The application of water below the evapotranspiration requirements of the plant is called deficit irrigation (DI). DI is a strategy that aims to stabilise yields and maximise water productivity while maintaining or increasing farmers' profits. Irrigation management shifts from maximizing net income per land unit to maximizing net income per unit of water used .
“Deficit irrigation is an optimisation strategy in which irrigation is applied during drought-sensitive growth stages of a crop. Outside these periods, irrigation is limited or even unnecessary if rainfall provides a minimum supply of water. Water restriction is limited to drought-tolerant stages, often the vegetative stages and the late ripening period. Total irrigation application is therefore not proportional to irrigation requirements throughout the crop cycle. While this inevitably results in plant stress and consequently in production loss, DI maximizes irrigation water productivity, which is the main limiting factor.”
Crop water use for production is termed evapotranspiration (ET). This is a combination of water lost by evaporation from the soil surface and transpiration by the plant, occurring simultaneously. It is difficult to separate the two processes but it can be done by modelling. The application of water below the evapotranspiration requirements of the plant is called deficit irrigation (DI). At this point it is important to introduce the concept of crop water productivity (WP) or water use efficiency (WUE) expressed in kg/m³. It is an efficiency term, expressing the amount of marketable product (e.g. kilograms of grain) in relation to the quantity of input (cubic meters of water) needed to produce that output . In other words WUE is the yield or net income per unit of water used in evapotranspiration.
The application of water below the evapotranspiration requirements of the plant is called deficit irrigation. Water can be saved from field irrigation and devoted to other uses. Deficit Irrigation especially allows economic optimization of water use in regard to crop output where water is limited, but it also involves structural adjustments (e.g. policies that support this kind of measure) in the agricultural system. Furthermore this watering technique is multifaceted, inducing changes at technical, socio-economic and institutional level .
One of the deficit irrigation strategies is the so-called partial root zone drying (PRD) . This technique is to allow one part of the root zone to dry while keeping the other half moist, and alternating the two zones every 2-3 weeks. Many plant species react to the signal sent by the root-zone exposed to the dry soil to the shoots which then induces stomata closure. This mechanism reduces water losses by transpiration and therefore increases water use efficiency [4, 5].
As agriculture accounts for 70% of freshwater withdrawals worldwide and, furthermore, most irrigation systems are very inefficient (only 30 to 50% of the water distributed is taken up by the plant), deficit irrigation is not only of high relevance in water-scarce areas or in dry seasonal periods; it also has the potential to optimise and reduce water use in irrigated systems .
Deficit irrigation techniques are very interesting when it comes to an efficient allocation of scarce resources like water. These techniques are maximizing water productivity, generally with good or unchanged harvest quality . It is particularly relevant for crops in which flowering and fruit development (like in mango) take place in the dry season, due to the application of relatively small amounts of water the harvest can be stabilized over time and it improves economic planning for farmers, which is increasingly interesting under climate change conditions where water resources are becoming scarce and rains erratic. Furthermore, since water use is reduced, the irrigated area can be increased and additional crops can be irrigated amplifying the diversity of the household production, which decreases the farmers' risk aversion.
The application of less water reduces the leaching effects of nutrients from the root-zone and agrochemicals, and the groundwater quality is preserved . Furthermore it reduces the risk of the development of certain diseases linked with high humidity (e.g. fungi) that are common in full irrigation systems.
Irrigation is applied to avoid water deficits that reduce crop production, but:
- farmers have to know the precise response of the specific crop to water stress; 
- deficit irrigation methods might cause decrease of production if the minimum water requirement of the crop is not guaranteed;
- the access to water has to be flexible during dry seasons with increased demand; 
- all irrigation water contains salts which accumulate in the soil profile as water evaporates; therefore salts have to be displaced below the root zone before they reach levels that influence crop production. Irrigation water above the evapotranspiration rate is beneficial in maintaining a positive salt balance in the soil .
Experiences: Deficit irrigation strategies in Thailand on mango trees
An experiment carried out in Thailand showed the positive effects of the use of partial root-zone drying (PRD) on mango trees (Chok Anan variety). The results below are from Spreer at al. 2007  and Spreer at al. 2009 .
In Thailand the mango crop is of high economic importance and it is produced in all agro-ecological zones of the country. Fruit development takes place during the dry season and high quality is guaranteed by irrigation. The study was comparing an irrigated group used as control group (fully irrigated, 100% of ET), a group with deficit irrigation (50% of ET), one with partial root zone drying (50% of ET alternatively applied to the sides of the root zones) and a last one without irrigation (rain feed). Analyses were conducted on the fruit development on the trees, at harvest and post-harvest.
- Pre-harvest (on-tree): the colour development of the fruit skin (exocarp) and the pulp (mesocarp) in the control was slightly better but it did not show significant differences between treatments. The same is valid for the sugar-acid ratio (the taste was not influenced either).
- Harvest: the best values of fruit size distribution were reached in the partial root-zone drying treatment. About 80% of the fruits were classified in the highest weight class, above 300g, and only 2% were not marketable with a weight below 200g, whereas in the control the unmarketable fruit rate was about 10%. Moreover, the fruits from the PRD group had a higher fraction of fruit pulp (mesocarp). Nevertheless, the amount of fruits in the control group was higher as in the other treatments but with a smaller fruit size. Yield differences between the control and the PRD treatment were not significant but led to a nearly doubled WUE.
- Post-harvest: the quality performance of the fruits from the deficit irrigation plots was not negatively influenced and no visible outer differences in between the treatments could be seen. Chemical composition was not influenced either.
Results showed that the fully irrigated field had higher yields because of higher crop load rather than because of greater fruit size. Fruits grown under PRD were bigger in size and had a higher fraction of edible parts compared to the other treatments (which can be very relevant when the fruits are processed), and the fraction of non-marketable fruits was reduced.
PRD is a method which is particularly relevant in years with a high fruit set, where fruit thinning with high labour cost is required to achieve larger fruits that meet premium prices.
It can therefore be concluded that deficit irrigation techniques are not negatively affecting the yield to a larger extent but can save considerable amounts of water.
1 Fereres, E., Soriano, M.A., (2007).Deficit irrigation for reducing agricultural water use J. Exp. Bot. 58, 147-158
2 English, M., (1990). Deficit Irrigation. I: Analytical Framework. J. Irrig. Drain. E.-ASCE 116, 399-412.
3 Kijne, J.W., Barker, R., Molden, D., (2003). Improving water productivity in agriculture: editor's overview. In: Kijne, J.W., Barker, R.M.D. (eds.), Water productivity in agriculture: limits and opportunities for improvement. International Water Management Institute, Colombo, Sri Lanka , p. xi-xix.
4 Dry PR, Loveys BR, Du¨ring H, Botting DG (1996) Effects of partial rootzone drying on grapevine vigour, yield composition of fruit and use of water. In: Stockley CS, Sas AN, Johnstone RS, Lee TH (eds) Proc. 9th Aust. WineInd. Techn. Conf., Adelaide, Australia. Winetitles, Adelaide, pp 128–131
5 Stikic R, Popovic S, Srdic M, Savic D, Jovanovic Z, Prokic LJ, Zdravkovic J (2003) Partial root drying (PRD): a new technique for growing plants that saves water and improves the quality of fruit. Bulg J Plant Physiol (Special Issue):164–171
6 Sadras V.O., Grassini P., Steduto P. (2007). Status of water use efficiency of main crops SOLAW Background Thematic Report - Status of Water use Efficiency of major Crops FAO. http://www.fao.org/docrep/017/i1688e/i1688e.pdf
7 Spreer, W., M. Nagle, S. Neidhart, R. Carle, S. Ongprasert and J. Müller (2007) Effect of regulated
deficit irrigation and partial rootzone drying on the quality of mango fruits (Mangifera indica L., cv. ‘Chok
Anan’). Agricultural Water Management 88 (1-3): 173-180
8 Pandey, R.K., Maranville, J.W., Chetima, M.M., (2000). Deficit irrigation and nitrogen effects on maize in a Sahelian environment. II. Shoot growth, nitrogen uptake and water extraction. Agric. Water Manage 46, 15-27.
9 Zhang, H., Oweis, T., (1999). Water-yield relations and optimal irrigation scheduling of wheat in the Mediterranean region. Agric. Water Manage 38, 195-211
10 Spreer, W., M. Nagle, S. Neidhart, R. Carle, S. Ongprasert and J. Müller (2007). Effect of regulated deficit irrigation and partial rootzone drying on the quality of mango fruits (Mangifera indica L., cv. ‘Chok Anan’). Agricultural Water Management 88 (1-3): 173- 180.
11 Spreer, W., Ongprasert, S., Hegele, M., Wünnsche, J. N., Müller, J. (2009). Yield and fruit development in mango (Mangifera indica L. cv. Chok Anan) under different irrigation regimes. Agric. Water Manage 96, 574-584.
Further reading and external links
http://www.fao.org/nr/water/aquacrop.html AquaCrop: the new crop water productivity model from FAO
Spreer et al (2012). Applicability of deficit irrigation strategies in lychee and longan production in Thailand.pdf
http://www.fao.org/nr/water/aquastat/water_use_agr/ AQUASTAT: the global information system on water and agriculture.
http://waterdata.iwmi.org/ The International Water Management Institute
http://www.icarda.org/ The International Center for Agricultural Research in the Dry Areas
http://www.waterforfood.org/ CGIAR challenge program on Water and Food
http://www.uco.es/investiga/grupos/agr119/dimas/htm/dimas.html European project on deficit irrigation