Evaluation of the Rainfall Components Distribution and the optimum Samples Size to Estimate Throughfall for Needleleaf and Broadleaf Stands in Zagros Forests, Ilam

Measuring rainfall and its components is essential in forest areas to manage the land's water resources. Throughfall is one of the most important components of the water cycle in forest ecosystems, which has many spatial variations due to the heterogeneous structure of canopy and variable rainfall patterns. However, few studies conducted to determine the optimum samples size for estimating this variable. This study aimed to estimate the number of collectors needed to estimate the average cumulative throughfall of Persian oak (Quercus brantii), pine (Pinus eldarica) and silver cedar (Cupressus arizonica) stands within certain error limit percentage in the semi-arid Zagros forests (Chaoghasabz Forest Park, Ilam County). By determining the appropriate number of collectors, it is possible to have a proper measurement of rainfall and its components in forest areas with acceptable accuracy and saving time and cost.

This research was conducted from October 2017 to February 2018, in natural stands of Persian oak, 30-year eldar pine and silver cedar plantations. The amount of rainfall was measured by five rain gauges located in the open space (outside the canopy), adjacent to the studied stands. The measurement of throughfall was carried out by 27 collectors in Persian oak underneath the canopy and 36 collectors in each eldar pine and silver cedar stands. The average stemflow of five trees in each stand was considered as the average stemflow of the stand. The amount of rainfall interception was estimated from the difference between the amount of gross rainfall and the sum of throughfall and stemflow.

During the study period, 20 rainfall events (in total, 258.2 mm) were measured, and on average, 80, 50 and 59% of it has passed through the canopy in the form of throughfall in each oak, pine and cedar stands, respectively. The mean values for rainfall interception were computed as 46.23 mm for Persian oak, 113.17 mm for eldar pine and 92.17 mm for silver cedar stands. The average number of required collectors estimated with 5, 10 and 15% error percentage of the cumulative average of throughfall at a confidence limit of 95% are 102, 25, and 11 for oak, 5, 10, and 15 for pine and 30, 8 and 3 for cedar, respectively.

According to the results of this study, a positive and strong relationship was observed between the amount of rainfall and throughfall for Persian oak (R2=0.9155), eldar pine (R2=0.8831) and silver cedar (R2=0.8967). On this basis, it can be said that with the increase of rainfall size, throughfall increases. Considering the rainfall regime and the importance of water in these areas, in forestry and modeling of ecohydrological processes in forest ecosystems, priority should be given to planting species with higher throughfall and less interception loss.
The estimated number of 27 collectors is enough to measure average cumulative throughfall of Persian oak with an error of 10%. In the eldar pine and silver cedar stands, where 36 collectors used, this number found to be optimum for measuring the cumulative average of throughfall with an error of 5%. Thus, in the Persian oak stand, the number of collectors should be increased in order to estimate the cumulative average of throughfall with a lower percentage of error, and in the other two stands, the number of collectors was found to be suitable.