1,530 research outputs found
AN IRRIGATION MODEL FOR MANAGEMENT OF LIMITED WATER SUPPLIES
A two-stage simulation/mathematical programming model is presented for determining the optimal intraseasonal allocation of irrigation water under conditions of limited water supply. The model is applied to a series of water shortage scenarios under both surface and center pivot irrigation. Economically efficient irrigation management is shown to involve the coordination of a number of managerial decisions, including irrigation scheduling, crop substitution, the adoption of improved irrigation labor practices, and idling land. The results indicate that significant opportunities exist for conserving water in the study area under both surface and center pivot irrigation.Crop Production/Industries, Resource /Energy Economics and Policy,
The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution
The stability properties of one-dimensional radiative shocks with a power-law
cooling function of the form are the main
subject of this work. The linear analysis originally presented by Chevalier &
Imamura, is thoroughfully reviewed for several values of the cooling index
and higher overtone modes. Consistently with previous results, it is
shown that the spectrum of the linear operator consists in a series of modes
with increasing oscillation frequency. For each mode a critical value of the
cooling index, , can be defined so that modes with are unstable, while modes with
are stable. The perturbative analysis is complemented by several numerical
simulations to follow the time-dependent evolution of the system for different
values of . Particular attention is given to the comparison between
numerical and analytical results (during the early phases of the evolution) and
to the role played by different boundary conditions. It is shown that an
appropriate treatment of the lower boundary yields results that closely follow
the predicted linear behavior. During the nonlinear regime, the shock
oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle.
The modes of oscillations during this phase do not necessarily coincide with
those predicted by linear theory, but may be accounted for by mode-mode
coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical
Journa
Use of soil moisture information in yield models
There are no author-identified significant results in this report
The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variables
The temperatures of electrons and ions in the post-shock accretion region of
a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass
flow rates or for sufficiently weak magnetic fields. At lower mass flow rates
or in stronger magnetic fields, efficient cyclotron cooling will cool the
electrons faster than the electrons can cool the ions and a two-temperature
flow will result. Here we investigate the differences in polarized radiation
expected from mCV post-shock accretion columns modeled with one- and
two-temperature hydrodynamics. In an mCV model with one accretion region, a
magnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, along
with a relatively generic geometric orientation of the system, we find that in
the ultraviolet either a single linear polarization pulse per binary orbit or
two pulses per binary orbit can be expected, depending on the accretion column
hydrodynamic structure (one- or two-temperature) modeled. Under conditions
where the physical flow is two-temperature, one pulse per orbit is predicted
from a single accretion region where a one-temperature model predicts two
pulses. The intensity light curves show similar pulse behavior but there is
very little difference between the circular polarization predictions of one-
and two-temperature models. Such discrepancies indicate that it is important to
model some aspect of two-temperature flow in indirect imaging procedures, like
Stokes imaging, especially at the edges of extended accretion regions, were the
specific mass flow is low, and especially for ultraviolet data.Comment: Accepted for publication in Astrophysics & Space Scienc
Superdiffusion in a Model for Diffusion in a Molecularly Crowded Environment
We present a model for diffusion in a molecularly crowded environment. The
model consists of random barriers in percolation network. Random walks in the
presence of slowly moving barriers show normal diffusion for long times, but
anomalous diffusion at intermediate times. The effective exponents for square
distance versus time usually are below one at these intermediate times, but can
be also larger than one for high barrier concentrations. Thus we observe sub-
as well as super-diffusion in a crowded environment.Comment: 8 pages including 4 figure
The Effect of Man on Water Yield, Peak Runoff and Sedimentation
Man can alter several facets of the hydrologic cycle appreciably. Those facets most easily altered are related to vegetative cover and surface geometry either on the land or in the drainage system. Changes brought about by man affect surface runoff and base flow, timing and peak discharges of flood flows, and water yields. Cover changes and more efficient drainage systems increase sediment delivery to streams. This paper attempts to define evidence of changes, and to comment on our present understanding of the extent of changes
A platform for efficient, thiol-stable conjugation to albumin's native single accessible cysteine
Herein we report the use of bromomaleimides for the construction of stable albumin conjugates via conjugation to its native, single accessible, cysteine followed by hydrolysis. Advantages over the classical maleimide approach are highlighted in terms of quantitative hydrolysis and absence of undesirable retro-Michael deconjugation
Anomalous Diffusion Induced by Cristae Geometry in the Inner Mitochondrial Membrane
Diffusion of inner membrane proteins is a prerequisite for correct functionality of mitochondria. The complicated structure of tubular, vesicular or flat cristae and their small connections to the inner boundary membrane impose constraints on the mobility of proteins making their diffusion a very complicated process. Therefore we investigate the molecular transport along the main mitochondrial axis using highly accurate computational methods. Diffusion is modeled on a curvilinear surface reproducing the shape of mitochondrial inner membrane (IM). Monte Carlo simulations are carried out for topologies resembling both tubular and lamellar cristae, for a range of physiologically viable crista sizes and densities. Geometrical confinement induces up to several-fold reduction in apparent mobility. IM surface curvature per se generates transient anomalous diffusion (TAD), while finite and stable values of projected diffusion coefficients are recovered in a quasi-normal regime for short- and long-time limits. In both these cases, a simple area-scaling law is found sufficient to explain limiting diffusion coefficients for permeable cristae junctions, while asymmetric reduction of the junction permeability leads to strong but predictable variations in molecular motion rate. A geometry-based model is given as an illustration for the time-dependence of diffusivity when IM has tubular topology. Implications for experimental observations of diffusion along mitochondria using methods of optical microscopy are drawn out: a non-homogenous power law is proposed as a suitable approach to TAD. The data demonstrate that if not taken into account appropriately, geometrical effects lead to significant misinterpretation of molecular mobility measurements in cellular curvilinear membranes
The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars II. Final Results
We present the final results of a ROSAT PSPC program to study the soft X-ray
emission properties of a complete sample of low quasars. The main results
are: 1. There is no evidence for significant soft excess emission or excess
foreground absorption by cold gas in 22 of the 23 quasars. 2. The mean 0.2-2
keV continuum of quasars agrees remarkably well with an extrapolation of the
mean 1050-350A continuum recently determined by Zheng et al. (1996), indicating
that there is no steep soft component below 0.2 keV. 3. The occurrence of warm
absorbers in quasars is rather rare, in sharp contrast to lower luminosity AGN.
4. The strongest correlation found is between the spectral slope, alpha_x, and
the Hb FWHM. This remarkably strong correlation may result from a dependence of
alpha_x on L/L_Edd, as seen in Galactic black hole candidates. 5. There appears
to exist a distinct class of ``X-ray weak'' quasars. These may be quasars where
the direct X-ray source is obscured, and only scattered X-rays are observed. 6.
Thin accretion disk models cannot reproduce the observed optical to soft X-ray
spectral shape. An as yet unknown physical mechanism maintains a strong
correlation between the optical and soft X-ray emission. 7. The well known
difference in alpha_x between radio-loud and radio-quiet quasars may be due
only to their different Hb FWHM. 8. The agreement of the 21 cm and X-ray
columns implies that He in the diffuse H II component of the Galactic ISM is
ionized to He II or He III (shortened abstract).Comment: 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol.
447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8
figures available at http://physics.technion.ac.il/~laor/rosat/paper.p
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