Irrigation scheduling with soil instruments: error levels and microprocessing design criteria

Two criteria for deciding when a crop should be irrigated are: (1) the depletion of water in the root zone to some predetermined amount, or (2) the decrease of water potential at some given depth to a predetermined level. The value one chooses for either of these criteria to indicate that irrigation is needed will depend on soil properties, crop rooting characteristics and stage of plant growth. Functional relations between these two criteria and produc-tion are not yet known quantitatively, thus one cannot say that either ap-proach is inherently better than the other. The effective application of either requires experience and judgment. Recent years have seen significant progress in scheduling irrigation using meteorological data to calculate the depletion of water in the root zone. The daily potential evaporation from a full cover reference crop can be calculated within a few percent using measurements of air temperature, humidity, solar radiation and wind run. • Given an appropriate crop coefficient curve, the evapotranspiration can also be estimate and the soil water depletion know