Water Management in Horticultural Crops (1+1)

Climatological approach using irrigation water depth and evaporation relationship

Evaporimeter

• Evaporimeters like United States Class-A open Pan Evaporimeter, Sunken Screen Open Pan Evaporimeter and atmometer may be used for irrigation control. They are employed to measure the evaporation loss, which is used to determine the consumptive use by crops by multiplying the evaporation values with crop coefficient values. The coefficient varies from 0.6 to 0.8 for most crops at their different stages. Irrigation is applied when crops consume the available soil water to certain limit, calculated on the basis of consumptive use rate as determined by evaporimeters. Sunken screen evaporimeter value can be used from the period of 25 per cent ground coverage by crops till their maturity. The values of pan evaporation for this purpose are found for various crops at their different growth stages under different soil and climatic conditions.
• To be filled up to 5cm below ring


Depth of irrigation

• Depth of irrigation is a function of the water retentive capacity of root zone soil and extent of soil water depletion at the time of irrigation. It refers to the depth to which the applied water would cover an area. The net depth of irrigation is decided by the amount of water required to bring the soil water content just before irrigation to field capacity in the root zone soil. The water content of soil just before irrigation must be known to calculate the net depth of water required to be applied. It is calculated by the following formula,


Where,

D= net depth of water to be applied or net irrigation, cm
Fci = field capacity of the i-th layer of soil in per cent by weight
Mbi = water content of the i-th layer of soil just before irrigation, per cent by weight
asi = apparent specific gravity of i-th layer of soil, g/cm3
di= depth of i-th layer of soil in the root zone, cm
n = number of soil layers in the root zone d

• Usually, the soil zone that accounts for 90 per cent of the root mass needs to be wetted by irrigation when the crop is fully grown, but for an actively growing crop the soil little below the actively growing roots should be made moist. The depth of irrigation required for different soil types when soil water is depleted to 50 per cent availability are given in the following table.
Irrigation depths required for different soils at 50 Per cent Soil Water Depletion
 
Soil class	Depth of irrigation in millimeters per metre depth of soil
Sandy soil	40
Sandy loam soil	60
Loam soil	80
Clay loam soil	100
Clay soil	125