Searching for Sustainable-Irrigation Issues of Clementine Orchards in the Syrian Akkar Plain: Effects of Irrigation Method and Canopy Size on Crop Coefficients, Transpiration, and Water Use with SIMDualKc Model

Abstract

Citrus is one of the most valuable crops in Syria, with the largest production areas in the Tartus and Latakia provinces. Water-saving policies have been adopted to modernize the irrigation systems and increase water productivity. Following dedicated research, this study aimed to evaluate the water balance in clementine trees irrigated with diverse methods and schedules using the SIMDualKc software model. Two experiments are reported: one with 10–14 years old trees irrigated with different methods (2007−2011) and the other with the same trees but now 18−20 years old, irrigated with different schedules (2015−2019). The SIMDualKc model successfully simulated the soil water contents measured in the various field plots, with root mean square error values lower than 0.004 m3 m−3 and modeling efficiencies up to 0.83. The model-calibrated standard basal crop coefficients (Kcb) were approximately constant throughout all growing stages, assuming values of 0.54−0.55 for the mature trees having smaller height (h) and fraction of ground cover (fc), and 0.64 for older trees with larger canopies, i.e., larger h and fc. With drip irrigation, single Kc had a higher value (1.14) at the end, non-growing, and initial stages, and a lower value (0.75–0.76) during mid-season (Kc mid), because precipitation was lesser then, contributing less to soil evaporation. On the other hand, Kc values were nearly constant with micro-sprinkler and surface irrigation techniques because the ground was fully wetted. The Kcb values derived from the fraction of ground cover and height (A&P approach) were similar to those obtained from the model, thus showing that the A&P approach represents a practical alternative to estimate Kcb in the practice of irrigation management. The soil water balance further revealed a large weight of the terms corresponding to the non-beneficial water consumption and non-consumptive water use when the fraction wetted was large and the application efficiencies were low. These terms were reduced, namely, evaporation losses when drip irrigation was used. This study, thus, provides a valuable tool for improving the irrigation management, water saving, and water productivity of Syrian citrus production systems.

Setting Irrigation Thresholds for Building a Platform Aimed at the Improved Management of Citrus Orchards in Coastal Syria

“…The application of this approach does not require calibration/validation if using the tabulated parameters in Pereira et al [19]. Nevertheless, when field data are available, a validation may be performed by comparing the K cb values derived from the A&P approach with those computed from field data, as already performed for the study area by Darouich et al [20].…”

“…Once this is achieved, it is possible to simulate the water balance of the citrus depending on the characteristics and the K cb of orchards, either in real time or relative to various water-demand scenarios. The companion paper by Darouich et al [20] followed the research referred to above and consisted of the first part of the study on improved irrigation of citrus orchards to be continued through this article. Using the software SIMDualKc, Darouich et al [20] developed a soil water balance of a clementine orchard, derived its crop coefficients (K c ) relative to various irrigation methods (drip, bubblers, micro-sprinkler, and ring basin), and assessed the performance of those methods, but focusing on drip irrigation positive impacts on the non-beneficial uses of water in citrus orchards, i.e., by decreasing evaporation losses during the dry summer season due to the small area wetted by drippers when compared with other methods.…”

“…The companion paper by Darouich et al [20] followed the research referred to above and consisted of the first part of the study on improved irrigation of citrus orchards to be continued through this article. Using the software SIMDualKc, Darouich et al [20] developed a soil water balance of a clementine orchard, derived its crop coefficients (K c ) relative to various irrigation methods (drip, bubblers, micro-sprinkler, and ring basin), and assessed the performance of those methods, but focusing on drip irrigation positive impacts on the non-beneficial uses of water in citrus orchards, i.e., by decreasing evaporation losses during the dry summer season due to the small area wetted by drippers when compared with other methods. That study also showed the impact of canopy cover, tree height, and irrigation method on crop coefficients, and the impact was confirmed later by the standard K cb of vineyards [21], as well as for citrus and various Mediterranean crops, using the published data of more than one hundred papers reporting good-quality field research [22].…”

“…The current study, which serves as a companion study to that of Darouich et al [20], aimed at further assessing appropriate issues to implement water-saving irrigation in the coastal Tartus and Latakia citrus orchards by focusing on irrigation scheduling in combination with drip irrigation. Considering the previous calibration and validation of SIMDualKc, as well as the careful definition of the orchards, water-saving thresholds, and atmospheric demand for water, the results of the simulations performed consist of effective information tools for irrigation scheduling thresholds to be used in practice.…”

“…The main objective of this study was, therefore, to further pursue the work performed by Darouich et al [20] by developing tools to improve the management of irrigation water in citrus orchards located in the Syrian coastal region. The specific objectives were (i) to derive K cb and K c values for the various citrus orchards, using the A&P approach; (ii) from those K cb and K c , to compute the crop transpiration (T c ), crop evapotranspiration (ET c ), and irrigation water needs in Syrian citrus orchards, while considering different crop densities, water-shortage conditions, and climate variability ; and (iii) to estimate yield reductions from adopting different water-saving irrigation strategies.…”