Border irrigation is a method of surface irrigation that is commonly used after irrigation furrow in agricultural fields. The main problem of the surface irrigation system is low efficiency. One of the important reasons for low efficiency of surface irrigation systems in our country, is deficiency of proper design and incorrect management of irrigation. Advance time is one of the important parameters of design. One of the popular models of prediction of advance time is the volume balance model. This model has two coefficients, one related to the surface flow and the other to the subsurface flow (infiltration). Constant considering these coefficients causes an error in calculating of advance time in the border. The purpose of this study is to provide a method for accurate determination of the surface shape factor ( ) and subsurface shape factor ( ). For this purpose, at 18 fields that irrigated by border method, soil texture, infiltration coefficients of the Kostyakov-Lewis equation, input discharge, slope of border, constant shape factors and roughne coefficient were determined. Using the numerical solution of the Saint-Vanant equations, the volume of surface and subsurface flow were calculated and the shape factors were determined at any time during the advance phase. The volume balance model was evaluated with introducing these variable factors. The results showed that firstly, the volume of surface and subsurface flows using the volume balance model with variable factors had a 1.77% error, secondly, the volume balance model with variable and constant factors had 4.5% and 13.8% error in prediction of advance time respectively. Also, using the volume balance model with variable factors, the sensitivity of the shape factor to the physical and hydraulic characteristics of the border in two light and heavy soil texture were investigated. The results showed that in both of texture, was sensitive to the infiltration coefficients. In the heavy texture, was not sensitive to any of the hydraulic properties and it was sensitive in the light texture to the input discharge. was ensitive to slope, roughness and input discharge respectively. Keywords : Border irrigation, Volume balance model, Surface and subsurface shape factors, Saint-Vanant equations.