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What is ETo?
Evapotranspiration (ET) is the combination of evaporation of water from the soil and plant surfaces and water transpired due to plant growth. ETo is evapotranspiration from standardized vegetation surfaces (grass or alfalfa) with known biophysical characteristics. A standard vegetation surface is required because different plants have different biophysical characteristics and it is not practical to formulate equations for each type of planting. The standard reference surface for CIMIS ETo estimation is well-irrigated, clipped grass.
How can I use ETo data for irrigation scheduling?
ETo indicates how much water a reference crop of grass needs over a certain time period for optimum growth and development. You derive daily Etc (crop ET) by multiplying ETo by Kc. Kc is a crop coefficient derived from field experience and research that converts ETo to ETc for a specific crop. By adding up daily ETc values and including losses due to inefficiencies in your irrigation system, it is possible to estimate how much water you need to apply and when it is time to irrigate. Although irrigation scheduling can be complicated, it can be analogous to balancing your checkbook. The analogy lies in that additions of irrigation water and precipitation can be considered deposits whereas ET, runoff, and deep percolation can be considered withdrawals. For more information on irrigation scheduling, click on "IRRIGATION" from the CIMIS Home Page and then select "Scheduling".
What is a crop coefficient (Kc) and how can I obtain it?
A crop coefficient is a factor that is used to convert ETo into estimated evapotranspiration (ETc) for a specific crop. It is a dimensionless number ranging in values roughly between 0.1 and 1.25. It depends on many factors including crop type, stage of growth, the equation used to calculate ETo, and moisture content of the soil surface. For most crops and landscape vegetation growing in California, you can obtain Kc values and information by clicking on "ETo" from the CIMIS Home Page and then clicking on "Crop Coefficients".
How come ETo from CIMIS is greater than Precipitation?
There is no direct connection between ETo calculation and Precipitation (P). ETo can be greater, less, or equal to P. ETo tells you the amount of evaporative demand that an atmosphere imposes on a given reference surface provided providing there is adequate moisture in the soil. If soil moisture is less than the atmospheric demand, then ETo has to be adjusted in the calculation of ETc. This adjustment is not done here because in irrigated agriculture one is not expected to stress the crops to the extent that the soil moisture is less than atmospheric demand. Generally speaking, factors such as clouds (during precipitation) may indirectly affect ETo by reducing solar radiation but cannot limit ETo to necessarily be less than P all the time. If rainfall is the only source of water, ETc (not ETo) should always be less or equal to P.
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