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close this book Forests, Climate, and Hydrology: Regional Impacts (1988)
View the document Acknowledgements
View the document Foreword
View the document Preface
Open this folder and view contents 1. Introduction
Open this folder and view contents 2. The living past: Time state of the tropical rain forest
Open this folder and view contents 3. Effects of tropical forest on water yield
Open this folder and view contents 4. Effects of forests on precipitation in India
Open this folder and view contents 5. The Influence of forests and forest reclamation practice on streamflow and water balance
Open this folder and view contents 6. Hydrologic process models
Open this folder and view contents 7. Modelling the behaviour of water
Open this folder and view contents 8. Review of general circulation models as a basis for predicting the effects of vegetation change on climate
Open this folder and view contents 9. Résumé and conclusions
View the document Glossary
View the document Workshop participants



absorptivity. acceptance by a body of incident radiation resulting in its temperature rising. It is usually taken to be numerically equivalent to emissivity (q.v.) for the given wavelength.

adiabatic cooling/expansion. A thermodynamic change of state of a system in which there is no transfer of heat or mass across the boundaries of the system. Compression always results in warming, expansion in cooling.

advection. The transfer of thermal energy by the ingress of air of different temperature or humidity to that of the local air by predominantly horizontal motion in the atmosphere.

aerodynamic (surface) roughness. Character of a surface that produces drag on a moving fluid; results in turbulent flow with efficient transfer of matter and energy.

air masses The concept of atmospheric dynamics in which distinct volumes of air are considered to move independently of each other.

albedo The ratio of the amount of radiation reflected by a surface to the amount incident upon it. It is usually referred to the visible part of the spectrum with the incident and reflected light normal to the surface.

angle of deviation. The horizontal inclination of wind to the isobars.

anticyclone. The disposition of barometric pressures with a central region of higher pressure.

architecture, plant. Classification of stem morphology and ontogeny.

assimilation. The uptake of carbon dioxide by green plants and subsequent photosynthesis producing carbohydrates.

baseflow. River stage (depth) after storm events and wet seasons cease to produce discernible flow effects.

biochore. The areal disposition of a vegetation type.

biogeocoenosis. An ecosystem, emphasizing the integration of the living and non-living entities at a location.

biota. Strictly, the species of plants and animals occurring at a location.

blackbody. The ideal object whose absorptivity and emissivity (q.v.) are perfect and taken as unity.

Bowen ratio. The partitioning of net radiation between latent heat of vaporization of water and sensible heat (q.v.). Expressed as the ratio of sensible to latent heat.

box-diffusion model. Conceptual approach to the dispersal of marine solutes between discrete volumes of the oceans.

bush fallow. A cropped area allowed to revert to secondary forest, restoring its fertility.

Cancer, Tropic of. Latitude 23.5°N where the sun appears overhead on northern midsummer's day.

canopy. The more or less continuous leafy cover of an area provided by the dominant trees.

canopy capacity. The maximum amount of water held by the forest canopy (q.v.) in a prolonged storm.

canopy saturation. See canopy capacity.

capillary potential The thermodynamic free energy of water in a porous medium, conventionally measured in joules, but usually expressed in pressure terms: pascals, bars, cm water, etc.

Capricorn, Tropic of. Latitude 23.5°S; the southern division between the tropics and subtropics (the Tropic of Cancer, q.v., is the northern boundary).

catchment (area). Specific basin or drainage area. See also watershed.

cauliflory. Bearing flowers directly on the stem.

cells The conceptual discrete volumes of a fluid that move almost independently of each other. See air masses.

Charney effect. The superficially anomalous behaviour of air masses subsiding over hot deserts because the albedo of the ground is large and the lower atmosphere is not warmed.

Charney mechanism. See Charney effect.

Charney model. See Charney effect.

chernozem. Grassland soils of western USSR and central USA, characterized by well incorporated organic matter giving a dark colour.

climax. The end point of vegetational succession, assumed to be chiefly determined by climate; often woodland unless the water balance is unfavourable.

cloud forest. Persistent mist associated with the vegetation that probably traps the fine droplets; often on high ground of coastal ranges.

coefficient of variation. A statistic expressing the standard deviation as a proportion (usually a percentage) of the mean.

coenosis. The integration of entities and functions at a site. See biogeocoenosis.

conservation. The law that states that in any isolated system the total amount of energy is constant; or, the law that states that matter is neither created nor destroyed during any physical or chemical change.

continental drift. Palaeogeography as ordered by plate tectonics in which sea floor spreading from mid-ocean ridges moves continents apart.

continental shelf. The relatively shallow region between the land and the deep ocean subject to deposition of terrestrial products.

continuity equation. Following from the laws of conservation (q.v.) there is mathematical correspondence of diffusion between adjacent systems.

contributing area. That portion of a catchment that transmits precipitation to stream outflow by reason of its small detention storage on a specific occasion.

convective rainfall. See precipitation, convectional.

convergence, atmospheric moisture. A cyclonic atmospheric pressure system towards which air masses with precipitable moisture flow and where they may ascend to produce precipitation.

Coriolis parameter. Force acting on the atmosphere due to the earth's rotation that modifies the circulation pattern.

cosmic rays. Electromagnetic waves incident on the earth from space with very high energy and penetrating power.

crown. The leafy upper shoots of a tree.

cumuliform clouds. As cumulus, that is thick, with massive excrescences above.

cyclonic. An atmospheric annular barometric situation with a low pressure centre producing convergent winds, upward air currents, and thus, often, precipitation.

density, forest stand A measure of the closure of the tree canopy (q.v.); the completeness of cover of the ground; a semi-quantitative classification from 0 (treeless) to I (complete cover).

depression. An atmospheric pressure system resulting in a falling barometer reading, as with a cyclone (see cyclonic).

derived savanna. An anthropogenic degradation of tropical and subtropical forests, usually through frequent fires, resulting in short, widely spaced trees with seasonal grasses between.

deterministic. A (mathematical) relationship incorporating the physical cause of the observed effect.

detritus. Organic remains deposited by water.

discharge, specific river. Volume flow per unit time per unit area of catchment (e.g. Iitres per second per square kilometre).

drag. The aerodynamic force component parallel to the surface opposed to the fluid flow and resulting from the aerodynamic roughness (q.v.).

ecosystem. All the living and non-living entities at a location together with their mutual influences.

edaphic. Pertaining to soil.

eddy diffusivity. The exchange of heat and mass in a direction at right angles to flow due to turbulence in the moving fluid.

e-folding depth. Depth in soil at which the daily fluctuation of soil temperature decreases to l/e, which is - 36% of the surface value.

emergent. A tree whose crown (q.v.) projects above the average forest canopy (q.v.).

emissivity The ratio of radiation at a given wavelength emitted per unit time and per unit area of a surface to that from a perfect (blackbody) emitter at the same temperature in the same environment.

energy, potential. The state of water that would produce flow if the conductivity of the medium would allow it.

epiphyte. Plant adapted to grow entirely above ground upon other plants.

eutrophication. The increasing fertility of water bodies (usually anthropogenic) through nutrients in drainage water, often leading to anaerobiosis and death of animal organisms.

evapotranspiration. The total water vaporized by all routes (plant transpiration, wet surface, from soil, and from litter) according to the meteorological drying power (potential evapotranspiration) as modified by moisture availability (actual evapotranspiration.)

fixation (of carbon dioxide). See assimilation.

flux density. Quantitative expression of fluid flow; that passing through unit area.

forcing. A condition imposed on a (model) system to discover its sensitivity or response.

frontal precipitation. See precipitation, frontal.

gap-phase regeneration. Dynamics of vegetation in which death or destruction introduces a local irregularity where the age and species of plants will differ from the average; a characteristic mosaic of vegetation results.

general (global) circulation model (GCM). A mathematical algorithm describing the dynamic physical relationships of the earth's atmosphere (conceptualized as discrete boxes on a 3-D grid) that may be used predictively.

geopotential. The projection of the geoid (the acceleration due to gravity) and the height above mean sea-level; it is usefully applied to isobars visualized in the vertical plane.

Gondwanaland The southern fragment of Pangea, a supercontinent in the Southern Hemisphere that split at the end of the Cretaceous epoch; see continental drift. Antarctica, S. America, Australasia, India, and Africa once composed Gondwanaland.

greenhouse effect. The effect of increasing carbon dioxide in the earth's atmosphere, decreasing radiative cooling of the planet by trapping more long-wave radiation and so raising the mean atmospheric temperature with possible consequences on the hydrological cycle.

Hadley circulation. The circulation of air by hot equatorial air rising, to move northwards in the upper layers (in the Northern Hemisphere). At about 30°N it descends and moves southwards as the NE trades towards the equator.

hydraulic conductivity, unsaturated. The reciprocal of the resistance to moisture diffusion per unit time, area, and potential difference, it varies with soil moisture content.

hydrography The time course of river stage (water depth).

hydrosphere. That region near the earth's surface in which the hydrologic cycle supervenes.

hygric. Adapted to life with a plentiful water supply.

infiltration. The process of water entering the ground surface.

infrared radiation. Portion of electromagnetic spectrum between wavelengths 0.7pm and 100 µm; includes thermal radiation.

instability rain. See precipitation, convectional.

interception capacity See canopy capacity.

interception (loss). The process of rain impinging on vegetation, wetting it, and evaporating; the last is interception loss.

interflow. The lateral movement of water below ground but above the water-table, caused by horizons of low permeability.

isobar. Line joining points of equal barometric pressure reduced to sea-level datum.

isotope. Form of a chemical element differing in nuclear mass.

isotropic. Having the same properties in every direction.

kinetic energy. The energy that a body possesses by virtue of its motion and the work that the body could do on being brought to rest.

lapse rate. The vertical temperature gradient above the earth's surface; that corresponding to adiabatic expansion of rising air is a reduction of 0.6° K per 100 m.

latent heat. The energy required to convert unit mass of a solid to liquid (or liquid to gas) at the same temperature; melting of ice, 80 cal g-1; vaporization of water, 540 cal g-1.

leaf area index. The area of leaf surface (usually upper or lower surface only) divided by the crown (q.v.) area projected on a horizontal surface.

liane. Plant adapted to climbing over other plants; initially, at least, rooted in the soil.

fife form. Classification of plants according to the exposure of the perennating shoot apices in the inimical season.

litter. branches, etc. accumulating on the ground surface after death and dehiscence.

mast year. Season in which irregularly reproducing trees bear fruit freely.

moisture potential, tissue. The thermodynamic free energy of water within the plant.

moisture pressure (potential), soil. A measure of the thermodynamic energy of the water in the soil.

morphogenic. Referring to the development of the external appearance of a plant.

multiple regression equation. A stochastic (q.v.) relationship between a dependent variable and more than one independent variable.

Newtonian (ideal) fluid. An incompressible fluid that remains unordered whilst flowing.

nuclei, condensation. Particles around which water molecules collect to form droplets or crystals.

occult precipitation. See precipitation, occult.

ombrophile. Plant adaptation to high rainfall habitats; see hygric.

Orographic precipitation. See precipitation, orographic.

Overland flow. The movement of water downhill without infiltrating (see infiltration) the soil, as when accession of water (perhaps by intense rainfall) exceeds the infiltration capacity of the soil.

pan (evaporation). The standard ("class A") water filled trough from which evaporation is measured by recording water depth changes.

parametrization. The quantification of objects and processes for insertion into mathematical models.

percolation. The process by which water moves through soil as influenced by gravity.

permafrost The soil condition under which the subsoil rarely, if ever, thaws in summer.

permeability soil. The rate at which water moves through soil by a given hydraulic or potential gradient.

petiole. The leaf stalk.

phyllode. A flattened, leaflike peticole (q.v.).

phytomass. The weight (probably fresh) of canopy (q.v.; probably not including twigs and branches) per hectare.

pinnate (leaf). Leaflets arranged on either side of the midrib to form a compound leaf.

pioneer species. Plants adapted to enter the gaps in open vegetation but that are unable to regenerate in climax (q.v.) woodland.

Planck function (constant). Ratio of a quantum of radiation to its frequency; 6.626 x 10-34 Js, 6.626 x 10 erg s.

potential energy. See energy, potential.

precipitation, convectional. Air masses that rise due to their lower density (higher temperature) relative to surrounding air, precipitate their moisture as they cool, saturate, and the water condenses.

precipitation, frontal. Where air streams of different densities (or temperatures) converge (a front), one will override the other; the adiabatic expansion (q.v.) of the former may result in precipitation.

precipitation, occult. Strictly, the condensation of water or ice from incoming saturated air as dew or frost, but often not distinguished from mist catching, guttation, and very local vaporization/condensation systems.

precipitation, orographic. Precipitation resulting from the topography causing air masses to rise, cool, and precipitate their moisture.

pressure field. The synoptic situation of barometric pressure in the vicinity.

primary production. The increase in (dry) weight of green plants due to (net) photosynthesis; see assimilation.

probability density function. Used to generate a random sequence of events conforming to a given mean value and frequency distribution.

pyrene. A small, hard fruit containing a single seed.

quickflow. See runoff, direct.

radiation cooling. Loss of heat by emitting at infrared wavelengths rather than by convection.

regolilh. An immature soil, usually thin and stony.

resistance, stomalal. The measure of obstruction to diffusion of water vapour passing out of leaves into the leaf boundary layer (see also stomata; transpiration).

rotation rare. The speed of vertical circulation of air between heat sources and sinks.

roughness length. The height above the ground surface at which the wind speed is assumed to be zero; it provides a quantitative measure of the roughness of a vegetative cover. See also aerodynamic (surface) roughness

runoff, direct. Water entering streams without infiltrating the soil.

runoff, surface. See overland flow.

secondary forest. The resultant vegetation after clearing primary forest and permitting natural regeneration.

secondary succession. The ensuing changes after vegetation disturbance that has not so degraded the site as would require primary succession from an abiotic site.

sensible heal. Heat energy that can be sensed (i.e. with a thermometer); used in contrast to latent heat.

sensitivity (of models). The degree of the response of the output of mathematical models to a change of the magnitude of the input parameter(s).

shola forest. Patchy forest among extensive grassland occurring on the Nilgiri plateau of southern India.

skyline logging. Using an overhead cable, block, and tackle to move felled trees, thus avoiding serious damage to ground vegetation and the soil.

snow pack. The accumulation of snow whose slow spring melt feeds the rivers.

snow-storage coefficient. The maximum ratio in winter of snow pack (q.v.) in forests to that in open fields.

sorptivity. The surface tension of moisture in the soil that reduces its thermodynamic free energy.

specific humidity. Mass of water vapour per unit mass of moist air.

stability, atmospheric (thermal). The comparison of the vertical temperature profile with the adiabatic lapse rate (neutral stability); super-adiabatic lapse rate = instability.

Stefan-Boltzmann constant. a = 5.670 x 10-4 erg cm-2s-1K-4, by which the fourth power of the absolute temperature is multiplied to give the total radiation from a perfect emitter.

stemflow. The water that reaches the ground in forests by trickling down the tree trunks.

stick-raked. Ground prepared by dragging a tree trunk with its branch snags over the surface.

stochastic. Mathematical relationship established through application of the laws of probability.

stomata The pores, more often on the underside of leaves, through which gases diffuse in and out of the leaves, but with variable aperture controlled by guard cells around the pore.

stomatal resistance. See resistance, stomatal.

storm hydrograph. The response of river stage to a storm event arbitrarily separated from the flow that would have occurred anyway (cf. runoff, direct).

storm intensity The rate of rainfall (e.g. mm hr-1).

sublimation. The change of phase from solid to gas without an intermediate liquid state, e.g. the ablation of snow.

subsidence. The downward movement of air masses where the ground conditions do not favour warming of the air column; precipitation is unlikely to ensue.

Suess effect. The dilution of atmospheric radiocarbon (14C) through the injection of carbon dioxide from fossil fuels.

surface runoff. See overland flow.

temperature potential. The temperature of air if taken adiabatically to sea-level, neglecting moist processes.

thermal infrared. Electromagnetic radiation at wavelengths longer than near infrared, within the waveband 3µm to c. 18pm.

thermal lows. Regions of low barometric pressure and convergence of air masses due to convective updraughts from a warm area of the earth's surface.

thermocline. The steep temperature gradient that separates the warm oceanic surface water from the cold deep ocean.

throughfall. The precipitation reaching the forest floor either without interception (q.v.) or by dripping from the canopy (q.v.); the contribution of streamflow (q.v.) may or may not be included but is usually relatively insignificant.

through-flow. See interflow.

trade winds. North-east winds (Northern Hemisphere) and South-east winds (Southern Hemisphere) that are persistent and blow from the 30° latitudes towards the doldrums just north of the equator, but shifting some 5a north and south with season.

transfer characteristics. The physical attributes of the system that determine the rates of exchange of heat and materials between the conceptual elements of box-diffusion models (q.v.).

transpiration. The process of evaporation and loss of water vapour from plants; to some extent the rate is controlled by the stomata (q.v.).

transpiration rate coefficient. The transpiration (q.v.) during the summer in m, per tonne (probably fresh weight) of leaf (see phytomass) to the hectare.

tritium. Hydrogen isotope (q.v.) with relative atomic mass 3.

troposphere. The zone of the atmosphere with a relatively steep temperature gradient in which weather phenomena originate; region below the stratosphere (which lies above 9 km at the poles and 16 km at the equator).

turbulent diffusion. See eddy diffusivity.

turbulent transfer. The transfer of heat, momentum, and matter (e.g. water vapour and carbon dioxide) in the atmosphere by bulk movement in pockets of air moved by turbulence.

upwelling. The movement of cold, mineral-rich, deep ocean water to the surface.

water holding capacity. Applied to the forest canopy; see canopy capacity. watershed. The drainage area (basin) of a stream network bounded by the topographic divide (sometimes called the watershed) or, more precisely, the phreatic divide. water use efficiency. The increase in biomass for unit of water evaporated or transpired. xeromorphic. Plants whose appearance suggests that they are adapted to drought. yarding. The conveying of felled logs to a convenient location.

zenith angle. The angle made between the sun, the point of observation, and the vertical at that point.

zonal. Geographic distribution (e.g. of soil types) that approximates to a relatively simple latitudinal climatic pattern, as opposed to azonal distribution.

zonobiomes. Vegetation types with their associated faunas whose distribution approximates to latitudinal zones.