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Modeling of runoff-producing rainfall hyetographs in Texas using L-moment statistics

by 1969- Asquith, William Harold

Abstract (Summary)
An instantaneous hyetograph (hyetograph), is the temporal distribution of rainfall occurring over a point or area during a storm. Synthetic hyetographs are estimates of the expected time distribution for a design storm and principally are used in smallwatershed hydraulic-structure design. Combining a hyetograph with a unit hydrograph provides the designer with a synthetic streamflow hydrograph. A data base of more than 1,600 observed cumulative hyetographs that produced runoff from selected small watersheds in parts of Texas provided estimates of parameters of a simple triangularshaped hyetograph model. The model provides an estimate of the average or expected hyetograph in dimensionless form for storm durations of 0–24 hr and 24 hr and greater (up to about 3 days). The modeled hyetographs are formulated, graphed, and tabulated to facilitate use in design applications. In this study, the expected dimensionless hyetographs of 0–12 hr and 12–24 hr durations were similar and were combined with minimal information loss. Also, dimensionless hyetographs are independent of the frequency level or recurrence interval of total storm depth. The frequency independence should enhance the suitability of dimensionless hyetographs for design applications. Introduction An instantaneous hyetograph, or simply a hyetograph, is the temporal distribution of rainfall occurring over a point or area within a storm. Other forms of the hyetograph 57 are common; a hyetograph integrated with time produces a cumulative hyetograph. The hyetograph in its various forms has several applications. For example, in order to time distribute the depth of a design storm, an expected or synthetic (design) hyetograph for a particular location is often used by engineers and hydrologists during the design process of hydraulic structures such as culverts or runoff detention basins in small watersheds. A design storm is characterized by the depth of rainfall having a specified duration and recurrence interval (exceedance probability) predicated by the design criteria; for example, the 12-hr 100-year storm. When a hyetograph is convoluted with a unit hydrograph, a synthetic streamflow hydrograph is produced (Chow and others, 1988, chap. 7). A convolution example is provided in chapter 7. Purpose and Scope The purpose of this chapter is to estimate synthetic dimensionless cumulative hyetographs for storms known to produce significant runoff in small watersheds in Texas. The synthetic hyetographs are estimated using a simple triangular model of the instantaneous hyetograph. A dimensionless cumulative hyetograph has units of percent storm duration on the horizontal axis and percent storm depth on the vertical axis. Dimension is easily restored to the hyetograph through multiplication of the storm duration and depth with the percentages of the horizontal and vertical axes respectively. The triangular model is appealing over more complicated functions or geometrical shapes because of its simplicity and ease of application. 58 A specialized hyetograph data base described in the next section provides the basis for the analysis. Three ranges of storm duration considered for this study were 0–12 hr, 12–24 hr, and 24 hr and greater (up to about three days). The majority of hydrologic design applications that require synthetic hyetographs for small watersheds have time scales on the order of 24 hr or less, but some might require longer durations. Hyetographs defined for these duration ranges are useful to practitioners of small watershed hydraulic design. Data Sources A data base of cumulative hyetographs for storm events known to produce runoff from small watersheds was compiled during the execution of multiple on-going (as of 2002) research projects that are sponsored by the Texas Department of Transportation (project nos. 0–4193 and 0–4194) and the U.S. Geological Survey (USGS). The projects are being performed by investigators at Texas Tech University, Lamar University, University of Houston, and USGS. The fact that the hyetograph data represents storms known to produce runoff is important for applications involving rainfall-runoff relations and distinguishes the analysis presented here. At this time over 1,600 events for 91 USGS streamflow-gaging stations are available. The locations of the stations are shown on figure 17; the figure is identical to figure 2. 59 Fort Dallas Big Worth Spring Waco Austin San Antonio Fort Worth Dallas Trinity River Basin Map Base Information Albers Projection Basins from Texas Water Development Board Cities from Texas Natural Resources Information System Waco Brazos River Basin Colorado River B asin
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School:The University of Texas at Austin

School Location:USA - Texas

Source Type:Master's Thesis

Keywords:rain and rainfall runoff texas

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