Improvement of irrigation efficiency in citrus orchards by lowering dripper emitter delivery rate

Abstract

Most of the world’s freshwater is used by the agricultural industry, with irrigation of crops being one of the main uses. In water scarce countries, like South Africa, it is important that this water is used sustainably. Citrus is one of the most irrigated fruit crops in South Africa at around 99 000 ha, making it a very large and important water user. The growth of the industry, 35 000 ha increase since 2014, has placed pressure on current water resources and growers are in search of more efficient irrigation methods to maximise the water at their disposal. Drip irrigation has emerged as a very effective method for the irrigation of citrus and recent advances in drip irrigation technology have reduced emitter delivery rates by 70% compared to conventional drip, in an attempt to increase irrigation efficiency by decreasing drainage and increasing water storage in the rootzone. These systems, commonly referred to as low flow drip (LFD), have been widely adopted in the citrus industry with little research on the effects they have on plant and soil water relations and general irrigation management. This study therefore attempted to determine the differences between conventional and LFD irrigation systems in both soil and plant water relations. Furthermore, current FAO-56 crop coefficient values were evaluated with two treatments where a 20% and 40% deficit of crop evapotranspiration (ETc) were applied to test the hypothesis that current crop coefficient values overestimate ETc for low flow drip systems. This was done by a randomised trial design consisting of five treatments in a mature Mandarin orchard (Citrus reticulata cv. ‘Nadorcott’) where treatment 1 (1.6 l h-1), treatment 2 (2.3 l h-1) and treatment 3 (0.7 l h-1) were irrigated with no deficit and treatment 4 and 5 (0.7 l h-1) were irrigated at -20% and -40% of ETc respectively. The results indicated that LFD decreased drainage (D) below the root zone and increased water stored in the root zone, which resulted in an increase in transpiration. When soil water content was evaluated from capacitance probe data the LFD treatments had the highest average water content in the active rootzone, with treatment 3 (0.7 l h-1) at 92% followed by treatment 4 (0.7 l h-1 -20%) and 5 (0.7 l h-1 -40%) at 88%, the conventional drip followed with treatment 2 (2.3 l h-1) at 87% and finally treatment 1 (1.6 l h-1) at 85%, indicating that LFD stored more water higher in the profile. There were also differences in wetted area between treatments, with treatment 2 (2.3 l h-1) creating the smallest wetted area with an average of 51% of the interpolated area having plant available water (VWC > 0.13 cm3 cm 3) compared to treatments 1 (1.6 l h-1) and 3 (0.7 l h-1) with an average of 56% and 59 % of the total area having plant available water (VWC > 0.13 cm3 cm 3) respectively. There were, however, no differences observed in stomatal conductance (gs) and both pre-dawn (Ψpd) and midday stem water potential (Ψsmd) between treatments, with all readings well below the thresholds for stress, indicating that all treatments were well watered at the time of measurements. There were no significant differences in yield and quality between treatments illustrating that LFD is an effective and viable option for citrus irrigation. Furthermore, this also confirms the hypothesis that current FAO-56 values are overestimated for LFD and that it could be reduced between 20-40% with no influence on yield, size and quality in this sub-tropical climate. The final part of this study reviewed irrigation and yield data of Mandarin orchards (Citrus reticulata cv. ‘Nadorcott’) from a wide variety of production areas in South Africa. The main objective was to determine water productivity (WP) benchmark values and to evaluate if there are distinct differences between irrigation methods. No distinct differences were observed between conventional drip and LFD irrigation systems in terms of WP, but there was a noticeable decrease in applied Kc values (season total irrigation ÷ season total ETo) with a reduction in emitter delivery rate, suggesting improved application efficiency. The findings of this study suggest that irrigation water productivity (WPi) for ‘Nadorcott’ in a winter rainfall area is ~ 9.0 kg m-3 and ~ 18 kg m-3 for summer rainfall areas. When the effective rainfall (Pe) was calculated the summer rainfall regions ranged between 60% -80% and the winter rainfall areas between 40%-60%. These values will ultimately impact the applied Kc, as was the case in this study with much lower applied Kc values realised than recommended in FAO-56 for citrus. Due the impact of climatic differences on irrigation requirements, a normalised crop water productivity (WPn) was proposed in this study, which does not only take total water used (TWU) into account but also ETo and the contribution of rainfall to irrigation. Further research is warranted to gain a deeper understanding of and make meaningful comparisons between summer and winter rainfall regions with regards to WPc and the contribution or utilization of rainfall.