42 research outputs found
High energy parton-parton amplitudes from lattice QCD and the stochastic vacuum model
Making use of the gluon gauge-invariant two-point correlation function,
recently determined by numerical simulation on the lattice in the quenched
approximation and the stochastic vacuum model, we calculate the elementary
(parton-parton) amplitudes in both impact-parameter and momentum transfer
spaces. The results are compared with those obtained from the Kr\"{a}mer and
Dosch ansatz for the correlators. Our main conclusion is that the divergences
in the correlations functions suggested by the lattice calculations do not
affect substantially the elementary amplitudes. Phenomenological and
semiempirical information presently available on elementary amplitudes is also
referred to and is critically discussed in connection with some theoretical
issues.Comment: Text with 11 pages in LaTeX (twocolumn form), 10 figures in
PostScript (psfig.tex used). Replaced with changes, Fig.1 modified, two
references added, some points clarified, various typos corrected. Version to
appear in Phys. Rev.
Changing climate both increases and decreases European river floods
Climate change has led to concerns about increasing river floods resulting from the greater water-holding capacity of a warmer atmosphere. These concerns are reinforced by evidence of increasing economic losses associated with flooding in many parts of the world, including Europe. Any changes in river floods would have lasting implications for the design of flood protection measures and flood risk zoning. However, existing studies have been unable to identify a consistent continental-scale climatic-change signal in flood discharge observations in Europe, because of the limited spatial coverage and number of hydrometric stations. Here we demonstrate clear regional patterns of both increases and decreases in observed river flood discharges in the past five decades in Europe, which are manifestations of a changing climate. Our results—arising from the most complete database of European flooding so far—suggest that: increasing autumn and winter rainfall has resulted in increasing floods in northwestern Europe; decreasing precipitation and increasing evaporation have led to decreasing floods in medium and large catchments in southern Europe; and decreasing snow cover and snowmelt, resulting from warmer temperatures, have led to decreasing floods in eastern Europe. Regional flood discharge trends in Europe range from an increase of about 11 per cent per decade to a decrease of 23 per cent. Notwithstanding the spatial and temporal heterogeneity of the observational record, the flood changes identified here are broadly consistent with climate model projections for the next century, suggesting that climate-driven changes are already happening and supporting calls for the consideration of climate change in flood risk management
The effect of soil moisture anomalies on maize yield in Germany
Crop models routinely use meteorological variations to estimate
crop yield. Soil moisture, however, is the primary source of water for plant
growth. The aim of this study is to investigate the intraseasonal
predictability of soil moisture to estimate silage maize yield in Germany. We
also evaluate how approaches considering soil moisture perform compare
to those using only meteorological variables. Silage maize is one of the most
widely cultivated crops in Germany because it is used as a main biomass
supplier for energy production in the course of the German Energiewende (energy transition). Reduced form fixed effect panel models are employed to investigate the relationships in this study. These models are estimated for each month of the growing season to gain insights into the time-varying effects of soil moisture and meteorological variables. Temperature, precipitation, and
potential evapotranspiration are used as meteorological variables. Soil
moisture is transformed into anomalies which provide a measure for the
interannual variation within each month. The main result of this study is
that soil moisture anomalies have predictive skills which vary in magnitude
and direction depending on the month. For instance, dry soil moisture
anomalies in August and September reduce silage maize yield more than
10 %, other factors being equal. In contrast, dry anomalies in May increase
crop yield up to 7 % because absolute soil water content is higher in May
compared to August due to its seasonality. With respect to the meteorological
terms, models using both temperature and precipitation have higher
predictability than models using only one meteorological variable. Also,
models employing only temperature exhibit elevated effects