TDR (Time-domain reflectometry) is an advanced method for monitoring the water content in fields. TDR measurements can provide accurate information about vegetation effects on both spatial and temporal soil water distribution. The objective of this study was to examine the effects of vegetation conditions on the distribution of the soil water content. Soil water content was measured to express the spatial distribution in the surface layer (0-0.2 m) in grass, corn, and non-planted fields. Additionally, using the TDR method with multiple probes with different lengths, the measurements were conducted on a temporal basis at 4 depths (0-0.1, 0.1-0.2, 0.2-0.3 and 0.3-0.5 m) to determine the soil-water content profiles in the 3 fields. A significantly higher water content in both the grass and cornfields was found, compared to the non-planted field. Moreover, in the cornfield, we observed that microtopographic features significantly affected the spatial water distribution. The lower the soil water content, the higher the coefficient of variation of the soil moisture content. These results indicated that the effects of the vegetation conditions on the soil water distribution in the fields were significant. From the observation of the temporal soil water profiles in the 3 fields, it was found that soil management, i.e. tillage or no-tillage, was a major factor affecting the profiles.Discipline: Agricultural engineering Additional key words: electromagnetic wave, Andisol, multiple probes with different lengths JARQ 37 (4), 243 -248 (2003)

In northeast Thailand, salinization caused by the discharge of salty groundwater is a serious problem. Locations where groundwater flows into surface water must be identified before measures against saliniza tion can be taken. Radon-222 (222Rn) is a useful indicator for determining areas of groundwater discharge. We measured 222Rn concentrations in surface waters in the region and found that groundwater flows into streams and lakes in low-elevation areas (below 180m). Our results agree with the results of published simulations of groundwater flow.

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