ESTABLISHING DIFFERENTIAL IRRIGATION LEVELS USING TEMPERATURE - TIME THRESHOLDS D. F. Wanjura, D. R. Upchurch, J. R. Mahan ABSTRACT. The requirements of precision crop management or limited water supply requires that either plant water status be varied during the growing season to optimize crop performance or a continuous level of limited irrigation must be practiced to match irrigation water availability. Canopy temperature (TC) has been successfully used to time irrigation applications for well-watered crop growing conditions. The cumulative daily time that TC exceeds a crop-specific temperature threshold, designated as stress time (ST), is used to indicate the need for irrigation. Manipulation of the ST value required to indicate the need for irrigation changes irrigation frequency and seasonal irrigation. The ST value that generates the irrigation signal is the time threshold (TT). This study developed a procedure for estimating the relationship between TT for cotton and seasonal irrigation rates. Data were analyzed from irrigation studies that measured the water application and cotton yield response to a range of average daily ST. All irrigation studies included a common control TT of 330 min/d above a canopy temperature threshold of 28°C as the criteria for the irrigation signal that maintained cotton in a well-watered status. In order to develop relationships between daily ST and final yield, or water input during the season, daily ST values were calculated and averaged over the irrigation season. The same procedure was used to calculate the average TC. The procedure for identifying control TT to establish different crop water status levels included the following steps. First, a linear relationship was defined between average daily ST and lint yield. Second, separate linear relationships were established between average daily ST and total water or irrigation input. Third, based on the slope of the trend line between average daily ST and yield, 1-h differences in average daily ST were selected to produce sufficiently large differences in total water and irrigation input to affect yield. These average ST values were 408, 468, and 528 min/d, and corresponded with control TT values of 330, 390, and 450 min/d. These control TT should result in different amounts of water application during the growing season and produce differences in cotton yield. Keywords. Irrigation scheduling, Water stress, Canopy temperature.
T
he increasing use of center pivot and more recently drip irrigation systems provide the capacity to uniformly apply water across a field. Scheduling becomes the next limiting factor to increasing yield and efficiency of water use with these irrigation systems. Subsurface drip irrigation systems typically apply daily irrigations to replace soil water depletion. A multi-year study with a linear move irrigation system equipped with Low Energy Precision Applicators (LEPA) applied irrigation to cotton beginning at first flower using irrigation intervals between 1 and 15 days (Bordovsky and Lyle, 1999). Intervals of 2 to 3 days were most beneficial for production, particularly at deficit levels of irrigation (amounts less than crop consumptive use). Irrigation scheduling methods either monitor soil and/or plant water status or compute a soil water budget to schedule irrigation based on the estimates of water depletion in the crop root zone (Fereres, 1999). An experimental procedure for timing irrigation, which has not yet been adopted by field
Article was submitted for review in February 2003; approved for publication by the Soil & Water Division of ASAE in October 2003. The authors are Donald F. Wanjura, ASAE Member Engineer, Agricultural Engineer, Dan R. Upchurch, Laboratory Director and Soil Physicist, and James R. Mahan, Plant Physiologist, USDA-ARS, Cropping Systems Research Laboratory, Lubbock, Texas. Corresponding author: Donald F. Wanjura, USDA-ARS, 3810 4th Street, Lubbock, TX 79415; phone: 806-749 -5560; fax: 806-723 -5272; e-mail:
[email protected].
crop production agriculture, views the plant as the integrator of its environment and measures canopy temperature as its indicator of water stress (Upchurch et al., 1996). This approach, named Biologically Identified Optimal Thermal Interactive Console (BIOTIC), daily accumulates time that canopy temperature is above a crop-specific temperature threshold and issues an irrigation signal when this time accumulation exceeds the locally calibrated amount of time (time threshold). The time when TC exceeds the temperature threshold is stress time (ST) for the crop since there is a deficit amount of water to fully meet transpiration demand. The temperature threshold for a crop is based on physiological parameters and the time threshold (TT) for well-watered conditions can be initially estimated with an energy balance model using local climatic data. Measured canopy temperature data can be used to check and refine TT values estimated from energy balance models (Evett et al., 2000; Wanjura et al., 1995; Wanjura and Upchurch, 1996, 1998). BIOTIC has been used to control irrigation timing for well-watered conditions. Four essential components for a systematic approach to irrigation management in precision agriculture were described by Colaizzi (1999). These components include (1) a pressurized irrigation system; (2) automated control, monitoring, and recording of water and other system inputs; (3) real -time feedback of soil, crop, and meteorological conditions; and (4) soil moisture modeling for future irrigation scheduling decisions and verification of components 2 and 3. BIOTIC has the potential of replacing components 3 and 4
Applied Engineering in Agriculture Vol. 20(2): 201-206
2004 American Society of Agricultural Engineers ISSN 0883-8542
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since it integrates the effect of soil, crop, and microclimate in developing the irrigation signal. The objective of this study was to develop a procedure for estimating the relationship of daily TT with cotton yield and seasonal water input. The sensitivity of TT to yield and water application is needed to extend its use to conditions where irrigation is limited. Data was analyzed from cotton irrigation experiments that used varying TT values above a temperature threshold of 28°C to control irrigation timing. This information should help to define a method for selecting TT levels that provide irrigation rates that create different water supply levels for crop production.
PROCEDURE IRRIGATION Information from prior field research that used TT to identify irrigation signals for controlling cotton irrigation was analyzed. Data were examined from studies using different irrigation levels that covered the range of water inputs from well watered, that maintained cotton in a fully watered condition, to limited water that subjected cotton to daily yield-limiting stress periods. Relationships or data that included irrigation and total water input, yield, and TT values that controlled timing of irrigation applications were examined. The first phase of the analysis examined existing relationships of TT values with cotton yield. Next, the relationship of TT to water input quantity was examined. In all analyses the stability of the relationships under environmental variation was considered by using data that included multiple years. Then a set of TT values was selected that correspond to different irrigation and yield levels. Performance of the control TT was evaluated in a field watered by a subsurface drip irrigation system. The data set included cotton irrigation studies conducted from 1997 to 1999 and 2001. The 1997 to 1999 studies were located three miles east of the Texas Agricultural Experiment Station at Lubbock, Texas (latitude 33° 41’ N, longitude 101° 46’ W, and altitude 988 m). The soil classification was Olton clay loam (fine, mixed, thermic, Aridic Paleustolls). The 2001 field study was located in the field of the Plant Stress and Water Conservation Laboratory, Lubbock, Texas (latitude 33° 35’ N, longitude 101° 53’ W, and altitude 958 m). The cotton cultivar Paymaster HS 2326 was planted on beds oriented north to south and spaced at 1 m at both locations. Following seedling establishment in all studies drip irrigation laterals were placed on each bed. The BIOTIC irrigation procedure was used to time irrigation events using a minimum irrigation interval of three days in all years. The irrigation interval increased in 1-d increments if an irrigation signal was not obtained on the third day (Upchurch et al., 1996). An irrigation signal that required the daily time accumulation of canopy temperature exceeding 28°C for 330 min/d resulted in soil moisture levels that maintained cotton in a well-watered condition (WW). Previous research (Wanjura et al., 1992) reported that water use for cotton irrigated with a 28°C temperature threshold was 86% of potential evapotranspiration. The 1997 study had three irrigation levels created by applying 21 mm (WW), 14 mm (0.66*WW), or 7 mm (0.33*WW) when an irrigation signal was generated in the WW treatment receiving 21 mm. The multi -year average potential evaporation for 3-d periods
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reported by the PET network for the Lubbock, Texas area was 21 mm for the summer irrigation period. The 1998 and 1999 studies also included the WW treatment that received 21 mm following each irrigation signal. The 1997 to 1999 studies included a treatment that only received rainfall (LW). The 2001 study used surface drip irrigation with a 21-mm WW treatment and a 10.5-mm 0.5*WW treatment. Irrigation timing in all treatments was determined by the WW treatment controlled by a 330-min/d TT to create the irrigation signal. The upper canopy surface temperature was continuously measured in all years with infrared thermocouples attached to poles in the rows center. The infrared temperature sensor viewed the canopy top from a nadir position. Average canopy temperature, dry and wet bulb temperatures, total radiation, net radiation, wind speed and direction, and rainfall, were recorded as 15-min averages by a CR7 Campbell Scientific data logger.
RESULTS AND DISCUSSION ANALYSIS OF CANOPY TEMPERATURES Average daily ST and average TC characteristic responses to different temperature thresholds and two irrigation levels during 2001 are presented in table 1. Temperature thresholds
Table 1. Average daily stress time (ST) and canopy temperatures for three periods defined by temperature threshold, air temperature (TA), and net radiation (Rn) for two irrigation levels, 2001.[a] Temperature Threshold 28_C
Statistic/Period
30_C
32_C
WW[b] Days with no time accumulation TC>28_C, TA>28_C, all Rn TC>28_C, TA>28_C, Rn>10 Wm -2 TC>28_C, TA>28_C, Rn>200 Wm -2 Stress time (ST), min d -1 TC>28_C, TA>28_C, all Rn TC>28_C, TA>28_C, Rn>10 Wm -2 TC>28_C, TA>28_C, Rn>200 Wm -2 Average TC, _C TC>28_C, TA>28_C, all Rn TC>28_C, TA>28_C, Rn>10 Wm -2 TC>28_C, TA>28_C, Rn>200 Wm -2
1 3 3
8 8 8
14 14 14
558 447 378
394 352 304
241 235 212
31.0 30.5 30.8
28.4 28.7 28.7
25.9 26.0 26.1
0.5*WW Days with no time accumulation TC>28_C, TA>28_C, all Rn TC>28_C, TA>28_C, Rn>10 Wm -2 TC>28_C, TA>28_C, Rn>200 Wm -2 Stress time (ST), min d -1 TC>28_C, TA>28_C, all Rn TC>28_C, TA>28_C, Rn>10 Wm -2 TC>28_C, TA>28_C, Rn>200 Wm -2 Average TC, _C TC>28_C, TA>28_C, all Rn TC>28_C, TA>28_C, Rn>10 Wm -2 TC>28_C, TA>28_C, Rn>200 Wm -2
0 1 1
0 1 1
0 2 2
660 495 414
549 474 401
441 414 365
35.3 36.4 37.2
36.5 36.5 37.3
37.8 36.8 37.4
[a]
The analysis period includes all days from DOY 187 through DOY 243. [b] A control TT of 330 min was used to determine irrigation signals in the well-watered treatment.
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of 28°C, 30°C, and 32°C for three daily periods illustrate the effect that these factors have on the magnitude of ST. The data included a 57-d interval from DOY 187 to DOY 243, when plant canopies were of sufficient size to measure TC without directly including the background soil through gaps in the canopy. The diurnal periods were determined by TC, air temperature (TA), and net radiation (Rn). Three periods were defined that satisfied the following conditions: (a) TC > 28°C, TA > 28°C, and all Rn levels; (b) TC > 28°C, TA > 28°C, and Rn > 10 Wm-2; and (c) TA > 28°C, TC > 28°C, and Rn > 200 Wm -2. These periods are progressively shorter portions of the total day that primarily result from the Rn values. For all diurnal periods, the measured TC must exceed 28°C and the conditions specified for each of the other two components must be satisfied before a time interval was added to the daily ST summation, which then had to exceed the designated TT value to produce an irrigation signal. Period (a) may include the entire 24-h period in some environments and is not a realistic condition for accumulating ST when solar radiation is zero. Period (b) includes the daylight period while the sun is above the horizon but the TC and TA limitations cause further reductions in the length. The Rn limitation of period (c) reduces the daylight period further and in some environments occasionally eliminates late afternoon periods while TC and TA remain above 28°C. Both periods (b) and (c) include all time when incident solar radiation is high, but the higher Rn value of period (c) increases the probability that high TT values will not be reached late in the growing season when day lengths are shortest. The number of days in 2001 when no time was accumulated above the TC thresholds increased as the temperature threshold increased in both irrigation treatments (table 1). The 0.5*WW water level had more days with time above each temperature threshold than the WW irrigation treatment. Average daily ST above temperature thresholds of 28°C, 30°C, and 32°C were calculated along with the corresponding average TC. The ST values decreased as the Rn levels of an accumulation period increased or the temperature threshold increased from 28°C to 32°C. There were small increases in average TC within a temperature threshold as Rn level increased. Both the ST and TC values were lower in the WW than the 0.5*WW treatment. The ST values linearly decreased as temperature threshold increased with a corresponding linear increase in average TC in both irrigation treatments (fig. 1). The difference in average daily 40
700 35
0.5*WW−TC
600
WW−TC
30
500 400
25
0.5*WW−ST
300
WW−ST
200
Average Daily Canopy Temperature (TC), 5 C
Stress Time (ST), min/day
800
20 27
28
29 30 31 Temperature Threshold, 5C
32
33
Figure 1. Temperature threshold relationships with stress time and average daily canopy temperature during period (b), 2001. Average values are based on all days between DOY187 and DOY243.
Vol. 20(2): 201-206
ST between the 0.5*WW and WW treatments was 48, 122, and 179 min as the temperature threshold increased from 28°C to 32°C in increments of 2°C. The information in table 1 and figure 1 can be used to answer additional questions about the use of time and temperature thresholds to optimize precision of irrigation control. The first question is how do ST values change as the temperature threshold increases from 28°C to 32°C within an irrigation level? For the WW treatment, ST decreased by 47% as temperature threshold increased from 28°C to 32°C in the proportion of -21% and -26%, from 28°C to 30°C and 30°C to 32°C, respectively (fig. 1). The ST in the 0.5*WW treatment also decreased, but only by 16% as the temperature threshold increased from 28°C to 32°C in the proportion of -4% and -12% from 28°C to 30°C and 30°C to 32°C, respectively. The lower sensitivity of ST in the 0.5*WW was due to its higher canopy temperatures which frequently exceeded the interval from 28°C to 32°C. Thus ST that maintained cotton in a well-watered condition should have lower stability than limited irrigated conditions where water stress exists. A related question refers to a situation where differences in irrigation levels result in significant differences in plant size. In this situation, how do ST of the different size plants change in relation to one another? The ST at each temperature threshold shows that the 0.5*WW values were always higher than for WW (fig. 1). The ST values of the 0.5*WW treatment were greater than the WW treatment by 11% at 28°C, 35% at 30°C, and 76% at 32°C. The WW treatment stress time treatment decreased faster with higher temperature thresholds than the 0.5*WW treatment and resulted in the increasing difference in ST values between the two water levels. If these data are representative of the water level effect that result from differences in crop plant size, the results can be generalized to stating that ST decreases at a faster rate in well -watered than in limited-water cotton, as temperature thresholds increase. The use of TT and temperature thresholds for controlling irrigation timing needs to consider if different water levels are most effectively achieved by (a) using variable temperature thresholds with a constant TT or with (b) a constant temperature threshold combined with variable TT? The quantity of irrigation applied by the WW and 0.5*WW scheduling treatments differed by 50% based on the WW treatment. The difference in ST between the two water levels was 48 min at 28°C, 122 min at 30°C, and 179 min at 32°C temperature thresholds (fig. 1). These numbers would indicate that as temperature threshold increased the differences in ST between WW and 0.5*WW also increased. The data suggests that small differences in ST for low temperature thresholds should produce water level differences equal to those of higher temperature thresholds with larger differences in TT. The important unknown is how consistent is yield and crop water status to a fixed temperature threshold that uses the same set of multiple TT in a given location over a series of years. The ST values in table 1 and those used in other analyzes were computed for the growing season period from DOY 187 to DOY 243, which includes most of July and the month of August. During this period plant canopies have usually reached sufficient size to enable accurate measurements of canopy temperature. Flowering begins during the early part of the period which leads to boll formation at some fruiting
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USE OF TIME THRESHOLDS TO ESTABLISH DIFFERENT YIELD LEVELS Diurnal period (b), determined by the condition of TA > 28°C and TC > 28°C with Rn > 10 Wm -2, was used in the TT analysis and selection procedure since it included most of the daytime period while the sun was above the horizon. Average daily ST for period (b) was used to examine the relationship between ST and lint yield. A linear relationship describes the association between average daily ST and lint yield for irrigated treatments in 1997, 1998, 1999, and 2001(fig. 2). The reduction in lint yield increased as average daily ST became larger, indicating that cotton was experiencing water stress for more time each day. The WW treatments in 1997, 1998, and 2001 plus the 0.66*WW 1997 treatment, were highly correlated with lint yield. The 1999 WW treatment deviated from the linear regression line as do the more limited irrigation treatments of 0.33*WW in 1997 and 0.5*WW in 2001. The 1999 WW treatment received vegetative damage from hail during a 7.8-cm rain on DOY 163, which was 21 days before the beginning of the averaging period for TT (Wanjura et al., 2002). The TT-yield curve in figure 2 will be used to select different irrigation levels that should result in different cotton yield levels. Based on the control TT of 330 min/d, the average daily ST over a period of approximately 60 d averaged 404 and 411 min/d, respectively, in 1997 and 1998. The average observed ST was 408 min/d (indicated by the intersection of the first vertical line and the regression line in fig. 2) during the two years, which is 78 min/d greater than the control TT of 330 min/d used to establish an irrigation signal. Control TT of 330, 390, and 450 min/d are chosen as irrigation signal criterion to establish different water levels that result in different yields. Assuming that an increase of 1600 1997
Lint Yield, kg/ha
1400 WW 1200
1998 WW
1997 0.66*WW
1000
1999 WW 2001 WW
1997 0.33*WW
800 600 400
2001 0.5*WW
2
LY = 5049 - 8.73*ST, R = 0.75 200 400
420
440
460
480
500
520
540
Average Daily Stress Time (DOY 187 − DOY 243), min
Figure 2. Cotton lint yield vs. average daily stress time during period (b) in 1997, 1998, 1999, and 2001. The seasonal period for the daily stress time averages was DOY 187 to DOY 243. Average stress times included all days of the seasonal period, including those days with zero ST accumulation about 285C. The three vertical lines are the estimated average daily ST for control TT of 330, 390, and 450 min/d.
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Table 2. Stress time and water application for time threshold (TT) controlled irrigation studies conducted between 1997 and 2001. Year Stress Time (ST) Water Application (cm)[a] Treatment 1997 Rain (LW) 0.33*WW 0.66*WW WW[b] 1998 Rain (LW) WW 1999 Rain (LW) WW 2001 0.5*WW WW
Interval (d)
Average (min/d)
Irrigation
Total
DOY 188-243
535 504 432 404
0.0 5.9 23.9 36.0
22.9 28.9 46.9 58.9
DOY 187-243
538 411
0.0 44.0
14.8 58.8
DOY 187-243
588 466
0.0 31.7
22.6 54.3
DOY 187-243
495 447
19.1 35.9
29.6 46.4
[a]
Water application amounts are for the interval from planting date through DOY 243. [b] The control TT for the WW treatment in each year was 330 min/d.
78 min/d would occur for each control TT, the average TT are 408, 468, and 528 min/d. The resulting yield levels based on the linear relationship in figure 2 are 1490, 965, and 440 kg lint/ha. Yields achieved in another year may vary from the estimated yields in figure 2 depending on growing season weather conditions; however, the relative yield differences should be proportional. TIME THRESHOLDS ESTABLISH DIFFERENT IRRIGATION LEVELS The previous section related average daily ST to yield. These ST are shown in table 2 and were correlated with total water applied (irrigation plus rainfall) from planting through DOY 243 (fig. 3) and then only with amount of irrigation for the same time interval (fig. 4). All irrigation treatments, except the 1999 WW treatment, were closely associated in a common linear relationship. The regressions in figures 2 and 3 indicate that the yield and total water applied to the 1999 WW treatment was higher than the trend line for all Total Water Applied ( Planting − DOY 243 ),cm
nodes that collectively represent potential yield, which ultimately results from the cumulative boll weight. Thus this period includes the establishment of potential yield (number of bolls) and the beginning of boll maturation that determines actual yield.
70 1997 WW
60
1998 WW
1999 WW 2001 WW
50 1997 0.66*WW
40
1997 0.33*WW
30 2001 0.5*WW
20 10 0
TW = 176 − 0.29*ST, R2= 0.79 400
420
440
460
480
500
520
540
Average Daily Stress Time ( DOY 187 − DOY 243 ), min
Figure 3. Estimated relationship of average daily time with total water applied for irrigated treatments, 1997 to 1999, 2001. The three vertical lines are the estimated average daily ST for control TT of 330, 390, and 450 min/d.
APPLIED ENGINEERING IN AGRICULTURE
Irrigation Applied ( Planting − DOY 243 ), cm
70 60 1998 WW
50
2001 WW
40
1999 WW
1997 WW
30
2001 0.5*WW
1997 0.66*WW
20 10
1997 0.33*WW
2
I = 150 - 0.27*ST, R = 0.68 0 400
420
440
460
480
500
520
540
Average Daily StressTime ( DOY 187 − DOY 243 )
Figure 4. Estimated relationship of average daily stress time with irrigation from 1997 to 1999, 2001. The three vertical lines are the estimated average daily ST for control TT of 330, 390, and 450 min/d.
treatments according to its average daily ST. The ST for the1999 WW treatment was greater than for the WW treatments in the other years, and its canopy size was smaller on DOY 187, probably due to the earlier hail damage sustained on DOY 163. This data suggests that the hail increased ST but had less adverse effect on yield. Using control TT of 330, 390, and 450 min/d to create irrigation signals for three separate water levels resulted in the average daily ST of 408, 468, and 528 min/d shown in figure 3. The total water applied was 59, 41, and 24 cm. Cumulative irrigation between planting and DOY 243 was 40, 24, and 7 cm for the same average daily ST according to the linear relationship between ST and applied irrigation given in figure 4. A comparison of figures 3 and 4 indicates that both total applied water and irrigation between planting to DOY 243 are linearly related with average daily ST between DOY 187 and DOY 243. The average daily ST reflect the crops’ water stress level, which is influenced by total soil water input consisting of rainfall and irrigation. The regression lines for average daily ST with total water applied and irrigation have similar slopes indicating that average daily ST is not differentiating between the two sources of water input. The irrigation scheduling program protocol recognized rainfall as part of the water input by assuming
Figure 5. Relationship of irrigation only or total water application with lint yield in irrigation treatments in 1997 to 1999, 2001.
Vol. 20(2): 201-206
75% of the quantity of a rain event was effective water available to the crop to substitute for an irrigation event. The relation of average daily ST with total applied water in figure 3 shows a high correlation for all treatments except 1999 WW. However, the correlation of average daily ST and irrigation applied shows more scatter of data points about the regression (fig. 4). All 1997 irrigation treatments are below the trend line and the other years are above. In the average daily -ST irrigation applied relationship, the 1999 WW treatment fits the pattern of 1998 and 2001 data. If the average daily ST are indicators of crop water stress, total water applied should influence water stress more closely than applied irrigation, which averaged 58% of the total water for all treatments. Total water should also be more highly related with yield than total irrigation, as indicated in figure 5. The average daily ST of the irrigated treatments calculated from measured canopy temperature, linearly correlated with lint yield (fig. 2). These average daily ST also had a negative linear correlation with total water and irrigation inputs, with R2 values of 0.79 and 0.68, respectively (figs. 3 and 4). Water inputs had a positive linear relationship with lint yield where total water was more highly correlated (R2 = 0.82) than irrigation input (R2 = 0.48) (fig. 5). The irrigation program protocol uses rain in the total water application to control crop water status, which should then determine yield more directly than irrigation. All relationships are linear due to the range of TT values. The resulting water inputs had a limited range because all treatments received some irrigation and rainfall within years was similar. In a more extended range of values these relationships would be expected to become more complex. A method for selecting TT values that create different levels of water input with irrigation timing can be identified from the relationships in figures 2 to 4. All average daily ST values resulted from a control TT of 330 min. The WW irrigation treatments in three out of the four years produced yields between 1152 and 1510 kg/ha with average daily ST ranging from 404 to 447 min/d (fig. 2). The WW treatments in 1997 and 1998 produced the highest and similar yields (fig. 2) from the same total water input (fig. 3). These two treatments were averaged to represent optimum well-watered conditions and their average TT of 408 min/d is shown as vertical lines in figures 2 to 4. The difference between average daily ST and the control TT is 78 min/d (408 330 min/d). The average daily ST values are larger than the control TT because the crop cannot maintain a constant canopy temperature by transpiring. Thus differences in daily environments will result in some variation in the time that canopy temperature remains above the temperature threshold of 28°C, even though soil moisture is adequate for supplying water to the plant. Aboveground environments also cause the driving force of atmospheric demand to fluctuate and affect canopy temperature. Based on the regression line slopes in figures 3 and 4, 1-h differences in average daily ST were judged to cause sufficiently large differences in total water and irrigation input to affect yield. Average daily ST values of 408, 468, and 528 are identified as vertical lines intersecting the trend lines in figures 2 to 4. These average daily ST assume that the difference of 78 min/d between average daily ST and control TT values apply to all TT values. The control TT values corresponding to these average daily ST are 330, 390, and 450 min/d.
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CONCLUSIONS
REFERENCES
The characteristic response of average daily ST and TC to different temperature thresholds, irrigation levels, and length of daily time period was demonstrated using weather conditions for the 2001 growing season.An increase in canopy temperature threshold was related to a decrease in average daily ST. Longer diurnal accumulation periods increased average daily ST but decreased average TC values. Smaller average daily ST values were associated with increased irrigation quantity. Negative linear relationships described the association between average daily ST with lint yield, total applied water, or irrigation. Average daily ST values are greater than their control TT that determine the time requirements for irrigation signals because well-watered cotton does not maintain a constant canopy temperature in fluctuating atmospheric environments. A method for selecting TT values that produces different water input levels with irrigation timing was delineated. First, a linear relationship was identified between average daily ST and lint yield. Second, a linear relationship was established between average daily ST and total water or irrigation input. Third, based on the slope of trend line between average daily ST and yield, 1-h differences in average daily ST were judged to produce sufficiently large differences in total water and irrigation input to affect yield. Control TT values of 330, 390, and 450 min/d resulted in corresponding average daily ST values of 408, 468, and 528 min/d.
Bordovsky, J. P. and W. M. Lyle. 1999. Evaluation of irrigation interval on high plains cotton production with LEPA systems. Proc. Beltwide Cotton Conf., 372-375. Memphis, Tenn.: National Cotton Council. Colaizzi, P. D. 1999. A four layer approach for irrigation management in precision agriculture. ASAE Paper No. 992172. St. Joseph, Mich.: ASAE. Evett, S. R., T. A. Howell, A. D. Schneider, D. R. Upchurch, and D. F. Wanjura. 2000. Automatic drip irrigation of corn and soybean. Proc. 4th Decennial National Irrigation Symp., 401-408. St. Joseph, Mich.: ASAE. Fereres, E. 1999. Irrigation scheduling and its impact on the 21st century. International Water & Irrigation 19: 24-28. Upchurch, D. R., D. F. Wanjura, J. J. Burke, and J. R. Mahan. 1996. Biologically-Identified Optimal Temperature Interactive Console (BIOTIC) for managing irrigation. United States Patent No. 5,539,637. Wanjura, D. F., D. R. Upchurch, and J. R. Mahan. 1992. Automated irrigation based on threshold canopy temperature. Transactions of the ASAE 35(5): 1411-1417. Wanjura, D. F., D. R. Upchurch, and J. R. Mahan. 1995. Control of irrigation scheduling using temperature-time thresholds. Transactions of the ASAE 38(2): 403-409. Wanjura, D. F., and D. R. Upchurch. 1996. Time thresholds for canopy temperature-based irrigation. Proc. Intl. Conf. Evapotranspiration and Irrigation Scheduling, 295-302. St. Joseph, Mich.: ASAE. Wanjura, D. F., and D. R. Upchurch. 1998. Evaluation of analytical methods for daily estimation of crop water stress. ASAE Paper No. 982122. St. Joseph, Mich.: ASAE. Wanjura, D. F., D. R. Upchurch, J. R. Mahan, and J. J. Burke. 2002. Cotton yield and applied water relationships under drip irrigation. Agric. Water Mgt. 55: 217-237.
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