Browsing by Author "van der Merwe, Johan Herbst"

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    Quantifying water movement in soils irrigated by continuous drip irrigation with citrus in the Western Cape
    (Stellenbosch : Stellenbosch University, 2022-04) van der Merwe, Johan Herbst; Hoffman, Eduard; Raath, Pieter; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: The development of low discharge rate emitters by Netafim opened new possibilities to approach irrigation and fertilization and introduced continuous low flow drip irrigation, allowing for intensive management of fruit production on difficult to manage soils. This entails decreasing the application rate of the irrigation system to balance with the maximum daily water use over the total consumptive period of the crop addressing many challenges in managing irrigation in an orchard. A key for efficient drip irrigation is to adjust the shape of the wetted soil volume created under an emitter to the main root distribution, which is well known for conventional drippers in different soil types, but not for low flow continuous drip irrigation. In this study the daily water requirement of young ‘Nadorcott’ mandarin citrus trees were applied in three continuous drip irrigation treatments with different drip discharge rates (T1-0.4L.hˉ¹, T2-0.7L.hˉ¹ and T3-1.6L.hˉ¹ allowing different irrigation cycle lengths, to study the effect on soil water content distribution, root distribution and crop response . T1-0.4L.hˉ¹ maintained an average higher SWC in the active root zone along the season followed by T2-0.7L.hˉ¹ and T3-1.6L.hˉ¹ respectively. Significantly higher SWC was observed close to the soil surface with T1-0.4L.hˉ¹ followed by T2-0.7L.hˉ¹ and T3-1.6L.hˉ¹ respectively. Relative average lower SWC with daily conventional irrigation was expected due to narrower spacing of drippers and lower volume of water applied per emitter. 2-Dimentional water distribution patterns parallel and across the ridge showed that T1- 0.4L.h resulted in a smaller wetted soil volume compared to T2-0.7L.h with less horizonal and vertical water distribution from an emitter, which may have encouraged fibrous roots to grow closer to the soil surface and to the tree. More overlap was observed in T2-0.7L.h parallel with the ridge resulting in greater vertical distribution of water between two adjacent drippers where wetted soil volumes merge which favoured roots to grow and proliferate deeper down the soil profile and further away from the tree. A wetted strip was formed under T3-1.6L.h due to narrower spacing of drippers, compared to the ‘pots’ formed with T1-0.4l.h and T2-0.7L.h, resulting in a dense root strip across the profile in the 20 to 40cm depth. Highest fine root density was recorded in T3-1.6L.h in the 20 -30 cm layer, followed by T2-0.7L.h in the 30 to 40 cm layer and T1-0.4L.h in the 20-30 cm layer respectively. A bimodal distribution of water was observed in T1-0.4L.h and T2-0.7L.h early in the morning when a shorter irrigation event took place characterized by a main wetted soil volume in the upper 50 cm and concentrated areas of higher soil water content in the lower 50 cm. Approaching midday as irrigation continued SWC in the main core of the wetted soil volume increased and lower SWC was observed in the rest of the soil profile indicating a ‘shrinking’ of the wetted bulb midday after which a bimodal water distribution resumed. T3- 1.6 showed very little fluctuation during the day. It seems as if the antecedent soil water content may contribute largely to soil water content fluctuation and distribution patterns. Overall little fluctuation of the wetted soil volume was observed daily, maintaining very constant SWC levels in the root zone.