Farm water requirement and determination of stream size

a. Water requirement:

• It is the quantity of water required by a crop in a given period of time for its normal growth under field condition at a given place.
• It includes losses during the application of water and the quantity of water required for special operation such as land preparation, transplanting, leaching, etc.

WR = ET or CU + Application losses + Special needs

Numerically,

WR = IR + ER + S

Where,

WR = Field water requirement

IR = Irrigation requirement

ER = Effective rainfall

S= soil profile contribution

b. Field Irrigation Requirement:

• It refers to the water requirement of crops exclusive of effective rainfall and contribution from soil profile and it may be given as ,

IR = WR – [ ER + S]

• It depends on irrigation need of individual crop, their area and the losses in the farm water distribution systems, mainly by the way of seepage.
• The irrigation requirement of an outlet command area includes the irrigation requirement of individual farm holdings and the losses in the conveyance and distribution system.

c. Effective rainfall:

• It is that portion of the total rainfall which directly satisfy crop water needs.

d. Field water Balance:

• It includes all gains, losses and changes of storage of water occurring in a given field within specified boundaries during a specified period of time.
• Gains includes precipitation, irrigation, accumulation of runoff from higher tracts of land.
• Losses includes surface runoff, deep percolation out of the root zone, evaporation from soil surface and transpiration from the crop canopy.

Change in Storage = gains – lossess

Or, Δ S + Δ V = ( P+I) – ( R+D+E+T)

Where,

P = precipitation

I= Irrigation

R= Run off from field

D = Downward drainage

E= Evaporation from soil

T = Transpiration by the crop canopy

ΔS = The change in soil water content of the root zone

ΔV = The Change in plant water content

e. Net Irrigation Requirement (NIR):

• It is the depth of irrigation water or the amount of water required to bring or drain the soil moisture in the effective root zone of the soil upto the field capacity.

NIR = £ⁿ_i=1  x A_i x D_i

 Where,

NIR = Net irrigation requirement

Mfci, Mbi = Moisture content % in Fc and before irrigation in the i^th layer in the soil

Ai = Apparent specific gravity

D = Depth of soil in the i^th layer

f. Field irrigation Requirement (FIR)

• It is the amount of water required to meet net irrigation requirement plus the water losses, percolation in he field water courses, field channels and the field application of water.

FIR = NIR/ η_a

Where η = water application efficiency

Note: Losses in the water conduct channel is called conveyances or transit losses.

g. Gross Irrigation Requirement:

• It is the sum of water required to satisfy the field irrigation requirement and the water lost as the conveyance losses in the distribution in the fields.

GIR = FIR/ η_c

Where, η_c = water conveyance efficiencies.