Water is essential for all living things including plants. Precipitation and irrigation are the two primary sources of water for agricultural crops and landscape vegetation. Plant leaves and soil surfaces temporarily retain parts of water directly applied to them. As solar radiation hits such surface, it breaks the retained water molecules apart and changes liquid water into water vapor. The water vapor is transported away from the surface by winds. The process by which the water molecule is converted from liquid phase into the gaseous phase (and removed from the surface) is called evaporation.
The water that is not retained by the plant and soil surfaces infiltrates into the soil. Plants extract the infiltrated water through their roots and transport it to their leaves for photosynthesis, a process by which plants produce glucose (sugar). In addition to water, plants need carbon dioxide (CO2) and light for photosynthesis. The light comes from the sun and CO2 comes from the atmosphere. In order to take atmospheric CO2 in, plants open their stomata, the microscopic pores on plant leaf surfaces, thereby exposing the water in the leaf to solar energy and wind. This results in a loss of water from within plant leaves to the atmosphere through the process known as transpiration.

Evapotranspiration (ET) is the term used to describe the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues). Accurate estimates of ET are needed in many applications. In agricultural and landscape irrigation, for example, estimates of ET are necessary for system design, irrigation scheduling, water rights, water transfers, water resources planning, and other water related issues.

As described above, three conditions must be met for ET to take place. First, water has to be present at the surface-atmosphere interface. Second, there must be some form of energy to convert the liquid water into a water vapor. This conversion requires large amounts of energy (about 540 Calories per gram of water at a temperature of 100 °C), which is usually provided by the sun. Third, there must be a mechanism to transport the water vapor away from the evaporating surface.

Introduction to ET provided by the Califronia Irrigation Management Information Systems (CIMIS)
For more information on CIMIS, please visit their website! 

The Groundwater Sustainability Agencies in the Kaweah Subbasin are providing a new service to members to assist with water management within the GSAs through a Daily Irrigation Management Tool.
For the last five years, Land IQ has been providing the GSAs with monthly field-by-field estimates of actual evapotranspiration (ET). While highly accurate, the results do not come quickly enough for irrigation management decisions, thus Land IQ developed a new tool that provides growers, managers, and the GSAs with access to daily ET and precipitation results within 1-2 days.

The purpose of this tool is for individual water management decision-making. The tool allows growers to track their water use on a daily basis in relation to a district, GSA, or grower-defined threshold. This will allow growers to adjust water management decisions during the year on a real time basis to achieve, and hopefully not exceed, this threshold.
For consistency, the results of the daily tool will be reconciled against and will match the proven results of the 30-day ET results that are currently being provided to the Subbasin.