But while tropical regions are experiencing increased rainfall, parts of Africa that have long struggled with drought are receiving even less rainfall. In a report published in 2011 in Climate Dynamics, scientists from the US Geological Survey (USGS) and the University of California, Santa Barbara, determined that in eastern Africa, a climate-change-related decrease in rainfall was putting 17.5 million people at risk. They suggested that the increased frequency of drought observed in eastern Africa, one of the world’s driest regions, over the past 20 years was likely to continue as long as global temperatures continued to rise.
Streamflow is more difficult to measure accurately and continuously than is stage. Streamflow for a gaging station is typically determined from an established , or rating curve. Individual streamflow measurements are made by USGS personnel at a gaging station by using standard procedures (Rantz and others, 1982); ideally, these measurements are made when the stage is not changing. A series of these measurements made over a range of flow conditions defines the rating curve, which is used to convert continuous measurements of stage to a continuous record of streamflow. Channel changes, resulting from scour, deposition, vegetation, or other processes, alter the stage-streamflow relation, so that streamflow measurements must be made routinely and continuously to ensure that the rating curve remains accurate.
A recent review of global rainfall led by researchers at the University of Adelaide and published in February last year, using data compiled from more than 8,000 weather-gauging stations around the world, concluded that for every degree increase in global atmospheric temperature, there was a seven per cent increase in extreme rainfall intensity. ‘If the global average temperature increases as expected by 3°C–5°C by the end of the 21st century, this could mean a very substantial increase in rainfall intensity,’ says Dr Seth Westra, the paper’s lead author.
The USGS and other agencies often refer to the percent chance of occurrence as an Annual Exceedance Probability or AEP. An AEP is always a fraction of one. So a 0.2 AEP flood has a 20% chance of occurring in any given year, and this corresponds to a 5-year recurrence-interval flood. Recurrence-interval terminology tends to be more understandable for flood intensity comparisons. However, AEP terminology reminds the observer that a rare flood does not reduce the chances of another rare flood within a short time period.
If trees are flooded by heavy rain or snow melt in late winter or early spring when the trees and shrubs are not actively growing, and the water recedes before growth begins, flooding usually is not a problem. Most tree species can withstand one to four months of flooding during the dormant season. However, when flooding occurs during the growing season, especially during warmer weather, one to two weeks of flooding can cause major, long-term damage to sensitive trees and shrubs, even death with some species. Other species can survive as long as three to five months in flooded situations.
The USGS and other agencies often refer to the percent chance of occurrence as an Annual Exceedance Probability or AEP. An AEP is always a fraction of one. So a 0.2 AEP flood has a 20% chance of occurring in any given year, and this corresponds to a 5-year recurrence-interval flood. Recurrence-interval terminology tends to be more understandable for flood intensity comparisons. However, AEP terminology reminds the observer that a rare flood does not reduce the chances of another rare flood within a short time period.
The data isn't going anywhere! "Short-Term" refers to the nature of the events for which we are collecting data, but the data is intended to be kept indefinitely...
Estimates of peak flows, which are outside the range of the established rating curve, may be made by an extrapolation of the rating curve to the peak stage. At some gaging stations, indirect methods of streamflow determination based on high-water marks, channel properties, and hydraulic principles may be used to obtain an independent estimate of discharge. These indirect methods generally require accurate field surveys to determine high-water marks, channel properties, and channel shape. The information obtained in the field is then processed using computer programs to determine the streamflow. Continued evaluation of these streamflow computations may result in some revision of previously determined peak flows.
Another factor to consider is the relation between the duration of the storm and the size of the stream basin in which the storm occurs. For example, a 100-year storm of 30-minutes duration in a 1-square-mile (mi2) basin will have a more significant effect on streamflow than the same storm in a 50-mi2 basin. Generally, streams with larger drainage areas require storms of longer duration for a significant increase in streamflow to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood.
Another factor to consider is the relation between the duration of the storm and the size of the stream basin in which the storm occurs. For example, a 100-year storm of 30-minutes duration in a 1-square-mile (mi2) basin will have a more significant effect on streamflow than the same storm in a 50-mi2 basin. Generally, streams with larger drainage areas require storms of longer duration for a significant increase in streamflow to occur. These and other factors determine whether or not a 100-year storm will produce a 100-year flood.
Trees in a woodland can be rated according to their crown classes which indicate where a tree’s crown is placed with respect to nearby competitors. Trees in the dominant crown class survive flooding much better than trees with a crown class of intermediate or suppressed.
Recurrence intervals for the annual peak streamflow at a given location change if there are significant changes in the flow patterns at that location, possibly caused by an impoundment or diversion of flow. The effects of development (conversion of land from forested or agricultural uses to commercial, residential, or industrial uses) on peak flows is generally much greater for low-recurrence interval floods than for high-recurrence interval floods, such as 25-, 50-, or 100-year floods. During these larger floods, the soil is saturated and does not have the capacity to absorb additional rainfall. Under these conditions, essentially all of the rain that falls, whether on paved surfaces or on saturated soil, runs off and becomes streamflow.
For metro New Orleans, where 47 percent (and rising) of residents are minorities, minority-owned business enterprises (MBEs) will play a significant role in driving job and wealth generation for the foreseeable future. Post-Katrina, New Orleans has enjoyed a boom in entrepreneurship, with startup rates eclipsing the national rate by 64 percent. However, the jury is out on whether the minority population of metro New Orleans has benefited equally from the recent startup renaissance. In this paper, we provide empirical data on how MBEs perceive the post-Katrina entrepreneurial ecosystem, including impressions on inclusiveness, supports, and access. We close with recommendations on policies and procedures that improve the ecosystem's ability to support MBE development.