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I I I I I I I I I I SECTION IV RAINFALL SEASONAL FYDROLOGY Most floods in the Trinidad area occur in the period April through August. Flooding is predominantly caused by frontal storms and thunder- storms; snowmelt rarely causes flooding except when augmented with heavy rains. Average annual rainfall at the Federal Aeronautics Adminstration (FAA) Office at Trinidad Airport, approximately 7 miles north of the Frijole Creek basin, is 12.RO inches. The average rainfall for July at that station is 1.86 inches. The total precipitation during July 19R1 measured 6.55 inches, 4.69 inches above the average. DESCRIPTION OF JULY 2-3 STORM I I I I I I I I I Between 10:00 p.m. July 2 and 2:00 a.m. July 3, 19R1, an intense thunderstorm inundated the Frijole Creek basin and surrounding area southeast of Trinidad, Colorado. The storm was centered over the upper portions of the Frijole Creek and San Francisco Creek basins, west of the Colorado and Southern Railway. Heavy flooding was reported through- out both basins by local residents; however, the runoff produced by the storm was much more severe in the Frijole Creek basin than in the San Francisco Creek basin. Precipitation There are no weather stations in the Frijole Creek basin. The nearest official weather reporting station is at the Trinidad Airport where a total of 3.66 inches of rain was reported from the storm. Based on interviews with local residents and data obtained from other sources, the storm in the Frijole Creek basin began at approximately 10:00 p.m. The period of intense rainfall was approximately four hours with very little precipitation preceding or following the heavy downpour. The rainfall distribution pattern is shown in Figure 3. Because of the intensity of the storm and remoteness of the area, measurements of total rainfall were sparse and consisted primarily of estimates by residents based on observations of storm intensity at different periods and bucket