Campbell Scientific Sentek TriSCAN® Manual v 1.2a - Manual (Page 29)

TriSCAN Manual Version 1.2a
Copyright © 1991 – 2004 Sentek Pty Ltd All rights reserved Page 22
Topography
Topography relates to the configuration of the land surface and is described in terms of differences in
aspect, elevation and slope. This impacts on plant -soil-water dynamics via influencing climatic conditions
including:
•
Rain shadows and sunshine hours
•
Rainfall and temperature patterns up slopes
•
Elluviation (washing-out) of clays from higher elevations and illuviation (washing-in and
accumulation) of clays at lower elevations
•
Relatively poorer drainage at lower elevations
Crop
Crop differences have an impact on crop water use and irrigation scheduling requirements. While all require
management between full point and refill point at most times, the depth of root extraction varies, as do
specialized requirements e.g. the deliberate stressing of horticultural crops during flowering.
Most plant tissues contain about 90% water and the rate of uptake of water from the soil solution by plant
roots is largely controlled by the rate of water loss through transpiration. Plant characteristics such as crop
type, size, age, vigour, variety, rootstock, development stage, leaf area, nutritional status, disease status,
crop load and harvest all affect crop water use. Specialized advice should be sought in this regard. A rough
guide to water use can be obtained from crop coefficients, which are widely available in the literature for
different stages of growth for most crops (Table 5). These express evapotranspiration as a ratio of reference
evaporation (in this case Penman-Monteith).
Table 5. Examples of crop coefficients (FAO)
Crop Kc (initial) Kc (mid) Kc (end)
Green beans 0.5 1.052 0.9
Artichokes 0.5 1 0.95
Cereals 0.3 1.15 0.4
Sugar cane 0.4 1.25 0.75
Alfalfa 0.4 0.95 0.9
Cultural Management
The impacts of cultural management (agronomic/horticultural practices) also need to be understood for
proper irrigation scheduling.
Soil preparation
Cultivation increases evaporation from the topsoil, reducing soil water available to the plant. It may also
reduce water run-off and improve the infiltration of rain and irrigation water, improving plant water
availability. The dynamics of nut rient availability are also affected. An example is the dependence of
mineralization rates, of particularly nitrogen, upon aeration.
Cover crop and mulch
Cover crops provide more competition for water, but reduce evaporation and facilitate infiltration of rain and
irrigation water, reducing run-off.
Mulch can improve the infiltration rate of the soil, reduce water run-off, encourage root growth near the soil
surface and increase the soil water holding capacity over time, through the accumulation of soil organic
matter, and reduction of soil temperature.
Oil spraying
Oily substances on leaves reduce water use by temporarily closing stomata. An example of this is mite
control in citrus.