Water Management in Horticultural Crops

Methods of estimating evapo-transpiration

• Various methods are employed to estimate the crop ET or CU.
  
• The methods may be grouped into (i) direct methods, (ii) empirical methods and (iii) pan evaporimeter method
I. Direct Methods

• Direct methods are the water balance or hydrologic methods and include
• (1) lysimeter, (2) field experimentation, (3) soil water depletion, and (4) inflow-outflow methods.
• They give reliable values, but require elaborate installations and precise measurements.
  
• They are however costly, laborious and time consuming.
1. Lysimeter method

• The lysimeter method is very important in measurement of not only the ET but also the various components of water balance.
  
• A lysimeter is a device by which an experimental soil located in a container is provided with hydrologic separation from the surrounding soil.
  
• The method involves growing crops in lysimeters installed in crop fields to provide the crop environment and measuring the water balance during the crop growing period.
  
• Measurements of different components for water balance studies such as water added to lysimeters through precipitation and irrigations, change in soil water storage, and water lost through evaporation, transpiration, run-off and deep percolation are made. This can be expressed as,


Where,

P=precipitation, cm
IR n = net irrigation requirement of crop, cm
ΔSW = soil water contribution (the difference between soil water contents at sowing and at harvest of crop in cm)
R = surface runoff, cm
PW = deep percolation, cm
or
CU or ET = ER+ IRn+ ΔSW
Where,
ER = effective precipitation, cm
ER = P- (R+PW) = effective rainfall, cm

• Increase with the development of canopy, the evaporation from the adjacent soil surface gradually decreases, while the transpiration and the resultant ET increase. Crop density influences the ET in the same way as the crop cover influences the ET. The plant population and other crop management practices that affect the net radiation at the soil surface, change the ET unless the soil surface and plants get constant water supply. With lower plant population, the ET is low. Plant height increases ET by greater interception of advective heat.
• Root spread governs the ET to the extent roots encounter water in the soil profile, when the soil water is limiting in upper part of the soil. This is quite important particularly in arid and semi-arid areas where deep-rooted crops have higher ET than shallow rooted ones.
• Lysimeters are installed in fields with a fairly large guarded area having the same crop as in the lysimeter. The guarded areas are irrigated whenever the lysimeter crop is irrigated. The soil is placed in the lysimeters as close to in - situ condition as possible.
• Both the weighing and nonweighing type lysimeters are used for measurement of ET. When very short period (daily or hourly) estimates are wanted, the weighing type lysimeter is installed. In weighing type lysimeters, the container is placed inside a tank containing some suitable liquids (water or ZnCl₂ solution) so that the lysimeter container remains floating to ease the weighing. An overhead portable balance may be used for weighing. The changes in the buoyancy or in the hydraulic load are calibrated to estimate the loss in weight of the lysimeter owing to evapotranspiration. The loss in weight gives the measure of the evapotranspiration. A deduction is made for any loss due to deep percolation.
• Estimates of consumptive use by nonweighing type lysimeters are made following the soil water depletion method as discussed later in this chapter. Soil water measurements may be made by neutron scattering meter or gravimetric method. The former is preferred. Determination of soil water content by gravimetric method requires replicated soil sampling. The change in soil water content is worked out by using equation.
2. Field experimentation method

• Field experiments with treatments having varying levels of irrigation are carried out to estimate the seasonal consumptive use of irrigated crops.
  
• The water table should be at a considerable depth (at least 3 metres deep for field crops).
  
• Measurements of water supplied to the crop through effective rainfall and irrigation and changes in the soil water reserve during the growing season are made.
  
• The water thus supplied to the crop under treatments of varying levels of irrigation is correlated with the yields obtained.
• The quantity of water used to produce the yield that appears most profitable is taken as the CU.
3. Soil water depletion method

• Consumptive use of crops may be determined by soil water depletion studies on a fairly uniform soil.
  
• Water table should be deep enough (at least 3 m deep) so that it does not influence the soil water fluctuations in the root zone.
  
• Soil water content in different layers of the root zone are measured just before and after irrigation or rainfall (immediately, as early as soil sampling is possible after irrigation) and during the period between two successive irrigations as frequently as possible depending on the degree of accuracy desired.
  
• Frequent soil water measurements give more accurate information.
  
• The soil water depletion during any short period is considered as the consumptive use (CU) and is obtained by summing up soil water depletion or losses of soil water during the different periods of measurements in the growing season.
4. Inflow-outflow methods

• The inflow – out flow method is applied for estimating the yearly CU over large area. It is also called water balance method. It may be formulated as follows.

Where,

CU = Yearly consumptive use over a large area, hectare metres
P= Yearly precipitation, hectare meters
GW= Change in ground water storage, hectare metres
R=Yearly outflow from the area , hectare metres

• The change in soil water storage in the profile is not included as it is considered negligible. It is assumed that the subsurface inflow into the area is the same as the subsurface outflow.