Coverage of the Studies
Currently, there are several known models for canopy rainfall retention
during rainfall (excluding stem flow which is no more than 10% of the
total interception). The most common models are the ones of Horton,
Merrian, Jackson, Gash, Fan (Gash et al., 1995; Pypker et al., 2012;
Klaassen et al., 1998; Link et al., 2004; Herbst et al., 2008; Muzylo et
al., 2009).
In the present study, the authors have carried out a set of experiments
on artificial creation of droplets on leaf surface of individual
branches, determination of the maximum mass of water retained and
calculation of the leaf surface area of the analyzed samples,
determination of the maximum sizes of droplets and their edge angles and
determination of empirical dependencies of the leaf surface area of
individual tree species on the maximum mass of water retained.
The Gash and Fan models are considered to be the most modern methods for
estimating canopy rainfall interception. With an actual interception of
a single rainfall event of 33.2% of the total amount of precipitation,
the estimated interception ratio from the Gash model was 35.9% and from
the Fan model 53.6%.
The common feature of all models is that they rely on empirical
observations of net rainfall rather than on physical modeling of
moisture retention on leaf surfaces. None of the models uses the
objective values of the droplet mass on the lamina and the leaf surface
area, which the authors of this study have managed to obtain by
artificial sprinkling of the branches and weighing of the branches,
followed by leaf area estimation.
The formation of the peak rainfall flood discharges to small
watercourses of the forest zone is determined by rainfall input to the
watershed surface, various losses, slope runoff formation and its
transformation into runoff hydrograph at the outlet (Golubtsov V.V.,
2010). The most significant initial losses are rainfall interception by
canopies of coniferous and deciduous trees, as well as rainfall
interception loss by evaporation off the leaf surface of the canopies.
In order to estimate the spatial rainfall distribution intercepted by
the canopies of coniferous tree stands, a set of experimental,
cartographic and laboratory studies as well as meteorological
observations on the estimation of water retention on the phytomass of
forest-forming species of the Middle Urals have been performed. The
forest watershed of the Reshetka River which is near the village
Novoalekseevskoe (watershed area F=32.0 km2) was
chosen as an experimental site. The watershed located in the Sverdlovsk
Region is 31 km west of the Ekaterinburg weather station and is equipped
with the Roshydromet observation station that has been operating since
1946 up to the present day (Fig. 1). Main hydrographic characteristics
of the watershed: weighted-average watershed slope 33.1%; forest cover
82% (72% accounts for pine; 8% - birch; 2% - other tree species
(spruce, aspen, linden, etc.); lake percentage- 0%; swampiness - 0%;
average height - 22 m. Tree species of the studied area belong to the
spring-summer-autumn-green phenotype with the period of winter-spring
dormancy. Vegetation lasts from the second decade of May to the first
decade of October. The green mass of deciduous trees reaches its maximum
growth by the end of the first decade of July. During this time the leaf
surface area is maximal and this period corresponds to the period of
maximum rainfall frequency. Within the reference watershed, a set of
field and desktop research was carried out to clarify the forest stand
characteristics (height and species composition) and hydrographic works.
Methods of estimating precipitation losses from the tree canopies during
the period of rainfalls of various intensities were developed and
tested.
The liquid of (rainfall or condensation) is retained on the tree leaf
surface in droplet form and the retention is provided by surface energy
at the phase boundaries between ”solid” and ”gas” and ”liquid”.
Due to the fact that most of the liquid precipitation is kept by
coniferous and leaf surfaces, the first task to be solved is to
determine the biometric indices of trees:
- leaf mass per 1 ha (F1, t/ha);
- leaf area index (ratio of leaf area to unit ground surface area) (LAI,
ha/ha);
- leaf area (total leaf or needle surface area per unit ground area)
(LA, ha);
- number and average size of leaves and needles.
The bonitet class of forests, trunk timber reserves and the degree of
canopy cover are taken into account.
When estimating the average size of needles and leaves, the approach
proposed by A.I. Utkin and others was used (A.I. Utkin et al., 2008). It
involves the formation of representative samples of needles and leaves
(100 units), and their measurements by automated means (scanning of
images and subsequent measurements of area, length, width by AutoCAD).
Based on the measurements, statistical parameters (average, the
coefficient of variation Cv, the coefficient of asymmetry Cs) of needle
and leave size of the analyzed species were determined (Table 2).
The phytomass values of leaves and needles are determined on the basis
of growth chart and tables of biological productivity of tree stands
(A.Z. Shvidenko et al., 2008), the results of field-forest studies on
the surface areas of forest plants obtained with the use of experimental
data (A.I. Utkin et al., 2008), as well as space images and large-scale
maps. A bonitet class is determined on the basis of field studies (data
on height and age of stand) using M.M. Orlov’s tables.
Using the databases and empirical dependencies established by A.I. Utkin
and others (Table 2), LAI was determined on the basis of the phytomass
value (F1, t/ha) of stand species as LAI = f(F1).
The total leaf surface area (LA, ha) is defined as:
, (1)
where S is the area of the forest plot (ha), k is the stand density (in
unit fractions).
The stand density and its bonitet on the model watershed of the Reshetka
River are determined on the basis of field survey, using the tables of
M.M. Orlov.
The phytomass value (F1, t/ha) for watershed areas with different
species composition has been determined depending on their age and a
bonitet class: M.M. Orlov’s tables have been put into analytical form,
and the estimation of forest age of a certain bonitet has been made on
the basis of stand height data (Fig. 2).
According to the fieldwork materials, the prevailing bonitet class for
the model watershed is II. On the basis of works (by Shvidenko A.Z. et
al., 2008), the relationship between the phytomass value (green mass and
the mass of branches) (F1, t/ha) and the age of tree stands of different
species composition having the bonitet class II was determined in the
analytical form. In the watershed area of the Reshetka River pine stands
prevail, the share of spruce is insignificant (not more than 10% of the
total area of coniferous plantations). Among deciduous species, birch
and aspen prevail. The forests with the share of deciduous or coniferous
trees not less than 25% are considered as a mix of deciduous or
coniferous forests. On the basis of these ratios, the dependency curves
of phytomass on the age of tree stands have been estimated (Fig. 3).
As can be seen, for the considered watershed under conditions of rather
homogeneous species composition of tree stands, the dependencies of F1
values on the forest age are very similar and can be described with a
single equation of the form:
F1=11.7-ln(T) - 22.8. (2)
Taking into account the above mentioned dependencies of tree height on
age, the estimating equation for F1 for bonitet class II
is:
(3)