17 Irrigation and Drainage
391
65 d after sowing. The highest spectral reflectance was measured where 135
kgNha- 1 was applied. Spectral reflectance was related to dry matter yield. The
technique could not discriminate between some of treatments in the experiment.
D'Urso and Menenti (1996 a) developed an equation, based on the PenmanMonteith combination formula, to calculate crop coefficients from spectral
radiances and meteorological observations.
17.3.4 Crop water stress - Thermal Infrared
Field irrigation experiments. The relation of the surface radiometric temperature
of foliage T f with crop transpiration and water stress has been demonstrated in
several field studies. Kumar and Tripathi (1990) studied the impact on wheat crops
of different irrigation treatments. These authors observed that environmental
factors other than soil water availability influenced T f and proposed a water stress
indicator which included vapour pressure deficit. The significance of T f as a crop
water stress indicator was confirmed by other studies (e.g. Inoue, 1991) who
investigated photosynthetic activity in wheat, maize, soyabeans and cotton. On the
other hand the results presented by Reicosky et al. (1994) indicate a limited
sensitivity of T f to reductions in canopy photosynthesis and evapotranspiration of
spring wheat. These authors observed that differences in evapotranspiration and
canopy temperature were small, while canopy photosynthesis was reduced by 52%
in the deficit-irrigation treatment compared with the full irrigation treatment.
Potential for irrigation scheduling. Reginato et al. (1987) reviewed the state-ofthe-art on remote sensing and irrigation to conclude that radiometric measurements
of T f could be used to determine when plants are under stress and to quantify that
stress for irrigation scheduling purposes. Jones (1994) evaluated different ways to
enhance the sensitivity of a crop water stress indicator. A higher sensitivity was
observed when T f was compared with the temperature of either a wet or a dry
reference surface. Overall sensitivity is greatest when incident radiation is high, at
low ambient humidity and at low windspeeds. In another review Kadam and
Magar (1994) emphasised once more the potential of improving rrngation
scheduling using radiometric measurements of T f. This conclusion relied on
studies with tomatoes, wheat and chickpeas in India.
Actual transpiration and irrigation scheduling. Ben Asher et al. (1992) carried
out similar experiments in California and Israel on tomato and cotton to assess the
applicability of radiometric thermal infrared measurements to improve management of high frequency drip irrigation systems. They concluded that this method
could not yet improve the efficiency of high frequency irrigation. Adequate
measurements of T f were only obtained when water treatments differ largely from
one another. On the other hand good agreement was obtained under optimal water
regime when daily lysimeter measurements were compared to TIR estimated
transpiration.
391
65 d after sowing. The highest spectral reflectance was measured where 135
kgNha- 1 was applied. Spectral reflectance was related to dry matter yield. The
technique could not discriminate between some of treatments in the experiment.
D'Urso and Menenti (1996 a) developed an equation, based on the PenmanMonteith combination formula, to calculate crop coefficients from spectral
radiances and meteorological observations.
17.3.4 Crop water stress - Thermal Infrared
Field irrigation experiments. The relation of the surface radiometric temperature
of foliage T f with crop transpiration and water stress has been demonstrated in
several field studies. Kumar and Tripathi (1990) studied the impact on wheat crops
of different irrigation treatments. These authors observed that environmental
factors other than soil water availability influenced T f and proposed a water stress
indicator which included vapour pressure deficit. The significance of T f as a crop
water stress indicator was confirmed by other studies (e.g. Inoue, 1991) who
investigated photosynthetic activity in wheat, maize, soyabeans and cotton. On the
other hand the results presented by Reicosky et al. (1994) indicate a limited
sensitivity of T f to reductions in canopy photosynthesis and evapotranspiration of
spring wheat. These authors observed that differences in evapotranspiration and
canopy temperature were small, while canopy photosynthesis was reduced by 52%
in the deficit-irrigation treatment compared with the full irrigation treatment.
Potential for irrigation scheduling. Reginato et al. (1987) reviewed the state-ofthe-art on remote sensing and irrigation to conclude that radiometric measurements
of T f could be used to determine when plants are under stress and to quantify that
stress for irrigation scheduling purposes. Jones (1994) evaluated different ways to
enhance the sensitivity of a crop water stress indicator. A higher sensitivity was
observed when T f was compared with the temperature of either a wet or a dry
reference surface. Overall sensitivity is greatest when incident radiation is high, at
low ambient humidity and at low windspeeds. In another review Kadam and
Magar (1994) emphasised once more the potential of improving rrngation
scheduling using radiometric measurements of T f. This conclusion relied on
studies with tomatoes, wheat and chickpeas in India.
Actual transpiration and irrigation scheduling. Ben Asher et al. (1992) carried
out similar experiments in California and Israel on tomato and cotton to assess the
applicability of radiometric thermal infrared measurements to improve management of high frequency drip irrigation systems. They concluded that this method
could not yet improve the efficiency of high frequency irrigation. Adequate
measurements of T f were only obtained when water treatments differ largely from
one another. On the other hand good agreement was obtained under optimal water
regime when daily lysimeter measurements were compared to TIR estimated
transpiration.