Under favorable environment, about 1000 m3 of water is needed to produce 1.0 t of wheat. About half of this is used in transpiration and the other half is lost by evaporation and unavoidable deep percolation. Under less favourable conditions, about 5000 m3 water is needed to produce 1.0 t of wheat. On an average, the water requirement of wheat ranges from 400-600 mm in different crop growing environments.
Normally, wheat is irrigated by surface methods of which check basin and border methods are most common. When water supply is limited or the topography is not suited to surface irrigation, sprinkler irrigation could be advantageous.
Problem Soils and Water Management:
Problematic soils (soil salinity, alkalinity) and brackish irrigation water require different treatment from water management point of view than irrigation scheduling under normal soil and water conditions.
Tolerance to salinity varies with crop. Wheat can tolerate about 12 dS m-1 EC of saturated extract for 50 per cent yield (EC threshold) and also for 50 per cent germination. Therefore, about 25-50 per cent higher seed rate is required for adequate stand establishment on such soils. It is known that salts accumulate on rides.
As such, furrow irrigation could help in obtaining better crop stand and yield under saline conditions. Alkali soils have low infiltration rate. As such, water should be applied at controlled rates of smaller depths at more frequent intervals. Light and frequent irrigations around 6 cm depth are ideal for wheat on alkali soils.
There are situations where limited quantities of good quality water are available in addition to saline water resources. These two supplies should either be mixed or used in alternate irrigations. Alternate use of saline and canal water is simpler than several issues relating to decision on mixing ratios.
Use of non-saline water for presowing irrigation and at early growth stage and poor quality water at other stages leads to adequate stand establishment for optimum yield. One irrigation with brackish water after two irrigations of quality canal irrigation water resulted in optimum wheat yield besides considerable saving (30 %) in canal irrigation water.
Water Management for High Water Table:
Good quality groundwater at shallow depths considerably reduces need for supplemental irrigations, which is not required for most crops if ground water table is as shallow as 50 cm. Under high water table conditions (50-100 cm) as in Tarai (UP) and northeastern parts of the country, irrigation at CRI was adequate for wheat crop.
Under medium water table situations (100-150 cm), 2 to 3 irrigations were adequate. At Hissar, wheat needed one irrigation only at CRI when water table was between 100 and 110 cm, 2 to 3 irrigations at IW/CPE ratio of 0.4 when it was between 120 and 200 cm and four irrigations at IW/CPE ratio of 0.6 when it varied between 220 and 250 cm.
Wheat was more remunerative crop under shallow water table conditions. Groundwater at 60, 90, 120 and 150 cm depths contributed 70, 53, 27 and 20 per cent towards water use by wheat respectively. If ground water is saline, it promotes soil salinisation with water table depth less than 60 cm.