Elsevier

Scientia Horticulturae

Volume 98, Issue 2, 18 April 2003, Pages 121-130
Scientia Horticulturae

Crop water stress index for watermelon

https://doi.org/10.1016/S0304-4238(02)00215-7Get rights and content

Abstract

This study was conducted to determine the suitability of a crop water stress index (CWSI) to schedule irrigation for watermelon (Citrullus vulgaris) grown with trickle irrigation. The effects of five irrigation levels (100, 75, 50, 25 and 0% replenishment of soil water depleted from 0.90 m soil profile depth) on watermelon yields and resulting CWSI were investigated. The highest yield and water use was obtained under fully irrigated conditions (100% replenishment of soil water depleted) in 2 years. The CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures and vapor pressure deficit values for five irrigation levels. The trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Unlike the yield, CWSI increased with increased soil water deficit. An average CWSI of about 0.41 before irrigation produced the maximum yield. The yield was directly correlated with mean CWSI values and the linear equation ‘Y=91.143−66.077 CWSI’ can be used for yield prediction. The CWSI value was useful for evaluating crop water stress in watermelon and should be useful for timing irrigation and predicting yield.

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 (TcTa) versus vapor pressure deficit (VPD) of the atmosphere for a crop transpiring at its potential rate. The lower limit (TcTa) versus VPD represents the measured temperature difference when the crop is well watered (minimal stress). The upper limit (TcTa) 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.

Section snippets

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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