Review paper: Many alternatives exist for scheduling irrigation. These includes the various soil and plant monitoring methods as well as the more common soil water balance, and simulation models. The use of the various scheduling tools is dependent on the various inputs – soils, crop, and weather data, as well as to the frequency of data collection. Limitation and requirements of each approach/technique is important in the selection of the appropriate scheduling method.
Available approaches of irrigation scheduling
Pan Evaporation-climatic approach:The potential advantages of scheduling irrigations using climatic data have been advocated by many researchers. Penman (1952), for example, analyzed this approach in 1952 as have others since then (Baver, 1954; Pierce, 1960; Pruit and Jensen, 1955; Van Bavel, 1960).
Doss et al. (1962) suggested that open pan evaporation may be used to estimate evapotranspiration and irrigation requirements of corn, provided a relationship between evapotranspiration and open pan evaporation has been established for a given area.
Evaporation from a U.S. standard open pan evaporimeter takes into account the effects of advective energy along with other parameters of the climate; it may be inferred that the pan evaporimeter may serve as a good guide for scheduling irrigation (Reddy et al. 1980/1981). Pan evaporation can be used as a guide to irrigation once the coefficient relating pan evaporation to crop water requirement is determined (Metochis, 1980).
Combination of IW/CPE (where, IW = depth of irrigation water applied, and CPE= cumulative open pan evaporation minus effective rainfall since previous irrigation) with growth stage may be an easy and reliable approach demonstrating the fact that co-efficient relating pan evaporation to crop irrigation water requirement must be soil type and location specific and the co-efficient should be determined under a high level of management (Ali, 1995).
Shortcoming: In IW/CPE approach, the frequency of irrigation varies with the initial depth of irrigation water (IW). Rafey et al. (1978) found ideal IW/CPE ratio of 0.75 for wheat when the depth of irrigation was 8 cm whereas when the depth of irrigation was reduced to 5 cm, the suitable IW/CPE ratio was found to be 0.9. Taking the initial depth of irrigation without considering soil moisture status, may result over or under irrigation.
Stage basis: Generally water stress reduces biological yield, but it does not always decreases economic yield; in fact in some cases it may increase it. Water stress, as with most other forms of stress, tends to hasten the rate of plant development. This reaction is probably an expression of plant survival. The effects of stress depend on its intensity and duration and also on the development stage of the plant (Dougherty,1973; Talukder et al. 1987; Talukder, et al. 1990). Economic yield, as a consequence, while normally increased by the alleviation of water stress, may in some cases be depressed. The effects on yield quality are variable, though generally water stress decreases quality. To implement the stage based concept, different physiological/ development stages is to be identified. According to this concept, irrigation is applied at different physiological stages of the crop. A fixed schedule based on the development stage of the crop may result in over or under irrigation because water is applied without considering the soil water content or water use by the crop.
Soil moisture basis: This approach relates irrigation scheduling with the available water depleted from the soil. Although scheduling of irrigation on soil water depletion is more scientific, it is difficult to ascertain the soil field moisture in the root zone (at farmers level).
Irrigation Scheduling Using Neutron Probe: The amount of water available to a crop and the water use rate by that crop are required to schedule an irrigation correctly. The neutron probe has been used to further simplify gathering of the required information. The neutron probe measurements of soil moisture depletion can be used to determine the values of transient water use by plants as well as transient water loss by drainage.