Standardization of Frequency Domain Reflectometry and Watermark Sensors for Soil Moisture Measurement at Field Level

Abstract

A field experiment with spilt plot design was carried out for standardization of Frequency Domain Reflectometry (FDR) and Watermark sensors in drip irrigated broccoli(Brassica oleracea var. italica). The experiment included three levels of irrigation frequencies: N₁ (once every day), N₂ (once every 2 days) and N₃ (once every 3 days) with three irrigation regimes of 100, 80 and 60 % of crop evapotranspiration (ET_c). For evaluating the performance of different soil moisture sensors, the sensors’ readings were taken from the plot on a daily basis and these readings were compared with gravimetric methods (standard). It was observed that the calibration of two sensors (FDR and Watermark) give a similar calibration equation with lowRMSE after field calibration. The value of coefficient of determination (R²) for FDR was observed 0.85, 0.86, 0.89 and 0.86 for 0-15, 15-30, 30-45 and 45-60 cm soil depth whereas, the value of coefficient of determination (R²) for Watermark sensor was observed as 0.76, 0.83, 0.84 and 0.85 for 0-15, 15-30, 30-45 and 45-60 cm soil depth, respectively. The Watermark sensor curves observed less sensitive at low soil water tension, whereas FDR sensors performed better in wet as well as dry conditions. That field calibration of soil moisture sensors is a prerequisite to measure soil moisture content in the soil.

Keywords : Broccoli, Crop evapotranspiration_, FDR, Watermark, Split-plot design