Surface soil environment was surveyed and characterized using a multi-frequency electromag-netic wave (EM) sensor. Traditional resistivity method needs multiple point electrodes in the field to investigate the deeper points, however, this method uses broadband electromagnetic data to obtain the apparent conductivity from different depth, i.e., the higher the frequency, the shallower data we can obtain. It is non-invasive and rapid, therefore, appropriate for wide area environmental survey. Five frequencies of 2310, 3870, 7950, 24510 and 47970 Hz were applied so that we obtained vertical profile of the domain. Forest under different management, winter-onded paddy field, orchard and tea farm in hilly mountainside and orchard in plain field were investigated using a broadband electromagnetic wave sensor sounding. Surface electrical con-ductivity (EC) obtained by Wenner array has the highest correlation with 47970Hz data, showing it is the appropriate frequency for surface soil investigation. Moreover, EC in the order of several tens milli-Siemens obtained by EM sounding corresponded very well with the EC obtained by traditional method. Conductivity data successfully showed management induced changes in soil environment by conductivity map along with vertical conductivity profile, which is advan-tage of broadband EM survey. Water content changes in forest after rainfall, effect of ponding management in paddy field, and the differences in fertilizer application practice in hilly farm, were clearly demonstrated by the broadband EM sensor.
An empirical calibration method for temperature dependence of a commercially available capacitance probe (ECH2O probe : Decagon Devices) was presented in this paper. A laboratory experiment was conducted to obtain probe outputs at different water contents (θ) and temperatures (T) using soil from Shaanxi Province, located in the Loess Plateau, China. The probe output showed strong positive response to T. A calibration equation describing the probe output as a function of θ and T was developed through an empirical curve fitting approach. The θ values were estimated by solving the calibration equation numerically. The root mean square errors between actual and estimated θ were 0.016 m3 m-3. We also discussed the theoretical background of the temperature dependence of the probe using the results from the experiment. The calibration equation was applied to field observation data in the Loess plateau. The equation significantly reduced the daily and seasonal fluctuations of the probe outputs to T. A linear calibration equation provided by the manufacturer, which neglects temperature dependence, caused incorrect interpretation of water movements, suggesting the importance for temperature calibration in arid environments.
Soil crusting is one of the major problems affecting the emergence of soybean seedlings in converted paddy fields in some regions of Japan. To identify the converted paddy fields in which soybean emergence is at risk from soil crusting, we collected twenty-seven samples of surface soils from paddy fields across Japan. Relationship among physical and chemical properties, clay mineralogy and surface crusting under simulated rainfall were studied. In terms of morphology, most of the soil samples showed crust after exposure to a rainfall of 37 mm/h. With regard to the relationship between crust properties and soil characteristics, some correlations were found between the decrease in infiltration and the hardness and area of the cracks when crust dries. The infiltration rates were correlated with the mean weight diameter of aggregates. The hardness indicated a rather obscure correlation between the silt contents and acid-oxalate-extractable aluminum (an indicator of Andie properties). The cracked areas were clearly correlated with the sand contents. With regard to clay mineralogy, in the soil samples which showed no or trace peak for kaolinite and vermiculite and a clear peak for smectite, hardness did not increase after drying.
In Northeast Thailand, permeable sandy soils are widely distributed. Most rainfall may infiltrate the ground and there may be little surface runoff. However, soil erosion caused by surface runoff has been reported. To investigate the mechanism by which surface runoff occurs, we selected a sloping area where the top soil consisted of a permeable sandy layer overlying a very low permeability clay layer. We defined the soil water storage capacity of the sandy layer at a particular time as the difference between the maximum observed water content and the observed water content at that time. We assumed that when the amount of rainfall exceeded the soil water storage capacity, the excess rainfall became surface runoff. As a result of monitoring of soil water content in the sandy soil layer over a period of one year, the total amount of runoff was estimated to be about 30 % of the total rainfall and the periods for which our analysis showed that soil water storage capacity was exceeded were followed by rises in the water level of a pond that surface runoff flowed into.
We improved the measurement apparatus for convective velocity of ponded water in paddy fields, proposed by Fujimaki et al. (2000). Since the effect of air bubbles accumulated along the sensor on the measurement values was observed, we developed a new equipment to prevent air bubbles from accumulating along the sensor. Another improvement was that the power sources of all measurement devices in the apparatus were exchanged from AC power supplies to DC power supplies. This improvement allowed us to measure convective velocity of ponded water in paddy fields without AC power supplies. The good performance of the improved apparatus was verified by the measurement of convective velocity of ponded water in a paddy field.
Measurement of temporal changes in hydrological condition such as a flooding depth is needed to understand green-house gas exchange between paddy rice fields and the atmosphere. We introduced a simple method to monitor the flooding depth using time domain reflectometry (TDR). The results of laboratory experiments indicated that water depths measured with TDR agreed well with those observed when the water depth was estimated assuming that the second reflection of TDR pulse occurs not at the end of the rod, but at the water surface with large water depths. The laboratory experiments revealed that TDR provided precise enough water depths so that field experiments were conducted. Water levels measured with a 20-cm long TDR probe agreed well with those measured with a pressure-type water gauge with a root mean square error (RMSE) of 0.28 cm. The water levels measured by TDR, however, might be affected by raindrop depositions.
In test method for particle size distribution of soils, sieve analysis result and hydrometer analysis result may not be skillfully connected. It is said that in hydrometer analysis, much error arises from many causes, and it'brings about the error for the values of percent finer by mass and particle size calculated using Stokes’ law. This fact was estimated by the trial, and instrumental error of hydrometer, density error of the water, viscosity coefficient error of the water was clarified. Hydrometer analysis result was corrected using these. It was confirmed that the corrected hydrometer analysis result agrees almost with the sieve analysis result using 45 μm sieve, 32 μm sieve and 20 μ msieve. Then, as convenient method, the method for moving particle size curve by hydrometer analysis parallel in the vertical direction was proposed so that the percent finer by mass of 32//m particle size by the hydrometer analysis may agree with the percent finer by mass of 32//m particle size by the sieve analysis, and the result was good.