Open Access ISSN: 2168-9768 Irrigation and Drainage Systems Engineering Research Article Volume 10:8, 2021 Determination of Optimal Soil Moisture Depletion Level for Stevia ( Stevia Rebaudiana Bertoni ) at Wondo Genet, South Ethiopia Abstract The study was conducted based on the objective to evaluate the optimal soil moisture depletion level of stevia (Stevia rebaudiana Bertoni). Six levels of soil moisture depletion (20, 30, 40, 50, 60, and 100%) with four replications were used in a randomized complete block design. Different levels of total available water depletion significantly (p<0.05) affected all recorded yield and yield components. Significantly highest plant height, fresh and dry leaf weight, fresh and dry stem weight, aboveground fresh and dry biomass, and water productivity were obtained as stevia was irrigated below 40% TAW depletion level. However, water productivity was decreased both at higher and lower depletion levels and higher was at 30 and 40% depletion levels. The pooled mean revealed that irrigating stevia under different soil moisture depletion levels had a significant (p<0.05) effect on fresh leaf weight, fresh stem weight, dry stem weight, fresh biomass, and water productivity. Moreover, plant height, dry leaf weight, and dry biomass of stevia were also highly significantly (p<0.01) affected due to different soil moisture depletion levels. The maximum plant height (23.95 cm), fresh leaf weight (3300.3 kg/ha), fresh stem weight (1213.3 kg/ha), fresh biomass (4513.6 kg/ha) and water productivity (1.92 kg/m3) were recorded at 30% TAW. Moreover, the maximum dry leaf weight (1155.8 kg/ha), dry stem weight (869.0 kg/ha) and dry aboveground biomass (2024.7 kg/ha) were recorded at 40% TAW. On the other hand, the minimum growth, yield and yield component, and water productivity were recorded at 100% TAW. The current study revealed that irrigating stevia after 40% of the total available water in the soil depleted could be practiced in the study area and similar agroecology and soil type as the wider irrigation interval could be achieved without significantly affecting yield and water productivity. Keywords: Essential oil • Soil moisture depletion • Stevia • Water productivity Elias Meskelu*, Henok Tesfaye, Ayele Debebe, Mulugeta Mohammed and Seble Bekele Wondo Genet Agricultural Research Center, P. O. Box 198, Shashemene, Ethiopia *Address for Correspondence: Elias Meskelu, Wondo Genet Agricultural Research Center, P. O. Box 198, Shashemene, Ethiopia; E-mail: emeskelu@yahoo.com Copyright: © 2021 Elias Meskelu, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Received 02 August 2021; Accepted 10 August 2021; Published 25 August 2021 Introduction Stevia (Stevia rebaudian Bertoni) is an herbaceous perennial plant that belongs to the family Asteraceae which is originated from Paraguay. Leaves of the stevia plant contain low-calorie sweetening agents that can be used as a natural alternative to sweetening different products. Moreover, the leaves are consumed in special human diets and for the treatments of various diseases. The sweetness level of fresh leaves stevia reported as 15 – 30 times than common sugar and the leaf extracts as 250 – 330 times the common sugar. The plant is adaptive to the Ethiopian climate for the production of sweet leaves and stevioside under different agroecology even though yield varies considerably from highest 3034 to lowest 992 kg/ha within a year [1]. Stevia yield was reported to range from 2.85 t/ ha to 3.48 t/ha for different varieties under different irrigation conditions including reduced irrigation in central Greece [2]. Reducing irrigation water application to 33 and 66% refill to field capacity level leads to a significant reduction in yield and yield component. However, application of full irrigation to field capacity level leads to 40% higher leaf dry yield than irrigated with 33% irrigation to field capacity level [3]. However, Vasilakoglou et al. (2016) reported that satisfactory dry leaf biomass yield could be achieved even when stevia is grown under reduced irrigation to the level of 75% of evapotranspiration [4] reported that water stresses significantly reduced stevia yield and yield components as compared with the full irrigation based on 100% evapotranspiration. Moreover, the same author reported that total steviol glycosides yield reduced by 37.66% when the irrigation level reduced to 50% than the full irrigation despite the total glycosides content increased by 24.71% due to the moisture stress than the full irrigation. Stevia is a shallow-rooted crop in which the plant obtains soil moisture and nutrient in the limited upper soil profile. This leads to being considered an inefficient root system and the applied irrigation water is at risk of leaching below the shallow rooting zone [5,6]. Different studies also recommend that stevia should be irrigated with shallow irrigation as its main roots tend to be located close to the surface of the soil surface [7]. Similarly, different research reports revealed that stevia is sensitive to water deficit which mainly affects its growth and yield despite it improves the content of steviol glycosides [4]. Beyond leaching of irrigation water below the root zone, shallow root in crops leads to leaching of essential nutrients like nitrogen, especially under high irrigation depth [8]. Reported that water productivity is affected by water dose and nitrogen level, as higher water dose leads to leach both water and nutrient from the shallow-rooted crops to affect yield. Therefore, like many crops, stevia production could be affected by different agronomic practices like population density and agroecology [9,10]. Availability of soil moisture is among the major factor that affects crop yield including stevia which the yield is affected under different irrigation level [3] Different findings revealed that optimum irrigation scheduling based on soil moisture depletion levels varies for different crops. Determination of the optimum level of soil moisture before the next irrigation leads to maximize crop yield and water productivity. This is important especially for shallow- rooted crops as the effect of low moisture content in the topsoil leads to reduced yield, since moisture below the root zone will not be available for crops. However, there is a limitation of information regarding optimum soil moisture depletion level for irrigation scheduling of stevia in the study area. Therefore, the current study was initiated to identify the optimum soil moisture depletion level of stevia for better yield and water productivity. Materials and Methods The Study Area A Field experiment was carried out at Wondo Genet Agricultural Research Center, Ethiopia latitude 8 O 25'59'', longitude 39 O 01'44'' and altitude of 1800 m. a. s. l. for three years during 2016/17 to 2018/19 dry season to determine