Origins of the Ambient Solar Wind: Implications for Space Weather

The Sun's outer atmosphere is heated to temperatures of millions of degrees, and solar plasma flows out into interplanetary space at supersonic speeds. This paper reviews our current understanding of these interrelated problems: coronal heating and the acceleration of the ambient solar wind. We also discuss where the community stands in its ability to forecast how variations in the solar wind (i.e., fast and slow wind streams) impact the Earth. Although the last few decades have seen significant progress in observations and modeling, we still do not have a complete understanding of the relevant physical processes, nor do we have a quantitatively precise census of which coronal structures contribute to specific types of solar wind. Fast streams are known to be connected to the central regions of large coronal holes. Slow streams, however, appear to come from a wide range of sources, including streamers, pseudostreamers, coronal loops, active regions, and coronal hole boundaries. Complicating our understanding even more is the fact that processes such as turbulence, stream-stream interactions, and Coulomb collisions can make it difficult to unambiguously map a parcel measured at 1 AU back down to its coronal source. We also review recent progress -- in theoretical modeling, observational data analysis, and forecasting techniques that sit at the interface between data and theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue connected with a 2016 ISSI workshop on "The Scientific Foundations of Space Weather." 44 pages, 9 figure

Effect of the accuracy of spatial rainfall information on the modeling of water, sediment, and NO3-N loads at the watershed level

In a given watershed, the accuracy of models in predicting the hydrologic and erosion behavior depends, to a large extent, on the quality of the knowledge in respect of the spatial rainfall. The hydrologic and erosion aspects of rainfall are often discussed without due regard to any resulting improvement in watershed modeling. Thus, there is a real need for streamlining raingauge networks in order to reflect rainfall variability and its effect on the prediction of water, sediment and nutrient fluxes at the watershed scale. In this study, such an impact was analyzed using 9-year data collected at the outlets of two watersheds encompassing a range of climates, surface areas and environmental conditions. The Soil and Water Assessment Tool (SWAT) was applied using as input data that collected from 1 to 15 precipitation gauges per watershed. At both sites the highest densities of raingauges were used for SWAT calibration. The differences between the highest gauge concentration and lower concentrations used for the estimation of sediment loads led to the conclusion that a high gauge concentration is necessary. At both watersheds, predictions using rainfall records from the national service stations produced inaccurate estimations. This was probably because the gauge concentration was too sparse. Finally, the general applicability of these results is proposed by displaying the possibilities of extrapolation to other watersheds or models. 0 2005 Elsevier B.V.. All rights reserved

Predicting water, sediment and NO3-N loads under scenarios of land-use and management practices in a flat watershed

Changes in land-use or management practices may affect water outflow, sediment, nutrients and pesticides loads. Thus, there is an increasing demand for quantitative information at the watershed scale that would help decision makers or planners to take appropriate decisions. This paper evaluates by a modeling approach the impact of farming practices and land-use changes on water discharge, sediment and NO3-N loads at the outlet of a 51.29 km(2) watershed of central Iowa (Walnut Creek watershed). This intensively farmed (corn-soybean rotation) watershed is characterized by a flat topography with tiles and potholes. Nine scenarios of management practices (nitrogen application rates: increase of current rate by 20, 40%, decrease of current rate by 20, 40 and 60%; no tillage) and land-use changes (from corn-soybean rotation to winter wheat and pasture) were tested over a 30 yr simulated period. The selected model (Soil and Water Assessment Tool, SWAT) was first validated using observed flow, sediment and nutrient loads from 1991 to 1998. Scenarios of N application rates did not affect water and sediment annual budgets but did so for NO3-N loads. Lessening the N rate by 20, 40 and 60% in corn-soybean fields decreased mean NO3-N annual loads by 22, 50 and 95%, respectively, with greatest differences during late spring. On the other hand, increasing input N by 20 and 40% enhanced NO3-N loads by 25 and 49%, respectively. When replacing corn-soybean rotation by winter wheat, NO3-N loads increased in early fall, immediately after harvest. Pasture installation with or without fertilization lessened flow discharge, NO3-N and sediment delivery by 58, 97 and 50%, respectively. No-tillage practices did not significantly affect the water resource and sediment loads. Finally, such realistic predictions of the impact of farming systems scenarios over a long period are discussed regarding environmental processes involved

Field Method for Measuring Mobile/Immobile Water Content and Solute Transfer Rate Coefficient

Numerous field and laboratory studies have documented the occurrence of preferential transport of solutes due to a fraction of the soil water being immobile and not taking part in the transport process. Domain models have been developed that describe these processes, but before we can apply them routinely, we need methods for measuring the required model parameters, particularly the fraction of immobile water to total water θlm/θ and the exchange coefficient between the mobile and immobile domains, α. We developed a field method for measuring both θlm/θ and α. The method uses a sequence of conservative anionic tracers consisting of Br−, pentafluorobenzoate, o-trifluoromethylbenzoate, and 2,6-difluorobenzoate infiltrated with time through a tension infiltrometer. Previous studies have confirmed that these tracers have very similar transport properties in a wide range of soils. The method was applied to an undisturbed loam and a greenhouse soil as an initial test of the approach. Calculated θim/θ fractions averaged 0.69 and ranged from 0.25 to 0.98, while calculated α values averaged 0.0081 h−1 and ranged from 0.0030 to 0.021 h−1. These values compare well with values reported earlier by other investigators. The method is simple and allows routine measurement of transport properties of field soils. The method can also be used to validate the applicability of domain models to specific soils.This article is published as Jaynes, D. B., S. D. Logsdon, and R. Horton. "Field method for measuring mobile/immobile water content and solute transfer rate coefficient." Soil Science Society of America Journal 59, no. 2 (1995): 352-356. Doi: 10.2136/sssaj1995.03615995005900020012x. </p

Cover crop effects on nitrogen load in tile drainage from Walnut Creek Iowa using root zone water quality (RZWQ) model

Studies quantifying winter annual cover crop effects on water quality are mostly limited to short-term studies at the plot scale. Long-term studies scaling-up water quality effects of cover crops to the watershed scale provide more integrated spatial responses from the landscape. The objective of this research was to quantify N loads from artificial subsurface drainage (tile drains) in a subbasin of the Walnut Creek, Iowa (Story county) watershed using the hybrid RZWQ-DSSAT model for a maize (Zea mays L.)-soybean [Glycine max (L.) Merr.] and maize-maize-soybean rotations in all phases with and without a winter wheat (Triticum aestivum L.) cover crop during a 25-year period from 1981 to 2005. Simulated cover crop dry matter (DM) and N uptake averaged 1854 and 36 kg ha-1 in the spring in the maize-soybean phase of the 2-year rotation and 1895 and 36 kg ha-1 in the soybean-maize phase during 1981-2005. In the 3-year rotation, cover crop DM and N uptake averaged 2047 and 44 kg ha-1 in the maize-maize-soybean phase, 2039 and 43 kg ha-1 in the soybean-maize-maize phase, and 1963 and 43 kg ha-1 in the maize-soybean-maize phase during the same period. Annual N loads to tile drains averaged 29 kg ha-1 in the maize-soybean phase and 25 kg ha-1 in the soybean-maize phase compared to 21 and 20 kg ha-1 in the same phases with a cover crop. In the 3-year rotation, annual N loads averaged 46, 43, and 45 kg ha-1 in each phase of the rotation without a cover crop and 37, 35, and 35 kg ha-1 with a cover crop. These results indicate using a winter annual cover crop can reduce annual N loads to tile drains 20-28% in the 2-year rotation and 19-22% in the 3-year rotation at the watershed subbasin scale over a 25-year period.Subbasin scale Crop rotation Winter wheat cover crop Nitrate leaching