Global 500-m (18-arcsec) resolution cropland suitability was estimated using recently developed high-resolution global cropland data and a digital elevation model. The high-resolution estimation more precisely represented topographical constraints to agriculture that were not adequately reflected in previous low-resolution estimates. It also successfully suppressed the overestimation of cropland suitability on areas with steep slopes. Furthermore, the distinction of rainfed and irrigated cropland removed suitability overestimation induced by human agricultural intervention and enabled more natural and realistic estimation of cropland suitability. The comparative analysis between the estimated land suitability and actual cropland distribution revealed that, if only natural condition is considered, it is possible to expand cropland area by 9.25 million km², which is more than needed in the future while socio-economic factors controlling cropland suitability should be considered for more practical assessment. The newly developed highest-resolution cropland suitability map is expected to contribute to solving upcoming water, energy, and food issues, by integrating with water resource models and biomass studies.

To study the spatial variability of water surface fluxes, turbulence measurements on a moving platform are useful. However, such measurements have only been carried out with large research vessels over the ocean. We tested the feasibility of flux measurements with a small excursion ship over Lake Kasumigaura, the second largest lake in Japan. After the formal application of coordinate rotations to account for the ship’s movements, we derived mean wind velocities as well as latent and sensible heat fluxes. They were compared with spatially interpolated wind velocities from meteorological stations and with fluxes estimated from the bulk method. Equally good agreements were found with those reported in previous studies over the ocean, indicating the feasibility of ship measurements in a lake. Possible error sources were identified for the improvement of the accuracy of flux estimation.

Extensive deforestation in tropical regions may significantly influence the hydrological cycle. However, subsurface runoff processes in thick soil layers in humid tropical forests are poorly understood; thus, the impact of land-use changes in such regions remains unclear. To understand runoff generation mechanisms in the humid tropics, we monitored groundwater and soil moisture dynamics in a forested hillslope in Sumatra, Indonesia. We also conducted field and laboratory experiments to determine soil hydraulic characteristics and used the results to simulate vertical infiltration and groundwater recharge. Although the soil is categorized as silty clay loam, the high infiltrability and high water retention capacity of the soil enabled infiltration during storm events and recharge to groundwater. Within the 4–5 m thick soil layer at the foot of the hillslope, the shallow groundwater table quickly responded to rainfall and did not drop below a depth of 2–3 m, possibly due to continuous flow contributions from the upslope. Overall, this study demonstrates the importance of subsurface flow and vertical infiltration in thick soil layers in humid tropical regions.

Low frequency (once a month) but long-term (ca. 6 years) sampling including snow-melt periods in a mountainous stream, the Okura River (Sendai, Japan), revealed that loadings of 5 parameters (COD, TN, TP, TOC and D-SiO₂) could be expressed exponentially using discharge (Q), while the coefficients for the 5 loadings were all about 1. Here, mathematically, the periodically averaged Q leads to approximation of that of load (L). We analyzed the bias of the spot Q to that of the periodical (30, 14 and 8 days) means. The results ensured the utilization of the spot Q instead of the periodical mean Q for estimating L because of the high correlation factors (0.872, 0.914 and 0.923 on 30-, 14-, 8-day mean Q analyses, respectively) and suggested the validity of the usage of the observed regression slopes of 1.06, 1.22, and 1.22 over 30, 14, 8 days for quantitative correction of L because the fact that the slopes are larger than 1 indicate that the usage of the spot Q instead of the mean Q leads to the overestimation of L. Both changing correlation factors and the regression slopes realized small improvements via shortening the periods from 14 to 8 days. The protocol proposed here is quite original and is applicable to designing sampling strategies at target sites based on quantification of the limitations and/or reliability of L estimations.

Floods are major natural disasters that have considerable consequences worldwide. As the frequency and magnitude of flooding are expected to be affected by ongoing climate change, understanding their past changes is important for developing adequate adaptation measures. However, the limited spatiotemporal coverage of flood gauges hinders detection of changes in flooding, particularly in poorly gauged regions. Here, we propose a method using surface water data of river floodplain inundation as a proxy of the magnitude and frequency of flooding. Surface water data − Aqua Monitor which represented the probability linear trend changes in land and water surface area based on 30-m Landsat images between 1984–2000 and 2000–2013 was used in this study. The changes in water surface area over the floodplain obtained from Aqua Monitor showed high correspondence with historical trends observed or simulated annual maximum daily discharge, indicating the potential to detect changes in frequency and magnitude of flood from satellite data. In regions where changes could be measured with sufficient satellite images, 29% showed an increase in water surface area in the flood plain, 41% showed a decrease, and 30% showed small or no changes.

A number of rainfall-runoff models have been developed for hydraulic and hydrological engineering with an emphasis on reproducing river discharge time series. Physically-based rainfall-runoff models have recently reached a certain level of achievement following the advancement of computers and the development of various geographical and meteorological datasets. However, it has been pointed out that the current physically-based models do not properly reflect observed hillslope water dynamics. The present paper proposes a methodology to examine the capability of a depth-discharge constitutive equation for physically-based rainfall-runoff modelling to simulate hillslope water dynamics. An application of the methodology suggested that 1) the targeted constitutive equation was capable of representing the depth-discharge relationship on hillslopes under the assumed conditions, 2) the runoff simulations with the constitutive equation described hillslope water flows, at least in the downward direction, and 3) there was a possibility that the parameters in the constitutive equation was determined from the internal structure of hillslope water dynamics.