Citation: Londra, P.A.; Gkolfinopoulou, P.; Mponou, A.; Theocharis, A.T. Effect of Rainfall Regime on Rainwater Harvesting Tank Sizing for Greenhouse Irrigation Use. Hydrology 2022, 9, 122. https://doi.org/10.3390/ hydrology9070122 Academic Editor: Giorgio Baiamonte Received: 12 June 2022 Accepted: 4 July 2022 Published: 7 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). hydrology Article Effect of Rainfall Regime on Rainwater Harvesting Tank Sizing for Greenhouse Irrigation Use Paraskevi A. Londra 1, * , Panagiota Gkolfinopoulou 1 , Anastasia Mponou 1 and Achilleas T. Theocharis 2 1 Laboratory of Agricultural Hydraulics, Department of Natural Resources Management and Agricultural Engineering, School of Environment & Agricultural Engineering, Agricultural University of Athens, 75 Iera Odos Street, 11855 Athens, Greece; stud617009@aua.gr (P.G.); stud617031@aua.gr (A.M.) 2 General Directorate of Food, Hellenic Ministry of Rural Development and Food, 2 Acharnon Street, 10176 Athens, Greece; atheocharis@minagric.gr * Correspondence: v.londra@aua.gr; Tel.: +30-210-529-4069 Abstract: The use of rainwater harvesting tanks to supply human water needs is an old and sustainable practice. In the case of covering irrigation demand in greenhouse agriculture, the potential is huge. Still, the relative research worldwide is low, while it is nearly absent in Greece. In this study, the rainwater harvesting tank size for irrigation use of greenhouse tomato cultivation was investigated by applying a daily water balance model in three regions of Crete Island (Greece) with significant greenhouse areas. Daily rainfall data from three representative rainfall stations of the study areas characterized by different rainfall regime for a 12-year time series were used. Additionally, the daily irrigation water needs for a tomato crop during an 8-month cultivation period were used. The greenhouse roof was defined as catchment area of the rainwater harvesting system and greenhouse areas of 1000, 5000 and 10,000 m 2 were studied. In all areas examined, a tank of 30–100 m 3 per 1000 m 2 of greenhouse area could reach approximately 80–90% reliability. Higher values of reliability (reaching 100%) could be achieved mainly with covered tanks. Tank size for 100% reliability in covered tanks, ranged from 200 m 3 (per 1000 m 2 of greenhouse area) in the study area with high mean annual rainfall depth (974.24 mm) and moderate mean longest dry period (87.67 days), to 276 m 3 (per 1000 m 2 of greenhouse area) in the study area with relatively low mean annual rainfall depth (524.12 mm) and high mean longest dry period (117.42 days). For uncovered tanks, a 100% reliability value could be reached only with a tank size of 520 m 3 (per 1000 m 2 of greenhouse area) in the study area with high mean annual rainfall depth and moderate mean longest dry period. Keywords: daily rainfall; dry period; water balance model; rainwater tank size; reliability coefficient; greenhouse; irrigation needs; tomato crop 1. Introduction The ever-increasing demands for water for domestic and agricultural use due to climate variability, growing population and increased food production, press the already burdened existing water resources, making the need to find alternatives and sustainable water sources [1–4]. Rainwater harvesting, although it is an ancient practice of water management, currently gains increasingly more ground as a sustainable source of water, providing autonomy in its management. With agriculture being the main consumer of water in several countries, many of them have highlighted the benefits of rainwater harvesting systems and promoted their use in agriculture [5]. In Greece, irrigated agriculture is also the main water consumer, while the adoption of rainwater harvesting technique is at low levels and there is no research on tank sizing and harvesting potential. Greenhouse agriculture is the main sector that uses the rainwater harvesting technique. Greenhouse area in Greece covers approximately 7100 ha [6]. Greenhouses on the island of Crete cover approximately 2800 ha [6]. Rainwater harvesting is popular among greenhouse holdings, both because of water scarcity and centuries of tradition. Thus, special care is taken, in the measures of the Water Framework Hydrology 2022, 9, 122. https://doi.org/10.3390/hydrology9070122 https://www.mdpi.com/journal/hydrology