Scientific irrigation must be based on an understanding of the soil- water-plant-atmospheric relationship. Irrigation needs of crops depend on the evaporation demand of the ambient atmosphere, soil-water regime in the crop root zone and the plant foliage.
Visual plant symptoms: Visual symptoms in plants to decide on the time to irrigate are changes in the colour of plants, curling of the leaves and tendencies of wilting. These are observed by looking at the crop as a whole and not individual plants. When the crop comes under water stress the appearance changes from vigorous growth to retarded growth. Under conditions of water stress the number of emerging leaves get reduced, and often there is a marked change in the colour of the leaf which gets darker and sometimes grey. Successful interpretation of crop stress requires keen observation and experience in crop fields. However, symptoms may sometimes be misleading due to changes in crop varieties and nutritional disorders as well as insect and pest incidence. Another limitation in resorting to the appearance of crop foliage is that by the time the symptoms are evident irrigation has already been withheld for too long a period for most crops and the yield losses have become substantial.
Plant water content and water potential: The status of water in the plant is generally indexed from the measurement of leaf-water content and leaf-water potential. Their values at any time of the day will depend on the lag between the evaporative demand of the atmosphere and the moisture uptake rate by the crop. When their values fall below certain critical limits specific to the plant species and their growth stages, important physiological and growth factors are adversely affected. Hence, these values can serve as reliable indicators for irrigating crops.
Plant temperature: Almost the entire radiation energy received on the leaf surface during the daylight hours is utilized for evapotrans- piration (ET). The remaining energy, if any, is used to heat the leaf tissues and the ambient air. When ET reduces due to water deficit in the plant, the energy saved in the process is partly used to raise the leaf temperature. Many investigations have shown that the leaf canopy temperature is a sensitive index in crops like soybean, oats, barley, lucerne (alfalfa), wheat, sorghum and maize (Prihar, S.S. and Sandhu, B.S., 1987).
Soil-water regime: A commonly recommended method to decide on irrigation schedules is soil moisture measurements in the field. When the soil moisture content has dropped to a certain critical level, say about 50% of field capacity level in the crop root zone, irrigation is applied. Normally irrigation is not delayed beyond 60% field capacity moisture content.
Irrigation schedules based on prediction of ET: Irrigations can be suitably scheduled on a farm if the allowable maximum water depletion in the crop root zone and the evapotranspiration (ET) for short periods during the growing period are known.
Irrigation scheduling based on pan evaporation: Prihar et al. (1974) suggested a 'practical approach' relating irrigation requirement to the cumulative evaporation during a time period as measured in a standard open pan. The relationship is expressed as IWEpan in which IW is the depth of irrigation water and Epan is the cumulative open pan evaporation. The rainfall during the period, if any, is subtracted from the value of Epan. The validity of the procedure will depend on the proper installation of the evaporation pan and the rain gauge and the precision in measurements of pan evaporation and rainfall. Further, the suitability of the method is situation-specific arid limited to the particular variety of a crops.