Discussions

Issues of estimating canopy rainfall retention have been investigated by various authors. However, most research is devoted to precipitation losses for long periods, but few publications focus on precipitation interception of a single rainfall event.
The first known studies on precipitation losses of a single rainfall event by forest canopies were carried out in the USA (Horton, 1919) and Switzerland (Buhler, 1891) in the late 19th–early 20th century. The studies relied on parallel observations of the field precipitation gauges installed both under the middle part of the canopies of the individual deciduous species and in open areas.
In 1917–1919, the Horton’s laboratory determined the values of maximum water retention by the canopies of 12 deciduous tree species (Norway maple (Acer platanoides L.), elm (Ulmus spp.), white willow (Salix alba L.), European ash (Fraxinus excelsior L.), beech (Fagus spp.), American linden (Tilia Americana L.), English oak (Quercus robur L.), Eastern hemlock (Tsuga canadensis (L.) Carrière), Scots pine (Pinus sylvestris L.), hickory (Carya spp.), European horse-chestnut (Aesculus hippocastanum L.), apple tree (Malus spp.), varying from 0.5 to 2.9 mm per canopy projection area. It was found that during the first minutes of the rain, from 70% to 100% of the precipitation is kept by the canopies (i.e., the proportion of precipitation leaked from the start of the rain to the moment of full water saturation of the tree canopies makes 25%–30%).
The general Horton model determines the amount of interception as the difference between the amount of precipitation flowing under the forest canopy and the amount of moisture retained by canopies and lost due to evaporation from the leaf surface.
The most important regularities of rainfall redistribution under the forest canopy were revealed by the beginning of the 20th century in the studies of Ebermeyer, Goppe, N.S. Nesterov, S.D. Okhlyabinin and others. The precipitation retention by vegetation cover and its subsequent evaporation are directly related to the size and properties of the wetted surface. The studies of different authors show that maximum canopy water retention differ significantly. According to Bele J. (Bele, 1975, 1980), spruce stands can intercept up to 3.6 mm of a single rainfall event. V.V. Rakhmanov (V.V. Rakhmanov, 1981) determined that the water retention capacity for coniferous stands (spruce, fir) is within 2-4 mm (in some cases - up to 6-8 mm).
The total interception is dependent on the age, composition, taxational stand volume determining the area of the leaf surface and the amount of precipitation. Light rains (0.5-1.5 mm) are completely intercepted by tree stand canopies; the maximum moisture retention capacity of high-density coniferous trees is 10-12 mm (Onuchin A.A., 2003).
Point estimates of the layer of single rainfall interception received by the authors slightly differ from the results obtained earlier. The difference stem from the fact that for understanding of hydrological processes the authors carried out artificial sprinkling of tree species and the volume of moisture retention was determined by weight experiment. In other studies, these values were determined by making observations of net and gross rainfalls which have a number of uncertainties related to the choice of precipitation measurement points under the canopies of individual trees.
In recent years, the research results on determining moisture retention values for such species as Ulmus processra, Platanus×acerifolia and Corymbia maculata per leaf surface area have been covered in works by Mariana D. Baptista et al. (2018). It was found that the interception value is determined by the leaf surface area and precipitation intensity. Thus, methods similar to those of the authors of this study were used to determine the leaf surface area. At the same time, interception storage capacity mapping within the whole river watershed has never been carried out.
In order to estimate the depth of precipitation distributed in space h (mm) and irrecoverably retained by tree canopies, the following ratio proceeding from the dependencies presented in Fig. 3 is used:
,
where k is the stand density, γ is specific water retention (water mass (kg) per 1 m2 of leaf surface area LA). Specific water retention value γ for forest forming species is calculated as weighted average proceeding from the share of species in the watershed area, based on empirical dependencies:
γ=167 - for coniferous stands, γ=92.7 - for deciduous stands, γ=108 - for mixed deciduous stands, γ=151 - for mixed coniferous stands.
In its turn, the LAI value was estimated for the main stand types by the formulas derived from empirical dependencies proposed by Utkin and calculated as weighted average for forest forming species of the model watershed (Table 4).
Based on the above-mentioned ratios, calculations and experimental works, the values of irrecoverable rainfall retention by canopies were calculated and mapped. All the previously obtained data related to the studied area were brought into a single thematic layer by methods of GIS-technologies and spatial analysis operations. After that, based on the data from the attribute table of a layer, the data were recalculated on the above-mentioned formula with the use of Field Calculator tools in ArcGIS 10.4. The obtained data have been classified by the method of defined intervals and are shown on the map (Fig. 8).
As can be seen from the results of the calculations, common larch has the highest water-retention capacity (up to 147 liters of water for a given bonitet and age), and aspen has the lowest - (13.3 liters of water). The layer of the initial rainfall interception reaches 5.9 mm per 1 ha of forest-covered area. With the change in age and bonitet class of natural tree stands, the precipitation interception layer can reach per 1 ha can be as high as 5-8 mm or even more. Due to the uneven distribution of forest-forming species in the watershed area, the amount of precipitation actually flowing under the canopy differs significantly from the amount of precipitation falling on the canopy. It was the first case in the world scientific practice when mapping (spatial distribution of precipitation losses by canopies) was carried out.