ALMOST 70 per cent of world water is used in irrigation, 20 per cent in industry and 10 per cent for drinking purposes. Whereas, in Pakistan, the canal command areas receive normally an average rain-fall of 53 mm in the Rabi season, the water requirement for winter crops is not more than 36.0 million acre feet. There remains a shortage of more than 50 per cent.

The problem is worsened because irrigation efficiency is between 30-40 per cent which can be increased up to 70 per cent by using standard techniques.

Our farmers irrigate crops without considering their stage of growth. As a result, they apply more water to crop without any benefit.

In case of wheat, moderate moisture during the early vegetative period does not have a substantial effect on yield; but farmers, even at this stage apply 75 acre mm of water per flood irrigation.

A sensitive stage is when no water is available for crops. After the crown root stage, the moisture stress is most critical. Many of heads fail to produce grains if there is moisture stress at this stage. The period between grain filing and maturity is also very sensitive. Yield is reduced the most when stress starts during soft dough or during or following heading.

More than 10 per cent yield losses may occur due to moisture stress during the grain maturity stage. The shortage of water and food goes side by side. Over or under-irrigation means reduction in crop output and for better crop, proper application of water is a must.

An efficient application of water, wheat yield goes up to 7907 kg per hectare as against average yield of around 2000 kg. The potential of higher yield can be realized by efficient water application and as such, water scarcity is becoming a critical issue.

In the year 1951, the per capita water availability was 5650 cubic meters per person which has dropped to 1350 cubic meter. To save a nation from joining the countries suffering from water shortage, 1000 cubic meter per person water availability is a must.

Irrigation managers can use water most efficiently for wheat growing by applying the water as per demand of the crop. It will reduce the amount of water used and increase water availability for the crop at its critical growth stages. The water thus saved may be used in other priority areas. The efficient use of water resources with optimum outputs should be a key objective of the farming community.

At present, application of 75 acre millimetres ( three acre inches) per irrigation is a general practice. It becomes about 300 acre millimetres if applied only at four critical stages and 450 acre millimetres under routine practices.

A judicious use of water may be developed on the concept of water use efficiency (WUE) which may be defined physiologically as the ratio of yield to evapotranspiration. In a more biological sense it is the amount of carbohydrates produced through photosynthesis from CO2, sunlight and water per unit of transpiration.

The concept of WUE provides a simple means water per unit of transpiration. The concept provides a simple means to assess whether the yield is limited by water availability or it is due to some other factors. The results of the experiments conducted on the Agronomic Research Area, University of Agriculture, Faisalabad to improve water use efficiency in wheat during 2001-02 and 02-03 indicated that wheat can be grown successfully without the application of so much irrigation water as is traditionally done.

For this purpose, the “maximum potential soil moisture deficit should be used as a criterion for irrigation. The amount of water applied must be equal to the difference between potential evapotranspiration (PET) and rainfall +irrigation applied in the previous week or apy specified period. A measured quantity of water is needed to be applied.

It was concluded that application of a measured quantity of water as per requirement of the crop not only saves the irrigation water but helps increase the crop yield. For example, grain yield of 6.02 tones per hectare was obtained by applying a total of 295 mm water excluding in addition to the 11 mm of rain which was received during 01-02 the wheat growing season (Nov-01—- March 02).

The average wheat grain yield is less than three tones even by the use of more irrigation water (450 mm). The total dry matter WUE for water applied (irrigation + rainfall) ranged from 5.53 to 7.44 gm‘2 mm”. Mean response of grain yield to water received (I+R) ranged from 1.72 gm mm‘ 1 to 2.31 gm-2 mm1.

In general, an increased supply of water reduced the WUE because additional water leaches down beyond the reach of most of the roots. It also causes the nutrients to leach down. That is the reason that for wheat seeds, a WUE of less than one gm 2 mmi is received by the utilization of 450 mm of water in a traditional manner and about one and two mm 7’ of applied water by the’ use of 300 mm of water applied at four critical stages of crop growth.

It indicates, we need not to irrigate our wheat crop heavily to get higher grain yields but application of proper amounts of water at proper time is the only requirement. To get more water use efficiency, we have to be careful about irrigation water from anthesis to physiological maturity of the crop. At this stage, the temperature increases day by day. The zygotes may die for want of water causing a reduction in the number of grains per spike.

The grains will be of smaller size due poor translocation of carbohydrates from source (the leaves, which loose chlorophyll rapidly due to shortage of water and nutrients) to the sink (the grains). In the end, flag leaf is required to be kept green to help the seed in the spike to accumulate maximum carbohydrate.

At this stage, the photosynthats produced by the flag leaf are mostly translocated to the seed/grain because of the minimum share of the stem at this stage. Mostly, the farming community fails to arrange for the supply of irrigation at this stage and the result is shrivelled grains in addition to reduced yield per unit area. This is rather a dilemma that in spite of bumper stand of wheat crop, yield per unit area is always low.

Conclusions: Apply irrigation water based on crop potential Evapotranspiration;

Critical stages of crop should be avoided to become water stressed;

Reproductive phase of crop, especially from anthesis to physiological maturity is very sensitive to moisture;

Irrigation system should be demand based and not the supply based one; There should be the small storages at the farm level. During the rainy season water should be stored in these storages and this stored water can be used as and when needed by the wheat crop during its season.