There are several methods to work out equations infiltration rate.as Darcy's law, General hydrologic budget, Horton's equation, Green-Ampt, Kostiakov equation and several others. However, the main problem with these method is a relatively accurate assessment of the infiltration characteristics of the soil is needed. It may be carried out by taking different parts of preclassified soil zone on the basis of texture. These samples may be tested in a lab for precise infiltration rates which may be averaged, if variation is not very large, the average rates may be generalised for respective soil zones. The USGS has charaterised soils into four hydrologic soils with different ranges of infiltration rates, if you are study an area where hydrologic soils groups are available, then there is no problem. There are also available instrument designed for field expermints to this effect.

Thank you for your answer. I will examine these models and see if they will be useful.

Search for Pedo-Transfer Functions related to infiltration. They should provide you a good rough estimate. If you are dealing with saturated conditions, look at the soils maps made by USDA and they list saturated hydraulic conductivity (Ksat). After the initial wetting of the soil profile, infiltration should be limited to the Ksat of the most limiting layer.

Thank you. This indeed seems promising. I will examine it more closely.