Introduction

The prevailing scarcity of water in the West Asia North Africa (WANA) region makes it one of the poorest regions in the world in terms of water resources, both globally and per inhabitant, particularly in the Near East. The total population of the region makes up 10 per cent of the world population, of which 50 per cent is rural. Population is increasing by 2.7 per cent per year and this will only exacerbate the situation in a region that receives only 3.5 per cent of the world's precipitation and has only 2.2 per cent of its renewable water resources. Many of the countries in the Near East suffer from 'severe water scarcity' (under 500 m³/capita year; Fig. 1) and all but Iraq are under some degree of water stress. It must not be forgotten that unlike other regions, the Near East region uses 80-90 per cent of its available water for irrigation of nearly 55 million hectares. Because of the increased demand on this vital resource, it must be used more efficiently. Surface irrigation is practiced on 87 per cent, sprinkler on 11 per cent and localized irrigation systems on less than 2 per cent of the total irrigated area of the region. High soil salinity is also widely prevailing in the region, from both intrusion of sea water and salinization, due to decades of irrigation. The per capita available water in the Near East countries is presented in Fig. 1.

The type of irrigation is of a great importance on water use efficiency as illustrated in Table 1. Traditional surface irrigation is characterized by a high volume of application, leading to water losses by leaching and evaporation. Sprinkler irrigation offers a higher efficiency, although not Research findings Optimizing Crop Nutrition comparable to that of drip irrigation, which is well adapted to semi-arid regions.

Potassium plays a key role in water relationships in plants. Its role on the control of osmotic pressure is vital under conditions of water stress. Potassium affects water transport in the plant, maintains cell pressure and regulates the opening and closing of stomata and is responsible for cooling and absorption of carbon dioxide for photosynthesis. It also acts as a catalyst, regulating enzymatic processes in the plant that are necessary for plant growth. Potassium is important for a plant's ability to withstand extreme cold and hot temperatures, drought and pests. All these different actions favor plant growth under healthy conditions and consequently have an impact on water use efficiency.

To improve water use efficiency, balanced fertilization, drip irrigation and fertigation techniques, and their combination need to be developed.

Efficiency in the use of water in irrigation may be defined in various ways, depending on the nature of the inputs and outputs to be considered. It can be considered as the financial return obtained from irrigation in relation to the investment made in the water supply. Agronomic criteria refer to the fraction of the water volume that is consumed by a crop, relative to the amount applied. Crop consumption consists of the amount of water actually absorbed by the crop, most of which is generally transpired to the atmosphere. Climatic conditions have a large influence on plant transpiration as well as potassium content in the plant tissues. Carbon dioxide for photosynthesis and transpiration occur concurrently through the same stomatal openings in the leaves, so the two processes are closely interrelated. The ratio between photosynthetic activity and transpiration is one way of measuring the water use efficiency (WUE) of a plant.

Improving the use of water in agriculture and adopting measures to alleviate the negative effects of salinity is a prerequisite to the future of this region. A research programme conducted by IPI and the Soil Science Department - Ege University, Bornova, Turkey illustrates the role of potassium on WUE (Kilic, 2005).

Experiment

In an experiment carried out from 2003 to 2005 in Izmir (Turkey) on Satsuma mandarin (Citrus unshiu) on two rootstocks (Troyer citrange and Poncirus trifoliata) evaluated the effect of salinity and potassium fertilization on the WUE of the trees by measuring their photosynthesis and transpiration rates. Salinity levels tested were zero (fresh water), saline (3.5 ds/m) and highly saline (6.5 ds/m). Potassium levels were zero K, optimum K (600 g K₂O/tree) and high K (1,200 g K₂O/ tree). Potassium fertilizer used in the experiment was potassium sulphate, 50 per cent K₂O.

The results show that Satsuma mandarin X Poncirus trifoliata was less affected by salinity, and at high salinity levels (6.5 dS/m) was more responsive to the application of potassium. Satsuma mandarin X Poncirus trifoliata had a higher WUE than Satsuma mandarin X Troyer citrange at all S and K levels, except when irrigated with fresh water and with no added K (K=0) (Fig. 2).

Salinity significantly affected both rootstocks' WUE (P=0.01) and was more pronounced when potassium was not applied. The ill effect of salinity on WUE was alleviated in both rootstocks when potassium was applied at 600 and 1,200 g K₂O/tree, especially in Satsuma mandarin trees X Poncirus trifoliata (K=600), where the application of potassium almost restored WUE.

Fertilization with potassium at rates of 600 and/or 1,200 g/tree (K₂O) significantly affected WUE (P=0.01) at all salinity levels. Under conditions without salinity (fresh water), application of K increased WUE by 11.8 and 27 per cent in Troyer citrange and Poncirus trifoliata, respectively. At the high salinity level (6.5 dS/m), K application was extremely effective increasing WUE by 68 and 58 per cent in Troyer citrange and Poncirus trifoliate, respectively. Additionally the effect of high K application (1,200 g K- 20/tree) was very pronounced in Satsuma mandarin X Poncirus trifoliata under high salinity (6.5 dS/m), where WUE was restored to its level of lower salinity (3.5 dS/m).

These findings demonstrate well the role of K in alleviating the damage to citrus production caused by high salinity. In general, Satsuma mandarin X Poncirus trifoliata was less affected by salinity responding better to K application, as compared to Satsuma mandarin X Troyer citrange. However, both combinations responded well to K application at all salinity levels.

As a conclusion, what criteria should be taken into consideration to improve water use efficiency?

In addition to improving WUE by optimizing physiological parameters, there are some other ways that are directly related to water management, such as using closed conduits to reduce conveyance losses, or reducing direct evaporation during irrigation by using drip irrigation or mini sprinkler under canopy. Runoff and percolation losses due to over irrigation can be reduced by correct timing and volume of water applied. Optimal tillage and mulching can also reduce evaporation.

Other factors are linked to plant management. It is advisable to select the most suitable crops and varieties for the region, and to use optimal timing for planting and harvesting. Balanced fertilization, preferably by fertigation, and proper plant protection control, assist the crop in resistance to water stress. Irrigation at high frequency and in the exact amounts needed to prevent water deficits, should be made taking into account weather conditions and crop growth stage.

Finally, soil conservation for long-term sustainability is also important: it is necessary to avoid salinization by monitoring water quality, using appropriate fertilizers and appropriate drainage.

References

• Darwish, T. and M. Nimah. Fertigation potentials in the Near East region. Proceedings of the IPI workshop on potassium and fertigation development in WANA region, 2004. International Potash Institute (IPI), Horgen, Switzerland. Also available online on http://www.ipipotash.org/speech/index.php?ev=50.
• FAO. 1997. Small-scale irrigation for arid zones. http://www.fao.org/docrep/W3094E/W3094E00.htm.
• Kilic, C.C. 2005. Potassium fertilization as a technique to control salinization in Citrus. IPI report on Citrus experiment, 2005.
• Osman M.E. 2004. Comparative Analysis of Agricultural Policies in Selected ESCWA Countries. Symposium on "Agricultural Policies in the Arab World" 18 June 2004, Amer. Univ. of Beirut, Lebanon.
• Khalifeh, F. 2002. New horizons in irrigation techniques. Professional and Technical Development of Agriculture in Lebanon.

Edited by E.A. Kirkby.