2. The water requirement of the crop is that
amount of water that is required to meet
the ET(evapotranspiration) rate.
Therefore in order to estimate the crop water
requirement we need to measure the ET
rate .
3. BASED ON DEMAND BASED ON SUPPLY
1. Lysimeter method
2. Estimation of ET losses.
3. Soil moisture depletion.
1. Field experimental
technique.
2. Water balance mathod.
4. Lysimeter are he containers in which a
known volume of soil is placed and crops
are grown as in the main field .
Which measures the loss of water supplied
to the crop in form of evapotranspiration
and percolation.
5. 01.WEIGHING TYPE 02.NON WEIGHING TYPE
Gives accurate measure of
quantity of water lost.
Non weighing type of lysimeter
works on the Archimedes
principle of liquid
displacement.
6.
7.
8. a. PAN EVAPORATION METHOD
It provides a measurement of combined
effect of temperature, humidity, windspeed,
sunshine on reference Eto.
Many types of evaporation pan being used .
The best one is class A evaporation pan
and sunken colorado pan.
9. Pan is installed in the field.
Pan is filled with the known quantity of
water.
Water is allowed to evaporate during a
certain period of time.
After 24 hrs remaining quantity of water is
measured .
Amount of evaporation per time unit is
calculated.
10.
11. ETo = Kpan *Epan
where,
ETo = reference crop
evapotranspirationKpan= pan co-efficient
Epan=pan evapotranspiration
12. If no measured data on pan evaporation are
available locally, a theoretical method to
calculate reference crop Eto has to be used.
It is a simple method using the temperature
only. Especially under extreme climatic
condition the method is inaccurate .
In windy, dry, sunny areas the ETo is
underestimated while in calm
, humid, clouded areas ETo is overestimated.
13.
14. ETo = p(0.46 Tmean +8)
Where,
ETo = reference crop
evapotranspiration
Tmean =mean daily temperature
p=mean daily % of annual daytime hrs.
15. Here, consumptive use is estimated by
measuring soil moisture depleted in
different layers in irrigated field crops .
FORMULA:
WR=Cu+ER+GWc-Wd. Where
ER= effective rainfall
GWc=ground water contribution
Wd= drainage.
16. The seasonal water requirement are
computed by adding measured quantities
of irrigation water, the effective rainfall
received during the season an the
contribution of moisture from the soil.
17. This sis also called inflow-outfloe method
and suitable for large areas like watershed
and micro command .
