Simulation of streamflow and sediment with the soil and water assessment tool in a data scarce catchment in the three gorges region, china.

The Three Gorges Region in China is currently subject to a large-scale land use change, which was induced by the construction of the Three Gorges Dam on the Yangtze River. The relocation of towns, villages, and agricultural areas is expected to affect the water balance and increase erosion rates and sediment yields in the affected catchments. Hydrologic and water quality models are frequently used to assess the impact of land use changes on water resources. In this study, the eco-hydrological Soil and Water Assessment Tool (SWAT) model is applied to the Xiangxi Catchment in the Three Gorges Region. This paper presents the calibration and validation of streamflow and sediment loads at Xingshan gauging station. The calibration of daily streamflow resulted in a satisfactory fit of simulated and observed data, which is indicated by Nash-Sutcliffe efficiency (NSE) values of 0.69 and 0.67 for the calibration (1981-1986) and validation (1988-1993) periods, respectively. In contrast, the model was not able to simulate the monthly average sediment loads correctly, as indicated by very low NSE values of 0.47 (calibration) and 0.08 (validation). This might be due to inadequate representation of spatial rainfall variability by the available climate stations, insufficient input data, uncertainties in the model structure, or uncertainties in the observed sediment loads. The discussion of these possible reasons for the incorrect prediction of sediment loads by SWAT reveals the need for further research in the field of hydrological and water quality modeling in China.

A spatio-temporal analysis of environmental fate and transport processes of pesticides and their transformation products in agricultural landscapes dominated by subsurface drainage with SWAT.

Pesticides are detected in surface water and groundwater, endangering the environment. In lowland regions with subsurface drainage systems, drained depressions become hotspots for transport of pesticides and their transformation products (TPs). This study focuses on detailed modelling of the degradation and transport of pesticides with different physico-chemical properties. The objective is to analyse complex hydrological transport processes, to understand the temporal and spatial dynamics of the degradation and transport of pesticides. The ecohydrological model SWAT+ simulates hydrological processes as well as agricultural management and pesticide degradation, and can therefore be used to develop pesticide loss reduction strategies. This study focuses on modelling of three pesticides (pendimethalin, diflufenican, and flufenacet), and two TPs, flufenacet-oxalic acid (FOA) and flufenacet sulfonic acid (FESA). The study area is a 100-hectare farmland in the northern German lowlands of Schleswig-Holstein that is characterized by an spacious drainage network of 6.3 km and managed according to common conventional agricultural practice. SWAT+ modelled streamflow with very good agreement between observed and simulated data during calibration and validation. Regarding pesticides, the model performance for highly mobile substances is better than for non-mobile pesticides. While the transport of the moderately to very mobile substances via tile drains played an important role in both wet and dry conditions, no transport via tile drains was modelled for the highly sorptive and non-mobile pendimethalin. In conclusion, the model can reliably represent the degradation of moderately to very mobile pesticides in small-scale tile drainage-dominated catchments, as well as surface runoff-induced peak loads. However, it has weaknesses in accounting for the subsurface transport of non-mobile substances, which can lead to an underestimation of the subsequent delivery after precipitation events and thus underestimates the total load.

Evaluating the contribution of subsurface drainage to watershed water yield using SWAT+ with groundwater modeling.

Drainage outflow from artificial subsurface drains can be a significant contributor to watershed water yield in many humid regions of the world. Although many studies have undertaken to simulate hydrologic processes in drained watersheds, there is a need for a study that first, uses physically based spatially distributed modeling for both surface and subsurface processes; and second, quantifies the effect of surface and subsurface parameters on watershed drainage outflow. This study presents a modified version of the SWAT+ watershed model to address these objectives. The SWAT+ model includes the gwflow module, a new spatially distributed groundwater routine for calculating groundwater storage, groundwater head, and groundwater fluxes throughout the watershed using a grid cell approach, modified in this study to simulate the removal of groundwater by subsurface drains. The modeling approach is applied to the South Fork Watershed (583 km2), located in Iowa, USA, where most fields are drained artificially. The model is tested against measured streamflow, groundwater head at monitoring wells, and drainage outflow from a monitored subbasin. Sensitivity analysis is then applied to determine the land surface, subsurface, and drainage parameters that control subsurface drainage. Simulated drainage flow fractions (fraction of streamflow that originates from subsurface drainage) range from 0.37 to 0.54 during 2001-2012, with lower fractions occurring during years of high rainfall due to the increased volumes of surface runoff. Subsurface drainage comprises the vast majority of baseflow. Results indicate surface runoff and soil percolation parameters have the strongest effect on watershed-wide subsurface drainage rather than aquifer and drain properties, pointing to a holistic watershed approach to manage subsurface drainage. The modeling code presented herein can be used to simulate significant hydrologic fluxes in artificially drained watersheds worldwide.