6,201 research outputs found
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Evapotranspiration Mapping for Forest Management in California's Sierra Nevada
We assessed the response of densely forested watersheds with little apparent annual water limitation to forest disturbance
and climate variability, by studying how past wildfires changed forest evapotranspiration, and what past evapotranspiration
patterns imply for the availability of subsurface water storage for drought resistance. We determined annual spatial patterns
of evapotranspiration using a top-down statistical model, correlating measured annual evapotranspiration from eddycovariance
towers across California with NDVI (Normalized Difference Vegetation Index) measured by satellite, and with
annual precipitation. The study area was the Yuba and American River watersheds, two densely forested watersheds in the
northern Sierra Nevada. Wildfires in the 1985-2015 period resulted in significant post-fire reductions in evapotranspiration
for at least 5 years, and in some cases for more than 20 years. The levels of biomass removed in medium-intensity fires (25-
75% basal area loss), similar to magnitudes expected from forest treatments for fuels reduction and forest health, reduced
evapotranspiration by as much 150-200 mm yr-1 for the first 5 years. Rates of recovery in post-wildfire evapotranspiration
confirm the need for follow-up forest treatments at intervals of 5-20 years to sustain lower evapotranspiration, depending
on local landscape attributes and interannual climate. Using the metric of cumulative precipitation minus evapotranspiration
(P-ET) during multi-year dry periods, we found that forests in the study area showed little evidence of moisture stress
during the 1985-2018 period of our analysis, owing to relatively small reliance on interannual subsurface water storage to
meet dry-year evapotranspiration needs of vegetation. However, more-severe or sustained drought periods will push some
lower-elevation forests in the area studied toward the cumulative P-ET thresholds previously associated with widespread
forest mortality in the southern Sierra Nevada
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Near-axis subsidence rates, hydrothermal circulation, and thermal structure of mid-ocean ridge crests
We systematically investigated near-axis subsidence on the ridge flanks of intermediate and fast spreading mid-ocean ridges using bathymetric data from well-surveyed portions of the Southeast Indian Ridge (spreading at 72-76 mm/yr), the northern East Pacific Rise (91-96 mm/yr), and the southern East Pacific Rise (144 mm/yr). In all three regions, the mean subsidence rate of young (< 1 - 1.4 Ma) seafloor is less than 220 m/m.y1/2. The distribution of individual estimates shows a distinct peak at 180 - 220 m/m.y.1/2 with few profiles having subsidence rates greater than 275 m/m.y.1/2. The observed subsidence rates are significantly lower than both the worldwide average (~350 m/m.y.1/2) and subsidence rates observed for older lithosphere at the same ridge segments. Intense hydrothermal circulation at the ridge axis can result in low subsidence rates on the adjacent ridge flanks provided the vigorous flow is confined to the immediate vicinity (< ~5 km) of the axis. According to our model, the extremely vigorous hydrothermal circulation ceases off-axis, and conductive heat flow becomes a primary mechanism of vertical heat transport on the ridge flanks. The very low geothermal gradient within the cooled portion of the uppermost lithosphere retards conductive cooling, and the cooled area needs to be heated from below before a geothermal gradient can be established which permits significant heat to be conducted out of the lithosphere. As a result, subsidence in very young (~0.1 - 1 Ma) lithosphere is suppressed. A simple one-dimensional thermal model with a Nusselt number parameterization was used to estimate the effect of hydrothermal circulation. An upper layer with a high Nusselt number and the half-space that it overlies are initially at a temperature of 1300°C and the surface is maintained at 0°C. After 0.1 m.y. of cooling (about 3.5 to 7 km from the spreading axis), the Nusselt number of the top layer is set to 1 so that normal conduction is simulated in the cooled layer. We used an explicit finite difference method to solve for the temporal changes in temperature with depth. This model produces subsidence rates in the range that we observe for Nu in the range of 15-30. Isotherms resulting from the modeling imply rapid lithospheric thickening very near the axis, which is incompatible with most current models for the formation of the bathymetric axial high observed at fast spreading mid-ocean ridges
Blazar Counterparts for 3EG Sources at -40 < decl. < 0: Pushing South Through the Bulge
Supplementing existing survey data with VLA observations, we have extended
ray counterpart identifications down to decl. = -40 using our
Figure of Merit approach. We find blazar counterparts for 70% of EGRET
sources above decl. = -40 away from the Galaxy. Spectroscopic
confirmation is in progress, and spectra for two dozen sources are
presented here. We find evidence that increased exposure in the bulge region
allowed EGRET to detect relatively faint blazars; a clear excess of non-blazar
objects in this region however argues for an additional (new) source class.Comment: ApJ accepte
How getting the right actors involved can help successful local policies spread across the country
One of the oft-cited advantages of the US system of government is that it gives state and local governments the ability to innovate in policymaking. But how can policies which are successful in one area be taken up elsewhere? In new research which examines the introduction by states of drug courts for non-violent offenders, James W. Douglas, Ringa Raudla, and Roger E. Hartley write that the spread of such policies is influenced by other governments who have already adopted the policy, state and local governments, and professional, national, and regional associations and lobby groups. Without the support of such key actors, they write, successful local policy innovations are unlikely to spread widely
Implementing tradable permits for sulfur oxides emissions : a case study in the South Coast Air Basin
Tradable emissions permits have important theoretical advantages over source-specific technical standards as a means for controlling pollution. Nonetheless, difficulties can arise in trying to implement an efficient, competitive market in emissions permits. Simple workable versions of the market concept may fail to achieve the competitive equilibrium, or to take account of important complexities in the relationship between the pattern of emissions and the geographical distribution of pollution. Existing regulatory law may severely limit the range of market opportunities that states can adopt.
This report examines the feasibility of tradable permits for controlling particulate sulfates in the Los Angeles airshed. Although the empirical part of the paper deals with a specific case, the methods developed have general applicability. Moreover, the particular market design that is proposed -- an auction process that involves no net revenue collection by the state -- has attractive features as a general model
Counts and Colors of Faint Galaxies in the U and R Bands
Ground-based counts and colors of faint galaxies in the U and R bands in one
field at high Galactic latitude are presented. Integrated over flux, a total of
1.2x10^5 sources per square degree are found to U=25.5 mag and 6.3x10^5 sources
per square degree to R=27 mag, with d log N/dm ~ 0.5 in the U band and d log
N/dm ~ 0.3 in the R band. Consistent with these number-magnitude curves,
sources become bluer with increasing magnitude to median U-R=0.6 mag at 24<U<25
mag and U-R=1.2 mag at 25 < R < 26 mag. Because the Lyman break redshifts into
the U band at z~3, at least 1.2x10^5 sources per square degree must be at
redshifts z<3. Measurable U-band fluxes of 73 percent of the 6.3x10^5 sources
per square degree suggest that the majority of these also lie at z < 3. These
results require an enormous space density of objects in any cosmological model.Comment: 17 pages, MNRAS in pres
Design and Performance of Arena Dam
Arena Dam is located in north-central Trinidad, West Indies. The dam forms a 35,000-acre-foot reservoir, which serves as the main raw water storage facility for Trinidad. The 1.6-million-cubic-yard earthfill embankment has a crest elevation 80 feet above the original streambed. The upstream-sloping core is composed of dispersive clay. The shells are composed of compacted fine sand and silty fine sand. The dam is founded on deep, stiff, fissured clay deposits interbedded with sand. The project is located approximately 12 miles from the El Pilar Fault, a major Caribbean fault with seismic activity comparable to that of the San Andreas Fault in the United States. Important design concerns included the dispersive clay core, residual strength properties of the foundation, embankment and control structure settlement, and the seismic environment. This paper discusses the design criteria and approach, and field performance data from foundation and embankment piezometers and survey monuments in the outlet conduit
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Thermal consequences of lithospheric extension: Pure and simple
Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two-dimensional conductive and advective heat transport through time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listric faults which sole into a sub-horizontal detachment. The pure shear results show that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall extension rate, a pure shear model with a narrow zone of extension can result in pressure release melting of the mantle long before low angle simple shear models. These results are compared with topographic and heat flow data from the northern Red Sea rift, a Neogene continental rift which is close to initiating seafloor spreading. The long wavelength topographic asymmetry across the Red Sea, which has been cited as evidence for simple shear extension of the lithosphere, is not matched by any of the models. The observed high heat flow anomalies in the Red Sea require a large component of pure shear lithospheric extension centered under the region of maximum crustal extension. In contrast, at the plate separation rate of the northern Red Sea, simple shear extension of the lithosphere along a shallow ( <30° ) dip detachment is ineffective in reproducing the observed heat flow anomalies. Only a narrowing region of pure shear extension can satisfy the width of the rift, and the peak heat flow values and generate pressure release meltin
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