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It should be noted that for totally dry initial conditions, total porosity minus water content is equal to effective porosity, which is generally about 75 to 97 percent of the total porosity. 7, FIELD MEASUREMENT OF INFILTRATION. Although many methods can and have been used to measure infiltration rates in the field, a common practice is through the use of a double ring infiltrometer. These consist of concentric cylinders approximately 40 cm high. Diameter of the inner ring is 28- 33 cm, while the outer ring is 50-60 cm in diameter. Concentric rings are used, with measurement confin,ild to the inner ring, in order to minimize error due to flow divergence in directions other than the vertical. In the field, the cylinders are driven into the soil surface to approxima.tely 15 cm of depth. Water is filled to a depth of about 1 0 cm in both cylinders and the lime and height of water in the inner cylinder are recorded. Measurements are taken at 10, 20, 30, 45, 60, 90 and 120 minutes and each hour for at least four hours. Additional water is added and measured to maintain a relatively constant head. It is important that the water level in the outer cylinder be kept at about the same level as the inner onE! and should never be filled higher that the inner cylinder. Recorded rates of infiltration am computed from the measurements and plotted versus time to define the infiltration curve. From this relationship, parameters for use in the infiltration methods described previously can be determined. 8. FLOOD RECONSITUTION Reconstituting historic or observed storm events can give the most accurate estimate for infiltration for a basin. If a basin has rainfalVrunoff data from historic storm events, infiltration rates can be determined by "backtracking" from the observed runoff volume, what Infiltration values have to be applied to the observed rainfall hyetograph to match the observed runoff volumes. The example below shows this concept. A USGS rainfalVstream gage located in the Irondale Gulch Basin in Montebello was used to determine historical infiltration rates during actual flood events in the basin. The gage used in the flood reconstitutions was USGS gage 06714310, titled "Sand Creek Tributary at Denver, CO. which is actually in the Montebello development in Irondale Gulch. The gage was located on the left bank of a concrete lined drainage ditch in the median of Andrews Dr. Parkway, 50 feet downstream of Troy Street. The basin has a 0.30 square mile drainage area, The three largest flood producing events were selected out of the twenty-two storms with available rainfalVrunoff data to determine historic infiltration rates. An iterative process was used to determine what infiltration rate for each storm would result in the simulated volume of runoff that would closest match the historic volume of Colorado Flood Hydrology Manual DRAFT 7.20