The following points highlight the top five methods used for estimating the water requirement of crops. The methods are: 1. Lysimeteric Technique 2. Water Balance Techniques 3. Soil Water Depletion Method 4. Field Experiment Plots Method 5. Estimation of Etc from Climatological Data.
Method # 1. Lysimeteric Technique:
Lysimeter was constructed by Philippe de La Hire (1640-1718). Lysimeter is a proven method to provide complete information on all the components of water balance. It can be used for measuring the amount of percolating water and precipitation as well as to quantify water balance in soil column.
A lysimeter is a large container through which water losses and gains can be measured in soil in which crops are grown. The container is fitted with suitable inlets for irrigation and outlets for drainage. The lysimeters are buried in the soil and are surrounded by the same crop as is grown inside. The size of lysimeter varies from small oil drums to large size and deep lysimeters. They can be either the non- weighing or weighing type.
For weighing lysimeters, actual evapotranspiration can easily be calculated using the following equation:
Eta = P – S ± ΔS
Where, Eta, actual evapotranspiration (mm); P, the precipitation (mm); S, the amount of seepage water (mm); and, ΔS, change in the amount of stored water (mm).
For non-weighing lysimeters, changes in water balance are measured volumetrically weekly or biweekly. No accurate daily estimates can be obtained. Weighing lysimeters can provide precise information on soil moisture changes for daily or even hourly periods. The lysimeter is placed inside another tank which is in contact with the surrounding soil.
Method # 2. Water Balance Techniques:
Water balance models were developed in the 1940s by Thornthwaite (1948) and were later revised. It is also known as inflow-outflow method and has been mostly used to make quantitative estimates of water resources and the impact of man’s activities on the hydrologic cycle suitable for large area over long period of time. On the basis of the water balance approach, it is possible to make a quantitative evaluation of water resources and its dynamic behaviour under the influence of man’s activities.
The basic concept of water balance is: input to the system – outflow from the system = change in storage of the system (over a period of time).
The basic equation may be formulated as follows:
CU = P + I + ΔGW – R
CU = yearly consumptive use of water over a large area (hectare meter)
P = yearly precipitation (area in hectare × precipitation in meter)
I = surface water inflow into the area (hectare meter)
ΔGW = change in the ground water storage (hectare meter)
R = yearly outflow (run off) from the area (hectare meter)
Method # 3. Soil Water Depletion Method:
In this method, soil moisture content is measured from various layers of soil before and after each irrigation cycle or whenever soil profile is recharged by effective rainfall. The total depletion of soil moisture after every irrigation cycle from the root zone of crop give Consumptive Use (CU) of crops.
Consumptive Use (CU) of crops can be estimated as follows:
CU = consumptive use of water
Mai = soil moisture (%) after irrigation in ith layer
Mbi = soil moisture (%) before irrigation in ith layer
Bdi = bulk density of ith layer of soil
Di = depth of ith layer of soil
Method # 4. Field Experiment Plots Method:
Field experiment with various levels of irrigation is carried out to estimate the consumptive use of crops. Water table should be at least 3 m deep for field crops. This is reliable and common method for determining water requirement of crops used throughout the county. In this method, measurement of water applied through irrigation and effective rainfall and change in soil water reserve is calculated.
In simple words, total quantity of water used to produce the most profitable yield is called consumptive use (CU) of crop.
The consumptive use of crop can be calculated from the following equation:
CU = ER + IRn + ΔSW
Where,
CU = seasonal consumptive use (cm)
ER = effective rainfall (cm)
IRn = net irrigation applied to realise most profitable yield (cm)
ΔSW= soil water contribution
Effective rainfall is the portion of rainfall falling during the growing period of a crop, which meets crop water requirement or crop evatranspirational requirement.
This method is, otherwise, effective but having some limitation to get correct estimation of crop water requirement, i.e. amount of actual soil water content before and after irrigation, deep percolation loss of water.
Method # 5. Estimation of Etc from Climatological Data:
Evapotranspiration (ET) is the loss of water from a vegetative surface through the combined processes of plant transpiration and soil evaporation (ET is equivalent to and frequently referred to as consumptive use). Both environmental and biological factors affect ET. Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. Apart from the water availability in the top soil, the evaporation from a cropped soil is mainly determined by the fraction of the solar radiation reaching the soil surface.
This fraction decreases over the growing period as the crop develops and the crop canopy shades more and more of the ground area. At sowing, nearly 100% of ET comes from evaporation, while at full crop cover, more than 90% of ET comes from transpiration. The evapotranspiration rate is normally expressed in mm per unit time.
1. Climatic factors (temperature + wind + humidity) + well watered grass = ETo
2. ETo × well watered crop (optimal agronomic condition) = ETc
3. ETo × water and environmental stress = ETc adjusted
ETo is a climatic parameter expressing the evaporation power of the atmosphere. ETc refers to the evapotranspiration from excellently managed, large, well-watered fields that achieve full production under the given climatic conditions. Due to suboptimal crop management and environmental constraints that affect crop growth and limit evapotranspiration, ETc under nonstandard conditions generally requires a correction.
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. ETo is the rate of evapotranspiration 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.