• Saturation capacity:

Water content of soil when all pores are filled with water.

• Field capacity:

Water content of soil after free drainage has taken place for sufficient period.

• Gravity water:

Rainfall or irrigated water flow down to water table under gravity.

• Capillary water:

Water held by surface tension against gravity.

• Hygroscopic water:

Water that cannot be extracted by plants.

• Soil moisture:

Water above water table.

• Permanent wilting point (PWP):

The water content at which plant can no longer extract sufficient water for growth and wilts up.

• Available moisture content (AMC):

Difference of water content between field capacity and permanent wilting point.

• Readily available moisture content (RAM):

Portion of AMC that is easily extracted by plants.

• Optimum moisture content (OMC):

The water content soil is not allowed to deplete below a certain value.

• Soil moisture deficiency:

Water required to bring soil at a given water content to its field capacity.

Soil moisture irrigation relationship:

Let,

d = Effective depth of root zone

FC = Field capacity

From definition of field capacity,

FC = Weight of water retained in certain volume of soil / Weight of dry soil of same volume

Considering unit area of soil

Weight of dry soil = γ_d * d

Weight of water retained = FC * γ_d * d

So, volume of water retained is,

V = (FC * γ_d * d) / γ_w

Where,

γ_w = Unit weight of water

γ_d  = Dry unit weight of soil

1.2 Soil moisture regime: Crop water requirement

• Consumptive use (Cu) or Evapotranspiration:

Defined as the total amount of water used by the plants in transpiration and evaporation from adjacent soil in specified time.

• Potential evapotranspiration (PET):

If sufficient moisture is available to completely meet need of plant.

• Actual evapotranspiration (AET):

Real evapotranspiration occurring in a specific situation.

• Estimation of consumptive use:

Consumptive use is estimated by penman’s equation.

PET = ( A* H_n + E_a * γ) / (A + γ)

Where,

H_n =  Net incoming solar radiation or energy expressed in mm of evaporable water per day

A = Slope of saturation vapor pressure versus temperature curve at mean air temperature

E_a = Parameter including wind velocity and saturation deficit in mm/day.

γ = Psychrometric constant = 0.49 mm of Hg/^oC

1.3 Factors affecting crop water requirement:

• Crop factor:
  • Variety
  • Growth stage
  • Duration
  • Plant population
  • Crop growing season
• Soil factor:
  • Structure
  • Texture
  • Depth
  • Topography
  • Soil chemical composition
• Climate factor:
  • Temperature
  • Sunshine hour
  • Relative humidity
  • Wind velocity
  • Rainfall
• Agronomic management factor:
  • Irrigation method use
  • Frequency of irrigation and its efficiency

1.4 Base period, kor period, duty, delta and their relationship.

• Crop period:

Time period that elapses from the instant of its sowing to the instant of its harvesting.

• Base period (B):
  • Time period that elapses between first watering of crop after sowing to last watering of crop before harvesting.
  • Crop period and base period are taken equal.
• Delta (Δ):
  • Total quantity of water required by a crop for full maturity.
  • Expressed in mm or cm.
• Duty (D):
  • Number of hectares of land irrigated for full growth of a given crop by the supply of 1 cumec water continously during base period.
  • Expressed in hectare/cumec.
  • Design discharge (Q) = Area (A) / Duty (D)
• Kor watering:
  • First watering which is given to a crop when crop is few centimeter high.
  • Maximum watering is done.
• Kor period:

Kor watering must be applied within the fixed limited period.

• Kor depth:

Depth of water supplied in the Kor period.

Relationship between duty, delta and base period.

Let, there be a crop of base period B days. Let 1 cumec of water be applied to this crop for B days.

Now, volume of water applied to this crop during base period.

    V = ( 1 cumec * B days)

or, V = 1 * 60 * 60 * 24 * B days

or, V = 86400B m³

The quantity of water mature D hectare of land.

So, area irrigated A = 10⁴ D, ha

Total depth of water applied to the field is given by

  Δ = Volume / Area = 86400B / 10⁴ D

or, Δ = 8.64B / D , m

or, , Δ = 864B / D , cm

Since design discharge is calculated based on kor depth and kor period.

Kor depth = (864 * Kor period) / Duty , cm

1.5 Command area of irrigation system – GCA, CCA and NCA.

• Gross commanded area ( GCA) :
  • Total area bounded within irrigation boundary which can be irrigated without considering the limitation of quality of water available.
  • Includes cultivable and uncultivable area like road, ponds, residential area etc.
• Cultural  or cultivable commanded area (CCA):
  • Includes  only cultivable area.
  • Divided into:

a. Cultural cultivated area

b. Cultural uncultivated area

• Net commanded area (NCA):
  • Cultivable commanded area deducting the area of canal network, supply ditches, bunds constructed in the field.

1.6 Principal crops, their season and water requirement

In irrigation a year is divided into two cropping season:

• Kharif season (Summer)
  • Starts from April 1 and ends on 30^th September.
  • Kharif crops: rice, bajra, maize, cotton, tobacco etc.
  • Rice is the most important kharif crop.
• Rabi season (Winter)
  • Starts from October 1 and ends on 31^st March.
  • Rabi crops: wheat, barley, mustard, potatoes etc.
  • Wheat is the most important rabi crops.

References:

• WECS (1998), Design Guidelines for Surface Irrigation in Terai and Hills of Nepal, (Vol. I and II)
• Michael, A.M.(2011). Irrigation theory and practice
• FAO(1977). Guidelines for Predicting Crop Water Requirements. FAO Irrigation and Drainage Paper No. 24.

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