Weather-based or ET controllers use daily evapotranspiration as the basis for irrigation
scheduling. Evapotranspiration is the loss of water to the atmosphere by the combined
processes of evaporation (from soil and plant surfaces) and transpiration (from plant
tissue). It is an indicator of how much water plants need for healthy growth and
productivity. Many parameters affect ET including: weather factors such as solar
radiation, air temperature, relative humidity and wind speed; soil factors such as soil
texture, structure, density and chemistry; and plant factors such as root depth, foliage
density and plant height. An ET based ‘Smart’ controller uses ET data downloaded from
publicly available sources such as California Irrigation Management Information Systems
or CIMIS (see: www.cimis.water.ca.gov), from weather station networks, or from on-site
weather monitors as well as actual site conditions such as rainfall, slopes, plant type,
irrigation emitter types and precipitation rate to adjust the run-times to apply only the