(F) = definition as in the 'Framework for land evaluation' (FAO 1976). cf. = compare q.v. = see term elsewhere in this Glossary.
ACTUAL CROP EVAPOTRANSPIRATION RATE (ETa): the rate of evapotranspiration equal to or smaller than predicted ETm (syn. ET crop) as affected by the level of available soil water, salinity, field size or other causes; mm/day or m/day.
AGROCLIMATIC ZONE: a land unit defined in terms of major climate (q.v.) and growing period (q.v.) which is climatically suitable for a certain range of crops and cultivars.
AKIOCHI: soil with an imbalance of nutrients associated with hydrogen sulphide toxicity.
ALKALI SOIL: a soil that contains sufficient exchangeable sodium to interfere with the growth of most crop plants, either with or without appreciable quantities of soluble salts. Same as a SODIC SOIL which is the preferred term.
ALKALINE SOIL: a soil that has an alkaline reaction, i.e. a soil for which the pH reading of the saturated paste is higher than 7.0.
AREA-SPECIFIC LAND DEVELOPMENT; improvements for irrigation or drainage specific to any land unit as opposed to improvements of general applicability on all land throughout the project.
AVAILABLE WATER CAPACITY: the volume or depth of water retained in the soil between the field capacity and permanent wilting percentage.
BASIN IRRIGATION: a system of surface irrigation (q.v.) in which water is ponded within earthen bunds or banks on four sides.
BENEFIT/COST RATIO: the present value of benefits divided by the present value of costs, with all values adjusted to a common time basis.
BORDER STRIP IRRIGATION: a system of surface irrigation (q.v.) in which water flows and spreads over sloping strips of land between two earthen bunds.
BULK DENSITY: the ratio of the mass of oven dried soil to its bulk volume. When expressed in g cm bulk density is numerically equal to apparent specific gravity or volume weight.
CAM: Crassulacean Acid Metabolism (CAM). A metabolic pathway used by plants including sisal and pineapple.
CATION EXCHANGE CAPACITY (CEC): the total quantity of cations which a soil can adsorb by cation exchange usually expressed in milliequivalents per 100 grams. Measured values of cation exchange capacity depend somewhat on the method used for the determination.
CLASS DETERMINING FACTOR: a variable affecting agronomic, management, land development, conservation, the environment, or socio-economic conditions that has an influence on the outputs and inputs of a specified kind of land use, and which is used to assess the suitability class in which a land unit should be placed for that use.
COMMON PROJECT COSTS: costs which relate, not to any particular land area, but to the service provided to the whole project.
COMPOUND LAND UTILIZATION TYPE: a land utilization type consisting of more than one kind of use or purpose, either undertaken in regular succession on the same land, or simultaneously undertaken on separate areas of land which for purposes of evaluation are treated as a single unit (cf. multiple land utilization type). (F)
CONDITIONALLY SUITABLE; a phase of the land suitability order Suitable, employed in circumstances where small areas of land within the survey are unsuitable or poorly suitable for a particular use under the management specified for that use, but suitable given that certain other land improvements or management practices are employed. (F)
CONDUCTIVITY: see ELECTRICAL CONDUCTIVITY and HYDRAULIC CONDUCTIVITY.
CONSERVATION REQUIREMENTS: the land use requirements (q.v.) largely or entirely related to conservation and sustained use (q.v.).
CONSUMPTIVE USE: see CROP WATER REQUIREMENTS.
CONVEYANCE LOSSES: losses due to evaporation, percolation or breaches in the network of irrigation canals or pipes between the source of water and the field.
CONVEYANCE EFFICIENCY: ratio between water received at the inlet to a block of fields and that released at the project's headworks; fraction.
CRITICAL LIMITS: the limits defining the boundaries between suitability categories of individual class-determining land use requirements and limitations, land qualities or land characteristics. Critical ranges are bounded by critical limits and both are expressed in terms of critical values.
CROPPING INDEX: the number of crops harvested in relation to the years in the cropping cycle. Expressed as C, in percent-
CROPPING REQUIREMENTS OR LIMITATIONS- the land use requirements or limitations specifically related to an individual crop.
CROP WATER REQUIREMENT: the depth or volume of water needed to meet the maximum evapotranspiration rate of the crop when soil water is not limiting (ETcrop or ETm). Note that ETcrop and ETm have the same meaning; the latter is now preferred.
CULTIVATION FACTOR: the number of years under cultivation as a percentage of the total cultivated and non-cultivated cycle. Expressed as R, in percent.
CURRENT LAND SUITABILITY CLASSIFICATION: a land suitability classification based on the suitability of land for a specified use in its present condition, without major land improvements (c.f. potential land suitability classification).
DEGREES OF LIMITATION: the scaling of a single factor (land use requirement, land quality or land characteristic) according to its adverse effects on a specified land utilization type (cf. factor rating).
DIAGNOSTIC CRITERION: a variable (land quality, land characteristic etc.) that has an understood influence on the outputs and inputs of a specified kind of land use, and which serves as a basis for assessing the suitability of the land for that use.
DISCOUNTING: the use of interest factors to adjust cash flows to a common time basis.
DOUBLE CROPPING: growing two crops a year in sequence.
DRIP IRRIGATION: a technique for achieving a low rate, high frequency or long duration water delivery through pipes to drip nozzles located near the plants.
ECONOMIC ANALYSIS: assessment of an investment's profitability to a whole society or economy regardless of who in the society contributes to the costs and receives the benefits, conventionally measured in 'opportunity' rather than 'market' prices.
ECONOMIC RENT: the surplus remaining to a project beneficiary after he receives the rewards necessary to attract physical inputs, labour, management and willingness to bear risk.
ELECTRICAL CONDUCTIVITY OF THE SATURATION EXTRACT (q.v.), ECe: a measure related to the salt concentration of the soil dS/m (at 25°C), formerly mmhos/cm.
ELECTRICAL CONDUCTIVITY (EC): the reciprocal of the electrical resistivity. The resistance in ohms of a conductor, metallic or electrolytic, which is 1 cm long and has a cross sectional area of 1 cm2. Hence, electrical conductivity was formerly expressed in reciprocal ohms per cm or mhos per cm and now in S1 units as dS/m.
EVAPOTRANSPIRATION: the rate of water loss through transpiration from vegetation plus evaporation from the soil surface or from standing water on the soil surface - mm/day or m3/day.
EXCHANGEABLE SODIUM PERCENTAGE (ESP): the degree of saturation of the soil exchange complex with sodium. It may be calculated by the formula:
FACTOR RATING: the suitability of a land unit for a land utilization type in terms of one, or a single group, of class-determining factors as indicated by land characteristics, land qualities, land use requirements, limitations or other indicators. 'The rating is denoted by s1, s2, s3, n1 or n2 corresponding to critical values of the factor (see Table 13).
FARM FINANCIAL VIABILITY: using farm gate prices and actual cash flow, a determination made to ascertain whether the farmer is sufficiently better-off with the project than without the project.
FIELD APPLICATION EFFICIENCY: the ratio of water made directly available to the crop and that received at the field inlet; fraction.
FIELD CANAL EFFICIENCY: the ratio between water received at the field inlet of a block of fields and that at the head of the canal system; fraction.
FIELD CAPACITY: the moisture content of soil in the field after rapid drainage has ceased, usually 2 to 3 days after a thorough wetting of the soil profile. Expressed as moisture percentage on a volume weight basis. Often assumed to be approximately represented by soil in equilibrium with 1/3 bar (or atmospheres) pressure equivalent to 330 cm of water suction or a water potential of -33 kpa (S1 units).
FIELD SUPPLY SCHEDULE: the stream size, duration and interval of water supply to the individual field or farm (volume, duration, rate).
FINANCIAL ANALYSIS: assessment of an investment's profitability to those people or entities contributing capital and directly benefitting from the returns, conventionally measured in 'market' prices (see Appendix 3).
FULL IRRIGATION SERVICE LAND: irrigable land which will receive its full water supply from one source (USBR definition).
GENERAL PURPOSE LAND EVALUATION: a land evaluation in which the potential land utilization types are not closely specified at the beginning of evaluations (cf. special purpose land evaluation).
GROSS CLASSIFICATION AREA: the area mapped and classified in a given survey (USBR definition).
GROSS IRRIGATION WATER REQUIREMENTS: the gross depth or volume of water comprising the net irrigation water requirements, plus any additional leaching over and above percolation.
GROSS MARGIN: the revenues from a farming enterprise (crop yields x prices) minus the variable costs.
GROUNDWATER: water in land beneath the soil surface, usually under conditions where the pressure in the water is equal to or greater than atmospheric pressure and the voids are filled with water.
GROWTH CYCLE: the period required for an annual crop to complete its annual cycle of establishment, growth and production of harvested part. Also see growing period.
GROWING PERIOD: the duration, in days, of the period when both temperature and soil moisture permit crop growth (cf. growing season, growth cycle). Note: growing period relates to the land, growth cycle to the crop.
GROWING SEASON: used in a general way, not as a technical term, to refer to the period of the year when (most) crops are grown, e.g. the rainy season.
HYDRAULIC CONDUCTIVITY: the proportionality factor in the Darcy flow equation, which states that effective flow velocity is proportional to the hydraulic gradient. Hydraulic conductivity, therefore, is the effective flow velocity at unit hydraulic gradient and has the dimensions of velocity (LT-1).
HYV; high yielding variety or cultivar of a crop, or MV - modern variety.
INFORMATIVE APPRAISAL: investigations of present land use, productivity, existing land development, water requirements, drainability, topography, etc. to provide information for the planning, development and operation of irrigation projects (USBR term).
INPUTS: the material inputs (e.g. seed, fertilizers, fuel, chemical sprays) and other inputs (e.g. labour hours) applied to the use of land (cf. levels of inputs, outputs).
INTERCROPPING: growing two or more crops at the same time.
INTERNAL RATE OF RETURN: the rate of discounting at which the present value of benefits becomes equal to the present value of costs.
IRRIGABLE LAND: land suitable for irrigation, that can receive irrigation water and which is classified according to an economic assessment of its suitability for irrigated agriculture, taking into consideration water supplies and the project development costs and benefits.
IRRIGATION: controlled applications of water to supplement the rainfall (note that flooded land is not termed 'irrigated' unless the water is in some way controlled).
IRRIGATION INTERVAL: the time between the start of successive water applications on the same field, days.
KIND OF LAND USE: this term refers to either a major kind of land use or a land utilization type (q.v.), whichever is applicable; where the meaning is clear it may be abbreviated to 'kind of use' or 'use'.
LAND: an area of the earth's surface, the characteristics of which embrace all reasonably stable, or predictably cyclic, attributes of the biosphere vertically above and below this area including those of the atmosphere, the soil and underlying geology, the hydrology, the plant and animal populations, and the results of past and present human activity, to the extent that these attributes exert a significant influence on present and future uses of the land by man. (F)
LAND AREA SPECIFIC COSTS AND BENEFITS: investments and benefits connected with improvements on specific areas of land e.g. for correcting deficiencies in topography, drainage, etc. (see Chapter 7).
LAND CHARACTERISTIC: an attribute of land that can be measured or estimated, and which can be employed for distinguishing between land units of differing suitabilities for use and as a means of describing land qualities.
LAND EQUIVALENT RATIO (LER): the ratio of the area under sole cropping to the area under intercropping needed to give equal amounts of yield at the same management level. It is the sum of the fractions of the intercropped yields divided by the sole-crop yields.
LAND EVALUATION: the process of assessment of land performance when used for specified purposes, involving the execution and interpretation of surveys and studies of land forms, soils, vegetation, climate and other aspects of land in order to identify and make a comparison of promising kinds of land use in terms applicable to the objectives of the evaluation.
LAND FACET: a land unit (q.v.) with climate, landforms, soils and vegetation characteristics which for most practicable purposes may be considered as uniform. A subdivision of a land system (q.v.).
LAND IMPROVEMENT: an alteration in the qualities of land which improves its potential for land use (cf. major land improvement, minor land improvement). (F)
LAND MAPPING UNIT: see Land Unit.
LAND SUITABILITY: the fitness of a given type of land for a specified kind of land use. (F)
LAND SUITABILITY CATEGORY: a level within a land suitability classification. Four categories of land suitability are recognized:
Land suitability order: a grouping of land according to whether it is Suitable or Not Suitable for a specified kind of use.Land suitability class: a subdivision of a land suitability order serving to distinguish types of land which differ in degree of suitability.
Land suitability subclass: a subdivision of a land suitability class serving to distinguish types of land having the same degree of suitability but differing in the nature of the limitations which determine the suitability class.
Land suitability unit: a subdivision of a land suitability subclass serving to distinguish types of land having minor differences in production characteristics or management requirements.
LAND SUITABILITY CLASSIFICATION: an appraisal grouping, or the process of appraisal and grouping, of specific types of land in terms of their absolute or relative suitability for a specified kind of use - (F)
LAND SUITABILITY ORDER, CLASS, SUBCLASS, UNIT: categories in land suitability classification; see definitions in text.
LAND SYSTEM: a land unit (q.v.) with relatively uniform climate and with a repeating pattern of landforms, soils and vegetation. A land system may be divided into land facets (q.v.).
LAND UNIT: an area of land possessing specified land qualities and land characteristics, which can be demarcated on a map. Note: in the Framework, this was termed a 'land mapping unit'.
LAND USE REQUIREMENT OR LIMITATION: the conditions of land necessary or desirable for successful and sustained practice of a given land utilization type cf. crop (agronomic), management, land development, conservation requirements or limitations.
LAND USE SYSTEM: a specified land utilization type practised on a given land unit and associated with inputs, outputs, and possibly land improvements.
LAND USE TYPE: the same as land utilization type.
LAND UTILIZATION TYPE (LUT): a kind of land use described or defined in a degree of detail greater than that of a major kind of land use (q.v.). (F) In the context of irrigated agriculture, a land utilization type refers to a crop, crop combination or cropping system with specified irrigation and management methods in a defined technical and socio-economic setting.
LEACHING: the process of removal of soluble material by passage of water through soil.
LEACHING REQUIREMENT: the fraction of irrigation water entering the soil that effectively must flow through and beyond the root zone in order to prevent a build up of salinity due to the addition of salt in the water. The value is the minimum value to control salts; fraction..
LIMITATION: see LAND USE REQUIREMENT OR LIMITATION. The term limitation is used for conditions which by their presence adversely affect the land utilization type.
LOCALIZED IRRIGATION: an irrigation system using drip, trickle, spot, minisprinkler and other techniques that localize the water application.
MAJOR CLIMATE: a broad climatic division, defined in terms of monthly temperatures, seasonality of rainfall, and temperature regime.
MAJOR KIND OF LAND USE: a major subdivision of rural land use, such as rainfed agriculture, annual crops, perennial crops, rice cultivation, irrigated agriculture, grassland, forestry, recreation.
MAJOR LAND IMPROVEMENT: a large non-current input in land improvement which causes a substantial and reasonably permanent (i.e. lasting in excess of about 10 years) change in the suitability of the land, and which cannot normally be financed or executed by an individual farmer or other land user (cf. minor land improvement). (F)
MANAGEMENT REQUIREMENTS: the land use requirements (q.v.) largely or entirely related to management of a land utilization type.
MATCHING: this term is employed in two senses, (i) broader and (ii) restricted, (i) The process of mutual adaptation and adjustment of the descriptions of land utilization types, inputs and land improvements and the increasingly known conditions of the land to improve suitability, (ii) The (specific) process of comparing land use requirements and limitations with the land units to produce factor ratings.
MAXIMUM EVAPOTRANSPIRATION RATE OF THE CROP, ETm: the maximum rate of evapotranspiration from a crop when soil water is not limited, also called the crop water requirement, and ETcrop.
MILLIEQUIVALENT (me): one thousandth of an equivalent: atomic weight divided by valence/1 000.
Me per 100 g soil = milligrams of a cation divided by its equivalent weight per 100 g soil.
MILLIEQUIVALENT PER LITRE: a milliequivalent of an ion or a compound in one litre of solution.
MINOR LAND IMPROVEMENT: a land improvement which has relatively small effects on the suitability of land, or is non-permanent, or which normally lies within the capacity of an individual farmer or other land user (cf. major land improvement). (F)
MIXED INTERCROPPING: growing two or more crops simultaneously with no distinct row arrangement.
MULTIPLE LAND UTILIZATION TYPE: a land utilization type consisting of more than one kind of use or purpose simultaneously undertaken on the same land, each with its own inputs, requirements and produce or other benefits. (F)
NET FARM INCOME: the combined gross margins (q.v.) from enterprises on a farm, minus the fixed costs.
NET INCREMENTAL IRRIGATION BENEFIT: a measure of the potential net increase in productivity of a unit area of land when developed for irrigation. It is expressed in economic (rather than financial) terms either as a net present value or as an annual equivalent value.
NET IRRIGATION WATER REQUIREMENTS: the depth or volume of water to meet the crop water requirements minus contributions in the field by precipitation, run-on, groundwater and stored soil water and plus field losses due to run-off, seepage and percolation.
NET PRESENT VALUE: the present value of benefits minus the present value of costs.
NON-SALINE SODIC SOIL: (same as NON-SALINE ALKALI SOIL). A soil that contains sufficient exchangeable sodium to interfere with the growth of most crop plants and does not contain appreciable quantities of soluble salts. In the USDA definition, the exchangeable sodium percentage is greater than 15 and electrical conductivity of the saturation extract is less than 4 dS/m (at 25°C). The pH reading of the saturated soil paste is usually greater than 8.5.
OPPORTUNITY COST OF CAPITAL: the opportunity cost of using investment resources in a project, rather than in their next best alternative use, usually expressed in the form of an interest rate.
OUTPUTS: the products (for rainfed agriculture, crops), services (e.g. water supply, recreational facilities), or other benefits (e.g. wildlife conservation) resulting from the use of land. Note: in the Framework, this was defined as 'produce'; the distinction between 'produce' and 'products' has been found confusing, so the term 'outputs' is now substituted (cf. inputs).
PARALLEL APPROACH: a land evaluation methodology in which economic criteria are included throughout the process of identifying land use requirements, and land suitability classification.
PEAK DEMAND RATE: the maximum volume or depth of water per unit time per unit area, litres/sec/ha or l/s/ha.
PERCOLATION: the downward flow of water through soil especially in saturated or nearly saturated soil at hydraulic gradients of one or less.
PERMANENT WILTING POINT: the soil moisture percentage, water content or water potential, at which plants wilt and fail to recover turgidity. It is usually determined with dwarf sunflowers, or assumed to be approximately represented by the 15 bar percentage (i.e. 15 atmospheres water suction, pF 4.2 or a soil water potential of -1.5 MPa).
PERMEABILITY: 1. Qualitative: the quality or state of a porous medium relating to the readiness with which such a medium conducts or transmits fluids. 2. Quantitative: the specific property governing the rate or readiness with which a porous medium transmits fluids under standard conditions. Also see hydraulic conductivity.
PET: potential evapotranspiration q.v. reference crop evapotranspiration.
POROSITY: the fraction of the soil volume not occupied by soil particles i.e. the ratio of the sum of the volume of the liquid and gas phases to the sum of the volumes of the solid, liquid and gas phases of soil. I
POTENTIAL LAND SUITABILITY CLASSIFICATION: a land suitability classification based on the suitability of land for a given use after specified major land improvements (q.v.) have been completed where specified.
PRECIPITATION: total amount of precipitation (rain, drizzle, snow, hail, fog, condensation, hoar frost and rime) expressed in depth of water which would cover a horizontal plane if there is no run-off, infiltration or evapotranspiration; mm/day.
PROJECT: the package of goods and services provided to accomplish specific purposes such as the provision of irrigation, hydro-electric power, etc.
PROJECT FORMULATION: logical, stepwise incremental project justification analysis by adding successive separable units to an already justified core project. Each added unit must meet the specified project justification standard.
PROJECT JUSTIFICATION: analysis and testing by benefit/cost, net present value or internal rate of return to determine whether a project meets an acceptable or pre-determined standard of economic worth.
PROVISIONALLY-IRRIGABLE LAND: land classified as suitable for irrigation provided that water can be supplied to it, in the absence of full knowledge about the water supply or the project and land development costs.
QUADRUPLE CROPPING: growing four crops a year in sequence.
RATOON CROPPING: re-growth from root or crown of cut back or harvested plant.
RELAY INTERCROPPING: growing two or more crops simultaneously during part of each one's cycle. A second crop is planted after the first crop has reached maturity.
REFERENCE CROP EVAPOTRANSPIRATION, ET: the rate of evapotranspiration from an extended 8 to 15 cm tall, green grass cover of uniform height, actively growing, completely shading the ground and not short of water; mm/day.
RETURN FLOW: drainage water that is returned to the irrigation supply system.
ROW INTERCROPPING: growing two or more crops simultaneously with one or more crops planted in rows.
RUN-OFF: losses of water from the field by lateral surface flow.
RUN-ON: gains of water by the field lateral surface flow from adjacent land.
SALINE SODIC SOIL: a soil with sufficient exchangeable sodium to interfere with the growth of most crop plants and containing appreciable quantities of soluble salts. In the USDA definition, the exchangeable sodium percentage is greater than 15% and the electrical conductivity of the saturation extract is greater than 4 dS/m (at 25°C). The pH reading of the saturated soil is usually less than 8.5.
SALINE SOIL: a non-sodic soil containing soluble salts in such quantities that they interfere with the growth of most plants. In the USDA definition, the electrical conductivity of the saturation extract is greater than 4 dS/m (at 25°C), and the exchangeable sodium percentage is less than 15. The pH of the soil is usually less than 8.5.
SATURATION EXTRACT: the solution extracted from a soil at its saturation percentage.
SATURATED SOIL PASTE: a particular mixture of soil and water. At saturation the soil paste glistens as it reflects light, flows slightly when the container is tipped and the paste slides freely and cleanly from a spatula for all soils except those with a high clay content.
SEEPAGE: losses of water by lateral flow through the bunds of rice fields.
SEQUENTIAL CROPPING: growing two or more crops in sequence on the same field per year. The succeeding crop is planted after the proceeding one has been harvested (no intercrop competition).
SODIC SOIL; same as NON-SALINE SODIC SOIL and ALKALI SOIL.
SODICITY: used to describe the condition of a sodic (alkali) soil, e.g. in sodicity hazard or sodicity problem.
SODIUM ADSORPTION RATIO (SAR or R): a ratio for soil extracts and irrigation water used to express the relative activity of sodium ions in exchange reactions with soil:
where Na, Ca and Mg are expressed in milliequivalents per litre. See FAO 1985 for revised method of calculating an adjusted SAR (adj. RNa).
SOIL WATER BALANCE (FIELD WATER BALANCE): the sum of all gains and losses of water over a given period of time; mm/period.
SOIL WATER CONTENT: (same as SOIL MOISTURE CONTENT or PERCENTAGE). 1. Weight of water lost from soil dried to a constant weight at a standard temperature, as a ratio. 2. Volume basis: the volume of water from a soil dried to a constant weight at a standard temperature expressed as a ratio of the soil volume before or after drying. 3. Depth basis: the equivalent depth of free water per 100 units of depth of soil. Numerically the value approximates the volume of water per 100 units of volume of soil.
SPATE IRRIGATION: irrigation from storm flows in rivers or wadis.
SPECIAL PURPOSE LAND EVALUATION: a land evaluation in which the potential forms of land use are limited in number and are closely defined in the objectives of the evaluation (cf. general purpose land evaluation).
SPRINKLER IRRIGATION: overhead irrigation using a piped water supply to various types of sprinkler nozzle.
STORED SOIL WATER: depth of water stored in the root zone from earlier precipitation or irrigation applications.
STRIP INTERCROPPING: growing two or more crops simultaneously in different strips wide enough to permit independent cultivation but narrow enough for the crop to interact agronomically.
SUITABILITY ORDER, CLASS, SUBCLASS, UNIT: abbreviations of land suitability order, class, subclass, unit, see definitions.
SUPPLEMENTARY IRRIGATION: irrigation used to supplement the water requirements of crops where partially supplied from rain falling direct in farmers' fields.
SURFACE IRRIGATION: irrigation where the water flows over the soil surface to reach its destination.
SUSTAINED USE: continuing use of land without severe or permanent deterioration in the resources of the land.
TRICKLE IRRIGATION: same as DRIP IRRIGATION. TRIPLE CROPPING: growing three crops a year in sequence.
TWO STAGE APPROACH: a land evaluation methodology in which a first approximation of land suitability is made on the basis of physical criteria, and in which economic and social analysis is carried out as a second stage on the land use alternatives which appear most promising on the basis of physical evaluation (cf. parallel approach).
VARIABLE COSTS: the farming costs which can be assigned to specific farm enterprises (cf. fixed costs).
WATERLOGGING: the saturation of soil in the root zone of crops.
WATER TABLE: the upper boundary for groundwater. The upper surface of a locus of points at which the pressure in the groundwater is equal to atmospheric pressure.
WILTING POINT: see PERMANENT WILTING POINT.
