Deficit irrigation (DI) is an optimization strategy whereby net returns are maximized by reducing the amount of irrigation water and crops are deliberately allowed to sustain some degree of water deficit without affecting the yield.

Farmers irrigate full (FI) crops even in water limited areas. In this method, Crops receive full evapo-transpiration (ETP) requirements in order to produce maximum yield. But in real practise farmers apply several folds of ETP in regular irrigation. The latter practice is Luxury irrigation (LI). In the case of luxury irrigation it will not result in maximum yield; in fact yield becomes lower than normal due to excess wetness of the rhizo- sphere.

Deficit Irrigation (DI) is a water saving irrigation method that can cut down water application to 50% compared to full irrigation (FI). The amounts of irrigation reduction in DI is crop- dependent and generally do not cause yield reduction and hence increase water productivity (WAPRO). Partial root-zone drying Irrigation (PRD) and Regulated Deficit irrigation (RDI) are two methods of applying DI.

The concept of DI originated around 1970. But it has not been into practical application because of the lack of Agronomic and Economic analyses of the outcome of DI in many crops. Now that the situation is critical regarding the water availability for irrigation, especially in the arid and semi-arid regions of the world, technologies like DI need to be brought into adoption. The climate change threats on Agriculture, is found to be mainly through lack of efficient water management in practice. It is here DI assumes an important role in years to come.

PRD has been successfully applied to Apple, Strawberry, Grape, Mango, Pomegranate and Citrus without compromising yield while reducing water consumption. It is interesting to note that no such studies on DI is done in India, and not only farmers but academics and scientists are not aware of the veracities and effects of DI. This article is written with the intention that DI should find a place in our water management methods. It is critical and urgent.

One should clearly understand that DI is not water stressing the plant as in the conventional sense. Benchmarking the yield response, irrigation is provided less than the normal ETP. How much deficit the crop can withstand is the amount of reduction of irrigation quantum to the point where yield is maintained as in 100 % ETP replacement.

Partial Root zone Drying (PRD)

PRD is a form of DI method which involves irrigating only one half of the root-zone in each irrigation event. The wetted half of root zone is allowed to dry to certain soil water content before re -wetting by shifting the irrigation to the dry side. During the PRD process, soil drying (partial soil drying) will stimulate root to leaf biochemical signalling that would reduce stomatal conductance and hence transpiration. In other words, DI thru PRD induces stomas to partially shut and thus reduce water loss from the crop.

Regulated deficit Irrigation (RDI)

It is another strategy where after estimating the ET for full irrigation (FI), a certain fraction (50%-70%) of ET is applied to the whole root zone.

Physiological mechanism of PRD induced water balance

While PRD is in progress, it also maintains the plant water potential (the differential required for water absorption from the drying soil) and reduces soil evaporation, lesser soil surface is wetted when only half the root zone is irrigated each time. It is also reported that PRD will reduce water consumption by enhancing abscisic acid production in the dry half of the root system. Water use efficiency is increased because the well- watered half of the root zone ensures maintenance of fruit growth while vegetative growth is reduced.

The deficit level that a crop can withstand has to be arrived at experimentally before incorporating PRD into its agronomy as given below:

1. PRD in Mango

Experiment was conducted on 10 year old Mango variety "Chok Anan" grown in Chiang Mai, Thailand. The trees were grown at 4 m x 4m and 196 trees were included in the study conducted in 2004.

In PRD the irrigated half root zone is alternated every two weeks. Similarly, the authors of the study attempted to remove any irrigation method effect (micro-sprinkler vs Drip) by irrigating half number DI 50 trees with micro sprinkler and the rest with drip method. Similarly, in PRD also half the number of trees received micro sprinkler and the other half drip.

The results of this study proved;

The study clearly demonstrated the value of deficit irrigation thru Partial alternate drying of root zone. Later the study was repeated in a commercial orchard with FI (100 % ET), PRD at 50 % ET alternated on either side of the tree trunk and PRD at 50% ET is developed as a part of Mango irrigation agronomy. It resulted in no yield reduction but increased WUE.

2. PRD in Citrus

A similar study to establish deficit irrigation technique in Citrus was conducted by California citrus industry in 2007. In this study they had 6 treatments. 1. FI (100 % ET), 2. 75% PRD (75 % ET), 3. 60 % PRD (60 % ET), with one emitter on each side of the trunk but operated one side of the trunk at a time (half root zone wetting): 4. 75% and 5. 60% ET with emitters on both side of the trunk operating at the same time (full root zone wetting). Very interesting results were obtained. PRD (treatments 2 and 3) as such reducing the water consumption by 25-40%. And significantly higher fruit yield was recorded in 60% PRD (treatment 3) compared to others.

3. PRD in Pomegranate

It was first studied in Thessaloniki in Greece. The study had 3 treatments- 1. 100 % ET (FI) 2. PRD1 at 100% ET (one part of the root zone irrigated at 100% ET while the other part experienced drying) and 3. PRD 2 at 50% (half the root zone with 50% ET application and the other half drying). Here PRD 1 (100% ET to half root zone) recorded higher fruit weight, aril weight and juice percentage.

4. PRD in Strawberry

Experiments were conducted at East Maling research station, Kent, England. The objective was to see whether the soil moisture environment created by PRD or DI can change the anti- oxidant contents (Ascorbic acid and Ellagic acid) in Strawberry fruits. The treatments were DI at 60%, 80% and 100% ET and PRD at 60%, 80 %, and 100 % ET. Both these sets of deficit irrigations were compared with FI at 120% ET. The results showed that while fruit weight remained unchanged the ascorbic acid and ellagic acid concentrations increased by 55% in PRD at 80% ET and 270 % in PRD at 60% ET. Fruit quality changed to the better when irrigation water was reduced and applied through Partial root drying method.

Table 1 Summarises the research done in DI with different strategies and its Economic impact.

Table 2 Research focusing on WUE by RDI or PRD techniques

It is clear after the detailed study of the research works done as summarised above in DI, that it is a technology to conserve water for agriculture and it increases the efficiency of irrigation water use. In drip irrigated crops DI can easily be practised.

Besides the above studies there are a whole lot of DI studies on physiological and biochemical changes arising out of DI in crop systems. I am not tabulating them in this paper as the consolidated effect of all such physiological variations due to DI would be integrated in production figures and water use.

Unfortunately, India being a front runner in the water scarcity scenario, DI irrigation strategies like RDI, PRD etc. should be field tested and applied in large number of irrigated crops. Fortunately the adoption of drip irrigation helps in easy implementation of DI. DI also decreases the rapidity of soil deterioration and negative climate change effects.Why not adopt DI in Indian small farms as long as it does not reduce crop productivity ?