**2.2 Actual or real evapotranspiration (ETa)**

Actual ET is the amount of water actually utilized by an extensive surface vegetated with grass, at an active growth stage, covering completely the soil surface. ETa is the quantity of water that is actually removed from a surface due to the processes

#### **Figure 1.**

*Schematic representation of the water motion in the soil-plant-atmosphere system under optimal development conditions (adapted from [15, 16]).*

*Advanced Evapotranspiration Methods and Applications*

situation, generally plot size of 250 m2

improves water productivity.

increasing population. In the meantime, scientists across the world to regions working on it and tried to invent improved technologies which could partition greater share of the total ET water to T by diverting share of E. The surface water in the region continue to be delivered through old traditional canal and on-farm conveyance networks those have earthen bunds and are unlined resulting in very low water use efficiency (30–50%). Generally, irrigators usually cut off the supply when the advance is complete without considering the additional irrigation water infiltrates at the entrance especially when the soil is opened up with pre-seeding tillage. Thus large non-uniformities in water application in addition to over-irrigation. In our

fine texture is recommended for wheat [5] for improving the water use efficiency. For cotton furrow irrigation could save 100–150 mm of irrigation water [6] as it cuts down the share of E. Even the broad beds spaced at 1.35 m and planting cotton in furrows in paired rows improved its yield by 44% and saved 40% irrigation water as compared with row spacing of 0.75 m in flatbed system [7]. Micro-irrigation especially drip, mini-sprinklers and sub-irrigation systems designed to apply small and frequent irrigations, are now emerging as ideal technologies which cut off E share and partition greater share of the ET to the T component. As the water is being applied to the root zone rather than entire field and both unproductive losses, viz*.* evaporation (E) and deep drainage (D) is considerably reduced, irrigation efficiencies as high as 95 and 80% are achieved with drip and sprinklers systems, respectively. The overall water savings with drip systems ranged between 50 and 65% in vegetables [8] along with higher fertilizer use efficiency and better quality of the product. The government has introduced subsidies to the extent of 75%, still,

the drip system does not seem to be adopted at the desired levels.

For upland crops, viz*.* wheat, maize, potato, etc., effects of E demand and rainfall concept for timing irrigation to crops was put forward in 1970s [9, 10]. It is a deficit-irrigation approach that induces deeper rooting for promoting utilization of profile stored water especially the sub-soil water. Heavy pre-sowing irrigation followed by irrigations at IW/PAN-E ratio of 0.75 [11] and last irrigation during mid-March by charging soil profile to 80–100% of water depletion [9, 10] further

Thus, efforts have also been made to compute crop sensitivity to water stress by relating yield with ET or T. Water deficits mainly damage the crops during meiosis of pollen mother cells or around anthesis. Therefore, sensitive stages in different crops need to be identified to mitigate the adverse effects of limited irrigation. Generally, at present scientists advocate different technologies, viz*.* mulching, crop diversification, correct T time of rice, bed planting, zero tillage, short duration crop cultivars to partition greater share of the ET water share to the T by depressing E in one or other way. These technologies have a substantial scope in improving irrigation efficiency and reducing energy for groundwater withdrawal. In the present chapter, we tried to understand the concept and consequences of evapotranspiration for sustainable crop production in the era of climate change. A detailed description of evaporation, transpiration and evapotranspiration and their importance for sustainable agriculture are highlighted by the following sub-heading.

**2. Concept of evaporation, transpiration, and evapotranspiration** 

Evaporation is the physical process through which liquid water is converted to water vapor. The rate of E depends on the saturated vapor pressure of the liquid and increases with increase in temperature until the atmospheric pressure at the boiling point [12].

**and its relation to crop productivity**

in coarse texture and 500 m2

in medium to

**96**

*Advanced Evapotranspiration Methods and Applications*

## **Figure 2.**

*Schematic representation of the ETo and ETp (adapted from [16, 18]).*

of E and T [17]. There is a relation between potential ET and actual ET. Crop water need can be estimated by the following equation: Crop water needs = potential evapotranspiration − actual evapotranspiration.
