SOME BASIC NUMERACY NEEDED FOR IRRIGATION AND PESTCONTROL CALCULATIONS

DEFINITIONS• Pressure: a force acting on an area, expressed in units of kg/cm2, bars,

atmospheres or PSI (Pounds per Square Inch).• Water head: the pressure at the base of a water column, expressed in

meter units. The head in the bottom of a water column 10 m. high, is 10 m. = 1 bar. ≈ 1 atm.

• Water amount: measured in volume units. In the metric system the common units are liter (l) and cubic meter (m3) (1,000 l = 1 m3).

• Water flow-rate (discharge): the amount of water flowing through a certain cross-section per time unit. In the metric system the units are: m3/h or liter/h (l/h).

• Wetting diameter: the diameter of a circle of soil surface, wetted by a certain sprinkler = twice the wetting radius of the sprinkler. Measured in meters.

• Sprinkler spacing: the spacing between the sprinklers along and between the sprinkler laterals. For example: 12 m x 18 m.

Units

APPLIED TO IRRIGATION

UNIT CONVERSIONS: : 1 kg =1000g AREA: 1 Ha= 10 000m2 (100x100m) 1km2 (1000x1000) = 100 ha LENGTH: 1m= 100cm=1000mm/

1 kg =1000g

1 tonne is 1000kg

1 cm= 0.01m 1mm=0.001m 1inch, 1"= 2.5mm 2" = 50mm/)

CAPACITY: 1 litre: 1000ml (=1 dm3); 1 m3= 1000 litres)

PRESSURE: 1 BAR =10 m head / FLOW: m3/h expl: 20m3/h= 333l/min=5.55l/s

3l/s = 10 800 l/hr = 10.8m3/hr

PERCENTAGES

One part of 100 is called 1 per cent.One block of the 100 blocks in the picture is black, so 1 per cent is black. This is written as 1%.If 5 blocks are black, how many per cent is that? If 50 blocks are black, how many per cent is that?If a fourth is black, this corresponds to 25%.

From 1000 m2to 500m2to 250m2

EXAMPLE FOR CROP WATER REQUIREMENTS

1mm depth of water = 1L applied to 1 square metre.

• IRRIGATION DEPTH Assuming a layer of water of 40mm from flood irrigation covering the whole field. So a farmer can say ‘I need an irrigation application of 40 mm’. this is a standard irrigation application we can convert millimeters into m3/ha. Written as a formula this gives:

Area is 2ha an irrigation depth is 40mm how much water need to be applied

40mm= 0.040m= Volume/20 000Volume: 0.040x 20 000= 800m³

IF all water is in one session of 3 hours you need a flow of 266 m3/h

150 x1000/3600 l/s = 41.7 l/s