TN-28 : Study of soil moisture using neutron probe

Abstract

Soil moisture studies provide potential information in the field cf agriculture, meteorology and hydrology. In the field of agriculture, Information on soil moisture is needed for many diverse applications Including improved yield forecasting and irrigation scheduling. In meteorology, the knowledge of soil moisture is required for understanding radiation components while in the field of hydrology, the soil moisture content is important for understanding the component of infiltration due to precipitation and irrigation. in such processes of hydrological balance, the moisture content of the surface layers control upward and downward flow and storage of water in the unsaturated zone.

In this report, investigations on the soil moisture content and its redistribution along a soil profile in the unsaturated zone using neutron probe are presented. Various methods for in-situ soil moisture measurements are touched upon. Basic principle, theory of neutronic method, design features, calibration procedure, merits and limitations of the probe- are briefly discussed. Applications of neutron probe for the measurement of soil water storage, soil water reservoir characteristics and recharge etc. are given.

Studies carried out at the campus of National Institute of Hydrology clearly show the capabilities of neutron moisture probe, for accurate measurements of soil moisture changes, in-situ, in undisturbed soil in the natural state.

Some typical soil moisture profiles were plotted showing depletion In moisture content and water gained. It is seen that there is not much variation in the moisture content below 120 cm depth. Soil returned to an almost identical moisture level within two-three days after a rain storm. Evaporation for the period 22nd April to 22nd May and 22nd May to 21st June 1985 was estimated as 5 cm and 1 .91 cm respectively.