Donald F. Wanjura

Cotton yield and applied water relationships under drip irrigation

Abstract Different irrigation scheduling methods and amounts of water ranging from deficit to excessive amounts were used in cotton ( Gossypium hirsutum L.) irrigation studies from 1988 to 1999, at Lubbock, TX. Irrigation scheduling treatments based on canopy temperature ( T c ) were emphasized in each year. Surface drip irrigation and recommended production practices for the area were used. The objective was to use the 12-year database to estimate the effect of irrigation and growing season temperature on cotton yield. Yields in the irrigation studies were then compared with those for the northwest Texas production region. An irrigation input of 58xa0cm or total water application of 74xa0cm was estimated to produce maximum lint yield. Sources of the total water supply for the maximum yielding treatments for each year averaged 74% from irrigation and 26% from rain. Lint yield response to irrigation up to the point of maximum yield was approximated as 11.4xa0kgxa0ha −1 xa0cm −1 of irrigation between the limits of 5 and 54xa0cm with lint yields ranging from 855 to 1630xa0kgxa0ha −1 . The intra-year maximum lint yield treatments were not limited by water input, and their inter-year range of 300xa0kgxa0ha −1 was not correlated with the quantity of irrigation. The maximum lint yields were linearly related to monthly and seasonal heat units (HU) with significant regressions for July ( P =0.15), August ( P =0.07), and from May to September ( P =0.01). The fluctuation of maximum yearly lint yields and the response to HU in the irrigation studies were similar to the average yields in the surrounding production region. The rate of lint yield increase with HU was slightly higher in the irrigation studies than in the surrounding production area and was attributed to minimal water stress. Managing irrigation based on real-time measurements of T c produced maximum cotton yields without applying excessive irrigation.

Crop water stress index relationships with crop productivity.

SummaryField experiments between 1983 and 1987 were used to study the effect of crop development on crop water stress index (CWSI) parameters and the relationship of CWSI with the yield of cotton and grain sorghum. The absolute slopes of nonstressed baselines (NSBL) generally increased until canopy cover reached 70% (Table 1). NSBL derived from data collected when canopy temperature exceeded 27.4 °C had greater absolute slopes and higher R2-values than NSBL that included all diurnal measurements (Table 1). Average CWSI values of cotton and grain sorghum grown under varying soil water regimes were negatively correlated with yield. Grain sorghum yield was more sensitive to CWSI values than was cotton lint yield (Figs. 1 and 2). Multiyear data analysis indicated that yields from cotton that experienced a completely stressed condition during part of each day during the boll setting period would be 40% of those from completely nonstressed cotton (Fig. 3). Negative values of CWSI computed for cotton growing under non-water stressed conditions were associated with uncertainties in calculations of aerodynamic resistance (raand in estimating canopy resistance at potential evapotranspiration (rcp).

Determination of temperature and time thresholds for BIOTIC irrigation of peanut on the Southern High Plains of Texas

The timely application of irrigation water to a crop is essential for optimizing yield and production efficiency. The “Biologically Identified Optimal Temperature Interactive Console (BIOTIC)” is an irrigation protocol that provides irrigation scheduling based upon measurements of canopy temperatures and the temperature optimum of the crop species of interest. One of the goals of this paper is to document the gradual development of the method and its implementation. Two threshold values are required to implement BIOTIC irrigation of a crop in a given region, a species-specific temperature threshold and a species/environment-specific time threshold. The temperature threshold, an indication of the thermal optimum for the plant, is derived from the thermal dependence of its metabolism. The time threshold, which represents the average amount of time each day that the canopy temperature of the well-watered crop will exceed the temperature threshold, is calculated from weather data. Interest in the use of BIOTIC for irrigation scheduling for peanut ( Arachis hypogaea L.) resulted in this study in which the temperature and time thresholds for peanut were determined on the Texas Southern High Plains. A temperature threshold value of 27°C was determined from the thermal dependence of the reappearance of photosystem II variable fluorescence (PSII Fv) following illumination. A time threshold of 405xa0min was calculated from an analysis of weather data collected over the course of the 1999 growing season. The determination of these threshold values for peanut provides the basis for the application of the BIOTIC protocol to irrigation scheduling of peanut on the Southern High Plains of Texas.

Canopy temperature and water stress of cotton crops with complete and partial ground cover

SummaryThe use of canopy and air temperature differences to compute a crop water stress index (CWSI) for assessing plant water status was investigated using cotton crop canopies that either fully or partially covered the ground. The complete ground cover canopy condition was studied in a well watered moisture regime in a rainout shelter with measurements made on six Texas cotton race stocks. The partial ground cover canopy situation was investigated in a well watered moisture regime of a commercial cotton variety ‘Paymaster 266’ grown in the field. The slope of the nonstressed baseline of the CWSI for a cotton canopy with about 50% ground cover was approximately one-half that reported for full canopies. Values of CWSI calculated with “theoretical” and “empirical” procedures agreed more closely under a complete canopy condition than under a partial canopy situation. Values of aerodynamic resistance (ra) and canopy resistance for well watered soil moisture conditions (rep)were estimated in order to use the theoretical procedure of computing CWSI. Values of raranged from 10 to 15 sm−1 and rcpfrom 50 to 60 sm−1. Both the theoretical and empirical procedures showed much promise, but more information is needed to develop techniques for evaluating raand rcpunder differing canopy and environmental conditions.

Behavior of temperature-based water stress indicators in BIOTIC-controlled irrigation

A subsurface drip irrigation study with cotton used canopy temperature to determine signals for irrigation control during 2002–2004. Timing of irrigation applications was controlled by the biologically identified optimal temperature interactive console (BIOTIC) protocol, which used stress time (ST) and a crop-specific optimum temperature to indicate water stress. ST was the cumulative daily time quantity when cotton canopy temperature exceeded 28°C. STs between 5.5 and 8.5xa0h in 1xa0h increments were irrigation signal criteria, which produced different irrigation regimes. This investigation examined the association among ST, daily average canopy temperature (Tc), canopy and air temperature difference (Tc−Ta), and the relative crop water stress index (RCWSI) including their relationship with lint yield. Number of irrigation signals decreased linearly with ST at the rate of −10.2 and −8.7 irrigations per 1xa0h increase of ST in 2003 and 2004. There were significant curvilinear relationships between ST and the average daily stress on days with irrigation signals and for days without irrigation signals across years. The percentage of positive daily (Tc−Ta) values increased with ST level. ST and Tc were positively related in all irrigation signal treatments with 5.5 and 6.5xa0h being significant in 2003 and 2004. Yield declined at the rate of 343xa0kgxa0lint/ha for each 1xa0h increase of ST for days with irrigation signals. ST, mathematically the most simple of the canopy temperature-based parameters, provided the most consistent estimate of crop water stress and correlation with lint yield. The power of ST to characterize water stress effects on crop productivity evolves from being an integrated value of time while canopy temperature exceeds a physiologically based threshold value.

Accounting for humidity in canopy-temperature-controlled irrigation scheduling

Abstract High moisture content in the air surrounding crop canopies can reduce transpiration and increase canopy temperature (Tc) independently of soil moisture. Humid conditions can affect the accuracy of irrigation signals produced by a canopy-temperature-based irrigation scheduling procedure that uses a time threshold (TT), which is the daily summation of time above the temperature threshold (To) defined as the midpoint of the crops optimum temperature range. Because historical crop canopy temperature data were unavailable, an energy balance model was used to simulate time threshold values for different climates. A limiting relative humidity (LRH) algorithm was added to the model to estimate whether canopy temperatures that exceed the (To) were affected by high humidity. The LRH was computed from Ta and δT, denoted as (To) - T wb ∗ , where T wb ∗ is the highest wet bulb temperature that does not increase Tc. Time periods of restricted transpiration were identified by calculating ambient relative humidity (RH) and comparing it to the LRH value. If RH > LRH, canopy temperature was assumed to be increased by a reduction in transpiration. In a humid climate the LRH criterion reduced the simulated average TT value by 27%, 51%, and 69%, respectively, for δT values between 3°C and 5°C. This same LRH reduced the TT values by 16%, 32% and 36%, respectively, in a semiarid climate. The LRH criterion had no effect on the average TT value in the and climate. Estimated TT values had the lowest variability among years for a AT value of 4°C in the humid and semiarid climates. A generalized curve described the TT versus ΔT relationship across a wide spectrum of climates. The LRH procedure produced consistent adjustments to TT; however, further refinements may be needed to improve the accuracy of estimating daily TT when weather conditions are highly variable.

Thermal environment of cotton irrigated using canopy temperature

The threshold canopy temperature method for controlling a drip irrigation system includes a physiologically based threshold temperature and irrigation application rate that responds to the environment. Energy input from the environment causes canopy temperature to exceed the threshold value and irrigation is then applied. This study evaluated temperature distributions, amount of optimum time, and the amount of irrigation control time for cotton where irrigation scheduling was controlled by different threshold temperatures during the years 1988 to 1991. Optimum time for cotton growth was defined as the accumulated time that canopy temperatures were between 25 and 31 °C and the time accumulated above different threshold temperatures was designated as irrigation control time. Threshold temperatures over a 26 to 32 °C range altered the frequency distribution of temperature within the optimum temperature range (25–31 °C) by reducing temperatures above the threshold. Frequency of canopy temperatures of a 28 °C threshold temperature treatment decreased in the 28 to 29 °C increment and then remained below air temperature. Irrigation control time was more sensitive than optimum time to changes in threshold temperature between 26 and 31 °C. Optimum time and irrigation control time of the 28 °C threshold temperature varied by 37% and 29%, respectively. Lint yields in 1988 and 1990 were high while those in 1989 and 1991 were low because of unfavorable weather. Irrigation amounts applied during DOY 198–273 that were above 20 cm in high yield years or 12 cm in low yield years did not increase yield.

COTTON LINT YIELD ACCUMULATION RATE AND QUALITY DEVELOPMENT

Abstract Crop production management during the period of boll setting and maturation of cotton (Gossypium hirsutum L.) is critical in determining lint yield and quality. A field investigation conducted at Lubbock, Texas, during 1977–1980 included three cultivars to study the effect of temperature on factors that influence the rate of cotton yield accumulation and the development of lint quality parameters. A linear regression of physiological days (heat units) explained 93% of the variation in prebloom period expressed in days. The number of physiological days needed to complete the prebloom period was negatively related to average daily temperature in a linear manner and included a significant cultivar effect. The length of the boll setting period was determined by temperature and plant moisture stress. In the absence of plant water stress, 20–25 physiological days were needed to complete boll setting. The length of time to fully mature bolls was related to air temperature in a negative exponential manner (r2 = 0.85) with little direct influence of plant water stress or effect of cultivars. Average boll weight of crop increments fluctuated, but the only trend across years was for the mean boll weight of the last two crop increments to be smaller than the average for the total crop. However, there was a direct correlation between the percentage of total bolls set and percentage of final yield. Fiber micronaire was correlated with temperature in a positive linear manner. In the absence of significant plant water stress, temperature explained 80–90% of the micronaire values of individual cultivars compared with 60% when moisture stress conditions were included. An average crop boll temperature of 27°C appears adequate to achieve maximum development of fiber micronaire. Fiber strength also displayed a linear increase as boll period temperature increased, with coefficients of determination ranging from 0.60 to 0.77 among cultivars. Plant water stress significantly reduced micronaire values but had little effect on fiber strength. Fiber length was not affected by the range of environmental conditions during the four-year period of the study.

Water status response of corn and cotton to altered irrigation

Abstract. Water is a primary limiting factor to crop production and thus crop water status is essential information for management decisions. Corn and cotton were grown in the field under two constant water regimes. The low water level (WL) was 0.66×PET (potential evapotranspiration) in corn and rainfall for cotton. The high water level (WH) was 1.0×PET for both crops. Two transient water treatments in each crop began as the two constant water level treatments but then the water inputs were reversed and the change in water status was monitored. When the transient water treatments were initiated, corn was at the V14 and V16 growth stages in the WL and WH treatments, respectively, and cotton was 2xa0weeks past first bloom for both water levels. The purpose of the experiment was to compare the sensitivity of leaf water potential (LWP) and crop canopy temperature to changes in irrigation rate. The transient water treatment of each crop that relieved water stress (TLH) changed from WL to WH and the treatment which induced water stress changed from WH to WL (THL). The LWP values of the transient water treatments reversed 5 and 8xa0days after reversing water input rates to corn in 1998 and 1999, respectively, and after 3xa0days in both years for cotton. A reversal in canopy temperatures, expressed as the amount of daily time that the temperature was above 28°C (DST), was not detected between the TLH and THL treatments of corn after 25xa0days in 1998 or after 13xa0days in 1999. The DST values of the cotton transient water treatments reversed after 4xa0days in 1998 and 5xa0days in 1999, when the values of THL became greater than for TLH. Corn tassels, which apparently transpire less than leaves, were forming at the beginning of the transient water treatments and their presence in the view of the infrared thermocouples may have reduced the apparent radiometric temperature difference between the transient water treatments. During the water status adjustment period following the initiation of the transient water treatments, there were significant linear relationships between LWP and DST in cotton in both years but only in 1998 in corn. Cotton canopy temperature could be used to rapidly monitor an entire field in contrast to LWP which accurately measures plant water status but cannot provide automated measurements across a large area.

Cotton phenology parameters affected by wind

Abstract Cotton plant ( Gossypium hirsutum L.) phenology was observed in a sheltered environment for six different planting dates and compared to cotton grown in an unsheltered area to isolate the effects of wind movement on cotton phenology and plant stress. The shelters consisted of a snow fence which provided a 35% reduction in wind velocity 0.2 m above the crop for the season. Partially sheltering the cotton from wind resulted in increased plant height, earlier squaring, earlier boll set and more bolls and biomass when compared to the unsheltered cotton. The increase in biomass and plant growth for the sheltered cotton required more water as the season progressed than the unsheltered cotton produced without supplemental irrigation. The study shows that wind and water uptake should be considered in crop models.