The required timing and amount of irrigation water needed is determined by the prevailing climate conditions, crop and stage of growth, soil moisture holding capacity as well as the extent of root development as determined by type of crop, stage of growth and soil. It can be finely defined by the use of evapotranspiration (ET) rates in combination with knowledge of the other factors listed above. However, if ET knowledge is not available, crop indicators such as change of color or leaf angle can be used although this information may appear too late to avoid a reduction in crop yield or quality. Also, methods of scheduling, which involve determining the plant water stress, soil moisture status or soil water potential along with stage of the crop and timing in the season can help determine irrigation needs.