This study was initiated to find out the occurrence of soil water repellency (SWR), implications of SWR on hydrology, and possible causes of SWR in a selected catchment in Okinawa Island, Japan. Coastal forests in subtropical Okinawa archipelago are an important source of water supply. This function may be affected by SWR as SWR causes poor water infiltration leading to overland flows. To assess the SWR, sessile drop method (SDM) and in-situ and in vitro water drop penetration time (WDPT) tests were used. Being among the major factors influencing SWR, total soil organic carbon (TOC), water soluble organic carbon (WSOC), and soil texture were assessed. Nine transects, which included natural forest, secondary climax forest, and young secondary forest blocks consisting of abandoned plantation forests were studied. SDM results showed the existence of potential SWR in all the transects. The results of WDPT test and SDM were not directly comparable in the forest soil samples. In vitro WDPT test results showed the existence of potential SWR only in natural forest: Hikanzakura, and Iju plantation forests. However, none of the sites showed SWR during the in situ WDPT test. High soil moisture content may have masked the existence of SWR. A strong correlation was observed between WDPT and TOC (r²=0.89; p<0.05). SWR occurrence and its severity appear to correspond well to the TOC when the latter is approximately 7-8% and above, in the soils of broad-leaved forests where trees are not specifically known for their role in causing SWR. These findings suggest that soil moisture and TOC have been the major determinants of the expression and occurrence of SWR respectively, for the catchment. However, if the regular moisture regime of the area continues to persist, the influence of water repellency on water infiltration and, hence on the hydrological processes will not be significant in the catchment.

  Cross-sectionally averaged one-dimensional shallow water equations (1-D SWEs) serve as a fundamental tool in simulating open channel flows. Accurate numerical simulation of transient flows is crucial for practical analysis of mass and momentum transport phenomena in open channels. Temporal discretization algorithm is the one of the most influential factors that controls accuracy, stability and computational efficiency of numerical simulation. This paper presents a new numerical method for the 1-D SWEs, referred to as SElective LUmping Method (SELUM), which is an intermediate method between the Finite Element/Volume Method (FEVM) and Dual Finite Volume Method (DFVM) previously developed by the authors. The SELUM selectively lumps the mass matrix of the continuity equation so that stable numerical solutions are obtained even under severe flow conditions where the FEVM or DFVM fail. An approximate mass matrix inversion method is incorporated into the SELUM to improve computational efficiency while not to degrade accuracy and stability. Numerical simulations of idealized and experimental problems are carried out to demonstrate its advantages over the existing numerical methods.

  This paper investigated the possible use of agricultural wastes including rice husk, rice straw and rice bran for attenuation nitrate- or chloride-induced problems such as nitrate pollution or soil salinization in porous media. Column experiments with 30 cm in length were carried out under saturated flow conditions using two different solutions of KNO₃ and NaCl. Each agricultural waste was mixed with silica sand and filled into the column homogeneously. Mixture percentages of the weight of each material to silica sand were varied in order to examine the change of the degree of solute dispersion, mobility and mass attenuation, which were identified using temporal moment analysis based on experimental breakthrough curves of the effluent. The results showed that use of agricultural wastes provides the slight increase of the dispersivity relative to the case without agricultural wastes except for the case with rice bran in chloride ion transport. Each anion showed its own degree of solute mobility associated with mixed agricultural wastes, due to the hydrophobic property of the surface of wastes. The results also revealed that the use of rice husk and rice bran leads to the degradation of total mass from the effluent, although the rice straw has an extremely low property attenuating the mass of nitrate and chloride. Furthermore, a new quantity index to synthetically evaluate the potential of agricultural wastes was proposed. The results indicated that rice straw has the highest potential in terms of the attenuation of nitrate- or chloride-induced adverse effects in porous media.

  An extended mathematical model governing shallow water flows in vegetated open channels, referred to as the 1-D extended shallow water equations (1-D ESWEs), is presented in this paper as a physically more consistent alternative to the conventional 1-D SWEs. Emerged and submerged aquatic plants in channels are considered in the 1-D ESWEs as momentum sinks with appropriately defined water fractions. The 1-D ESWEs improve the double-counting problem on the momentum losses in the 1-D SWEs originating from the additivity assumption of the wall friction and vegetation drag forces without considering the water fraction. The 1-D ESWEs are applied to steady and unsteady numerical simulation of water flows in an agricultural drainage canal in Japan. The computational results demonstrate advantages of the 1-D EWSEs over the 1-D SWEs, reducing overestimation of the water depth. Impacts of the vegetation modeling on the flows in the canal are also assessed through the unsteady simulation.

  Since the structure of the weathered soil of tuff, which is generated as excavated soil by farm road improvement projects in Mt. Daisen area in Tottori, is easily destroyed, it cannot be reused and is dumped as industrial waste. The soil also easily becomes muddy and pollutes rivers. In this study, we made a baked granulated substance of the soil (BGD) and evaluated its adsorptive capacity for dissolved phosphorus and clarified the following points. 1) The phosphate absorption coefficient of BGD was changed by sintering treatment temperature and became maximum: 6.22gP₂O₅/kg at 500℃. 2) We found that active aluminum played an important role in the phosphorus adsorption of BGD. 3) The concentration of the dissolved phosphorus in the solution was decreased by immersion treatment of BGD. However, the effect failed to continue in repeated uses. 4) BGD can be reused in farmland after adsorption treatment because the amount of available phosphorus in BGD reached 70 to 80% of the amount of the absorbed phosphorus.

  We have practically tested small-scale solar pump systems, which are stand-alone PV systems including a DC pump, so as to develop methods for designing and operating them with low lifecycle cost. The application of the systems is drip irrigation for sloping field or orchard of approximately 10 ares. Two series of tests have been conducted. In one series, the pump discharge pressure head is regulated to keep 60 meters with a regulation valve. In the other series, the systems are used in practice at a citrus orchard and the pressure head is also approximately 60 meters. The results are as follows. The initial performance of the systems shows that the devised designing method is appropriate. In order to suppress the deterioration of batteries and pumps, intermittent pump drive, e.g. 15 minutes pumping every 45 minutes, is substantially effective.

  It is essential to study the mechanisms of the decomposition of anaerobic organic matter such as denitrification, iron reduction and sulfate reduction caused by oxygen depletion when considering the conservation and restoration of a closed water area under organic contamination. In this study, we observed the water quality of a reservoir with significant organic contamination due to humic acid, and evaluated quantitatively the water quality changes over time with respect to the oxidation reduction potential (ORP) as well as sulfides, ammonium nitrogen (NH4⁺-N), phosphate-phosphorus (PO₄^3--P), and sulfate ions (SO₄^2-) during an anoxic period. The study showed that the changes of the ORP under anaerobic conditions can be modeled using a logistic regression based on the number of consecutive anoxic days. It was suggested that the higher the initial concentration of nitrate nitrogen was. the smaller declining rate of the ORP was from an oxidized state to a reductive state. Furthermore, it was shown that the increase of NH₄⁺-N,PO₄^3--P, and sulfides over time, as well as the reduction of SO₄^2-, can be evaluated using a linear regression based on the number of days elapsed in which both an ORP level of less than -200 mV and a dissolved oxygen level of 0 mg/l are sustained. The dynamics of inorganic nitrogen, phosphorus, and sulfur compounds can be estimated and predicted using the two above-mentioned regression models without any mathematical model on the complex biochemical processes. This is valid information for the analysis of aquatic environments, which can contribute to the conservation and sustainable management of closed water areas.

  From 2000 until 2004, Japanese black pine growing on Sakurajima Volcano has been devastated by pine wilt disease. Most of the pines on the Taisho and Showa lava flows suffered high mortality rates after being invaded by pine wilt disease starting in 2004. We investigated a relationship between the growth of Japanese black pine forest, and the surrounding environment and water infiltration rates into soil of Japanese black pine forest after this invasion and the associated morality. We installed vegetation survey plots in the areas covered by the Taisho and Showa lava flows, which erupted from Sakurajima Volcano in 1914 and 1946, respectively, and conducted field surveys in 2013. In the study plots, for trees with the same diameter at breast height, pines on the Taisho lava flow grew taller than pines on the Showa lava flow; also, the pines on the Taisho lava flow grew faster and with a larger amount of diameter growth than pines on the Showa lava flow. In the study plots, the development of forest soil on the Taisho lava flow was better than that on the Showa lava flow because the Taisho lava flow has a better growth environment for pine, and therefore the water infiltration rate on the Taisho lava flow was larger than that on the Showa lava flow.

  This paper addresses the effect of revegetation on forest soil development and water infiltration rates in the shallow landslide scars of slopes covered by volcanic ash and pumice fall. The number of trees and tree species peaked approximately 10 years after the occurrence of the shallow landslides, and then declined with time. Approximately 40 years after the occurrence of the landslides, an apparent steady state was achieved. Based on the temporal variation of the Fisher-Williams' index of diversity, the forest located in the shallow landslide scars reached a climax approximately 40 years after the occurrence of the landslides. Revegetation developed on the forest soil and had grown to a thickness of approximately 40 cm by the time the forest reached a climax approximately 40 years after the landslides. After 58 years, the water infiltration rates and forest soil thickness in the shallow landslide scars were approximately 70 and 65 %, respectively, compared with those measured in the headwater forest of broad-leaf trees.