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Abstract:Soil layers play an important role in hydrological processes, particularly in forested basins. For example, thick soil layers can store rainwater temporarily and drain it gradually. In the Mekong River Basin, especially in Cambodia, evergreen and deciduous forests are widely distributed. However, we have no data on soil conditions such as thickness and hardness that affect water storage capacity in forested areas. To clarify the water storage role of forest soils in the lower reaches of the Mekong River Basin, we investigated the distribution of soil thickness using a handy dynamic cone penetrometer in three Cambodian provinces: Kampong Thom, Kratie, and Mondul Kiri. Soil thickness was greater than 8 m in a deciduous forest in Kampong Thom. In some evergreen forests, apparent soil thickness varied from the rainy to dry seasons because of specific grain size distributions; for example, the soils were rock hard in the dry season and soft in the rainy season. In Kratie, deciduous forests prevailed and soil thickness was shallow at less than 3 m. In Mondul Kiri, forests were mainly formed on a basaltic plateau, and soil thickness was greater than 7 m. Soil thickness and hardness differed among the soil types in these three provinces, as did water storage capacity. The maximum water storage capacity was 1350 mm in an evergreen forest distributed on Acrisols in Kampong Thom. In contrast, the minimum water storage capacity was 225 mm in a deciduous forest distributed on Leptosols in Mondul Kiri. We constructed a soil thickness map for the study area in Kampong Thom that was mainly covered with evergreen forest; the distribution of soil thickness was related to elevation, with soil thickness increasing 0Ð13 m per 1 m increase in elevation, except in a deciduous mixed swamp forest area.

We investigated radioactive cesium (Cs) export in stream water during a typhoon flood event (No. 1204, Guchol) in a small forested headwater catchment (1.2 ha), which was contaminated by the Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster. The experimental watershed is covered with hardwood and conifer trees. Hourly stream water samples were collected by an automatic water sampler, and turbidity was measured every 10 min. Downstream transport of radioactive Cs was recorded through the typhoon, during which the total rainfall was 168 mm. Total cesium-134 ( 134 Cs), cesium-137 ( 137 Cs) and suspended solid (SS) concentrations and turbidity were elevated on the rising limb of the hydrograph. When maximum rainfall intensity, maximum runoff peak were observed, and maximum radioactive Cs and SS concentrations occurred. Total 134 Cs and 137 Cs concentrations were 1.5 and 2.4 Bq L −1 , respectively, at peak flow. However, concentrations of radioactive Cs, SS and turbidity dropped sharply on the receding limb. After filtration (0.7 μm), the concentrations of 134 Cs and 137 Cs were either not detectable (ND) or below 0.09 Bq L −1 . Concentrations of dissolved radioactive Cs in stream water were very low, and radioactive Cs was mainly discharged with suspended materials. Total 134 Cs and 137 Cs exports from the study watershed were 43 and 72 Bq m −2 , respectively. These exports represented approximately 0.07% of the 134 Cs and 137 Cs fallout around the study site (180 kBq m −2 ). We therefore concluded that discharge of radioactive Cs from forested headwaters was negligible even though a typhoon flood event.

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