Rainfall interception by two deciduous Mediterranean forests of contrasting stature in Slovenia

Measurements of precipitation above the canopy, throughfall and stemflow were made on the south and north-facing slopes of a deciduous forest on the experimental watershed of the Dragonja river in SW Slovenia. The Dragonja watershed was chosen for the experimental watershed, being of interest because of intensive natural reforestation in the last decades that caused a decrease in minimum and maximum flows. At the same time no noticeable precipitation and temperature changes were observed. Two forest plots were selected. One is located on the north-facing slope (1419 m2) and the other on the south-facing slope (615 m2). Analyses and modelling were made for a 1-year period from October 2000 to September 2001. The leaf area index (LAI) was estimated by three methods, one direct and two indirect ones. The obtained values of LAI with the direct method were 6.6 and 6.9 for the south and north slopes, respectively. Measurements and regression analyses gave the mean annual throughfall value (±standard error) on the south plot 67.1 (±9.6)% of gross precipitation, and 71.5 (±11.6)% on the north plot. The average stemflow values were 4.5 (±0.8)% of gross precipitation on the south plot and 2.9 (±0.6)% on the north plot. The average annual interception losses amount to 28.4 (±4.1) and 25.4 (±4.0)% for the south and north slopes, respectively. In the study a significant influence of the south-east wind was proven. With regression analyses and the classification decision tree model it was established that at the events with more than 7 mm of precipitation and south-east wind with a speed higher than 4 m/s an unusually low amount of throughfall occurred and thus high interception losses. The analytical Gash model of rainfall interception (Gash, 1979 and Gash et al., 1995) was successfully applied. The results of the modelling corresponded well to the observed values and were within the limits of the standard error of the observed values.