Journal of the Japanese Society of Soil Physics

In this study, we investigated the optimum irri-gation regime for a sand dune soil field using a numerical simulation model. For this purpose, we used HYDRUS-1D to simulate the soil water regime in the root zone of an irrigated sand dune soil and evaluated the effects of irriga-tion scenario (matric potential threshold for irrigation and daily irrigation depth) on the cumulative irrigation amount, cumulative water discharge from the root zone, ratio of cu-mulative irrigation amount over potential evapotranspira-tion, and ratio of transpiration reduction using the triggered irrigation module in HYDRUS-1D. When a soil water characteristic parameter determined from undisturbed soil samples in the laboratory was used, volumetric water con-tents simulated by HYDRUS-1D differed from field mea-surements (RMSE = 0.031−0.039). Estimation accuracy was improved using a soil water characteristic parameter derived from an inverse analysis (RMSE = 0.007−0.009). Based on the results of scenario simulation with differ-ent matric potential thresholds for irrigation and daily ir-rigation depths, the matric potential threshold set at −75 cm and the daily irrigation depth of 3.2 mm are recom-mended to decrease transpiration reduction and water dis-charge from the root zone.

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After devastating nuclear disasters, the Bec-querel meter should be very important for monitoring the contamination of our surrounding environment and judg-ing the risk of exposure. However, the commercial Bec-querel meters are too expensive for the citizens to pur chase. Even the availability for the administrative agency is not ensured in emergency. Considering these risks, a self-produced Becquerel meter using a commercially avail-able scintillation survey meter is proposed. The sample for measurement should be set in a cylindrical bottle having a diameter of 3 cm and the height of 5 cm. Although the shielding of the system with lead is not perfectly given, the subtraction of the background dose enables the mea-surement of the radiocesium concentration of the contam-inated soil samples with sufficient accuracy. The theoretic-cal error analysis revealed the relative error depends on the total counts and the contribution of the background radi-ation. The counts required for the measurement with 10 % of error is 4500 if the contribution of the background radiation is 80 % of the total counts, while the required counts reduced to 600 if the contribution of background is 50 %. The correction methods to apply this system to the sample with variable thickness in the bottle is also pre-sented. Because the relationship between the counts rate and the radiocesium concentration depends on the propor-tion of 134Cs to 137Cs, the calibrations are generally re-quired at each measurement. However, no significant dif-ference was revealed for the calibration lines measured at Jan. 2013 and Aug. 2013 during which the proportion of 134Cs highly decreased. Thus, any frequent calibrations are not necessary for the practical use of the system. When the aforementioned conditions are satisfied, the proposed Becquerel meter meets the practical demand for knowing the radioactivity of soil samples.

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