United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

Temperate forest health in an era of emerging megadisturbance.

Abstract: Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and uncharacteristically severe wildfire are resulting in forest mortality beyond the levels of 20th-century experience. Additional anthropogenic stressors, such as atmospheric pollution and invasive species, further weaken trees in some regions. Although continuing climate change will likely drive many areas of temperate forest toward large-scale transformations, management actions can help ease transitions and minimize losses of socially valued ecosystem services.

Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer

Abstract: [1] We develop a forecast model to reproduce the distribution of main shocks, aftershocks and surrounding seismicity observed during 1986–2003 in a 300 × 310 km area centered on the 1992 M = 7.3 Landers earthquake. To parse the catalog into frames with equal numbers of aftershocks, we animate seismicity in log time increments that lengthen after each main shock; this reveals aftershock zone migration, expansion, and densification. We implement a rate/state algorithm that incorporates the static stress transferred by each M ≥ 6 shock and then evolves. Coulomb stress changes amplify the background seismicity, so small stress changes produce large changes in seismicity rate in areas of high background seismicity. Similarly, seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates. Thus a key constituent of the model is the background seismicity rate, which we smooth from 1981 to 1986 seismicity. The mean correlation coefficient between observed and predicted M ≥ 1.4 shocks (the minimum magnitude of completeness) is 0.52 for 1986–2003 and 0.63 for 1992–2003; a control standard aftershock model yields 0.54 and 0.52 for the same periods. Four M ≥ 6.0 shocks struck during the test period; three are located at sites where the expected seismicity rate falls above the 92 percentile, and one is located above the 75 percentile. The model thus reproduces much, but certainly not all, of the observed spatial and temporal seismicity, from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades. Finally, we offer a M ≥ 5 earthquake forecast for 2005–2015, assigning probabilities to 324 10 × 10 km cells.

Groundwater depletion: A global problem

Abstract: In the past half-century, ready access to pumped wells has ushered in a worldwide “explosion” of groundwater development for municipal, industrial, and agricultural supplies. Globally, groundwater withdrawals total 750–800 km/year (Shah et al. 2000). Economic gains from groundwater use have been dramatic. However, in many places, groundwater reserves have been depleted to the extent that well yields have decreased, pumping costs have risen, water quality has deteriorated, aquatic ecosystems have been damaged, and land has irreversibly subsided. Groundwater depletion is the inevitable and natural consequence of withdrawing water from an aquifer. Theis (1940) showed that pumpage is initially derived from removal of water in storage, but over time is increasingly derived from decreased discharge and/or increased recharge. When a new equilibrium is reached, no additional water is removed from storage. In cases of fossil or compacting aquifers, where recharge is either unavailable or unable to refill drained pore spaces, depletion effectively constitutes permanent groundwater mining. In renewable aquifers, depletion is indicated by persistent and substantial head declines. Excessive groundwater depletion affects major regions of North Africa, the Middle East, South and Central Asia, North China, North America, and Australia, and localized areas throughout the world. Although the scope of the problem has not been quantified globally, on-going analysis by the senior author indicates that about 700–800 km of groundwater has been depleted from aquifers in the US during the 20th century. One of the best documented cases is the 450,000 km High Plains aquifer system in the central US, where the net amount of water removed from storage during the 20th century was more than 240 km—a reduction of about 6% of the predevelopment volume of water in storage (McGuire et al. 2003). In some of the most depleted areas, use of groundwater for irrigation has become impossible or cost prohibitive (Dennehy et al. 2002). In some cases, removing the most easily recoverable fresh groundwater leaves a residual with inferior water quality. This is due, in part, to induced leakage from the land surface, confining layers, or adjacent aquifers that contain saline or contaminated water. In coastal areas, where many of the world’s largest cities are located, the available volume of fresh groundwater is reduced by seawater intrusion and upconing, which in turn are caused by head declines in the aquifer. As depletion continues worldwide, its impacts worsen, portending the need for objective analysis of the problem and its possible solutions. This essay examines future options for evaluating and managing groundwater depletion in a changing physical and social landscape.