Understanding patterns and seasonal variations of excessive nutrients in surface water from anthropogenic activities is important for pollution control. In this study, we developed an integrated biogeochemical modeling framework for nitrogen exchanges among the atmosphere, terrestrial, and aquatic ecosystems. A land surface model, a terrestrial nitrogen cycle model, and a riverine hydrodynamics model incorporated with a river temperature model were consolidated and driven by multiple nitrogen sources related to anthropogenic activities. We estimated the global nitrogen loading and transporting in global rivers, with consideration of seasonal variations, and the validation demonstrates the reliability of the proposed model. The total dissolved inorganic nitrogen (DIN) flow rate is accumulated following rivers and it has high total DIN loads even in regions with low population density but large basin area, such as those at high latitudes. This study successfully improves estimation of nitrogen loading on global scale with consideration of seasonal variation. Our results show consistent trends with the observed data of DIN concentrations in global rivers, where all above variables are greatly affected by seasonal variations. The results also reflect the monthly-variant nitrogen inputs help produce closer DIN concentration estimates to observations, which will possibly stress the need for further study on seasonal variability of anthropogenic emissions.

Changes in stem circumference (SC) were observed in teak (Tectona grandis) in a tropical region during the dry season after tree-ring formation had stopped. We hypothesized that these SC changes were caused by water absorption and emission from the outer bark surface. To test this hypothesis, we measured SC, heat pulse velocity (HPV), and leaf number using time series images in a teak tree plantation in northern Thailand. We also performed laboratory experiments to observe changes in the weight and thickness of teak bark blocks under various vapor pressure conditions. Increases in teak tree SC were observed after rainfall during the dry season, when defoliation was almost complete and HPV was low. The weight and thickness of the bark blocks, on which all surfaces other than the outer bark were sealed, varied with water vapor content. These results suggest that water vapor absorption and emission through the outer bark surface can affect SC during the dry season. However, SC continued to increase after the vapor pressure deficit increased, and decreased more rapidly in the tree with higher HPV, suggesting that water exchange between the xylem and inner bark also contributes to changes in SC.

Recently, Hyogo Prefecture in Japan has expanded its paddy field dam activity within the prefecture using wood weir plates. A wood weir plate was placed at the drainage outlet of a paddy field to limit drainage from the field, thus enabling the paddy fields to store rainwater to a certain extent. As the effect on flood mitigation was not appropriately estimated, we carried out a field experiment using a wood weir plate to estimate the weir coefficient, which determines the drainage discharge from the paddy according to the water depth. Then, using the estimated weir coefficient, we developed a 3,000 m² paddy field dam model. Thereafter, we applied the model to the Suse region (approximately 33 ha) considering the 2009 and 2011 rainfall events. It was observed that the paddy field dam has a relatively large effect on reducing the peak discharge (approximately 25.2% for 2009 and 48.5% for 2011) within the proximal downstream areas of the paddy dam site. However, the simulation showed that when the rainfall is extreme, as was the case in 2009, the paddy dam activity may cause erosion of the paddy levee due to the increase in the paddy water depth.

Simulating streamflow under both high-and low-flows is required for versatile eco-hydrological modeling. Typical streamflow simulators require hydrological data such as river geometry and observed river discharge/water level as upstream/downstream boundary conditions. However, these are not always available in data-sparse regions. Furthermore, because of the potential inaccuracy of digital elevation model (DEM) data around water surfaces, this data has not generally been utilized in streamflow simulations. Therefore, this study explores the potential applicability of DEM data to extract river cross-sections, focusing on the upstream Hiikawa River, Japan. A 1-D streamflow simulation was performed using river cross-sections extracted from a 5 m LiDAR DEM and the observed dam discharge from 2018 to 2020 as the upstream boundary condition. The simulated water depths with Manning’s roughness coefficients of 0.03 to 0.05 m^–1/3 s reproduce the observation results with Nash-Sutcliffe coefficients of 0.91–0.97 for the whole period and 0.60–0.97 for a flood event. The accurate results for both low and high flows were considered to reflect the reasonable representations of the river cross-section. Finally, the velocity-based suitability index for Ayu (P. altivelis) was evaluated. We demonstrate applicability and several possible limitations of DEM data for eco-hydrological modeling of data-scarce rivers.

The drought impacts in the Dakbla watershed were assessed based on a combination of hydrological modeling and drought indices. Three drought indices, the Standardized Precipitation Index (SPI), Standardized Soil Moisture Index (SSI), and Streamflow Drought Index (SDI) were utilized to evaluate the drought features of meteo-hydrology and agriculture. The results indicated that these indices are well adapted to the local conditions, especially the 12-month time scale. Evaluations of drought features on the watershed scale could provide more specific information regarding drought risk than regional-scale/district-level assessments, because a watershed is a hydrologically fundamental unit to consider water resources management. Additionally, evaluations of drought impacts using the SSI showed longer and higher trends than those using the SPI and SDI in terms of drought duration and frequency. Considering the spatial distribution of drought frequency, the areas predominated by agricultural land in the target watershed had higher drought risk. Thus, assessment of agricultural droughts along with meteo-hydrological droughts is extremely important to support realistic local drought management strategies by considering water availability, water balance, and soil characteristics, especially in specific agricultural areas.