Linjing Qiu

SWAT-based runoff and sediment simulation in a small watershed, the loessial hilly-gullied region of China: capabilities and challenges

Abstract Model calibration and validation are necessary before applying it for scenario assessment and watershed management. This study presented the methodology of evaluating Soil and Water Assessment Tool (SWAT) and tested the feasibility of SWAT on runoff and sediment load simulation in the Zhifanggou watershed located in hilly-gullied region of China. Daily runoff and sediment event data from 1998–2008 were used in this study; data from 1998–2003 were used for calibration and 2004–2008 for validation. The evaluation statistics for the daily runoff simulation showed that the model results were acceptable, but the model underestimated the runoff for high-flow events. For sediment load simulation, the SWAT performed well in capturing the trend of sediment load, while the model tended to underestimate sediment load during both the calibration and validation periods. The disparity between observed and simulated data most likely resulted from limitations of the existing SCS-CN and MUSLE methods in the model. This study indicated that the modification of SWAT components is needed to take rainfall intensity and its duration into account to enhance the model performance on peak flow and sediment load simulation during heavy rainfall season.

Potential impacts of climate change on carbon dynamics in a rain-fed agro-ecosystem on the Loess Plateau of China

Although many studies have been conducted on crop yield in rain-fed agriculture, the possible impacts of climate change on the carbon (C) dynamics of rain-fed rotation systems, particularly their direction and magnitude at the long-term scale, are still poorly understood. In this study, the sensitivity of C dynamics of a typical rotation system to elevated CO2 and changed temperature and precipitation were first tested using the CENTURY model, based on data collected from a 30-year field experiment of a corn-wheat-wheat-millet (CWWM) rotation system in the tableland of the Loess Plateau. The possible responses of crop biomass C and soil organic C (SOC) accumulation were then evaluated under scenarios representing the Representative Concentration Pathways (RCPs) 4.5 and 8.5. The results indicated that elevated CO2 and increased precipitation exerted positive effect on biomass C in CWWM rotation system, while increasing the temperature by 1°C, 2°C and 4°C had negative effects on biomass C due to opposite responses of corn and winter wheat to warming. SOC accumulation was enhanced by increased CO2 concentration and precipitation but impaired by increased temperature. Under future RCP scenarios with dynamic CO2, the biomass C of corn exhibited decrease during the period of 2046-2075 under RCP4.5 and the period of 2016-2075 under RCP8.5 due to reduced precipitation and a warmer climate. In contrast, winter wheat would benefit from increased CO2 and temperature and was projected to have larger biomass C under both RCP scenarios. Although the climate condition had large differences between RCP4.5 and RCP8.5, the projected SOC had similar trends under two scenarios due to CO2 fertilizer effect and precipitation fluctuation. These results implied that crop biomass C and SOC accumulation in a warmer environment are strongly related to precipitation, and increase in field water storage should be emphasized in coping with future climate.

SWAT-DayCent coupler

Process-based numerical models in environmental science can help understand and quantify terrestrial material cycles in nature. However, the existing models usually focus on the cycles of one or more elements (e.g., water, carbon, or nitrogen). For example, hydrological models such as Soil and Water Assessment Tool (SWAT) focus on the water cycle and nutrient loadings at watershed scale, whereas biogeochemical models such as DayCent (i.e., daily CENTURY) emphasize carbon/nitrogen storage and fluxes of ecosystems at landscape scale. Therefore, using either one of the two categories of models is not enough for understanding/solving the current complex environmental issues that involve multiple aspects. Although use of both models (SWAT and DayCent) could be an expedient way toward treating the problem, creating separate model projects for a single area could be challenging and time consuming, and integration/analyses of model results have some limitations due to the non-uniformity of input spatial data between models. To overcome this issue, we developed an integrated model implementation coupler that aims to drive SWAT and DayCentthe two representative models in hydrology and biogeochemistry, respectivelyjust using a users SWAT project without the need of any extra efforts such as developing a framework or preparing input data for DayCent modeling. This software is easy to use and would be promising for conducting comprehensive environmental impact assessment involving hydrological and biogeochemical cycles at watershed scale. SWAT-DayCent Coupler was developed to drive both SWAT and DayCent simultaneously.The software can help users integrate both model results on the HRU mapping units.The software can be readily used for an existing SWAT project without extra efforts.

Spatiotemporal response of the water cycle to land use conversions in a typical hilly-gully basin on the Loess Plateau, China

The hydrological effects of the “Grain for Green” project (GFGP) on the Loess Plateau have been extensively debated due to the complexity of the water system and its multiple driving factors. The aim of this study was to investigate the response of the hydrological cycle to the GFGP measures based in a case study of the Yanhe Basin, a typical hilly–gully area on the Loess Plateau of China. First, we analyzed the land use and land cover (LULC) changes from 1990 to 2010. Then, we evaluated the effects of LULC changes and sloping land conversion on the main hydrological components in the basin using the Soil and Water Assessment Tool (SWAT). The results indicated that cropland exhibited a decreasing trend, declining from 40.2 % of the basin area in 1990 to 17.6 % in 2010, and that the woodland and grassland areas correspondingly increased. With the land use changes from 1990 to 2010, the water yield showed a decreasing trend which was mainly due to decrease in surface runoff. In contrast, evapotranspiration (ET) showed an increasing trend over the same period, resulting in a persistent decrease in soil water. The conversion of sloping cropland to grassland or woodland exerted negative effects on water yield and soil water. Compared with the land use condition in 2010, the negative effects were most evident where cropland with a slope ≥ 15 was converted to woodland, with decreases in surface runoff and soil water of 17.1 and 6.4 %, respectively. These results suggest that the expansive reforestation on sloping land in the loess hilly–gully region decreased water yield and increased ET, resulting in reduced soil water. The results of this study can be used to support sustainable land use planning and water resource management on the Loess Plateau in China.

Nutrient stoichiometry of three plant species under a natural nutrient gradient of a semiarid small watershed

Abstract Current understanding of plant nutrient stoichiometry mainly focus on C, N, and P and in humid and tropic regions. This study was conducted to investigate nutrient stoichiometry in a semaird small watershed. Three dominant plants, Medicago sativa, Stipa bungeana, and Artemisia sacrorum, sampled along a natural nutrient gradient, were analyzed with respect to plant nutrient stoichiometry (N:P, N:K, and K:P). The results showed that nutrient stoichiometry varied greatly between plants and within the same plant at the small watershed scale. At the natural nutrient gradient, N:P of the N limited plant is mainly determined by plant N, while that of P or P+N limited plants were dominated by plant P. The N:K and K:P of both N limited and P or N+P limited plants were mainly controlled by plant K with weak relations to nutrient limitations. Aboveground biomass directly influences nutrient stoichiometry, while extractable soil nutrients exert their impacts on stoichiometry through aboveground biomass.

Bioenergy production and environmental impacts

Compared with the conventional fossil fuel, bioenergy has obvious advantages due to its renewability and large quantity, and thus plays a crucial role in helping defend the energy security. However, the bioenergy development may potentially cause serious environmental alterations, which remain unclear. The study summarizes the environmental impacts of bioenergy production based on the compilation and published data. Our analysis shows that more and more attention is being paid to the environmental protection as the development of bioenergy, and among the influencing terms of bioenergy production, water issues (i.e., water quantity and quality) gain the greatest concern, whereas the least attention has been given to soil erosion. Although we recognize that the bioenergy production can indeed exert negative effects on the environment in terms of water quantity and quality, greenhouse gas emissions, biodiversity and soil organic carbon, and soil erosion, the adverse impacts varied greatly depending on biomass types, land locations, and management practices. Identifying the reasonable cultivation locations, appropriate bioenergy crop types, and optimal management practices can be beneficial to environment and sustainable development of bioenergy. In this field, Chinese bioenergy production has lagged behind and does not match its rising energy consumption, but it has a great potential of and demand for biomass-based energy especially under its urbanization, in spite of the negative environmental impacts. Therefore, this article is expected to serve as a reference and guideline on what has been done in the bioenergy-oriented countries that might stimulate development of more effective and environmentally sound guidelines for promoting bioenergy production in China and other developing countries as well.