How to Determine the Frequency of Irrigation – Problem & Solution

How to Determine the Frequency of Irrigation

Mathematical Problem

Following data are given, determine the frequency of irrigation

  • Field capacity of soil = 45 %
  • Permanent wilting point = 19 %
  • Density of soil = 1.5 g/cm3
  • Depth of root zone = 65 cm
  • Daily consumptive use of water = 18 mm

Solution:

Available moisture = Field Capacity – Permanent wilting point = 45 – 19 = 26

Let readily available moisture = is 75 % of the available moisture,

So, readily available moisture is = 26 × 0.75 = 19.5 %

Optimum moisture content = 45 – 19.5 = 25.5 %

Now, by applying irrigation water the moisture content is to be raised from 25.5 % to 45 %.

Calculate the depth of water to be applied in each watering, by using the following equation:

D_{w} = \frac{W_{s}\times d}{W_{w}}\times [F_{c}-M_{0}]

Where,

  • Ws = Unit Weight of soil or density of soil = 1.5 g/cm3
  • Ww = Density of water = 1g/cm3
  • d = Depth of root zone = 65 cm
  • Fc = Field capacity = 45 %
  • M0 = Optimum moisture content = 25.5 %

After putting the all value,

D_{w} = \frac{1.5\times 65}{1}\times\left [ 0.45 - 0.255 \right ]

Or, D_{w} = 97.5 \times 0.195 = 19.01 cm

Now, we can calculate the frequency of irrigation by using the following equation

f_{w} = \frac{D_{w}}{C_{u}}

Where,

  • fw = Frequency of watering or frequency of irrigation
  • Cu = Daily consumptive use of water =
  • Dw = Depth of water to be applied in each watering

After puting the all value, the frequency of irrigation

f_{w} = \frac{19.01}{1.8} = 10.56 = 11 days (say)