Integrated assessment of land-to-river Cd fluxes and riverine Cd loads using SWAT-HM to guide management strategies.

In 2011, China invested US$9.8 billion to combat the severe heavy metal pollution in the Xiang River basin (XRB), aiming to reduce 50% of the 2008 industrial metal emissions by 2015. However, river pollution mitigation requires a holistic accounting of both point and diffuse sources, yet the detailed land-to-river metal fluxes in the XRB remain unclear. Here, by combining emissions inventories with the SWAT-HM model, we quantified the land-to-river cadmium (Cd) fluxes and riverine Cd loads across the XRB from 2000 to 2015. The model was validated against long-term historical observations of monthly streamflow and sediment load and Cd concentrations at 42, 11, and 10 gauges, respectively. The analysis of the simulation results showed that the soil erosion flux dominated the Cd exports (23.56-80.14 Mg yr-1). The industrial point flux decreased by 85.5% from 20.84 Mg in 2000 to 3.02 Mg in 2015. Of all the Cd inputs, approximately 54.9% (37.40 Mg yr-1) was finally drained into Dongting Lake; the remaining 45.1% (30.79 Mg yr-1) was deposited within the XRB, increasing the Cd concentration in riverbed sediment. Furthermore, in XRB's 5-order river network, the Cd concentrations in small streams (1st order and 2nd order) showed larger variability due to their low dilution capacity and intense Cd inputs. Our findings highlight the need for multi-path transport modeling to guide future management strategies and better monitoring schemes to restore the small polluted streams.

Integrated assessment of climate change impacts on multiple ecosystem services in Western Switzerland.

Climate change can affect the provision of ecosystem services in various ways. In this study, we provide an integrated assessment of climate change impacts on ecosystem services, considering uncertainties in both climate projection and model parameterization. The SWAT model was used to evaluate the impacts on water regulation, freshwater, food, and erosion regulation services for the Broye catchment in Western Switzerland. Downscaled EURO-CORDEX projections were used for three periods of thirty years: base climate (1986-2015), near future (2028-2057), and far future (2070-2099). Results reveal that in the far future, low flow is likely to decrease in summer by 77% and increase in winter by 65%, while peak flow may decrease in summer by 19% and increase in winter by 26%. Reduction in summer precipitation reduces nitrate leaching by 25%; however, nitrate concentrations are projected to increase by 14% due to reduced dilution. An increase in winter precipitation increases nitrate leaching by 44%, leading to an increase of nitrate concentration by 11% despite increasing discharge and dilution. Yields of maize and winter wheat are projected to increase in the near future but decrease in the far future because of increasing water and nutrient stress. Average grassland productivity is projected to benefit from climate change in both future periods due to the extended growing season. This increase in productivity benefits erosion regulation as better soil cover helps to decrease soil loss in winter by 5% in the far future. We conclude that water regulation, freshwater and food services will be negatively affected by climate change. Hence, agricultural management needs to be adapted to reduce negative impacts of climate change on ecosystem services and to utilize emerging production potentials. Our findings highlight the need for further studies of potentials to improve nutrient and water management under future climate conditions.

Discharge permit market and farm management nexus: an approach for eutrophication control in small basins with low-income farmers.

The economic concerns of low-income farmers are barriers to nutrient abatement policies for eutrophication control in surface waters. This study brings up a perspective that focuses on integrating multiple-pollutant discharge permit markets with farm management practices. This aims to identify a more economically motivated waste load allocation (WLA) for non-point sources (NPS). For this purpose, we chose the small basin of Zrebar Lake in western Iran and used the soil and water assessment tool (SWAT) for modeling. The export coefficients (ECs), effectiveness of best management practices (BMPs), and crop yields were calculated by using this software. These variables show that low-income farmers can hardly afford to invest in BMPs in a typical WLA. Conversely, a discharge permit market presents a more cost-effective solution. This method saves 64% in total abatement costs and motivates farmers by offering economic benefits. A market analysis revealed that nitrogen permits mostly cover the trades with the optimal price ranging from $6 to $30 per kilogram. However, phosphorous permits are limited for trading, and their price exceeds $60 per kilogram. This approach also emphasizes the establishment of a regional institution for market monitoring, dynamic pricing, fair fund reallocation, giving information to participants, and ensuring their income. By these sets of strategies, a WLA on the brink of failure can turn into a cost-effective and sustainable policy for eutrophication control in small basins.

Global assessment of nitrogen losses and trade-offs with yields from major crop cultivations.

Agricultural application of reactive nitrogen (N) for fertilization is a cause of massive negative environmental problems on a global scale. However, spatially explicit and crop-specific information on global N losses into the environment and knowledge of trade-offs between N losses and crop yields are largely lacking. We use a crop growth model, Python-based Environmental Policy Integrated Climate (PEPIC), to determine global N losses from three major food crops: maize, rice, and wheat. Simulated total N losses into the environment (including water and atmosphere) are 44TgNyr-1. Two thirds of these, or 29TgNyr-1, are losses to water alone. Rice accounts for the highest N losses, followed by wheat and maize. The N loss intensity (NLI), defined as N losses per unit of yield, is used to address trade-offs between N losses and crop yields. The NLI presents high variation among different countries, indicating diverse N losses to produce the same amount of yields. Simulations of mitigation scenarios indicate that redistributing global N inputs and improving N management could significantly abate N losses and at the same time even increase yields without any additional total N inputs.

A global and spatially explicit assessment of climate change impacts on crop production and consumptive water use.

Food security and water scarcity have become two major concerns for future human's sustainable development, particularly in the context of climate change. Here we present a comprehensive assessment of climate change impacts on the production and water use of major cereal crops on a global scale with a spatial resolution of 30 arc-minutes for the 2030s (short term) and the 2090s (long term), respectively. Our findings show that impact uncertainties are higher on larger spatial scales (e.g., global and continental) but lower on smaller spatial scales (e.g., national and grid cell). Such patterns allow decision makers and investors to take adaptive measures without being puzzled by a highly uncertain future at the global level. Short-term gains in crop production from climate change are projected for many regions, particularly in African countries, but the gains will mostly vanish and turn to losses in the long run. Irrigation dependence in crop production is projected to increase in general. However, several water poor regions will rely less heavily on irrigation, conducive to alleviating regional water scarcity. The heterogeneity of spatial patterns and the non-linearity of temporal changes of the impacts call for site-specific adaptive measures with perspectives of reducing short- and long-term risks of future food and water security.

A water resources threshold and its implications for food security.

Cereal import has played a crucial role in compensating local water deficit. A quantitative account of water deficit and cereal import relations therefore is of significance for predicting future food import demand and formulating corresponding national and international policies. On the basis of data for countries in Asia and Africa, we estimated a water resources threshold with respect to cereal import. Below the threshold, the demand for cereal import increases exponentially with decreasing water resources. There appeared to be a declining trend in the threshold, from 2000 m3/(capita year) in the early 1980s to 1500 m3/(capita year) by the end of the 1990s. Until recently, most countries below the threshold were oil-rich and thus were able to afford cereal import. However, the next 30 yr may see many poor and populous countries dropping below the threshold in association with their rapid population growth and the depletion of fossil groundwater. Water deficit-induced food insecurity and starvation could intensify because cereal import may not be affordable for these countries.