Nitrogen (N) pollution is a global environmental problem that has greatly increased the risks of both the eutrophication of surface waters and contamination of ground waters. The majority of N pollution mainly comes from agricultural fields, in particular during rice growing seasons. In recent years, a gradual shift from the transplanting rice cultivation method to the direct seeding method has occurred, which results in different water and N losses from paddy fields and leads to distinct impacts on water environments. The N transport and transformations in an experimental direct-seeded-rice (DSR) field in the Taihu Lake Basin of east China were observed during two consecutive seasons, and simulated using Hydrus-1D model. The observed crop N uptake, ammonia volatilization (AV), N concentrations in soil, and N leaching were used to calibrate and validate the model parameters. The two most important inputs of N, i.e., fertilization and mineralization, were considered in the simulations with 220 and 145.5kgha-1 in 2008 and 220 and 147.8kgha-1 in 2009, respectively. Ammonia volatilization and nitrate denitrification were the two dominant pathways of N loss, accounting for about 16.0% and 38.8% of the total N input (TNI), respectively. Both nitrification and denitrification processes mainly occurred in the root zone. N leaching at 60 and 120cm depths accounted for about 6.8% and 2.7% of TNI, respectively. The crop N uptake was 32.1% and 30.8% of TNI during the 2008 and 2009 seasons, respectively, and ammonium was the predominant form (74% of the total N uptake on average). Simulated N concentrations and fluxes in soil matched well with the corresponding observed data. Hydrus-1D could simulate the N transport and transformations in the DSR field, and could thus be a good tool for designing optimal fertilizer management practices in the future.

In the recent decade, increasing costs of labor, water, and fertilizers around the world led to a change in the method of crop establishment from traditional transplanted rice (TPR) to direct-seeded rice (DSR) and to a substantial rise in the DSR-managed area. Since water management in areas with DSR is quite different from those with TPR, vertical water movement and water and nutrient losses during the crop season may be different as well. Water flow and water losses in a DSR field in the Taihu Lake Basin of east China were monitored and evaluated using Hydrus-1D during two seasons with different rainfalls and irrigation managements. While during the 2008 season, irrigation accounted for 57% of the total water input (TWI), during the 2009 season, it accounted for only 32%. Due to large rainfall during the wet, 2009 rice season, surface runoff accounted for about 17.0% of TWI. During the much drier 2008 rice season with higher irrigation inputs, surface runoff (4.6% of TWI) could be controlled much better. Modeled evapotranspiration during the 2008 and 2009 seasons accounted for 54.6% and 44.6% of TWIs, respectively. Measured and simulated results indicate that water leaching (approximately 42.7% and 34.9% of TWIs in the 2008 and 2009 seasons, respectively) was the main path of water loss from the DSR fields, which implies that frequent irrigation increases water leaching. The plough sole layer played a major buffering role for water flow during both dry and wet seasons. Water productivities evaluated from TWIs during the 2008 and 2009 seasons were 0.71 and 0.59kg/m3, respectively; they were 1.30 and 1.33kg/m3 when evaluated from modeled evapotranspiration fluxes. Pressure heads and vertical fluxes simulated using Hydrus-1D matched measured data well. The Hydrus-1D can be used to simulate water flow and water balance in the DSR fields. © 2014 Elsevier B.V.

As an important part of a transition zone surrounding a lake, lakeshore plays a critical role in connecting hydrology and biochemistry between surface water and groundwater. The shape, slope, subsurface features, and seepage face of a lakeside slope have been reported to affect water and nutrient exchange and consequently the water quality of near-shore lake water. Soil tank experiments and Hydrus-2D model simulations were conducted to improve our understanding of the influence of slope revetment materials (SRMs) on water flow and solute transport in a lakeshore zone. The low hydraulic conductivity of SRMs affected flow patterns in the lakeshore zone and resulted in a local increase of the groundwater table near the slope face. Water and solute flux distributions on the slope face under bare-slope conditions followed an exponential function. Fluxes were concentrated within a narrow portion of the slope surface near the intersection point between the lake water level and the slope face. Surface pollutants (for example from fishponds and paddy fields surrounding a lake) were transported into the lake along shallow groundwater through both unsaturated and saturated zones. The SRMs on the slope face affected the ratio of water and solute fluxes in the unsaturated zone, increasing along with the decline of the hydraulic conductivity of SRMs. Furthermore, as the hydraulic conductivity of SRMs decreased, the retention time and the potential for oxygen reduction correspondingly increased, which affected the nitrogen transport and transformations in the lakeshore zone. Simulated and experimental results indicate that if concrete along the shoreline of Lake Taihu is replaced with a relatively high-conductivity lime or the slope is left bare, water fluxes will increase less than solute fluxes, which will rise significantly, in particular in the unsaturated zone and along the seepage face. On the other hand, the largest water and solute fluxes along the shoreline for the bare and lime-slope conditions will be located higher at the slope than for the concrete-slope conditions. Hydrus-2D provided a good description of complicated hydrodynamic and solute transport/transformation conditions in the lakeshore zone.