Measuring Temporal Changes in Pondnig Depth in a Rice Paddy Field Using Time Domain Reflectometry.

Global warming is a great concern. Rice paddy fields are important human-induced sources of methane (CH₄) and nitrous oxide (N₂O), whose emissions are influenced by soil redox potential. The temporal changes in ponding depth and soil water content need to be measured because they affect redox potential. Soil water content is usually measured by time domain reflectometry (TDR) with multiplexers. If ponding depth can be measured with TDR, it will be convenient for conducting field experiments because the numbers of measuring instruments would be reduced. Therefore, we introduced a simple method to monitor the ponding depth using TDR. First we verified whether the ponding depth could be measured with TDR in the laboratory, using various lengths of TDR probes (10, 15, 20cm long). A linear relationship between ponding depths and those measured with TDR probes was observed when ponding depth was larger than 0.3cm. The laboratory experiments have revealed that ponding depths can be measured using TDR method so that we conducted field experiments in a rice paddy field, using a 20cm-long TDR probe. The ponding depths measured with TDR agreed well with those measured with a pressure-type water gauge with a root mean square error (RMSE) of 0.29cm when the ponding depth was larger than 1.0cm. Although ponding depths measured with TDR might be affected by the deposition of rain drops, they were little affected by changes in dielectric constant by the variation in surface-water temperature. It was concluded that the ponding depth can be measured with TDR.