Water Requirement of Crop
Water requirement of crop is the quantity of water regardless of source, needed for normal crop growth and yield in a period of time at a place and may be supplied by precipitation or by irrigation or by both.
Water is needed mainly to meet the demands of evaporation (E), transpiration (T) and metabolic needs of the plants, all together is known as consumptive use (CU). Since water used in the metabolic activities of plant is negligible, being only less than one percent of quantity of water passing through the plant, evaporation (E) and transpiration (T), i.e. ET is directly considered as equal to consumptive use (CU). In addition to ET, water requirement (WR) includes losses during the application of
irrigation water to field (percolation, seepage, and run off) and water required for special operation such as land preparation, transplanting, leaching etc.
WR = CU + application losses + water needed for special operations.
Water requirement (WR) is therefore, demand and the supply would consist of contribution from irrigation, effective rainfall and soil profile contribution including that from shallow water tables (S)
WR = IR + ER + S
Under field conditions, it is difficult to determine evaporation and transpiration separately. They are estimated together as evapotranspiration (ET). IR is the irrigation requirement.
Factors influencing Evapotranspiration (ET): ET is influenced by atmospheric, soil, plant and water factors.
A) Atmospheric factors:
3) Wind velocity
5) Relative humidity
B) Soil factors:
1) Depth of water table
2) Available soil moisture
3) Amount of vegetative cover on soil surface.
C) Plant factors:
1) Plant morphology
2) Crop geometry
3) Plant cover
4) Stomatal destiny
5) Root depth
D) Water factors:
1) Frequency of irrigation
2) Quality of water ET.
Water requirement of any crop depends on crop factors such as variety, growth stage, and duration of plant, plant population and growing season. Soil factors such as temperature, relative humidity, wind velocity and crop management practices such as tillage, fertilization, weeding, etc. Water requirement of crops vary from area to area and even field to field in a farm depending on the above-mentioned factors.
Estimation of Evapotranspiration (ET):
Climate is the most important decides the rate of ET. Several empirical formulas are available to estimate ET from climate date. FAO expert group of scientists has recommended four methods for adoption of different regions of world.
1) Blaney and Criddle method
2) Radiation method
3) Pan evaporation method
4) Modified penman method
Estimation of ET Involves Three Important Steps:
a) Estimation of PET or evapotranspiration (ET) by any four above methods.
b) Estimation of crop co-efficient (KC) and
c) Making suitable adjustments to local growing conditions.
a) Reference Evapotranspiration (ETO):
ETO can be defined as the rate of evapotranspiration of an extended surface of an 8 to 15 cm tall, green cover, actively growing completely shading the ground and not short of water.
Selection of a method for estimation of ETO depends on availability of metrological data and amount of accuracy needed. Among four methods for estimation of ETO, modified Blaney-Criddle method is simple, easy to calculate and requires data on sunshine (S.S.) hours, wind velocity (WV), relative humidity (RH) in addition to temperature (T).
Among these methods, modified penman method is more reliable with a possible error of 10% only. The possible errors for other methods are 15, 20 and 25% of pan evaporation, radiation and modified Blaney-Criddle methods respectively.
Modified Blaney method:
ETO = C [P (0.46 T + 8)] mm/day Where ETO = Reference crop ET in mm/day for the month considered T = Mean daily temperature in oC over the month considered P = Mean daily percentage of total annual day time hours of a given month and latitude (from standard table) C = Adjustment factor depends on minimum R.H., Sunshine hours and day time wind estimates.
Pan evaporation method:
ETO = Kp | Epan Where Kp = Crop factor Epan = mean pan evaporation (Epan pan evaporation)
Modified penman method:
ETO = C [W.Rn + (1-w). f (U). (ea – ed)] Where Rn = Net radiation in equivalent evaporation expressed as mm/day W = temperature of altitude related factor F (U) = Wind related function Ea – ed= Vapour pressure deficit (mili bar) C = the adjustment factor (ratio of U day to U night) Rn (0.75-Rns)
Ea =Saturated vapour pressure (m.bar) Ed = Mean actual vapour pressure of the air (m. bar)
Crop co-efficient is the ratio between evapotranspiration of crop (Etc) and potential evapotranspiration and expressed as T (crop) = Kc X ETo
Irrigation requirement is the total quantity of water applied to the land surface in supplement to the water supplied through rainfall and soil profile to meet the water needs of crops for optimum growth.
IR = WR – (ER + S)
Net irrigation requirement:
The net irrigation requirement is the amount of irrigation water just required to bring the soil moisture content in the root zone depth of the crops to field capacity. Thus, net irrigation requirement is the difference between the field capacity and soil moisture content in the root zone before application of irrigation water.
Gross irrigation requirement:
The total amount of water inclusive of water in the field applied through irrigation is termed as gross irrigation requirement, which in other words is net irrigation requirement plus application and other losses.
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