Crop water stress index for watermelon
Introduction
Irrigation scheduling methods are generally based on measurement of soil water content or meteorological parameters for modeling or computing evapotranspiration. Irrigation scheduling based upon crop water status should be more advantageous since crops respond to both the soil and aerial environment (evaporative demand). Plant stress measurements with hand-held infrared thermometers (IRTs) have become increasingly popular in the last 10–15 years (Hatfield, 1990).
Plant stress associated with water deficits under field conditions have been quantified by using the crop water stress index (CWSI), defined by Idso et al. (1981) who developed empirical linear relationships for canopy–air temperature difference (Tc−Ta) versus vapor pressure deficit (VPD) of the atmosphere for a crop transpiring at its potential rate. The lower limit (Tc−Ta) versus VPD represents the measured temperature difference when the crop is well watered (minimal stress). The upper limit (Tc−Ta) represents the temperature difference occurring when the crop transpiration rate approaches zero (maximum stress) (Reginato, 1983, Stegman and Soderlund, 1992, Stockle and Dugas, 1992).
Many studies have been reported on the determination of CWSI or different crops. For example, Jackson (1982), and Stegman and Soderlund (1992) suggested that irrigation should be applied when the CWSI for wheat is in the range 0.3–0.5. Fangmeier et al. (1989) reached the highest yield in the wettest treatment with average CWSI values near 0.1 for cotton. Also, Ödemiş and Baştuǧ (1999), reported that the CWSI values could be used to determine irrigation time and irrigation should be applied when the CWSI was about 0.45 for cotton in Turkey conditions. Although, minimal yield reductions of corn were observed at a threshold CWSI value of 0.33 under Texas conditions (Yazar et al., 1999). This value was determined as 0.21 for Adana (Turkey) conditions (Gençoǧlan and Yazar, 1999).
The purpose of this study was to determine the variation in CWSI of watermelon grown with different rates of trickle irrigation and to evaluate the relationships amongst CWSI, yield, water stress, water applied and the soil water content.
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Materials and methods
Experiments were conducted in 1999 and 2000 at the research field of the Viticultural Research Institute of Tekirdaǧ (semi-arid climate region) in Turkey, at 40°59′N latitude, 27°29′E longitude and 4 m altitude. Some climatic factors of the region during the experimental years are summarized in Table 1. The soil type in the plot area are generally deep, heavy textured, well drained and the available water holding capacity within 1.20 m of the soil profile is approximately 0.18 m. The electrical
Results and discussion
Irrigation frequency and the amount of irrigation water applied in 1999 were lower than those in 2000 because of the different climatic conditions and the total soil profile water content (Table 1, Table 2). The seasonal evapotranspiration (ET) in treatment T1 was the highest in both years, suggesting that the irrigation water applied was adequate to meet the full crop water requirements. This treatment was used, therefore, to determine non-stressed CWSI baseline. Other treatments underwent
Conclusions
These results suggest that the CWSI could be used to measure crop water status and to improve irrigation scheduling for watermelon. The upper and lower baselines, and CWSI values determined during this study in the years of 1999 and 2000 were slightly different. These differences can be due to several factors mentioned earlier. Based on these results an average CWSI of about 0.41 before irrigation will produce the maximum yield. However, we cannot conclude that this CWSI value should be used
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