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  • Author or Editor: Suat Irmak x
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Evaporation pans continue to be used extensively throughout the world to measure free-surface water evaporation (Epan) and to estimate evapotranspiration for irrigation scheduling and water management for agronomic and horticultural crops. Epan is also being used extensively to estimate evaporation rates from lakes, wetlands, rivers, reservoirs, and other water bodies for management of wildlife and ecological habitat. A reliable method is needed to estimate missing daily Epan data. Determination of a reliable method for the estimation of Epan would also be useful in modeling of crop growth, and hydrological and ecological systems. Five methods [Penman (Penman, 1948), Kohler-Nordenson-Fox (KNF) (Kohler et al., 1955), Christiansen (Christiansen, 1968), Priestley-Taylor (PT) (Priestley and Taylor, 1972), and Linacre (Linacre, 1977)] for estimating Epan were compared with the historical (23-year) measured daily values to determine the suc- cess of accurate and consistent Epan estimations under humid climatic conditions in Florida. The root mean square error (RMSE) was used as the criteria to judge the accuracy and reliability of a given method. An RMSE value of <0.5 mm·d-1 (0.02 inches/d) between the measured and estimated Epan was considered as an acceptable error for daily estimations. The standard deviation (sd) values, and percent error (%E) between the estimated and measured values were also considered in the performance evaluations. Performance evaluations of the Epan estimates of the methods were made on a daily, monthly, and annual basis. Results indicated that the KNF method provided the best Epan estimations. The Linacre method yielded the poorest estimates. The second, third, and fourth best methods were the Penman, PT, and Christiansen, respectively. The RMSE and sd of Epan estimates were lowest when using KNF method. The mean value of the %E of daily, monthly, and annual estimations were 27%, 27%, and 26% for Christiansen; 6%, 6%, and 4% for KNF; 33%, 32%, and 26% for Linacre; 24%, 24%, and 21% for PT; and 19%, 17%, and 11% for Penman methods, respectively. The weekly, monthly, and annual total of Epan estimates from KNF method were also compared to the measured values of the two selected years of data (1981 and 1983). The annual rainfall totals were significantly lower than the 23-year mean in 1981, and higher in 1983. The %Es of weekly, monthly, and annual total Epan estimates were 9%, 9%, and -1% in 1981; and 11%, 5%, and 4% in 1983, respectively. The KNF method underestimated Epan in 1981 (dry year) and the underestimations were higher in summer months. The underestimations in a dry year, especially in summer months, might be due to the fact that the sensible heat advection is not effectively accounted for in the KNF equation causing underestimations of Epan. Overall results indicated that the KNF method should be the first choice, among the methods tested, for estimating daily Epan for irrigation scheduling and for estimating the missing Epan data in humid areas.

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This research study evaluates the effectiveness of a recently introduced irrigation-plant production system, multipot box system (MPBS), for moderating root zone temperature (RZT) compared with the conventional nursery containers. The study also deals with the development, calibration, and validation of a series of models that can be used to predict maximum (max) and minimum (min) RZTs using commonly available input variables. The Viburnum odoratissimum (Ker.-gawl.) was used as the test plant. Models were calibrated in the fall growing season and validated during the summer. The RZT was used as the dependent variable while the max and min air temperatures (Tmax and Tmin) and/or incoming solar radiation (Rs) were used as independent variables. The color of the MPBS had an effect on plant growth. Plants grown in the white MPBS had higher growth indices, shoot and root dry weights, and number of stems as compared with the plants in the black MPBS or the conventional (control) system (CS). White MPBS maintained cooler RZTs than the max air temperature during both seasons. Also, white MPBS maintained cooler RZTs than the black MPBS and CS during the two seasons. In both seasons, water temperature in the black MPBS was higher than the temperature in the white MPBS contributing to the high RZTs in the black MPBS. The RZT of the black MPBS and CS exceeded the critical value (40 °C), which is cited in the literatures as negatively impacting root growth, water and nutrient uptake, leaf area, plant survival, root and shoot dry weights, water status, and photosynthesis. The RZT in the CS was above 45 °C for most of the summer season and plants were exposed to this extreme temperature for a few hours a day during most of the summer. The white MPBS provided a better environment and enhanced plant growth. For regions where ambient air temperature ranged from 2 to 41 °C, the white MPBS can provide adequate and effective RZT protection for plants grown in No. 1, 3.8-L standard black conventional containers. Predicted RZT values were well correlated with measured values in all systems. Rs did not have an effect on predicting RZTmax in the MPBS treatments. Wind speed did not contribute to predicting RZT in any production systems. The root mean square error between measured and predicted RZT was relatively low ranging from 0.9 to 2.8 °C. Models were able to explain at least 74% of the variability in RZTs using only Tmax, Tmin, and/or Rs. Models developed in this study should be applicable for estimating RZTs when similar management and cultural practices are present. Models of this study are practical, simple, and applicable to predict RZTs where ambient air temperature ranges from 1.9 to 40 °C. Model results should not be extrapolated beyond these limits.

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Two colors (white and black) of a recently introduced irrigation-plant production system [multi-pot box system (MPBS)] for container-grown nurseries were researched and results were compared with those obtained from the sprinkler-irrigated conventional (control) system (CS). Experiments were carried out in summer and fall of 2001 in Gainesville, Fla. Plant growth [growth index (GI), growth rate (GR), and dry matter] and stress parameters [stomatal resistance (rs), crop water stress index (CWSI), plant water potential (PWP), and substrate temperature (ST)] were measured and analyzed for Viburnum odoratissimum (Ker-gawl). In both seasons, plants grown in the white MPBS had significantly higher GI and GR as compared to the plants in the black MPBS and CS. In summer, plants in the white MPBS reached marketable size about 17 days and 86 days earlier than those in the black MPBS and CS, respectively. In fall, they reached marketable size about 25 and 115 days earlier than those plants in the black MPBS and CS, respectively. Plants in the white and black MPBSs showed exponential growth rate in summer with plants in the white MPBS having significantly higher growth rate (greater slope) than the other two treatments. In both seasons, plants in the white MPBS produced the highest amount of dry matter. In general, plants in the white MPBS had lower rs values to vapor transport compared to the other two treatments, and the black MPBS treatment had lower rs values than the CS in both seasons. The CWSI values of the plants in both white and black MPBSs were significantly lower than the CS. In both seasons, ST in the black MPBS and CS exceeded the critical value of 40 °C several times. The ST of >40 °C is often reported to significantly reduce the plant growth and cause root death and/or injury for container-grown plants. Overall, the white MPBS provided a better environment for root development and plant growth under these experimental conditions. Results strongly suggest that there is a potential opportunity of using MPBS for irrigation and production of nursery plants. These important findings suggest that, in practice, producing nursery plants in a shorter period of time by using white MPBS will result in significant savings of energy, water, chemicals, and other inputs and thereby reducing the costs and increasing profits.

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