Rainwater harvesting gains more and more ground as a modern, relatively inexpensive and simple water-saving technology, and as a sustainable water management practice, which saves water, and reduces stormwater runoff and peaks and non-point source pollution. In this paper, in order to determine the optimal size of rainwater harvesting tanks, two methods, the daily water balance method and the dry period demand method, are used in 75 regions of Greece to meet 30, 40 and 50 % of total water demands of households of 3 to 5 residents. The daily water balance method was developed based on a heuristic algorithm which uses the daily rainfall data, the rainfall collection area, the runoff coefficient, the available storage volume and the water demands, allowing excess water to overflow and setting public water supply to zero. The dry period demand method is based on meeting demand for the longest annual average dry period. According to the daily water balance method, in the majority of the 75 Water Resour Manage Greece regions studied, tank sizes up to 50 m 3 can meet a 240 L/day demand (40 % of total daily demand of 4 residents) with roof area not exceeding 300 m 2 . More than 50 m 3 tank size is needed to meet demands of 300 L/day (40 % of 5 or 50 % of 4 residents) or 375 L/day (50 % of 5 residents). Results demonstrate that the tank size is strongly affected by the dry period length; small dry periods lead to small tanks, with the exception of low rainfall-high demand (300-375 L/day) case, where low rainfall increases sizes, having the dominant role. Comparison among the dry period demand and the daily water balance methods showed that in all cases, the dry period demand method calculates smaller tanks, with the exception of areas with medium-high rainfall and high dry period or low-medium demand (135-225 L/day) and high roof areas (more than 300 m 2 ). Therefore, the main conclusion is that the rainwater harvesting tank capacity is strongly affected by various local variables and cannot be formulated. However, the method presented here can be programmed in a spreadsheet with no much effort, making harvesting tank computations easy.
Rainwater harvesting is an ancient water management practice that has been used to cover potable and non-potable water needs. In recent years, this practice is adopted as a promising alternative and sustainable source of water to meet irrigation needs in agriculture in arid and semi-arid regions. In the present study, a daily water balance model was applied to investigate the size of rainwater tanks for irrigation use in greenhouse begonia and tomato cultivation in two regions of Greece with significant greenhouse areas. For the application of the water balance model, daily rainfall depth values of a 12-year time series (2008–2020) from representative rainfall stations of the study areas were used, as well as the daily water needs of the crops. The greenhouse roof was assumed to be the water collection area of the rainwater harvesting system with values ranging from 1000 to 10,000 m2. The analysis of the results showed that in the case of the begonia crop, the covered tanks ranged from 100 to 200 m3 per 1000 m2 greenhouse area with a reliability coefficient that ranged from 65 to 72%, respectively, to meet the water needs of plants. Further increase of the reliability coefficient was carried out with disproportionately large volumes of tanks. In the case of the tomato crop, covered tank volumes ranged from 100 to 290 m3 per 1000 m2 of greenhouse area, and had a reliability coefficient of 90% to 100%, respectively, while uncovered tanks had a maximum reliability coefficient of 91% for a critical tank volume of 177 m3 per 1000 m2 of greenhouse area and decreased for any further increase of tank volume.
Rainwater harvesting is an ancient practice aiming to cover water needs for domestic, irrigation and livestock uses. In this study, the rainwater harvesting tank size was investigated to meet five water-need levels of a mixed goat–sheep farm using a daily water balance method. This method was applied using daily rainfall data for a period of 16 years from six meteorological stations in selected regions of Greece, characterized by different rainfall regimes and well-developed livestock activity, taking into account, among other parameters, the water needs of animals, the rainwater collection area and the runoff coefficient. There is a great variation in the rainwater harvesting tank size among the stations studied due to differences in the annual rainfall and the maximum dry period. Results showed that meeting full demands (100% reliability) requires tank sizes ranging from 20 m3 for short dry period stations–low demand scenario (320 L/day) to 115 m3 for long dry period stations–high demand scenario (576 L/day), assuming a maximum collection area of 450 m2. Correspondingly, reliability analysis showed that very high values of reliability (95%) can be obtained with tank sizes ranging from 10 to 85 m3, respectively.
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 m2 were studied. In all areas examined, a tank of 30–100 m3 per 1000 m2 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 m3 (per 1000 m2 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 m3 (per 1000 m2 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 m3 (per 1000 m2 of greenhouse area) in the study area with high mean annual rainfall depth and moderate mean longest dry period.
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