Correcting Inaccurately Recorded Data due to Faulty Calibration of a Capacitance Water Content Probe

Measuring soil water content by capacitance probes requires rigorous calibration to achieve acceptable accuracy. Some of the capacitance probes' users might take several readings using the default device calibrations or other prestored calibrations by mistake. This can lead to logging of faulty readings for periods of up to months or years. This study aimed to (1) study the importance of probe calibration and the level of error that results from using flawed calibrations and (2) to develop a mathematical method to correct the faulty recorded data. This research involved studying eleven scenarios of faulty calibrations including errors in the air/water calibration and in the in-soil calibration. A mathematical method was developed to correct the faulty recorded data and comparisons were made for the data after and before correction. Results indicated that using the manufacturer's default calibration within the software resulted in substantial error values especially for heavy textured soils. It is recommended that users and especially researchers should perform rigorous in-soil calibration wherever the probe is installed, and they should repeat the calibrations whenever the soil structure changed. 1. Introduction Accurate estimations of soil water content are required for precise agriculture and for agricultural research such as determinations of crop water requirements, water use efficiency, and irrigation scheduling. Soil water content estimations are also used in field hydrology [1]. The direct method of soil water content measurement is the gravimetric method, which involves taking a physical sample of the soil, weighing it before any water is lost, and then drying it in an oven at 105°C before weighing it again [2, 3]. The soil water mass is measured as the mass difference between the two weights (before and after oven-drying). Normally, water content is expressed as the mass ratio of water to dry soil matter called mass basis , or volume ratio of water to soil called volume basis . The measure is usually used for comparative purposes, especially when the compared soil samples are not consistent in volume as in studying the tillage effect on water movement in soil or when the soil changes its volume as it dries (like some clay soils). On the other hand, is widely applied in a wide range of research fields especially in irrigation studies. Although the gravimetric method is accurate and reliable, it is slow and laborious; however, it does not allow continuous measurement of water content for a particular place as the sample is destroyed during the