Estimating storm erosion index in southern region of I. R. Iran

A universal soil loss equation is used for predicting soil erosion. Rainfall erosivity (EI) in this equation is related to storm type, amount, and intensity, so it should be calculated from rainfall characteristics. In the present research, data of 13 recording rain gauge stations at the southern region of the Islamic Republic (I.R.) of Iran were analyzed and single storm, daily, monthly and annual erosion indices (MJ.mm.ha^-1.h^-1) were calculated and estimated by different simple models. For a single storm erosion index, the model EI=a P_e^b/D^gwas modified. A coefficient of a  for stations without snowfall and with an elevation of less than 1000 m, or higher than 1000 m, and b for stations without snowfall were 0.51, 0.21 and 2.0, respectively. The values of a and β for stations with snowfall and g for both types of stations may be determined based on the station elevation using the proposed equations. For a daily erosion index, a power function (EI=ahb) was derived (h is daily rainfall, mm). The value of a was dependent on station conditions, but an average value of 1.61 was obtained for b. For monthly erosion index, a simple model was proposed as
 where H is the elevation (m), and P_m24 is monthly,  maximum  daily rainfall  (mm).  The coefficients of the Arnoldus model were modified for the study region to estimate the annual erosion index using annual maximum daily rainfall (P_a24, mm), monthly (P_i, mm) and annual rainfalls (P, mm) as follows: 
                            
Using simple available rainfall data for 65 weather stations throughout the study region, the annual erosion indices were calculated by the above equation and an iso-erosivity map for the study region was prepared.