Field Determination of Permanent
E.R. Norton, Plant Sciences Department
J.C. Silvertooth, Plant Sciences Department
Water is a vital resource for cotton production in the desert Southwest.
One method of managing irrigation water is through the use of a "checkbook"
approach to irrigation scheduling. This involves irrigating based upon the
percent depletion of plant available water (PAW) from the soil profile. In
order to effectively utilize this method of irrigation scheduling soil water
content values at field capacity (FC) and permanent wilting point (PWP) must be
defined. In this study the PWP values were characterized for two different soil
types, one at Maricopa, AZ and another at Marana, AZ. The possibility of having
different values for PWP as a function of crop stage of growth was also
investigated in this study. Results demonstrated differences in both FC and PWP
values between the two locations. Differences were also observed as a function
of crop growth stage in the pattern of soil water extraction at the Maricopa
There are several important inputs that go into producing a successful crop of
cotton. In order to attain maximum economic yield all inputs must be managed in
an optimal fashion. The first limiting factor and most critical input in desert
agriculture is water. If water is not managed in an optimum fashion, management
of other inputs such as fertilizers, PGR's, insecticides, etc. become much less
significant. The goal in managing irrigation water is to apply a sufficient
amount at the proper time to meet crop needs. There are several different
approaches that can be taken to schedule irrigation events. The use of infra-red
thermometry, pressure-bomb readings, computer models, etc. are all methods that
have been used in scheduling irrigations. In this study we wanted to investigate
some important parameters used in what is called a "checkbook" method to
determine irrigation timing.
The "checkbook" method is based upon a very simple premise. The amount of
water entering and leaving the system is monitored by keeping track of inputs
(irrigation and rainfall events) and water leaving the system
(evapotranspiration). Irrigation events are then scheduled based upon a
percent depletion of the plant available soil water (PAW). A very
important point to remember is that the total amount of water applied to the
system is not necessarily the amount of water that is available to the crop.
Due to the physical nature of the soil, the amount of PAW is usually much less
than the total, and is highly dependent on soil texture. Plant available water
(PAW) is defined as the difference between field capacity (FC) and permanent
wilting point (PWP). In general terms, PAW is defined (Miller and Donohue, 1995)
as the difference between soil water held at -33 kPa (FC) and -1500 kPa (PWP).
However, PWP is more functionally defined as the point (i.e. soil water content)
at which plants wilt but do not recover overnight (Taiz and Zeiger, 1991).
These terms are very general and will actually differ among soil types. The FC
however, is more easily determined and is essentially the soil water content
directly after an irrigation when gravity has drained water out of the profile.
PWP is somewhat more difficult to identify. Since PWP is best defined by the
condition of the plant, there are several factors that could affect PWP,
including; crop species, soil type, and crop stage of growth. In order to
effectively use the checkbook method for scheduling irrigations PAW and thus the
FC and PWP points must be clearly identified. Once these points are identified,
then irrigations can be scheduled based upon a percent depletion of PAW.
Depletion or evapotranspiration can be calculated from a reference ET multiplied
by the appropriate crop coefficient resulting in a crop ET. For Arizona crop
production crop ET values can be obtained from local AZMET weather stations
maintained by the University of Arizona. This method for irrigation scheduling
is used quite extensively in crop production situations. It has even been taken
to level of computer automation in programs like AZSCHED that was developed at
the University of Arizona (Fox et al., 1992). The information generated by this
type of research could easily be used to improve and validate such computer
models. Table 1 outlines some general PAW values
for several different soil types and Figure 1
graphically represents this concept. The objectives of this study were to
identify PWP soil water content values for two different soil types and to
identify how these values might change as a function of crop stage of growth.
Materials and Methods
Results from the 1997 study were very similar to the results found in the
1996 study (Norton and Silvertooth, 1997). In order to accomplish the stated
objectives two separate studies were established, one at the University of
Arizona Maricopa Agricultural Center on a Casa Grande sandy loam and the other
was at the University of Arizona Marana Agricultural Center on a Pima clay loam
(Table 2). Four blocks of 12, 40 inch rows
extending the full length of the irrigation run (approximately 600 feet) were
established at each location. Deltapine 33B was planted on 19 March and 17
April at Maricopa and Marana respectively. Two neutron probe access tubes were
placed down the center of each block spaced approximately 200 feet apart at both
locations. Neutron probe readings were conducted before and after each
irrigation. Irrigations were terminated at different stages of growth for each
of the four blocks. The first block received only a water-up at irrigation at
Maricopa and only a pre-irrigation at Marana. The second block was terminated
just prior to first bloom and received no further irrigations. The third block
was terminated after peak bloom receiving no more irrigations. The fourth block
was taken into cut-out before irrigations were terminated.
Table 3 provides a list of irrigation events
for all treatments at each location. PWP was identified visually for
each block. PWP is defined by Taiz and Zeiger (1991) as the point at which the
plant does not recover overnight from a water stress induced wilt. This was
the criteria used to identify PWP. Soil water content was then identified at
that point for each block using neutron probe readings.
Results and Discussion
At the Maricopa location, values for volumetric soil water content varied
according to stage of growth (Figure 2).
Treatment 1, which reached PWP just prior to first bloom still had a high
volumetric water content below 2 feet. As treatments 2 and 3 were brought to
PWP, more of the soil water below 2 feet was extracted.
These results demonstrate the importance of realizing from what depths the
crop is extracting water at different stages of the season. Water
extraction patterns at Marana differed slightly than those at Maricopa.
An increase in depth of extraction was not observed as the crop increased
in stage of growth. Several irrigations at Marana did not completely fill
the profile. Water penetration during these irrigation events reached to
approximately three feet. Since the fourth and fifth feet were very seldom
replenished the effect of increasing depth of extraction was not observed.
Tables 4 and 5
list the volumetric soil water content associated with the identification of
PWP for each of the three treatments at Maricopa and Marana respectively.
Also shown in these tables is the calculated PAW based upon FC and PWP for each
of the three treatments. The first and most obvious result to notice is the
differences in the amount of PAW between Maricopa and Marana. Marana, Pima
clay loam soil, has a much higher water holding capacity than the sandy loam at
Maricopa. A comment should be made regarding PAW values at lower depths at
Marana. It is possible that values for PAW are overestimated due to the
extremely low water contents at the fourth and fifth foot depths (particularly
treatment 2). If irrigations were being scheduled based on percent depletion of
PAW an optimum irrigation regime would call for irrigations more frequently at
Maricopa than Marana. This study will be continued at both Maricopa and Marana
for the 1998 growing season.
- Fox Jr., Fred A, T. Scherer, D.C. Slack and, L.J. Clark. 1992. AZSCHED
Arizona Irrigation Scheduling. User's Manual. The University of Arizona.
Cooperative Extension. Agricultural and Biosystems Engineering. Version 1.01.
- Miller, R.W. and, R.L. Donahue. 1995. Soils In Our Environment.
Prentice Hall Inc. A Simon and Schuster Co. Englewood Cliffs, NJ.
- Norton, E.R. and J.C. Silvertooth. 1997. Field determination of permanent
wilting point. Cotton. A college of agriculture report. Series P-108. pp 185-191.
- Taiz, L. and E. Zeiger. 1991. Plant Physiology. The Benjamin/Cummings
Publishing Co., Inc. Redwood City, CA.
This is a part of publication AZ1006:
"Cotton: A College of Agriculture Report," 1998, College of Agriculture,
The University of Arizona, Tucson, Arizona,
85721. Any products, services, or organizations that are mentioned, shown, or indirectly
implied in this publication do not imply endorsement by The University of Arizona.
The University is an Equal Opportunity/Affirmative Action Employer.
This document located at http://ag.arizona.edu/pubs/crops/az1006/az10065d.html
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