crop water requirement Flashcards
1.Crop Water Requirement
Crop water requirement is the water required by the plants for its survival, growth, development and to produce economic parts.
This requirement is applied either naturally by precipitation or artificially by irrigation.
- What are included in the crop water requirement
a) Transpiration loss through leaves (T)
b) Evaporation loss through soil surface in cropped area (E)
c) Amount of water used by plants (WP) for its metabolic activities which is
estimated as less than 1% of the total water absorption. Hence the ET loss is taken as crop water use or crop water consumptive use.
d) Other application losses are conveyance loss, percolation loss, runoff loss,
etc. , (WL).
e) The water required for special purposes (WSP) like puddling operation,
ploughing operation, land preparation, leaching, requirement for the purpose
of weeding, for dissolving fertilizer and chemical, etc
Hence the water requirement is symbolically represented as:
WR = T + E + WP + WL + WSP
Water requirement
Water requirement=Consumptive use of water ET+ Application loss + Special needs
Consumptive use is the volume of water used by plants for growth, development and maturity plus the water transpired along with the evaporation from the adjacent soil
- depends plant type, diff place, climatic variables & soil differences)
Application loss is the volume of water lost during the conveyance or application.
- depends on irrigation system, soil texture & management practices
Special needs refer to the volume of water required for special operations like leaching excess salts, wetting soil prior to tilling or puddling
- depends on moisture status, salinity status & crop species)
1.The crop water requirement varies from :
place to place,
crop to crop and
agro-ecological variation
crop characters.
2.features influencing the crop water requirement
Crop factors
a) Species, b) Variety c) Growth stages d) Duration
e) Plant population f) Crop growing season
2) Soil factors
a) Structure b) Texture c) Depth d) Topography
e) Soil chemical composition
- Agronomic management factors
a) Irrigation methods used
b) Frequency of irrigation and its efficiency
c) Tillage and other cultural operations like weeding, mulching etc / inter-cropping,etc. - Climatic factors
a) Temperature
b) Sunshine hours
c) Relative humidity
d) Wind velocity
e) Rainfall
how does climatic factor influence the water requirement
temperature- high temperature more water
Humidity- low humidity more water
windspeed- windy more water needed
sunshine-sunny more water needed
- The influence of the climate on crop water needs is given by the……………………
- what is ETo
1.reference crop evapo-transpiration(ETo).
The ETo is usually expressed in millimetres per unit of time, e.g. mm/day, mm/month, or mm/season.
Grass has been taken as the reference crop.
2.ETo is the rate of evapo-transpiration from a large area, covered by green grass, 8 to 15 cm tall, which grows actively, completely shades the ground and which is not short of water
Methods to determine ETo
a. Pan evaporation method
Many different types of evaporation pans are being used. The best known pans are the Class A evaporation pan (circular pan),Sunken Colorado pan (square pan)
The principle of the evaporation pan
the pan is installed in the field
the pan is filled with a known quantity of water (the surface area of the pan is known and the water depth is measured)
the water is allowed to evaporate during a certain period of time (usually 24 hours). For example, each morning at 7 o’clock a measurement is taken.
The rainfall, if any, is measured simultaneously
After 24 hours, the remaining quantity of water (i.e. water depth) is measured
The amount of evaporation per time unit (the difference between the two measured water depths) is calculated;
This is the pan evaporation: E pan (in mm/24 hours)
The E pan is multiplied by a pan coefficient (K pan) to obtain the ETo.
Formula: ETo = K pan × E pan
(ETo: reference crop evapotranspirationK pan: pan coefficient (constant numerical)E pan: pan evaporation)
If the water depth in the pan drops too much (due to lack of rain), water is added and the water depth is measured before and after the water is added. If the water level rises too much (due to rain) water is taken out of the pan and the water depths before and after are measured.
- Determination of K pan
2. The pan coefficient (K pan), depends on:
When using the evaporation pan to estimate the ETo, in fact, a comparison is made between the evaporation from the water surface in the pan and the evapotranspiration of the standard grass.
The water in the pan and the grass do not react in exactly the same way to the climate.
Therefore a special coefficient is used (K pan) to relate one to the other.
2.the type of pan
pan environment: if the pan is placed in a fallow or cropped area
climate: the humidity and windspeed
how is k pan determined for high and low value
For the Class A evaporation pan, the K pan varies between
For the Sunken Colorado pan, the K pan varies between
- The k pan is high if the
pan is placed in fallow area
the humidity is high
the windspeed is low
the k pan is low if the pan is placed in cropped area
the humidity is low
the windspeed is high
For the Class A evaporation pan, the K pan varies between 0.35 and 0.85. Average K pan = 0.70.
For the Sunken Colorado pan, the K pan varies between 0.45 and 1.10. Average K pan = 0.80.
Formula: ETo = K pan × E pan
Blaney-Criddle Method
The Blaney-Criddle method is simple, using measured data on temperature only.
It should be noted, however, that this method is not very accurate; it provides a rough estimate or “order of magnitude” only.
Especially under “extreme” climatic conditions the Blaney-Criddle method is inaccurate:
In windy, dry, sunny areas, the ETo is underestimated (up to some 60 percent),
while in calm, humid, clouded areas, the ETo is overestimated (up to some 40 percent).
The Blaney-Criddle method always refers to meanmonthlyvalues, both for the temperature and the ETo.
For example, if it is found that T mean in March is 28°C, it means that during the whole month of March the mean daily temperature is 28°C.
The Blaney-Criddle formula
The Blaney-Criddle formula: ETo = p (0.46 T mean + 8).
ETo = Reference crop evapotranspiration (mm/day) as an average for a period of 1 month.T mean = mean daily temperature (°C)p = mean daily percentage of annual daytime hours.
Determination of crop factor (Kc)
- To determine the Kc it is necessary for each crop to know ……………
- Per crop four factors have to be determined, one crop factor for each four growth stage:
The actual amount of water used by a crop or crop evapo-transpiration(ET crop) is related to reference crop evapo-transpiration (ETo) by a crop coefficient (Kc).
The relationship between the reference grass crop and the crop actually grown is given by a crop factor (Kc) as shown in formula:
ETo x Kc= ET crop ( crop evapo-transpiration)
Or ET= ETo x Kc
- the total length of the growing season and the lengths of the various growth stages.
- Per crop four factors have to be determined, one crop factor for each four growth stage: initial stage, crop development stage, mid season stage and late season stage.
Crops, soil & water demand
Soil types differ in ability to retain water
Water that is held by the soil and can be taken up by plant is called “available soil moisture” (ASM)
Depends on texture, structure & porosity of soil
Sand & course sandy loam retain less ASM , frequent irrigation is required
Fine texture soil retains more ASM in rooting zone, heavier and less frequent irrigation is required
