The following points have to be considered for the design of the storage basin:
• The area of greenhouses and the vacant area available for a storage basin.
• The distribution of precipitation and the amount of rainfall.
• The crop water requirement.
• Whether the storage basin is only to be used for storing rainwater or for mixing rain and salty water.
Normally, daily frequencies of precipitation should be taken into consideration for the calculation of the storage volume. Those values are unknown in most cases. Therefore, monthly precipitation can be used to estimate the storage volume in a first approximation.
The monthly collected amount of precipitation is.
CVm = Pre x fC (l/m2month), (14.4)
Pre (l/m2month) = mean monthly precipitation fC = 0.9: Collecting factor for greenhouse roofs.
The collecting factor is the ratio of possible amount of collected rain water to the precipitation.
If rain water is to be used for irrigation, the monthly storable precipitation is: STPm = CVm – CWRm – EVpond (l/m2month). (14.5)
The evaporation of the storage basin surface EVpond can be neglected if the basin is covered by a swimming plastic cover, for example.
If STPm is positive, the storage will be filled, if STPm is negative, the storage will be emptied.
The yearly storable precipitation is:
STPy = positive STPm (l/m2month). (14.6)
The yearly deficit is:
Defy = negative STPm (l/m2month). (14.7)
The yearly storage balance is:
STBy = STPy – Defy (l/m2month). (14.8)
The following cases have to be distinguished:
1. If STBy > 0 or STPy > Defy, the storable precipitation is sufficient for irrigation throughout the year. The storage volume becomes:
VST = Defy (l/m2).
One can enlarge the volume, if the monthly variation of precipitation is high. The storage volume is:
VST = Defy(1 + Vc),
Vc = coefficient of variation for precipitation (see Chap. 2).
2. If STBy < 0 or STPy < Defy, the storable precipitation is not sufficient for irrigation. two cases have to be distinguished:
(a) If the storable precipitation is higher than the maximum monthly collected precipitation CVmmax If STPy > CVmmax, then
VST = STPy
VST = STPy(1 + Vc) (lm2)
(b) If STPy < CVmmax, then
VST = CV mmax (1 + Vc)