This study aims to develop a simulation model that provides the direction of how to design the capacity of a small-farm reservoir (SFR) for supplemental irrigation in rainfed agriculture. The model describes the water balance of an agricul-tural system that consists of an SFR, and the catchment and irrigation areas where peanuts or rice is cultivated. Simulations were conducted for two crop seasons during October 2013 and June 2014 with climate data in Gondangrejo, Central Java, Indonesia. The SFR capacity was either adequate, that is, the SFR supplied sufficient irrigation water throughout the crop seasons, or inadequate, the SFR failed to supply irrigation water. The optimum SFR capacity was defined as the minimum of the adequate capacity for given catchment and irrigation areas. Simulation results were compared with actual cases of five SFRs constructed in the simulation site. The results clarified the range of the adequate SFR capacity for peanut or rice cultivation in the first crop season during the rainy season and in the second crop season partly during the dry season. The model outputs can be utilized for evaluating and designing SFRs in rain-fed agricultural systems.

  This paper presents a stochastic process model for optimal strategies of harvesting and predator extermination to sustain Plecoglossus altivelis (Ayu) population in stochastic river environment. Human activities, which are extermination of Phalacrocorax carbo (Great Cormorant) from the river and fishing activity to harvest P. altivelis in the river, are taken as control variables in the model. An optimal management problem to maximize the profit from harvesting P. altivelis and simultaneously to minimize the cost to exterminate P. carbo while to sustain P. altivelis, is then formulated on the basis of the dynamic programming principle. The problem to be solved mathematically reduces to approximating solutions to a Hamilton-Jacobi-Bellman equation. Sensitivity of the optimal management strategy to three critical components of the present model, which are the intrinsic growth rate, environmental noise that makes the dynamics in a river be inherently stochastic, and the two human activities, is numerically verified focusing on the case of Hii River, Japan where most of the population of P. altivelis is thought to be maintained by release of juveniles.

  In this study, intermediate scale dye tracer experiments were conducted in a 1m length, 1m height and 0.03m thickness sandbox to quantify macrodispersion phenomena in stratified porous formations and to elucidate the effect of the layering on the degree of macrodispersion. Spatial and temporal moment approaches based on a snapshot of tracer distribution and NaCl concentration at a point, respectively, were applied to identify the macrodispersivities in longitudinal and lateral directions. The experimental results indicated that the longitudinal macrodispersivity depends on the travel distance of solute and the degree of heterogeneity in stratified formations with two or four layers, while the transverse macrodispersivity decreases with the increase of travel distance and depends on the magnitude of the initial solute distribution. A difference between longitudinal macrodispersivity estimates using spatio-temporal moments was also clarified.

  North-eastern Thailand has little rainfall and requires efficient irrigation development to enhance stable sugarcane production. However, identifying the highest priority areas for irrigation development is complex because the benefit derived from irrigation development depends on rainfall, available irrigation water, and soil characteristics. We used the CANEGRO model to simulate the sugarcane yield of existing cultivation areas under rainfed and irrigated conditions, taking into account actual weather and soil type. We then calculated the benefit of the irrigation development using the simulation results and actual data for groundwater well capacities, sugarcane prices, and irrigation development and running costs. We then analysed the results of the benefit calculation by ABC analysis and the decision tree method. The decision tree analysis confirmed that well capacity most influenced benefit. Areas with higher rainfall had high yields under rainfed condition, so the benefit from irrigation was small (or even negative as the cost of irrigation exceeded the increased income). A notable finding was that low soil available water content resulted in low yields in both rainfed and irrigated conditions, and high available water content resulted in high yields under rainfed conditions ; therefore, both low and high available water content resulted in low benefit from irrigation development.

  Field surveys in 2003 and 2016 were conducted to clarify a temporal change in the woody vegetation and water infiltration rate on the hillslopes of Sakurajima Volcano. We installed a total of two 100-m² survey quadrats on slopes at distances of about 2.0 and 3.0 km from the Minami-dake Crater of Sakurajima Volcano. A comparison of the 2003 and 2016 investigation data of the two survey quadrats showed that the numbers of trees and tree species, the Fisher-Williams’ Index of Diversity and water infiltration rate in 2016 were smaller than those in 2003. In addition, the thickness of the volcanic ash layer deposited since the Taisho eruption of Sakurajima Volcano in 1914 and dry density of the surface volcanic ash layer soil in 2016 were larger than those in 2003. The quality of successional woody vegetation and the growth environment of the herbaceous vegetation were poorer in 2016 than those in 2003. The deterioration of local environmental conditions that were unfavorable for growth following volcanic activity during 2003 and 2016 were believed to have caused the deterioration and failed ecological succession of woody vegetation.

  The Kushiro Wetland, which is designated for inclusion in the Ramsar List of Wetlands of International Importance, is located in the northeastern part of Hokkaido Prefecture, Japan. This study is associated with the environmental isotope-based investigations of hydrochemical aspects of distributed riparian ponds close to the Chiruwatsunai River, which is one of major natural rivers in the Kushiro Wetland. The observation results of dissolved radioisotope radon-222 in the ponds water showed that the discharge of deep groundwater below the underlying clay layer in the wetland might occur in the upstream area of the Chiruwatsunai River and the border area of low-lying wetland and hilly plateau areas. It is found from environmental isotope-based investigations that the distributed ponds in the wetland can be classified into two different types; a peatland water-fed type that is mainly formed by rainwater and peatland water, and a deep groundwater-fed type that receives the supply of groundwater from deeper aquifer in addition to rainwater and peatland water. It is also shown that the deep groundwater-fed type ponds have a small evaporative loss of ponds water compared with the peatland water-fed type ponds, and therefore maintain relatively steady hydrochemical environments by the contribution of the groundwater discharge.