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This paper reviews the various methods available for irrigation scheduling, contrasting traditional …

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- Irrigation scheduling: advantages and pitfalls of plant-based methods


Table 1. A summary of the main classes of irrigation scheduling approaches, indicating their main advantages and disadvantages



I. Soil water measurement    
(a) Soil water potential (tensiometers, psychrometers, etc.) Easy to apply in practice; can be quite precise; at least water content measures indicate ‘how much’ water to apply; many commercial systems available; some sensors (especially capacitance and time domain sensors) readily automated Soil heterogeneity requires many sensors (often expensive) or extensive monitoring programme (e.g. neutron probe); selecting position that is representative of the root-zone is difficult; sensors do not generally measure water status at root surface (which depends on evaporative demand)
(b) Soil water content (gravimetric; capacitance/TDR; neutron probe)    
II. Soil water balance calculations    
(Require estimate of evaporation and rainfall) Easy to apply in principle; indicate ‘how much’ water to apply Not as accurate as direct measurement; need accurate local estimates of precipitation/runoff; evapotranspiration estimates require good estimates of crop coefficients (which depend on crop development, rooting depth, etc.); errors are cumulative, so regular recalibration needed
III. Plant ‘stress’ sensing    
(Includes both water status measurement and plant response measurement) Measures the plant stress response directly; integrates environmental effects; potentially very sensitive In general, does not indicate ‘how much’ water to apply; calibration required to determine ‘control thresholds’; still largely at research/development stage and little used yet for routine agronomy (except for thermal sensing in some situations)
(a) Tissue water status It has often been argued that leaf water status is the most appropriate measure for many physiological processes (e.g. photosynthesis), but this argument is generally erroneous (as it ignores root–shoot signalling) All measures are subject to homeostatic regulation (especially leaf water status), therefore not sensitive (isohydric plants); sensitive to environmental conditions which can lead to short-term fluctuations greater than treatment differences
    (i) Visible wilting Easy to detect Not precise; yield reduction often occurs before visible symptoms; hard to automate
    (ii) Pressure chamber ({psi}) Widely accepted reference technique; most useful if estimating stem water potential (SWP), using either bagged leaves or suckers Slow and labour intensive (therefore expensive, especially for predawn measurements); unsuitable for automation
    (iii) Psychrometer ({psi}) Valuable, thermodynamically based measure of water status; can be automated Requires sophisticated equipment and high level of technical skill, yet still unreliable in the long term
    (iv) Tissue water content (RWC, leaf thickness [{gamma}- or ß-ray thickness sensors], fruit or stem diameter) Changes in tissue water content are easier to measure and automate than water potential measurements; RWC more directly related to physiological function than is total water potential in many cases; commercial micromorphometric sensors available Instrumentation generally complex or expensive, so difficult to get adequate replication; water content measures (and diameter changes) subject to same problems as other water status measures; leaf thickness sensitivity limited by lateral shrinkage
    (v) Pressure probe Can measure the pressure component of water potential which is the driving force for xylem flow and much cell function (e.g. growth) Only suitable for experimental or laboratory systems
    (vi) Xylem cavitation Can be sensitive to increasing water stress Cavitation frequency depends on stress prehistory; cavitation–water status curve shows hysteresis, with most cavitations occurring during drying, so cannot indicate successful rehydration
(b) Physiological responses Potentially more sensitive than measures of tissue (especially leaf) water status Often require sophisticated or complex equipment; require calibration to determine ‘control thresholds’
    (i) Stomatal conductance Generally a very sensitive response, except in some anisohydric species Large leaf-to-leaf variation requires much replication for reliable data
        – Porometer Accurate: the benchmark for research studies Labour intensive so not suitable for commercial application; not readily automated (though some attempts have been made)
        – Thermal sensing Can be used remotely; capable of scaling up to large areas of crop (especially with imaging); imaging effectively averages many leaves; simple thermometers cheap and portable; well suited for monitoring purposes Canopy temperature is affected by environmental conditions as well as by stomatal aperture, so needs calibration (e.g. using wet and dry reference surfaces)
        – Sap-flow sensors Sensitive Only indirectly estimates changes in conductance, as flow is also very dependent on atmospheric conditions; requires complex instrumentation and technical expertise; needs calibration for each tree and for definition of irrigation control thresholds
    (ii) Growth rate

Probably the most sensitive indicator of water deficit stress

Instrumentation delicate and generally expensive

Comments that relate to all methods in a section are not repeated in subsections.


Source: Journal of Experimental Botany 2004 55(407):2427-2436

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