ICARDA CARAVAN 11

A Little Goes a Long Way

It's the hardest task of all for the farmer anxious to grow a successful crop. To determine how much irrigation water to apply, and when, to get the optimum benefit from this scarce resource in the dry areas, ICARDA is experimenting on spreading the same amount of water widely for maximum returns.

By Theib Oweis

producing wheat in the dry areas of WANA is about 0.35 kg grain/m3, although this can be increased to 1 kg grain/m3 with good management and favorable rainfall. Water used for supplemental irrigation can be much more efficient.
         ICARDA research has demonstrated that a cubic meter of water applied at the right time when the crop is suffering from moisture stress, and combined with good management, is more than twice as water-efficient than rainfed production (Fig 1). Why is the water-use efficiency so high? It can be attributed mainly to the effectiveness of applying even a small amount of water to alleviate severe moisture stress during the most sensitive stages of crop growth and seed-filling. Applying the additional water before stress reaches a peak will allow the plants to reach their high yield potential.
         Does supplemental irrigation really outperform full irrigation in its water-use efficiency? Again, the results tell the story. In fully irrigated areas with good management, wheat grain yield could reach 6 t/ha when using about 800mm of water. That gives a WUE of about 0.75 kg grain/m3--one third of that under supplemental irrigation.
         However, it remains the case that most farmers irrigate earlier than they need to, and then apply too much water for an individual crop's needs. In part, this is because the cost of irrigation water is low and there is little incentive for the farmer to use less. ICARDA is           

Harvested yield across a greater area of land is much improved by using supplemental irrigation in conjunction with appropriate varieties and a good fertilizer program.

         Supplemental irrigation (SI) trials were carried out at the ICARDA research station in northern Syria. Water-use efficiency in SI is a function of the amount of irrigation water applied. It was found that the maximum water-use efficiency is reached when one- to two-thirds of the full irrigation water is applied. Given that many farmers over-irrigate, at least one-third of the full irrigation requirement can be saved without significant losses in productivity.
         The actual amount of water that should be applied will vary according to the actual rainfall in the rainfed areas. Research in the eastern Mediterranean region has shown that the amount of rain falling before the end of March is a good indicator of what will happen later in most years. One to three supplemental irrigations between late March and early May are usually enough for a wheat crop.
     It is important to remember that the crop will normally produce some yield without irrigation because of the influence of even limited seasonal rainfall. These rainfed crops should not, therefore, be irrigated at times when rain is already catering for their immediate needs, or irrigated in an attempt to provide moisture-stress-free conditions throughout the growing season. The prime aim should be to ensure there is a sufficient amount of water available during critical stages of crop growth to permit optimal instead of maximum yield.
        In the Mediterranean-type climate, moisture is usually short in the spring when crops need it for accelerated growth. Evapotranspiration rates are higher and soil moisture in the root zone is depleted as plants increase their uptake. Before long, in the absence of rain, moisture stress begins and can continue until the end of the season with severe effects on final yield. Wheat may have a yield potential in West Asia and North Africa of more than 5-6 tonnes/ hectare but the average yield of rainfed wheat is only about 1 t/ha, although it can range from 0.5 to 2 t/ha, depending on rainfall, soil fertility, management and crop variety.
         The average water-use efficiency (WUE) of rain in

Full irrigation (FI) has potential to increase wheat yield per hectare to a creditable 6 t/ha but at the expense of applying 800 mm of water. The water-use efficiency in producing this grain is much less than that of supplemental irrigation (SI) which spreads the water over a wider crop area before critical periods of moisture stress.

investigating various ways of helping the farmer make better judgments about issues such as timing, either by recording rainfall and linking it to soil and crop conditions, or by using 'indicator' plots containing other water-sensitive crops to give early warning of impending stress.
Farmers are also reminded of the importance of other farm management practices in getting best crop performances in rainfed systems. These include variety choice and planting date. Lack of nitrogen is a common problem that can lead to lower yields and poor water-use efficiency. Other nutrient deficiencies will also hamper crop response to nitrogen and to water if they are left uncorrected.
           The aim of the ICARDA research is to produce a package of measures and recommendations that will help as many growers as possible to gain the maximum benefit from available water. It is water, not land, that is the limiting factor in the dry areas. Any improvement in water-use efficiency is reflected across the whole agricultural sector.

Dr Theib Oweis is Water Management/Supplemental Irrigation Specialist at ICARDA.

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