Experiments on Maximum Rainfall Retention by the Canopies of Tree Stands. Modeling of Rainfall Interception by Conifer Needles
In order to estimate the water retention value, a set of experiments on artificial sprinkling of the parts of tree canopies (branches) was carried out. The experimental research focused on the study of 9 deciduous and coniferous species growing in the watershed area, e.g. birch, aspen, lime tree, pine and fir, in the period of their maximum green mass growth (the end of June, 2018). Branches for being sprinkled and weighed in order to determine moisture mass were selected from different parts of the canopies of the analyzed tree species. The age range of the trees for analysis was from 40 to 60 years old and the height was from 16 to 20 m. From each tree 10–20 branches weighing from 0.4 to 5.0 kg were used. Natural branch weight was determined under dry weather conditions accurate within 5 g (with the use of accuracy class III scales, OKP 42 7451, GOST 29329-92 with permissible measurement error of +/- 5g, error in measuring water mass on the leaves at the moment of maximum retention didn’t exceed 11.1% with the average 4.3%). Then, a branch was artificially sprinkled until maximum mass is stabilized. The maximum moisture retention value was determined on the basis of the difference between the masses of the branch under dry and wet conditions.
Leaf surface area was calculated as follows: the number of leaves on the analyzed branch was counted manually, then a sample set of 50 leaves from different parts of the branch was formed, and the average leaf area in the analyzed sample set was calculated (with the measurement error of no more than 2 mm2). Statistical error of determining the area didn’t exceed 3%. The area and vein length were measured for each leaf in a sample set, followed by statistical parameter calculations. As for conifer needles, only area and length were measured. The average value was used for calculations (Table 1).
The observations made under natural and laboratory conditions showed that water retention by the tree canopy occurs only due to droplets and wetting branches (no more than 10% of total water retention).
Based on the experiments on artificial sprinkling of different-sized branches, the empirical dependency of maximum rainfall retention on leaf surface area was obtained (Fig.3).
The dependencies are characterized by good quality (with correlation coefficients greater than 0.9 in all cases) and grouped into separate bundles. The dependency analysis allowed us to conclude that specific water retention of deciduous trees (the first coefficient presenting the mass of water per 1 m2 of the leaf surface) is determined by the leaf venation type of the analyzed species. Leaves with pinnately-netted venation have larger retention values as compared to leaves with pennate venation.
In order to confirm the assumption, vein length of the leaf samples under analysis was measured by means of computer programs (AutoCAD 2007). The obtained measurement results were published in the scientific papers by Klimenko et al 2018, 2019). The average leaf vein density and its dependency on specific moisture retention for a given species were determined on the basis of information on the total length of visible veins and “normal” (average for representative sample) leaf area. The dependency is considered to be of good quality (correlation coefficient 0.89).
Using both biometrical indices of the studied tree species and obtained empirical dependencies, estimation of the values of maximum rainfall retention on the canopy in the form of mass retention (kg) and the amount of precipitation per canopy projection area (m2) was made (Table 3).