G. Van Drecht

Global river nutrient export: A scenario analysis of past and future trends

[1] An integrated modeling approach was used to connect socioeconomic factors and nutrient management to river export of nitrogen, phosphorus, silica and carbon based on an updated Global NEWS model. Past trends (1970–2000) and four future scenarios were analyzed. Differences among the scenarios for nutrient management in agriculture were a key factor affecting the magnitude and direction of change of future DIN river export. In contrast, connectivity and level of sewage treatment and P detergent use were more important for differences in DIP river export. Global particulate nutrient export was calculated to decrease for all scenarios, in part due to increases in dams for hydropower. Small changes in dissolved silica and dissolved organics were calculated for all scenarios at the global scale. Population changes were an important underlying factor for river export of all nutrients in all scenarios. Substantial regional differences were calculated for all nutrient elements and forms. South Asia alone accounted for over half of the global increase in DIN and DIP river export between 1970 and 2000 and in the subsequent 30 years under the Global Orchestration scenario (globally connected with reactive approach to environmental problems); DIN river export decreased in the Adapting Mosaic (globally connected with proactive approach) scenario by 2030, although DIP continued to increase. Risks for coastal eutrophication will likely continue to increase in many world regions for the foreseeable future due to both increases in magnitude and changes in nutrient ratios in river export.

Global nitrogen and phosphate in urban wastewater for the period 1970 to 2050

[1] This paper presents estimates for global N and P emissions from sewage for the period 1970–2050 for the four Millennium Ecosystem Assessment scenarios. Using country-specific projections for population and economic growth, urbanization, development of sewage systems, and wastewater treatment installations, a rapid increase in global sewage emissions is predicted, from 6.4 Tg of N and 1.3 Tg of P per year in 2000 to 12.0–15.5 Tg of N and 2.4–3.1 Tg of P per year in 2050. While North America (strong increase), Oceania (moderate increase), Europe (decrease), and North Asia (decrease) show contrasting developments, in the developing countries, sewage N and P discharge will likely increase by a factor of 2.5 to 3.5 between 2000 and 2050. This is a combined effect of increasing population, urbanization, and development of sewage systems. Even in optimistic scenarios for the development of wastewater treatment systems, global N and P flows are not likely to decline.

Global modeling of the fate of nitrogen from point and nonpoint sources in soils, groundwater, and surface water

[1] We present a global model that describes the fate of nitrogen (N) from point and nonpoint sources in the hydrological system up to the river mouths at the 0.5° by 0.5° spatial and annual temporal resolution. Estimates for point sources are based on population densities, per capita human N emissions, and data on sanitation coverage and wastewater treatment. For nonpoint sources, we use spatial information on land use, climate, hydrology, geology, and soils, combined with data on N inputs (fertilizers and animal manure, biological N fixation, and atmospheric deposition), and outputs (N removal in harvested agricultural products, ammonia emissions). Denitrification in the root zone and nitrate leaching to groundwater are calculated with a model that combines the effect of temperature, crop type, soil properties, and hydrological conditions. The nitrate concentration of the outflow for shallow and deep groundwater layers is based on historical inputs of fertilizer N and the effects of residence time and denitrification. In-stream N retention is based on a global estimate of 30% of the N discharged to surface water. Calculated and reported total N concentrations of discharge near the river outlet agree fairly well. However, our model systematically overestimates total N concentrations for river basins with mean annual temperature >0°C.

Exploring changes in river nitrogen export to the world's oceans

[1] Anthropogenic disturbance of river nutrient loads and export to coastal marine systems is a major global problem affecting water quality and biodiversity. Nitrogen is the major nutrient in rivers. On the basis of projections for food production and wastewater effluents, the global river N flux to coastal marine systems is shown to increase by 13% in the coming 3 decades. While the river N flux will grow by about 10% in North America and Oceania and will decrease in Europe, a 27% increase is projected for developing countries, which is a continuation of the trend observed in the past decades. This is a consequence of increasing nitrogen inputs to surface water associated with urbanization, sanitation, development of sewerage systems, and lagging wastewater treatment, as well as increasing food production and associated inputs of N fertilizer, animal manure, atmospheric N deposition, and biological N fixation in agricultural systems. Growing river N loads will lead to increased incidence of problems associated with eutrophication in coastal seas.

Mapping contemporary global cropland and grassland distributions on a 5 x 5 minute resolution

This paper describes the development of agricultural land cover maps with 5 × 5 minute resolution based on satellite data and agricultural statistics from the Food and Agriculture Organization (FAO) for the period 1990–2000. Consistency with the FAO data allows for reconstructing past changes and developing scenarios for future changes in land cover. Two base maps were used: (1) the International Geosphere–Biosphere Programme (IGBP) map based on DISCover data using the IGBP classification; (2) the Global Land Cover (GLC) map based on the Global Land Cover 2000 VEGA2000 data. The underlying DISCover data from the seasonal land cover regions were used to allocate the areas of cropland and grassland for the IGBP map. For the GLC map no such data were available, so a trial and error approach was used. While neither of the two base maps completely matched the FAO country data, combination of the IGBP and GLC maps resulted in a satisfactory match with FAO data for all countries. Apart from noise in the data, interpretation problems and uncertainties in the ancillary data used in interpretation, a major problem in allocating grasslands is the broad definition used by FAO, allowing for important differences between countries in the type of grassland included in the statistics. Comparison with Holdridge life zones showed that a large part (∼40%) of global grassland occurs in semi deserts, deserts and sub-polar tundras, regions with unfavorable climates with low productivity and low carrying capacity. However, the distribution of grasslands over life zones varies widely in different parts of the world. Cropland occurs in more favorable climates.

Global pollution of surface waters from point and nonpoint sources of nitrogen.

Global 0.5- by 0.5-degree resolution estimates are presented on the fate of nitrogen (N) stemming from point and nonpoint sources, including plant uptake, denitrification, leaching from the rooting zone, rapid flow through shallow groundwater, and slow flow through deep groundwater to riverine systems. Historical N inputs are used to describe the N flows in groundwater. For nonpoint N sources (agricultural and natural ecosystems), calculations are based on local hydrology, climate, geology, soils, climate and land use combined with data for 1995 on crop production, N inputs from N fertilizers and animal manure, and estimates for ammonia emissions, biological N fixation, and N deposition. For point sources, our estimates are based on population densities and human N emissions, sanitation, and treatment. The results provide a first insight into the magnitude of the N losses from soil-plant systems and point sources in various parts of the world, and the fate of N during transport in atmosphere, groundwater, and surface water. The contribution to the river N load by anthropogenic N pollution is dominant in many river basins in Europe, Asia, and North Africa. Our model results explain much of the variation in measured N export from different world river basins.

Surface N balances and reactive N loss to the environment from global intensive agricultural production systems for the period 1970-2030

Data for the historical years 1970 and 1995 and the FAO-Agriculture Towards 2030 projection are used to calculate N inputs (N fertilizer, animal manure, biological N fixation and atmospheric deposition) and the N export from the field in harvested crops and grass and grass consumption by grazing animals. In most industrialized countries we see a gradual increase of the overall N recovery of the intensive agricultural production systems over the whole 1970–2030 period. In contrast, low N input systems in many developing countries sustained low crop yields for many years but at the cost of soil fertility by depleting soil nutrient pools. In most developing countries the N recovery will increase in the coming decades by increasing efficiencies of N use in both crop and livestock production systems. The surface balance surplus of N is lost from the agricultural system via different pathways, including NH3 volatilization, denitrification, N2O and NO emissions, and nitrate leaching from the root zone. Global NH3-N emissions from fertilizer and animal manure application and stored manure increased from 18 to 34 Tg·yr−1 between 1970 and 1995, and will further increase to 44 Tg·yr−1 in 2030. Similar developments are seen for N2O-N (2.0 Tg·yr−1 in 1970, 2.7 Tg·yr−1 in 1995 and 3.5 Tg·yr−1 in 2030) and NO-N emissions (1.1 Tg·yr−1 in 1970,1.5 Tg·yr−1 in 1995 and 2.0 Tg·yr−1 in 2030).

European-scale modelling of groundwater denitrification and associated N2O production

This paper presents a spatially explicit model for simulating the fate of nitrogen (N) in soil and groundwater and nitrous oxide (N(2)O) production in groundwater with a 1 km resolution at the European scale. The results show large heterogeneity of nitrate outflow from groundwater to surface water and production of N(2)O. This heterogeneity is the result of variability in agricultural and hydrological systems. Large parts of Europe have no groundwater aquifers and short travel times from soil to surface water. In these regions no groundwater denitrification and N(2)O production is expected. Predicted N leaching (16% of the N inputs) and N(2)O emissions (0.014% of N leaching) are much less than the IPCC default leaching rate and combined emission factor for groundwater and riparian zones, respectively.