Study of the parameters of radiation and water balance of Earth clearly shows that these parameters differ with latitudes. The interactions of these parameters of radiation and water balance are responsible for particular meteorological conditions in different regions of Earth which, in turn, result in climatic zones. Further, the biosphere of Earth, particularly the plant cover over land surface is characteristically dependent on the climatic conditions of a particular region. Thus, a number of distinct biogeographical zones can be recognized over Earth’s surface based on the characteristic climate, soil and plant cover. Dokuchavev for the first time observed that the boundaries of biogeographical zones are largely determined by climatic factors and are particularly dependent on moisture conditions. His classical studies included estimates of the ratios of precipitation to potential evaporation for major biogeographical zones of Earth. Various studies after him examined the relationship of soil and forest types with ratios of precipitation to potential evaporation. Grigoriyev and Budyko in comprehensive studies of global environment identified the most important parameters of radiation and water balance which determine the biogeographical zonality and gave the Law of Geographical Zonality.
LAW OF GEOGRAPHICAL ZONALITY
Budyko showed that equations for mean yearly heat and water balances of land may be written in the following form:
R/Lr = E/r + P/Lr and 1 = E/r + f/r
(the members of heat balance equation are divided by Lr and those of water balance equation by r).
The linkage equation of water and heat balance may then be added to these relations:
E/r = @ (R/Lr)
These equations link four relative values of components of heat and water balances. Therefore, it is sufficient to know any one of them for determining the others. Owing to special form of the linkage equation, ratio R/Lr or P/Lr may be selected as the parameter that determines all relative values of the components of heat and water balances. The ratios E/r and f/r can not be decisive factor for the first two variables in case of dry climates when small changes in E/r or f/r produce large changes in R/Lr or P/Lr. Further, it seems more appropriate to select R/Lr as the principal parameter determining the relative values of the components of heat and water balances. This parameter may also be viewed as the ratio of potential evaporation R/L to precipitation r, or else as a ratio of the radiation balance of a moistened surface to heat expended on the evaporation of the total yearly precipitation
While the relative values of components of heat and water balances are determined by one parameter R/Lr, the determination of absolute values of these components requires two parameters namely R/Lr and R.
It has been shown by Budyko that it is possible to calculate potential evaporation from the radiation balance at the Earth’s surface. The method for calculating potential evaporation from air humidity deficit as various other empirical methods yield less accurate results.
A world map of the radiation index of dryness (R/Lr) has been constructed by Budyko (1955). A comparison of this map with geobotanical and soil maps confirms that the positions of isoclines of the index of dryness confirm well to the distribution of major climatic and biogeographical zones. It has been shown that the radiation index of dryness accords well with the boundaries of major natural climatic and biogeographical zones. In general, values of the radiation index of dryness (< 1/3) correspond to tundras, values between 1/3 and 1 to forest zone, from 1 to 2 to the steppe zone, more than 2 to semi-desert zone and more than 3 to the desert zone. Further, within particular zones at various latitudes substantial differences in conditions determining the development of natural processes can be observed. These differences derive from the fact that the energy base of natural processes may be characterized through the magnitude of the radiation balance R and it differs at various latitudes. Accordingly, while it may be possible to make use of single parameter R/L in characterizing general zonal conditions of natural processes, the characterization of absolute values of the intensity of natural processes requires the use of two parameters i.e. R/Lr and R which determine the absolute magnitudes of the elements of heat and water balances.
The relation of biogeographical conditions with the parameters R/r and R plotted along its axes and on which major biogeographical zones are divided by straight lines. A schematic form of such a graph is given in Fig.
While the use of more accurate precipitation normals found with help of currently available measuring instruments introduce certain changes in the characteristic values of moisture indices, this does not alter the general patterns of the relation of moisture indices to biogeographical zones. The close relationship of parameters R/Lr and R with biogeographical zones has been examined and named the Law of Geographical Zonality (Grigoriyev and Budyko; 1956, 1962). It was specifically noted that within each latitudinal belt there exists a definite correspondence between the boundaries of natural zones and the isoclines for particular values of R/Lr. Table- represents the overall structure. In the compilation of the table, use was made of the values of R relating to the conditions of moistened surface and of more detailed data on precipitation and plant cover in various regions. To each section in the table, characterizing gradations of moisture conditions correspond to specific values of the coefficients of river run-offs. At the same time, yearly run-off normal in each section increases as radiation balance increases. However, this does not apply to desert areas where the run-off at low latitudes is close to zero. Important conclusions of the Law of Geographical Zonality are discussed below.
1. R/Lr and botanical zones: Similar types of plant cover correspond to each section in the table. In conditions of excessive moisture, forests are prevalent at all latitudes except for areas with substantial excess of moisture i.e. R/r < 2/5. In such cases, forests are replaced by tundras at high latitudes and by swamps at lower latitudes. Since R/r < 2/5 over vast territories is found only at relatively high latitudes, swampy areas at low latitudes can not be viewed as autonomous biogeographical zones. It should be noted that the ordinate represents values of the radiation balance for actual state of Earth’s surface, which differ from the values of the radiation balance for moistened surfaces. The continuous line on the graph bounds the region of actually occurring values of R/r and R (except for mountainous regions) within whose ranges specific values of R/Lr (shown as vertical lines) delimit the major botanical zones. Despite common features of plant covers in the forest zone, large changes in the values of radiation balance correspond to perceptible geobotanical changes within these zones.
Each gradation in moistening is characterized not only by a definite type of plant cover but also by a specific value of its productivity. In the studies of Grigoriyev and Budyko (1956, 1962), it was assumed that the productivity of natural vegetation increases as the moisture conditions approach optimal ones for a given radiation balance. Similarly, productivity increases with increase in radiation balance for given moisture conditions.
2. R and R/Lr and soil zones: It has long been established that there exists a close relationship between the zonality of plant covers and the zonality of soils. Therefore, the conclusions concerning the relation of botanical zones to specific values of parameters R and R/Lr can be fully applied to soil zones as well. It can be established that as the values of parameter R/Lr increase, soil types change in the following sequence:
(i) Tundra soils;
(ii) Podzols, brown forest soils, yellow soils, red soils and laterite soils (the diversity of soil types within that group corresponds to changes in the parameter R within the broad range);
(iii) Chernozems and black soils of savannah regions;
(iv) Chestnut brown soils;
(v) Grey soils.
General relation of soil zonality to climatic indices R/Lr and R may be represented as a graph similar to the graph for botanical zones. Considering the Table-, to each section of the table, there corresponds a specific sequence of changes in soil types which is largely similar within each section. For example, the third section is characterized by following sequence of soil types: tundras, podzols and brown forest soils, subtropical red soils and yellow soils, tropical podzol red soils and laterites. For the fourth section, the sequence is chernozems and dark chestnut soils, black soils and brown soils, weakly leached subtropical soils, red brown tropical soils etc. To each such sequence there correspond specific values of quantitative characteristics of the process of soil formation. The region of eternal snow occupies a special place within the table. It is described by negative value of R and R/Lr due to practical absence of plant and soil covers. Similarly, the subzone of Arctic deserts has negligible values of yearly radiation balance and a high humidity.
3. R/Lr and R and zonality of hydrological regime of land: The relation of the zonality of hydrological regime of land to parameters R/Lr and R may be established on both quantitative and qualitative terms. From the linkage equation it follows that to each gradation in value of parameter R/Lr there corresponds specific gradation in the value of run-off coefficient. A consideration of the influence of energy factors makes it possible to explain the zonal changes in run-off coefficients in quantitative terms. The absolute values of total run-off are determined by two parameters, namely R/Lr and R. Graph in Fig. , which is similar to that in Fig. represents the distribution of yearly total run-off and characterizes the absolute values of run-off in different biogeographical zones.
CAUSES OF PATTERNS OF GEOGRAPHICAL ZONALITY
From the above discussion, it is evident that the patterns in the Table-6 of Geographical Zones may be explained in terms of the following factors:
1. Latitudinal difference in Earth’s surface radiation balance: Owing to the spherical shape of Earth, its surface is divided into several latitudinal belts that differ in terms of radiation energy inflow to the surface of Earth.
2. Differences in moisture conditions: Within each of these belts (except for the region of eternal snow) there exist different moisture conditions ranging from excessive to highly insufficient moisture. These differences in moisture conditions within each latitudinal belt are responsible for the zonality within the belt..
Within different latitudinal belts, geographical conditions with similar moisture indices have a certain number of common characteristics, which are combined with differences resulting from different inflows of radiation energy. These common characteristics are periodically repeated as regions of two latitudinal belts in which humidity (or dryness) increases) are compared with each other.
SEASONAL CHANGES IN CLIMATIC FACTORS OF BIOGEOGRAPHICAL ZONALITY
The influence of climatic factors on biogeographical zones may be represented more clearly by considering the specific features of the climatic regimes in different seasons. In specifying the climatic conditions of particular seasons, following major types of climatic regimes may be identified:
1. Arctic climatic regime: It is characterized by the sow cover, negative air temperatures and negative values of radiation balance or else values close to zero.
2. Tundra climatic regimes: Such climatic regimes have average monthly temperatures ranging from zero to 10o C and a small positive radiation-balance.
3. Climatic regimes of forest zones: These have average monthly temperatures of more than 10o C and a positive radiation balance as well as sufficient moisture, when evaporation exceeds one half of the potential evaporation.
4. Climatic regimes of arid zones (Steppes and savannahs): These have a positive radiation balance and actual evaporation ranging from 1/10 to ½ of the potential evaporation.
5. Climatic regimes of deserts: These have positive radiation balance and an evaporation less than 1/10 of potential evaporation.
Important features of seasonal changes in climatic factors determining biogeographical zonality are:
(i) It has been shown that the type of climatic conditions in each month corresponds with the type of natural zone throughout the year. However, in most biogeographical regions, several types of climatic regimes replace each other during the year.
(ii) There are substantial differences in seasonal changes of climatic regimes at various longitudes. These are especially felt at middle and low latitudes where types of climatic regimes depend on moisture conditions.
(iii) In Africa and Europe, a regime of low humidity exists within a wide latitudinal belt that shifts to the north in summers and to the south in winters. This corresponds to the most favourable conditions in the subtropics in the winter and spring and at tropical latitudes in the summer. In East Asia and North America, the structure of moisture regime differs considerably from the first scheme because of substantial differences in circulation processes.
(iv) In most regions and at all longitudes there is either insufficient moisture (regimes 4 and 5) or insufficient heat (regime 1 & 2) during a major part of the year. Only in the narrow belt close to Equator, conditions corresponding to regime 3 exist throughout the year. Evidently under regimes 2 and 4 and especially under regimes 1 and 5, productivity of natural plant cover is reduced.
(v) Under conditions of insufficient heat the type of biogeographical zone is determined by climatic regime of the period in which productivity of natural plant cover is greatest even if that period is relatively short. For example, the zonal landscape of tundra is determined by conditions of the warm season, which may last no longer than 1/5th to 1/4th of the year. In such cases, climatic regime of the cold season, which extends over most of the year does not determine the landscape’s zonal character.
(vi) In regions of insufficient moisture also a regularity similar to the above one is observed. The most humid period of the year plays a determining role in establishing the type of zone, even though it may be shorter than the period with insufficient moisture. In natural zones with insufficient moisture, there may be short periods of either sufficient or excessive moisture which do not result in development of typical forest growth.
The above described regularities relating the type of biogeographical zone to seasonal changes in the climatic factors determining the zonality, in fact, complement the concept of the influence of climatic factors on biogeographical zonality described earlier in the Law of Geographical Zonality.