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A low-cost, simple to use portable rainfall simulator is developed for use over a 5 m2 plot. The simulator is easy to transport and assemble in the field, thereby allowing for necessary experimental replicates to be done. It provides rainfall intensities of between 20 and 100 mm/h by changing the number and type of silicon nozzles used. The Christiansen coefficient of uniformities obtained in the field are appropriate and vary from 79 to 94% for rainfall intensities ranging from 30 to 70 mm/h. In addition, the median volumetric drop diameters measured for rainfall intensities of 30, 50, and 70 mm/h are in the lower range of that of natural rainfall and equal to 1.10±0.08, 1.69±0.21, and 1.66±0.20 mm, respectively. The velocities of the raindrops with diameters less than 1.2 mm reached terminal velocities, while raindrops less than 2.0 mm achieved velocities reasonably close to the terminal velocity of natural rainfall. Furthermore, the average time-specific kinetic energy (KET) for rainfall intensities of 30, 50, and 70 mm/h are 257.7, 760.1, and 1645.2 J/m2/h, respectively accounting for about 78.0 and 86.5% of the KET of natural rainfall for 50 and 70 mm/h rainfall intensity, respectively. The applicability of the portable rainfall simulator for herbicide transport study is investigated using two herbicides (atrazine and metolachlor); herbicide losses in runoff and sediment samples are in the ranges reported in the literature. As a percentage of the amount of herbicide applied, 5.29% of atrazine and 2.15% of metolachlor are lost due to combined water and sediment runoff. The results show that the portable rainfall simulator can be effectively used in studying processes such as pesticide runoff, infiltration mechanisms, and sediment generation and transport at a field plot scale with an emphasis on how surface characteristics such as slope and soil properties affect these processes.

Climate change and anthropogenic activities are affecting the hydrological conditions of rivers and may have altered nutrient and suspended sediments released into coastal seas. However, testing this hypothesis is difficult, confounded by the lack of observational data and the unavailability of globally accepted suspended sediment concentration (SSC) algorithms. Here, we analyzed the trends in SSC (2000-2020) at the mouths of 10 major Asian rivers using 10 available satellite-SSC algorithms. We identified spatially distinct trends, with SSC decreasing at the mouths of the Yellow, Pearl, and Indus rivers, and increasing trends at the mouths of the Narmada and Ganges-Brahmaputra rivers, while there were no significant trends at the mouths of the remaining rivers. River discharge, dams, and land use changes in basins individually did not suffice, but reproduced the observed SSC trends when used together. Our results imply that anthropogenic activities threaten the marine ecosystem more than climate forcing on Asian coasts.

This study aimed to develop and validate a Dynamic in-River Agrochemical Fate and Transport (DRAFT) model simulating one-dimensional advective and dispersive pesticide transport processes under unsteady flow regime in a riverine system. The DRAFT model was coupled with two other modeling components, the PCPF-B model and the land use based tank model, which simulated hydrological/pesticide process in paddy fields and hydrological process in other land uses such as city, agricultural field and forest, respectively. The PCPF-B/DRAFT model was fed with the spatial information of the target catchment by incorporating the Geographical Information System (GIS). For the model validation, a full catchment monitoring data of a rice herbicide, mefenacet, along the Kose River, Fukuoka, Japan was utilized. After model calibration, hourly river discharge and daily mefenacet concentration were simulated by the PCPF-B/DRAFT model at individual observed points of the Kose River and model performance was evaluated by graphical assessment and multiple statistical indices (e.g. Nash-Sutcliffe efficiencies were 0.84–0.86 for streamflow and 0.16–0.72 for herbicide, respectively). The predicted mefenacet concentrations were strongly affected by: (1) water managements practiced in rice fields and (2) intensive rainfall events. The former concentrations were characterized by broad peak while for the latter the peak concentration was sharp and narrow. We used the PCPF-B/DRAFT model to further evaluate the applications of 7 days of water holding period after herbicide application in paddy fields, which was shown to effectively reduce the total loss of mefenacet from 18.9 to 12.8% of applied mass. Consequently, the broad peak concentrations of mefenacet in the Kose River decreased remarkably while the water management practice was less effective to reduce the sudden and sharp peak concentration resulting from intensive rainfall events.

A Bayesian method involving Markov Chain Monte Carlo (MCMC) technique was implemented into a pesticide fate and transport model to estimate the best input parameter ranges while considering uncertainties included in both the observed pesticide concentrations and in the model. The methodology used for integrating the MCMC technique into a pollutant fate and transport models was detailed. The uncertainties encompassed in the dissolution rate and in the adsorption coefficient of the herbicide mefenacet were greatly reduced by the MCMC simulations. In addition, an optimal set of input parameters extracted from the MCMC simulations accurately reproduced mefenacet concentrations in paddy water and paddy soil as compared to the original published dataset. Consequently, by simultaneously optimizing multiple parameters of environmental models and conducting uncertainty analysis, MCMC technique exhibits powerful capability for improving the reliability and accuracy of computer models. The main strengths of the MCMC methodology are: (1) the consideration of uncertainties from both input parameters and observations and (2) the prior distributions of the input parameters which can be reformulate when additional knowledge is available.

Model applications for delivering reliable information on soil water content (θv) and soil temperature (Tsoil) specific to Andosols (Kuroboku) are still limited despite their large area coverage (0.84% of the global terrestrial surface) and great potentials for improving agricultural production. The performance of the HYDRUS-1D model was therefore evaluated by comparing the predicted θv and Tsoil with field observations gathered from the vadose zone of a volcanic ash soil located in Fuchu (western suburb of Tokyo) representative of a temperate monsoon climate. The necessary soil properties to operate the model were obtained from both field and laboratory experiments while climatic data used for the meteorological submodels were extracted from a nearby weather station. A sensitivity and uncertainty analysis leveraging a Monte Carlo method was conducted to identify the soil hydraulic parameters critical to precise and reliable θv, vapor flow, and heat flow simulations. The temporal dynamics of simulated θv and Tsoil in the vadose zone of Andosol during the 365-day period were consistent with the values monitored under field condition demonstrating overall great performance of HYDRUS-1D. Specifying soil layer-specific hydraulic parameters significantly improved the goodness of fits between predicted and measured θv compared to the default simulation in which the surface soil layer was employed for the entire depth assuming a homogeneous profile. Water vapor influence on the total water flux and Tsoil dynamics was negligible during the whole period. Although the magnitudes of sensitivity and the contributions of soil hydraulic parameters to the water flow varied with soil profile and soil water regime, the foremost proportions of uncertainties were from the parameters w2, α2 and n2. While all remaining soil hydraulic parameters significantly contributed to substantial change in the predicted θv, their overall influence was relatively small. The HYDRUS-1D model can be used as effective tool for predicting θv and Tsoil in the vadose zone of Andosols in temperate monsoon environments for decision supporting in agriculture and other sectors such as to optimize water, crop yield and quality. The performance of the model can be greatly increased by setting soil layer-specific hydraulic parameters and focusing on the calibration of w2, α2 and n2.

Maintaining soil water content within the readily available range is optimum concerning crop production and water use. In this regard, the continuous monitoring of soil water content is a crucial element for identifying the key parameters for sustainable agricultural water management. In this study, volumetric soil water content (θv) was monitored and analyzed in a bare soil agricultural field (Sakaecho experimental field of Tokyo University of Agriculture and Technology). The field consisting of volcanic ash soils was monitored from June 2016 to September 2017 using capacitance sensor (CS). The optimal range of readily available water for plant uptake was defined as the range between field capacity and depletion level (θDep). These values were determined from the soil water potentials and θv values measured using pressure plate and soil cores, respectively. The 16-month period monitoring result revealed substantial temporal variability in θv in response to rainfall, evaporation and deep percolation. The monitored values of θv were above the θDep (0.307 cm3 cm−3 measured at suction (pF) value of 3.0) throughout the monitoring period for the 10–20 and 20–30 cm soil layers. In contrast, for the surface soil (0–10 cm), the θv fell below θDep for 27% of the monitoring period despite the high rainfall during those periods owing to high evaporation and deep percolation. The below θDep results for the surface soil suggests the need to conduct continuous θv monitoring, to support decision for planning efficient irrigation water management to avoid yield loss of shallow-rooted crops and deep-rooted crops at their earliest growth stages as well as quality reductions due to moisture stresses.

Summary The objective of this study was to develop, verify, and validate a new GIS-based model for simulating the fate and transport of rice pesticides in river basins. A plot scale model simulating pesticide fate and transport in rice paddies (PCPF-1) was incorporated into the Soil and Water Assessment Tool (SWAT) basin scale water and pollutant transport model. The new combined model, PCPF-1@SWAT model, was first used on some base-case scenarios to verify that the PCPF-1 algorithm and the routing of variables were correctly implemented. Next, the PCPF-1@SWAT model was calibrated and validated on the Sakura River basin (Ibaraki prefecture, Japan) using mefenacet concentrations measured during the rice growing season in 2008. The modeling procedures for simulating pesticide fate and transport in a Japanese river basin were demonstrated by providing model parameters related to hydrology, land use, pesticide fate, and rice field managements methods. The water flows predicted by the PCPF-1@SWAT model in the Sakura River basin were accurate throughout the whole simulation year, with R 2 and E NS statistics exceeding 0.74 and 0.71, respectively for daily flow. The use of different seepage rates had appreciable influence on the simulations. High seepage rates gave a slight overestimation of the predicted base flow during the rice growing period, whereas the base flow predictions using lower seepage rates were comparable to measured data. The PCPF-1@SWAT model successfully simulated the fate and transport of mefenacet in the Sakura River in which measured mefenacet concentrations peaked soon after the initial herbicide application in May, and decreased gradually during the months of June and July. Occasional major precipitation events caused the mefenacet concentration in streams to peak quickly due to a corresponding loss of mefenacet from paddy areas, and then rapidly decrease due to dilution by excess rainfall discharge. The simulation using a seepage rate of 0.12 cm day −1 had the most accurate prediction of mefenacet concentration in river water with an R 2 of 0.61 and an E NS of 0.65.