Soil Moisture Measurement through Time Domain Reflectometry (TDR). lrrigation Application

Abstract

Knowledge of how moisture varies in the soil is essential to determine the actual rate of evapotranspiration in crops and, thus, the amount of water necessary for irrigation. Lisimeters, tensiometers and the neutron probe have been widely used for precise measurement of soil moisture in order to calibrate models of irrigation scheduling or to calculate volumes of irrigation water. Despite the usefulness of such measurements, these methods have limited use because of problems like high cost of lisimeters and neutron probes as well as their strict regulation, limited range of validity of tensiometers. Moreover; the measurements obtained from these methods generally describe noncontinous points in time. Time Domain Reflectometry (TDR) is a viable alternative to such methods because of its precision and the continuity of its measurements.

This study evaluates soil moisture, using the neutron and TDR methods in maize crop irrigated by gravity in closed furrows. The results show that even though the absolute determinations of moisture are different in the 30-90 cm profiles, moisture variations were similar in all cases and respond consequently to the additions of water and to evapotranspiration. Neutron probe drawback becomes evident on its non-continuous measurements contrasting with the continuous measurements of TDR, which allows a detailed analysis of the infiltration and evapotranspiration phenomena at instantaneous time scales.