Description: The 85th percentile is a 24-hour rainfall total. It represents a value such that 85% of the observed 24-hour rainfall totals will be less that that value.CREATING THE SURFACEA collection of 56 precipitation gage points was developed with 85th percentile precipitation values based on multiple years of gage data. A raster surface (grid of cells with values) was interpolated from that set of points. The surface initially did not cover all of the County's jurisdiction. A total of 13 dummy points were added. Most of those were just outside the County boundary to enable the software to generate a surface that covered the entire County. A handful of points were added to enforce a plausible surface. In particular, one point was added in the desert east of Julian, to enforce a gradient from high precipitation in the mountains to low precipitation in the desert. Three points were added near the northern boundary of the County to adjust the surface to reflect the effect of elevation in areas lacking sufficient operating gages. Several methods of interpolation were considered. The method chosen is named by ESRI as the Natural Neighbor technique. This method produces a surface that is highly empirical, with the value of the surface being a product of the values of the data points nearest each cell. It does not produce peaks or valleys of surface based on larger area trends, and is free of artifacts that appeared with other methods.CALCULATION OF 85 PERCENTILE FOR GAGE POINTSThe method of calculating the 85th% is to produce a list of values, order them from smallest to largest, then pick the value that is 85% of the way through the list. For the purposes of water quality work, you want to look at 24-hour rainfall values that are capable of producing runoff. Lacking a legislative definition of rainfall values capable of producing runoff, some jurisdictions in California have attempted to limit the list to 24-hour rainfall values that are capable of producing significant runoff and have restricted the list to rainfall values greater than 0.10”. This results in rather large values for the 85th% and potential oversizing of catchment basins. Flood Control staff in San Diego County have observed that the point at which significant runoff begins is rather subjective, and is affected by land use type and soil moisture. In highly-urbanized areas, the soil has a high impermeability and runoff can begin with as little as 0.02” of rainfall. In rural areas, soil impermeability is significantly lower and even 0.30”of rain on dry soil will frequently not produce significant runoff. For this reason, San Diego County has chosen to use the more objective method of including all non-zero 24-hour rainfall totals when calculating the 85th%. To produce a statistically significant number, only stations with 30 years or greater of daily rainfall records are used.
Service Item Id: 2d948ee3c2ae4e7aaee6af1a6a1190ba
Copyright Text: Fred McCamic, County of San Diego, Department of Public Works, Cartographic Services. Rand Allan, County of San Diego, Department of Public Works, Flood Control Engineering.