Two climates for two hydrologic cycles

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Contemporary hydroclimatology recognizes that the atmosphere has a central role in delivering moisture to a specific location. In addition, the atmosphere provides the climatic framework for the Earth's surface energy and moisture fluxes and balances that drive the hydrologic cycle. Understanding the role of these components is required for a comprehensive view of hydroclimatology that extends beyond the traditional context of climatology and beyond the boundaries of hydrometeorology. A dynamic atmosphere within the context of the climate system provides a transport function for energy and mass identified as the atmospheric branch of the hydrologic cycle. This atmospheric transport component serves as the driving force for the global hydro-logic cycle and makes possible closure of the terrestrial branch of the hydrologic cycle. Consequently, it is convenient to identify two hydrologic cycles in terms of an atmospheric branch and a terrestrial branch. Obviously, both branches are necessary to form the global hydrologic cycle. Recognizing the coupled roles of the atmospheric branch and the terrestrial branch of the hydrologic cycle provides a rational construct for the total hydrologic cycle.

The differences between the atmospheric branch and the terrestrial branch of the hydrologic cycle suggest that climate expressed as fluxes of energy and mass may be different for these two entities. The atmospheric branch of the hydrologic cycle is most closely related to a dynamic and mobile atmosphere characterized by both vertical and horizontal energy and mass transfers. The terrestrial branch of the hydrologic cycle has an Earth surface orientation and energy and mass fluxes are dominantly vertical. The energy and mass fluxes and the transport role embraced by the atmospheric and terrestrial branches of the hydrologic cycle do not lend themselves readily to traditional climatology designations because they overarch approach and scale concepts commonly employed as criteria in climatological designations. Furthermore, just as there is a rationale for distinguishing between the atmospheric and terrestrial branches of the hydrologic cycle a rationale exists for distinguishing between the climate that is dominantly atmospheric and the climate that is dominantly associated with the Earth's surface. To facilitate ease of reference these two climatic constructs are identified as "climate of the first kind'' and "climate of the second kind'', respectively.

The atmospheric transport role is the basis for equating "climate of the first kind'' with the atmospheric branch of the hydrologic cycle. Atmospheric moisture transport is a central component of the hydrologic cycle in the same way that latent heat provided by phase changes of water is a significant contributor to the atmospheric redistribution of energy and moisture. A "climate of the second kind'' is aligned with the disposition of precipitation as outputs of evapotranspiration, streamflow, and storage of soil moisture and groundwater at the Earth's surface. This is the realm of the terrestrial branch of the hydro-logic cycle. The energy and mass fluxes related to "climate of the first kind'' and "climate of the second kind'' drive other natural systems and reflect the intermingling of climate and hydrology which emphasizes the coupling of the two branches of the hydrologic cycle.

1.5.1 Climate of the first kind

The atmosphere is responsible for moving energy and moisture globally with regular schedules and patterns. Storm delivery occurs as a specific component of the dynamic atmosphere. The persistence of the atmospheric time and space patterns gives rise to an expected state frequently expressed in terms of averages or probabilities commonly equated with climate. These statistics reflect the popular interest in periods longer than those characteristic of weather events. The widespread use of the climatic normal is an expression of long-term conditions of temperature and precipitation related to the general circulation of the atmosphere. The climatic normal is the average for a recent 30-year period, for example 1971-2000. The period used in this computation is updated every decade as more recent data become available, but as Bryson (1997) asserts there are no true climatic normals. An atmospheric focus is the basis for climate of the first kind that is fundamentally a circulation-based paradigm.

Sketches Hydrologic Cycle


Fig. 1.4. The Earth and atmosphere radiation balance and related non-radiative energy flows. Units are percentage of global mean solar radiation (100 units = 342 Wm~2).


Fig. 1.4. The Earth and atmosphere radiation balance and related non-radiative energy flows. Units are percentage of global mean solar radiation (100 units = 342 Wm~2).

Climate in the atmosphere is characterized by the transmission, absorption, and reflection of solar and terrestrial radiation (Fig. 1.4). Solar energy is the main input, but clouds and atmospheric gases reflect more sunshine away to space than they absorb. Note the units in Figure 1.4 are relative quantities based on 100 units of solar radiation at the top of the atmosphere. Most of the absorbed input of solar radiation takes place near the bottom of the atmosphere. Terrestrial radiation is generated continually by all surfaces that receive radiant energy whereas insolation is periodic. The greenhouse effect of the atmosphere selectively absorbs nearly all terrestrial radiation and radiates a larger quantity of atmospheric radiation back to Earth. The total flow of terrestrial radiation actually exceeds the total for insolation because the terrestrial flux is sustained by both insolation and atmospheric longwave radiation. These processes are examined in more detail in Chapter 2.

The Earth and the atmosphere are heated unevenly in both space and time by the fluxes of insolation and longwave radiant energy. Tropical locations receive more radiation than they emit (Fig. 1.5), and the atmosphere becomes warm and


Fig. 1.5. Daily averaged radiation fluxes for the South Pole (light stippled column), for a northern Australia (11.5° S) tropical grassland (gray column), and for a mid-latitude (44° N) meadow (heavy stippled column). Fs is incoming solar radiation, Fr is reflected solar radiation (albedo), Fa is incoming atmospheric longwave radiation, Ft is outgoing terrestrial longwave radiation, and Fn is net radiation. (Compiled from data in Gay, 1979; Linacre and Geerts, 1997; and Beringer and Tapper, 2002).

moist in response to the storage of heat and vapor fluxes from the surface. Polar latitudes receive smaller quantities of radiant energy than tropical locations, and they radiate less intensely. Nevertheless, these regions emit more radiant energy than they receive, and the atmosphere is relatively cold and dry. Furthermore, Figure 1.5 shows that the long hours of daylight during the high-sun season at mid-latitude locations can produce high solar radiation values and larger net radiation than at lower latitudes during specific periods. The global imbalance of energy and moisture in the atmosphere creates atmospheric pressure gradients that initiate global scale atmospheric motion. The global atmospheric circulation triggered in this way provides the systematic framework for global air transport more extensive and persistent than features responding to local or regional influences. These topics are addressed in Chapter 7.

The fundamental control of climate of the first kind is atmospheric circulation with energy and moisture fluxes imposed within the circulation pattern. Other influences commonly cited, such as latitude, elevation, and distance from water, are associated features, or false controls, rather than causal factors. The associated features help to explain the fluxes of energy and moisture responsible for many of the global atmospheric pressure features that initiate


: n

Fig. 1.5. Daily averaged radiation fluxes for the South Pole (light stippled column), for a northern Australia (11.5° S) tropical grassland (gray column), and for a mid-latitude (44° N) meadow (heavy stippled column). Fs is incoming solar radiation, Fr is reflected solar radiation (albedo), Fa is incoming atmospheric longwave radiation, Ft is outgoing terrestrial longwave radiation, and Fn is net radiation. (Compiled from data in Gay, 1979; Linacre and Geerts, 1997; and Beringer and Tapper, 2002).

Fs Fr Fa Ft Fu

Fs Fr Fa Ft Fu air circulation, but the movement of the atmosphere is the foremost factor responsible for time and space variations in temperature and precipitation. Evapotranspiration is conspicuously absent from a prominent role in climate of the first kind because it is a small quantity unless it is accumulated for periods greater than several days.

In recent years, considerable attention has been devoted to the development and refinement of equation-based general circulation models (GCMs). GCMs have been employed to study ancient climate simulations and to simulate future climate scenarios. These computer models rely on time and space averaging of data to an extent that air masses and fronts are averaged out of existence. This may be the ultimate expression of the statistical character of climate of the first kind. However, experimentation with these models has revealed they are quite sensitive to continental hydrologic budgets, and they require inclusion of energy and moisture fluxes between the surface and the atmosphere to achieve realistic simulation of existing temperature and precipitation fields (Szilagyi and Parlange, 1999). The important role of atmospheric circulation is not diminished, rather these models suggest a concept of climate is needed that extends beyond the atmosphere to incorporate the role of the Earth's surface processes and the atmospheric redistribution of heat and moisture transferred into the lower atmosphere from the surface.

1.5.2 Climate of the second kind

The interface between the atmosphere and the various material systems that lie beneath it is the focus for climate of the second kind. The climate of the interface is primarily forced by vertical endowments, storages, and return fluxes of energy and moisture. This distinctive perspective of climate at the interface has emerged largely from investigations into energy budget and hydrologic cycle questions addressing the coupling of climate and hydrology. The interface is conceptually a zone extending from a few meters below the Earth's surface and into the atmosphere to the top of the atmospheric boundary layer that has a typical depth of about one kilometer (Andrews, 2000). This zone is inhabited extensively by vegetation that extends upward into the atmosphere and downward into the soil creating a transition zone for fluxes of energy and mass between the land and the atmosphere. Strong vertical discontinuities of physical properties characterize this transition zone. These sharp contrasts are responsible for the strong vertical divergences in radiative fluxes of energy and the conversion of energy between radiation, sensible heat, and latent heat. As a consequence, there are large vertical gradients of sensible heat and latent heat and considerable differences in phase changes of water at the interface. Also, a strong vertical flux of water exists. Consequently, climate of the second kind includes surface, plant, soil, and gravity water. Groundwater in dead storage is outside the conceptual boundaries of the interface.

Climate of the second kind provides a means of estimating the outputs of evapotranspiration and streamflow and storages of soil moisture and groundwater. Quantification of these fluxes and storages is a fundamental requisite for developing budgets of inputs, storages, and outputs for consecutive periods of days, weeks, months, or seasons. The budgets deal mainly with exchanges in the vertical across the interface, and they employ accounting procedures based on the principals of the conservation of energy and mass. The climatic activity at the interface is principally vertical, and there is little that is atmospheric about climate of the second kind. Energy received at the interface as radiant energy is greatly affected by the atmosphere, but a radiation budget would exist in the absence of the atmosphere. The Moon is a convenient illustration of this reality.

Hydroclimatology is a dynamic budget of water fluxes vertically together with the energy endowment that drives evapotranspiration or the return flow of moisture to the atmosphere. Precipitation and evapotranspiration are the main contestants for dominance of the flow of water at the Earth's surface. The terrestrial branch of the hydrologic cycle is a manifestation of precipitation gains versus evapotranspiration losses at the surface. While evapotranspiration is tiny on a daily basis and is difficult to measure, it accumulates with time to attain an important stature in problems that deal with a period of a week, month, season, or year.

Water resources are seasonally replenished and discharged by climate of the second kind. Even the behavior of lakes and oceans is dependent upon this conception of climate's seasonal timing. The ecology of natural vegetation, agriculture, and forestry is timed by both the availability of soil moisture and the energy to extract it for plant processes as predominantly evapotranspiration. Plants keep only a tiny portion of the moisture extracted by roots. The great bulk of the moisture is passed into the atmosphere. Irrigation of agricultural crops is an attempt to overcome the water deficiency resulting from climate of the second kind.

Controls for climate of the second kind are the forcing functions that drive natural systems at the Earth-atmosphere interface. Fundamentally, these forcing functions are energy and moisture availability and the coincidence of the energy demand for moisture and the moisture supply. Moisture storages in the soil and as groundwater are important related processes that can be considered as secondary controls. While these storages are influenced by physical characteristics of the soil and strata, they respond to the supply and demand of energy and moisture at a specific site.

Evapotranspiration ascends to a prominent role in climate of the second kind and is the primary control for this climate concept. Evapotranspiration is driven by radiant energy, but the presence of moisture is required for transformation of

Fig. 1.6. Sketch of precipitation partitioning at the Earth's surface and plant utilization of soil water.

the radiant energy endowment at the Earth-atmosphere interface. For short intervals, evapotranspiration is a small quantity and is difficult to measure convincingly. As an alternative, numerous empirical techniques are available to estimate evapotranspiration or the climatic demand for water. Recognizing that the climatic moisture demand may exceed available moisture, the idea of potential evapotranspiration (ETp) was introduced almost simultaneously by Thornthwaite et al. (1945) and Penman (1948) to identify the maximum possible moisture loss limited only by the energy endowment. Numerous methodologies have appeared subsequently to quantify energy-driven evapotranspiration. In contrast, actual evapotranspiration (ETa) is the transfer of moisture from the surface to the atmosphere in response to both the energy demand and the available moisture supply.

While ETa is an upward directed flow of moisture away from the Earth's surface, the source of this moisture is precipitation that is a downward vector toward the surface in the hydrologic cycle. Precipitation is capricious and spatially variable, but the energy demand for moisture is more regular in both space and time. Moisture storage in the upper soil layers plays an important role in supplying moisture between precipitation events (Fig. 1.6). Soil moisture storage occurs from the Earth's surface downward for a few meters in the zone where water and air share the void spaces among solid soil particles. The moisture balance of the soil layer and the average soil moisture content are critical to land area climate (Hartmann, 1994). The soil layer and associated vegetation determine whether precipitation is quickly evaporated, absorbed by the soil, or becomes streamflow.

Climate of the second kind is clearly a manifestation of the hydrologic cycle and the important role of climate as the forcing function underlying the fluxes and storages of moisture expressed as precipitation, evapotranspiration, soil moisture, groundwater, and stream discharge. With the development of expanded data networks and satellite technology for remote data assessment, the water balance provides an analytical framework for examining the spatial and temporal characteristics of wetting and drying periods, and annual variations in the water resources (Hartmann, 1994).

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