CHARACTERIZING TIMES BETWEEN STORMS IN MOUNTAINOUS AREAS

Storm Simulation models are sometimes used to provide input to watershed models where there are no precipitation data. However, a basic understanding of the seasonal and spatial characteristics of parameters that comprise the inputs to the models is needed, and then methods for estimating the parameters must be developed. This study is an investigation of the behavior and estimation of two parameters for storm-occurrence modeling, average time between storms (ATBS) and minimum time between storms (MDPD), and how they vary with average annual precipitation (P(ann)), average monthly precipitation (P(mo)), and elevation (E) in mountainous terrain where snow is a major component of precipitation. Data used came from the dual-rain-gauge network in the USDA-ARS Reynolds Creek Experimental Watershed (RCEW) in southwest Idaho. Median ATBS and MDPD data show consistent trends over the RCEW with narrow variability. The exponential distribution adequately models times between storms in snowfall-predominant mountainous areas except for very long dry periods. The power equation form generally fitted the data best for regressions where there was a statistically significant regression compared with a linear equation. For four of the regressions found in this article (P(mo) vs. E, ATBS vs. P(mo), ATBS vs. E, and MDPD vs. ATBS), parameters were correlated, suggesting parameter mapping may be useful in their estimation. ATBS can be estimated by using the power equation for ATBS vs. P(mo) and ATBS vs. E (both inversely correlated). MDPD vs. ATBS correlations were positive, showing that as the average time between storms increases, so does the minimum time between storms. Guidance is given for estimating ATBS and MDPD for a specific location. Magnitudes of often-used fixed values of MDPD (e.g., 6 h) are too small to be representative of times between storms, and are not representative of the seasonal variability of times between storms. The results in this article for characterizing ATBS and MDPD in the mountainous 239-km(2) RCEW area follow closely those found for the 225,000-km(2) Colorado plains area in another study. Thus, equation forms identified in this article can suggest those that might be expected in general and may be used as guidance for estimating ATBS and MDPD in ungauged areas, including areas with snowfall as a major component of precipitation.