Regional snow depth frequency curves for avalanche hazard mapping in central Italian Alps

Here, the distribution of H-72 is investigated for a network of 40 gauging stations in the Alpine part of Lombardia region, in the central Italian Alps, using a data set dating back to 1985. A regional approach based on the index value is developed to estimate the T-years return period quantiles of H-72 at each single site i, H-72i(T). The index value is the single site sample average, (mu H72i). For each site, the values of H-72i scaled, i.e. divided, by mu(H72i), H-72i* are calculated. These are dimensionless quantiles with given return period. Some standard statistical tests are carried out, showing that the distribution of H-72i* is homogeneous in space, i.e. it is similar for each site in the considered area, which can then be viewed as a homogeneous region. Therefore, it is possible to use a regional approach for the frequency estimation of H-72*. The values of H-72i* are then grouped together in only one sample and their frequency of occurrence is accommodated by a general extreme value, GEV, probability distribution. The proposed distribution, valid in each site of the study area, can be used to assess the T-years return period quantiles of H-72i*. Then, by multiplying H-72i* by the single site sample index value mu(H72i), an estimate of the T-years quantile of H-72i at the single site is obtained. A sampling strategy is also suggested to estimate the single site index value, i.e. the average mu(H72i), critical for the evaluation of the single site distribution of H-72i. The final result is an estimate of the 3-day snow fall depth with a given return period, H-72i(T) for each site. Also, the standard error of estimation of H-72i(T) is given. It is shown that the value of H-72i(T) estimated by the regional approach is far more reliable than that calculated by the distribution fitting carried out using the data at each single site, particularly for high return periods.