testing the four models for prediction of gully head advancement (case study: hableh rood basin- iran)

gully erosion is one of the most complicated and destructive forms of water erosion. in order to prevent this erosion, the important factors advancing gully head must be recognized. nowadays, several models have been proposed in measuring gully head advancement and identifying the severity of erosion. these models must be calibrated for each country to see whether they are applicable or not. so it has been tried to study the necessities of the calibration in this research. this has been done in one of the sub¬¬-basins of hableh rood basin called dehnamak in arid and semi-arid climate of iran. three aerial photos of 1956, 1967 and 2000 years have been used to measure the gully head advancement in different periods of time including 1956-1967, 1967-2000 and 2000-2005. then in order to calibrate four models: 1- thompson, 2- scs (?), 3- scs (??) and 4- fao, all factors have been measured and studied. statistics studies such as relative error percent, absolute error percent and change variable percent have been used. the results of the mathematical study show that scs (??) and fao model have a relative error percent and absolute error percent with amounts of 37.3 and 7.51 and, 40.06 and 18.21, respectively. regarding to change variable percent, only 0.51% can be seen as a different between scs (??) and fao models, because of the usage of same factors and coefficients, each of two models are close to each other. finally, the best models in the studied area are scs (??), fao and scs (?) respectively, and thompson model cannot be proposed.