TR-12 : Field measurement of soil moisture movement in ponding condition

Abstract

Soil moisture movement studies provide potential information in the field of agriculture and hydrology. In the field of agriculture information is needed for many diverse applications including irrigation scheduling while in the field of hydrology, the soil moisture movement study is important for understanding the mechanism of recharge through the soil and to provide soil moisture storage data for the water balance study.

In this report, investigations on the movement of soil moisture in the unsaturated zone under ponding condition using depth moisture neutron gauge are presented. Various other methods for soil moisture movement studies using environmental and artificial radioisotopes are touched upon with an specific purpose to point out and summarize the isotopic techniques which may aid in investigations of the unsaturated zone in general and understanding of movement of soil moisture in ponding conditions in particular.

Studies carried out at the Campus of National Institute of Hydrology, Roorkee, clearly show the capabilities of neutron moisture probe, for accurate measurements of soil moisture changes under ponding conditions in the unsaturated zone. Small infiltrometer of size 2m x 2m x lm was used to measure infiltration rates into the soils followed by studies of the soil water flow mechanism. Initially infiltration rates were equally high but after few hours it decreased considerably. The soil absorbed an average of 1. 51 cm water during ponding time ( i.e. 92 minutes). Ponded water infiltrated into the soil at a high rate for 4 hours allowing adsorption of about 80. 37% of the applied water.

The variation of soil moisture is analysed for the ponding to provide basic understanding of the processes occurring within it. Various graphs are presented to understand the variation of soil moisture in the soil profile and the contribution of the ponded water to the unsaturated and saturated zone. It is observed that about 100% of ponded water infiltrated in 8 hours 45 minutes. The data is expressed in the form of Philip's two-parameter equation I=St ½+At, where I is cumulative infiltration 7 t is time; and 'S' and 'A' are the two parameters.Tnis equation could be adequately matched to the data so that the variability of the infiltration could be expressed in terms of the parameters S and A.