Calculation of rain collection amount and tank volume in multi-span greenhouse in humid region

Abstract: As rainfall is large in humid region of southern China, greenhouse rainwater harvesting should be designed to collect rainwater economically and moderately. The capacity of the rainwater tank needs to be determined according to rainwater harvesting plan and demand for irrigation. In order to determine a proper volume of rainwater harvesting tank and search for correlative calculation method, a concept describing the level of rainwater collection, named Rain Collection Volume Modulus is proposed in this paper. Rain Collection Volume Modulus is defined as the rain-collecting facility volume in per 1000m2 of the rain-collecting area, the unit is mm. We collected meteorological observation data from 1993 to 2012 in the experimental district of Lanxi city Zhejiang Province, and then used exponential decay function to establish fitting equation with Rain Collection Volume Modulus as the independent variables. According to the equation, we can establish a query table containing capacity of rainfall collection, rate of rainwater collection, volume of tank and re-storage times under different Rain Collection Volume Modulus from 5-100mm (spacing 5mm) under the condition of 1000m2 rain-collecting area. In order to calculate the volume of tank in actual practice, we can know the amount of rainwater that can be collected under 1000m2 rain-colleting area according to the actual rain-collecting area and required rainfall. Therefore, the corresponding Volume Modulus could be found in the table. And the volume of tank could be calculated by multiplying the mentioned Rain Collection Volume Modulus and actual rain-collecting area. The same Rain Collection Volume Modulus means the same rain-collecting rate and same re-storage times. Meanwhile, different Rain Collection Volume Modulus could satisfy different economical demand of rainwater harvesting project. The error analysis shows that there is no obvious relationship between the calculation error with rainfall, and there is no significant discrepancy between Rain Collection Volume Modulu and the relative error, but the former is in proportion to absolute error. The study shows that the accuracy of calculated value is influenced by rainfall intensity, therefore, unusual rainfall that deviates from the average over the years will cause large error. Rainfall is too concentrated in 4 years among 20 years, causing a great loss of rainwater, so the error is higher. The result shows that the relative errors are basically within 10%. Nearly half of the errors were less than 5%, and the maximum absolute value of them was 26.2%.The absolute value of 20-year average relative error was 6.6%, which almost matched the difference of rainfall.Our study was completed in the average rainfall of 1500mm, which showed that the accuracy of calculation had no significant relationship with rainfall, so the calculation method can be applied to this locality, and also it is an important reference to most parts of the South China where the rainfall is between 1 000 mm to 2 000 mm.