Page 222 - Biosystems Engineering

200 Cha pte r S i x

leaf tissue, and the cut portion of the petiole will appear dry. How-
ever, when that leaf is placed within the chamber and the chamber is
pressurized slowly, the sap is squeezed back out through the leaf
petiole that is exposed to the atmosphere. The pressure at which the
sap begins to reappear on the surface of the cut petiole is considered
to be equal to the leaf water potential. The higher the leaf water
potential, the more the plant is water stressed. Due to the destructive
nature of this method, it is restricted to research only.

Leaf Color Analysis Method Methods have been developed to analyze
the color spectrum of leaf images obtained by digital cameras (Zakaluk
and Sri Ranjan 2007; Zakaluk and Sri Ranjan 2008). It is well known
that when grass turns bluish-green it is experiencing severe water
stress, and it is time for irrigation. However, digital image analysis
can detect subtle differences in reflectance in response to water stress
much earlier than is visible to the naked eye. Thus, irrigation could
be triggered at a much earlier stage of soil water depletion before the
plant suffers severe water stress. With advances in digital image
acquisition technology, the inexpensive hardware necessary to imple-
ment this method is within reach of irrigation equipment makers.
Biosystems engineers have a major role to play in developing this
further for wide adoption by farmers. It will lead to conservation of
water and savings in nutrients.

6.4 Need for Irrigation
The primary purpose of irrigation is to supply the crop water demand
beyond what is supplied by precipitation. Assuming an average of
5 mm/day ET and a 100-day ET demand period, areas receiving more
than 500 mm of precipitation during the growing season do not
require irrigation. Areas receiving less than 250 mm of precipitation
definitely need irrigation. Areas that receive precipitation between
250 mm and 500 mm will require irrigation to meet their supplemen-
tal needs. In addition, there are secondary benefits to irrigation as
well.
6.4.1 Meeting the Crop Water Demand
The primary purpose of irrigation is to supply water to plants. Areas
receiving precipitation in the form of snow will have soil moisture
accumulating within the root zone due to spring snowmelt infiltra-
tion. Soil moisture storage at the beginning of the season is used by
plants during the initial germination stage. This is also the most criti-
cal stage because of the shallow depth of roots that have a smaller soil
volume to store the water. The smaller volume of soil will require
more frequent irrigation than that is required during later stages of
growth with deeper roots. However, the water stored within the soil
profile will migrate toward the root zone to replenish the water