Effect of initial soil moisture content on the vertical infiltration process — A guide to the problem of runoff-ratio and loss

Abstract

Initial soil moisture content plays an important role in hydrologic processes. In this paper, the relationships among the initial soil suction (an index of the soil-moisture content), total amount of rainfall, rainfall intensity, runoff loss, runoff ratio and lag between rainfall and runoff occurrence are examined with a simplified one-dimensional analog of the infiltration process based on a cylindrical lysimeter with artificial rainfalls. It is conclusively shown that the loss of infiltrated rainfall is uniquely correlated with the initial soil-moisture content measured by tensiometers and is independent of the total amount of rainfall as well as of the rainfall intensity, if it does not exceed the infiltration rate of the soil. Consequently, the runoff ratio is a function of the initial soil-moisture content and total rainfall, but not a function of the rainfall intensity. The nondimensional propagation velocity of the wetting front or inversely the nondimensional lag of runoff occurrence defined by T★ 1 = T 1r H ( T 1 = lag between rainfall and runoff; r = infiltration rate, in this case, rainfall intensity; and H = depth of the lysimeter or distance from the surface to the less pervious layer) is also well correlated with the initial soil moisture. These results are further proven and extended through the use of numerical experiments, solving the Richards equation of unsaturated infiltration. An approximate analytical solution has been derived for the runoff ratio and rainfall loss.