Efficiency assessment of the EGEM to estimate gully erosion in Iky-Aghzly watershed of Golestan province

An important form of gully erosion which usually develops in farmlands is ephemeral gully erosion. Researches have shown that soil loess due to ephemeral gullies is around 2 to 90 m3.h1.yr1. Given the high soil loess of loess deposits and intensive cultivated lands in Golestan province, assessment of factors affecting the development of ephemeral gullies is of great importance. The main objective of this study is the efficiency assessment of the Ephemeral gully erosion model (EGEM) to estimate gully erosion in the Iky-Aghzly watershed.
The area of Iky-Aghzly watershed is around 703 hectares, located in the basin of the Gorganroud (Golestan province). Its mean annual precipitation and temperature is 424 mm and 15.7 ºC, respectively. Through field reconnaissance in three stages (seasonally), 12 ephemeral gullies were identified and their locations recorded by GPS. Morphometric characteristics recordings and soil sampling were conducted in the head of gullies as well. Apart from measuring of morphometric characteristics and soil physicochemical properties, physical specifications of watershed adjacent to each gully was calculated. EGEM efficacy was validated by Chi-square test in R software, by comparing the observed variables (mean annual soil erosion and width of ephemeral gullies) with the same variables estimated by the model.
The average soil loss caused by ephemeral gully erosion was around156.7 t yr-1 for all ephemeral gullies. The depth and width of gullies was between 10 to 45 cm and 15 to 60 cm in the last measurements. The average soil loss per each ephemeral gully was around 3.76 to 12 t h-1.
The results showed that there is a significant difference at the 5% level between the measured annual average gully erosion rate and estimated erosion by the model. But the annual average width of ephemeral gullies corresponding to the model output variable, was not significantly different. The rate of ephemeral gullies erosion estimated by The EGEM is not acceptable in the Iky Aghzly watershed, while the width of ephemeral gullies (EGEM) was near the observed width.
Generally, according to the results of this research, it can be concluded that the ephemeral gullies were formed coinciding with the autumn rains and tillage (4 ephemeral gullies) and in the late winter (8 ephemeral gullies (.The reasons for ephemeral gullies emerging are rainfall intensity, antecedent soil moisture content, tillage along the slopes in the study area. According to these results, for accurate assessment of EGEM as event-based model in Iky-Aghzly watershed, individual participations is recommended.