract data and performing statistical analysis in two areas of Ismail Abad and Najm Abad villages in Alborz province. Identification of the gullies’ location and mapping of gullies distribution were performed by using the google earth database, interpretation of aerial orthophotos of the 1980s and 1990s, digital topographic maps (25,000). Also the present location of head cuts were determined by field survay in 2017 with a GPS device. The length, width and depth of gullies, the slope of the upstream channel leading to the gully and the slope of the gully channel bed and the landuse condition were recorded during field surveying. At the end soil samples were taken from the gulleis’ walls to determine the physicochemical attributes of the soil. Using geographic information system (GIS), the longitudinal, surface and volume growth of the gullies over the three time periods of 1988-99, 1999-2017 and 1988-2017 in two study areas estimated directly. By multiplying the surface of each gully by the average depth obtained from the field surviving, the volumetric growth was also determined. Multiple regression analysis was applied into the data to determine the significant factors (soil and topography and upland contribution area and road density).

The growth estimation results showed that the average annual longitudinal growth over the 1988-1999, 1999-2017 and 1988-2017 in Ismailabad and Najmabad were 1.24, 1.18, 1.2 m/Y and 75.03, 30.78, 3.75, respectively. The average surface growths are 4.96, 4.46 and 4.65 m2/Y in Ismailabad region and 464.2, 18.73, and 187.7 m2/Y in Najmabad region. Also, the average volume growths are 4.59, 3.38, and 3.84 m3/Y in Ismailabad region and 513.86, 20.70 and 207.76 m3/Y in Najmabad region. The gully volumetric growths of Ismail Abad and Najmabad during the period of 11 years (88-99) are 45.31% and 93.82%, respectively. Also, the volume growths during the period of 99-2017 (18 years) are 54.69%, 6.18% respectively in the two regions of Ismailabad and Najmabad, and the average gully growth over the 29-year period is more than the 18-year period.

The volume growth in both regions is higher in the first phase, the average gully development in the long-term time span (1988-2017) is more than the short-term period (1988-1999), which can be due to the greater growth of the head cuts in the initial phase of formation, landuse changes, and heavy low-frequency rainfalls. Also, due to factors such as slope, type of land use, soil, and humane construction activities, the average gully growth in Najmabad region is higher than Ismailabad one. The growth (longitudinal and surface) of gullies in Najmabad region has been much higher than in Ismailabad region. In Ismailabad region, due to the conditions of hilly rugged land, the contribution upstream area of the head cuts were less than Najmabad region (alluvial plain) and ttherefore, the amount of runoff will be less and consequently the gully head development is more in Najmabad. Comparing the results of this research with other researches shows that the reason for the high growth of headcuts in this research can be related to the sensitivity of the soil, the dry climate and the human developing constarctions such as roads construction.