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Many regions have a high water surplus for the irrigation of plants during rainy seasons and a deficit of water in dry seasons. When growing plants in greenhouses, the rainwater running off the roofs of greenhouses can be collected and used for irrigation. If salty water is available, this can be mixed with rain water and then used as irrigation water. It is necessary to build greenhouses with sufficiently large gutters and storages for the collection of rain water. The crop water requirement has to be known in order to calculate the storage for rainwater and for irrigation systems.

The crop water requirement CWR can be calculated by use of the reference evapotranspiration ET0 (mm/day) according to FAO–Penman–Monteith with adapted parameters for unheated greenhouses (see Chap. 13).

The actual evapotranspiration AET of the crop inside the greenhouse is

$$ {\hbox{AE}}{{\hbox{T}}_{\rm{C}}} = {\hbox{ET0}} \times {k_C}\quad ({\hbox{mm}}/{\hbox{day}} = {\hbox{l}}/{{\hbox{m}}^2}{\hbox{day}}). $$
(14.1)

The daily crop water requirement is.

$$ {\hbox{CW}}{{\hbox{R}}_{\rm{d}}} = {\hbox{AET}}(1 + {l_{\rm{I}}}) \times {A_{\rm{crop}}}/{A_{\rm{G}}}\quad ({\hbox{mm}}/{\hbox{day}}), $$
(14.2)

where

  • l I=0.03–0.1 loss factor for the drip irrigation system.

  • A crop/A G=0.9 for vegetables and flowers on ground beds.

The monthly crop water requirement CWR is

$$ {\hbox{CW}}{{\hbox{R}}_{\rm{m}}} = {\hbox{CW}}{{\hbox{R}}_{\rm{d}}} \times {d_{\rm{m}}}\quad {\hbox{(mm}}/{\hbox{month),}} $$
(14.3)

  • d m=days in the month

14.1 Calculation of the Storage Volume

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.

$$ {\hbox{C}}{{\hbox{V}}_{\rm{m}}} = {\hbox{Pre}} \times {f_{\rm{C}}}\quad ({\hbox{l}}/{{\hbox{m}}^2}{\hbox{month}}), $$
(14.4)

where

  • Pre (l/m2month)=mean monthly precipitation

  • f C=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:

$$ {\hbox{ST}}{{\hbox{P}}_{\rm{m}}} = {\hbox{C}}{{\hbox{V}}_{\rm{m}}} - {\hbox{CW}}{{\hbox{R}}_{\rm{m}}} - {\hbox{E}}{{\hbox{V}}_{\rm{pond}}}\quad ({\hbox{l}}/{{\hbox{m}}^2}{\hbox{month}}). $$
(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:

$$ {\hbox{ST}}{{\hbox{P}}_{\rm{y}}} = \sum {\hbox{positive ST}}{{\hbox{P}}_{\rm{m}}}\quad ({\hbox{l}}/{{\hbox{m}}^2}{\hbox{month}}). $$
(14.6)

The yearly deficit is:

$$ {\hbox{De}}{{\hbox{f}}_{\rm{y}}} = \sum {\hbox{negative ST}}{{\hbox{P}}_{\rm{m}}}\quad ({\hbox{l}}/{{\hbox{m}}^2}{\hbox{month}}). $$
(14.7)

The yearly storage balance is:

$$ {\hbox{ST}}{{\hbox{B}}_{\rm{y}}} = {\hbox{ST}}{{\hbox{P}}_{\rm{y}}} - {\hbox{De}}{{\hbox{f}}_{\rm{y}}}\quad ({\hbox{l}}/{{\hbox{m}}^2}{\hbox{month}}). $$
(14.8)

The following cases have to be distinguished:

  1. 1.

    If STBy>0 or STPy>Defy, the storable precipitation is sufficient for irrigation throughout the year. The storage volume becomes:

    $$ {\hbox{VST}} = {\hbox{De}}{{\hbox{f}}_{\rm{y}}}\quad ({\hbox{l/}}{{\hbox{m}}^2}). $$

    One can enlarge the volume, if the monthly variation of precipitation is high. The storage volume is:

    $$ {\hbox{VST}} = {\hbox{De}}{{\hbox{f}}_{\rm{y}}}({1} + {V_{\rm{C}}}), $$

    V C=coefficient of variation for precipitation (see Chap. 2).

  2. 2.

    If STBy<0 or STPy<Defy, the storable precipitation is not sufficient for irrigation. two cases have to be distinguished:

    1. (a)

      If the storable precipitation is higher than the maximum monthly collected precipitation CVmmax

      If STPy>CVmmax, then

      $$ {\hbox{VST}} = {\hbox{ST}}{{\hbox{P}}_{\rm{y}}} $$

      or

      $$ {\hbox{VST}} = {\hbox{ST}}{{\hbox{P}}_{\rm{y}}}{(}1 + {V_{\rm{C}}}{)}\quad ({\hbox{l}}{{\hbox{m}}^2}) $$
    2. (b)

      If STPy<CVmmax, then

      $$ {\hbox{VST}} = {\hbox{C}}{{\hbox{V}}_{\rm{mmax}}} $$

      or

      $$ {\hbox{VST}} = {\hbox{C}}{{\hbox{V}}_{\rm{mmax}}}({1} + {V_{\rm{C}}}) $$

14.2 Example 1: Storage Volume for Climate Conditions in Bangalore (India)

Table14.1: Calculation of the storage volume for collecting rainwater for irrigation in Bangalore (India). Precipitation Pre (Müller 1996); Monthly storable precipitation STPm with (14.5) and EVpond=0; ET0 see Fig.13.1;

Table14.1 Data for the calculation of the storage volume in Bangalore (India)
Full size table

CWRm=ET0×k C(1+l i)×A Cr/A G×d m

For tomato, mean k C=1.1; (1+l i)=1.05; A Cr/A G=0.9

k C(1+l i)×A Cr/A G=1.04;

CWRm=1.04×ET0×d m

From Table14.1 can be seen:

  • STPy=Σpos STPm=164.9l/m2year

  • Defy=Σneg STPm=371.2l/m2year

Case 2(a): Storage volume per m2 greenhouse area VST=STPy=165(l/m2)

The storage is empty from February until July.

Looking at the accumulated sum of STPm, starting in August, the crop water requirement can be covered for 6 months from August to January by rainwater with a storage volume of 0.165m3 per m2 greenhouse area.

14.3 Example 2: Storage Volume for Climate Conditions in Antalya (Turkey)

Table14.2: Calculation of the storage volume for the collection of precipitation in Antalya (Turkey). Precipitation Pre (Müller 1996); STPm with (14.5) and EVpond=0; ET0 from Fig.13.1. CWRm=1.04×ET0×d m (see Example 1)

Table14.2 Data for the calculation of the storage volume in Antalya (Turkey)
Full size table

STPy=Σpos STPm=608.2l/(m2year)

Defy=Σneg STPm=1,651.4l/(m2year)

STPy>CVmmax:

Case 2(a): Storage volume VST=STPy=608l/m2

The storage is empty from July until October. The crop water requirement can be covered for 8 months from November to June by rainwater with a storage volume of 0.61m3/m2 greenhouse area.

14.4 Design of Rainwater Storage Basins

Different types of storage basins can be built, if enough space is available near the greenhouse:

  • Simple basins, dug in the soil, if the soil at the bottom of the basin is sufficiently watertight.

  • Earth basin lined with plastic film.

  • Concrete basins; durable, need less maintenance, but are very expensive.

All storage basins should be covered at the surface by swimming plastic film, for example, to avoid too high evaporation.

Figure14.1 shows the arrangement of a plastic-film water basin with water tubes from the gutters to the basin (von Zabeltitz and Baudoin 1999). To collect heavier rainfall, the gutters and tubes leading to the storage basin must have an adequate diameter. The tubes leading the water to the basin should have a slope of about 1:50–1:100. The following diameters are recommended:

Fig.14.1
figure 1

Rainwater basin and rainwater run off from greenhouse gutters

Full size image

Greenhouse floor area (m2)

Tube diameter (mm)

<400

100

400–700

125

700–1,200

150

If the level of the storage basin is deep enough, the rainwater can be led via open gutter lined with plastic film.

The basin has to be situated at the deepest point of the site. If this is not the case and the greenhouses are placed deeper than the basin, the rainwater can be conducted into the basin by a siphon system (Fig.14.2). Watertight tubes are installed sloping downward to a deepest point near the basin, and from there into the basin. The gutters have to be above the water level of the basin. When rainwater in the tubes rises above the water of the basin, positive pressure develops, and the water flows from the tube into the basin.

Fig.14.2
figure 2

Rainwater conducted to the basin by a siphon system

Full size image

When digging out the basin, the soil is thrown up around the basin as an embankment (Fig.14.3).The angle of the embankment is about 34°, or it has a ratio of 1:1.5. Thus it is within the range of frictional angles of most types of soil.

Fig.14.3
figure 3

Measurements of a rainwater basin

Full size image

In the case of a 2m-deep square basin, the measurements a, b, c and d have the following values (m) for different quantities of water:

Water quantity(m3)

a(m)

b(m)

c(m)

d(m)

200

7.25

13.03

12.8

0.75

400

11.7

17.5

17.25

09

600

15.05

20.8

20.6

1.0