Evapotranspiration and water use of full and deficit irrigated cotton in the Mediterranean environment in northern Syria
Highlights
► Deficit irrigation has moderate impact on cotton water productivity. ► Rationalizing water application can contribute to natural resources conservation. ► Sufficient fertilization can contribute to natural resources conservation in water scarce areas.
Introduction
Cotton (Gossypium hirsutum L.) is grown in many countries, under both rainfed and irrigated conditions. In the semi-arid Mediterranean region, in places like southern Spain, northern Syria and western Turkey, the crop is entirely irrigated (Janat and Somi, 2001) with very limited rainfall during the summer growing season. Cotton requires large quantities of water, while water supplies are declining. Clearly, there is an urgent need for technical options as well as policy measures to encourage environmentally sustainable, yet economically viable practices. This study focuses on Syria, where cotton is vital to the national economy, while simultaneously, shallow aquifers are being depleted at an alarming rate. Water is increasingly pumped from deeper groundwater and the two most important production inputs – irrigation and fertilizers – are becoming the two largest production costs (Janat and Somi, 2001).
Cotton production in Syria dates back to ancient times. Cotton is economically more competitive than any other summer crop. It is a major source of income for one-fifth of the country's economically active population. Most fields range from 2 to 25 ha. Cotton is grown in a two year rotation with wheat, sugar beet, potato, legumes, and vegetables. Typically, wheat is harvested in June; land is plowed twice and left fallow. A third plowing is done in February or March of the following year, and cotton planted in April and May. Planting is mostly by hand, and quality seeds are available from the government. The harvest starts in September and lasts through December; the entire crop is handpicked in two or more pickings. Water is conveyed to the fields mainly through earthen ditches. The most common practice (seen on 75% of all cotton fields) is flooding of small basins, followed by furrow irrigation (24% of the fields). Irrigation applications are double (or even higher) the crop requirements, with a national average of about 1500 mm per season. Cotton accounts for an estimated 25% of Syria's total agricultural water use.
The national average for seed cotton yield is around 4 t ha−1. Productivity per unit area is at acceptable levels, but yield per unit of applied water is extremely low because of inefficient irrigation systems and improper water management. On a global level, acceptable yield of irrigated cotton is 4–5 t ha−1 seed cotton with water productivity (WPET) values of 0.4–0.6 kg per m3 of depleted water. This range is also inferred from the recent reviews of Grismer (2002) and Zwart and Bastiaanssen (2004), for data from multiple countries. Average WPET was reported as 0.65 and 0.23 kg m−3 for seed cotton and lint cotton, respectively, with a large variability ranging from 0.41 to 0.95 kg m−3 for seed cotton and 0.14–0.33 kg m−3 for lint cotton. The large variability is due to many factors, including differences in climate, soil, and irrigation and nutrient management, but suggests opportunities for maintaining or increasing production with less water. In Syria, WPET in the traditional surface irrigated cotton is only 0.2–0.25 kg m−3 of seed cotton and 0.07–0.09 kg m−3 for lint cotton. These values are only half to one-third of those achieved in most major cotton producers such as Argentina, Turkey, and USA (Hunsaker et al., 1998, Ayars et al., 1999, Howell et al., 2004, Dagdelen et al., 2006). They are also lower than the minimum values reported in global reviews (Grismer, 2002; Zwart and Bastiaanssen 2004).
More than 60% of irrigated lands in Syria use groundwater (Salman, 2004) and are not part of government irrigation schemes. These irrigators enjoy on-demand irrigation water, and thus potentially have the flexibility and control to implement an effective on-farm water management through scheduling, reallocation and other means. Currently, most farmers aim to maximize yield (and presumably profit) by maximizing irrigation. This is both unwise and unsustainable in basins where water is being withdrawn faster than it is being replenished. A better alternative is a targeted demand management strategy that may include water rationing or deficit irrigation (Farahani et al., 2006). Deficit irrigation, either voluntarily or regulated, is an option that may increase WPET (Kijne et al., 2002), and would most certainly improve resource sustainability.
Cotton is an indeterminate perennial shrub that is suitable for conditions of limited water and tolerant to salinity. Past research, dating back to at least the 1930s (DeTar, 2008), documents various aspects: physiological and morphological responses to water, deficit irrigation and its economics (English et al., 1990), and water use and yield relationship (Wanjura et al., 2002, Howell et al., 2004, Dagdelen et al., 2006). Drip irrigated cotton data from Wanjura et al. (2002) show reduced yields for deficit as well as for over-irrigation. Falkenberg et al. (2007) reported that irrigation at 75% of ETc did not reduce cotton yield. Data from Howell et al. (2004) and DeTar (2008) do not show any gains in WPET due to deficit irrigation. For a range of irrigation regimes starting at about 50% of the optimum application of 654 mm, DeTar (2008) observed reduced yields due to deficit irrigation. At the optimum application, WPET averaged 0.219 kg m−3 for lint cotton, which was reduced in deficit irrigated plots, as well as in over-irrigated plots where application exceeded requirements by 30%. However, past research clearly shows cotton yield reductions due to excess water (Wanjura et al., 2002 and Karam et al., 2006). The literature reports mixed results on the impact of water rationing on WPET of cotton, but a reduction in WPET due to over-watering is most certain. An increase in WPET due to deficit irrigation is neither obvious nor universally observed as it is a complex interaction of many factors including timing and duration of the stress in addition to variations in application methods and efficiencies.
Literature from Syria includes comparison of irrigation methods and fertilizers on yield, but no study of deficit irrigation is available. Local production functions are needed, especially since results from literature are mixed and difficult to transfer with certainty. To determine yield response to water, precise implementation of irrigation regimes and accurate measures of crop water use (ETc) and yield are needed. This was the objective of this study that was implemented in drip-irrigated cotton in northern Syria under four levels of water rationing regimes and three levels of nitrogen fertilizer during the period 2004–2006. Results quantify yield response to water and are useful for potential development of water demand management strategies and economic analysis.
Section snippets
Site description and field practices
The field study examined the effects of varying soil water and fertility regimes on production and water productivity of drip-irrigated cotton (Gossypium hirsutum L.) in the typical Syrian practice of two-year cotton-wheat rotation. This study was conducted at Tel Hadya research station (36°01′N, 36°56′E, and 284 m above mean sea level), the headquarters of the International Center for Agricultural Research in the Dry Areas (ICARDA), located 35 km south of Aleppo in northern Syria. The study was
Rainfall and temperature
The climate during the three growing seasons was very similar (Table 3). There was practically no rainfall during the three months of June, July and August, and only a few millimeters of rain in May and September. The monthly Class-A pan evaporation and total for the three seasons show little variations with an average value of 1841 mm over the three seasons. These variations are typical for the Mediterranean climate that is characterized by a long hot and dry summer, during which full
Discussion
Water productivity is computed as yield per ETc (WPET) and yield per applied irrigation water (WPiw). The former is more a biological indicator while the latter is influenced by the performance of the irrigation system and the degree of water losses beyond transpiration. Mean WPET decreased with increasing water rationing (from more than 0.39 kg m−3 at 100% irrigation to about 0.32 kg m−3 at 40% irrigation) while WPiw increased with increasing water rationing (from 0.43 at 100% irrigation to 0.48 kg m
Conclusions
Both yield and water productivity (WPET) of cotton increase with an increase in irrigation level. This was most pronounced at optimum N fertility levels. From a biological view point, deficit irrigation may not necessarily improve water productivity. However, economically, the conclusion may be different depending primarily on the cost of irrigation and/or water and the sale value of the produce. The WPET values obtained in this study are double the current cotton WP values in Syria, suggesting
Acknowledgements
The authors wish to thank Pierre Hayek, Jihad Abdullah, Ali Haj Dibo, Issam Halimeh, and other ICARDA field staff for managing the field trials and for carrying out the tedious and labor intensive soil water content and plant measurements. Appreciation is also extended to Dr. Murari Singh for his generous help with statistical design and data analysis.
References (21)
- et al.
Subsurface drip irrigation of row crops: a review of 15 years of research at the Water Management Research Laboratory
Agric. Water Manage.
(1999) - et al.
Water-yield relation and water use efficiency of cotton and second crop corn in western Turkey
Agric. Water Manage.
(2006) Yield and growth characteristics for cotton under various irrigation regimes on sandy soil
Agric. Water Manage.
(2008)- et al.
Remote sensing of biotic and abiotic stress for irrigation management of cotton
Agric. Water Manage.
(2007) Regional cotton lint yield, ETc and water value in Arizona and California
Agric. Water Manage.
(2002)- et al.
Cotton response to high frequency surface irrigation
Agric. Water Manage.
(1998) - et al.
Water use and lint yield response of drip irrigated cotton to the length of irrigation season
Agric. Water Manage.
(2006) - et al.
Optimizing supplemental irrigation: tradeoffs between profitability and sustainability
Agric. Water Manage.
(2009) - et al.
Cotton yield and applied water relationships under drip irrigation
Agric. Water Manage.
(2002) - et al.
Review of measured crop water productivity values for irrigated wheat, rice, cotton and maize
Agric. Water Manage.
(2004)