Climate Based Evapotranspiration
Water use from a grass cover, the basis of all climate based irrigation scheduling is called Reference evapotranspiration or (ET). (ET) represents the measure of the water lost from either a grass or alfalfa cover. The standard equations for calculating (ET) are Penman, Penman-Moneith, Hargrave-Samani, the latter only requiring air temperature data, whereas the Penman equations require solar radiation, air temperature, humidity, and wind speed.
A crop coefficient is a number that is figured by determining the percent of the ground that is covered by a specific crop. The crop coefficient is multiplied with an (ET) to equal the water use of a specified crop in a given climate.
The Climate Center at New Mexico State University has developed an irrigation scheduling model that will generate an approximation on when to irrigate using New Mexico climate data for the area and length of time you specify. The irrigation scheduling model should be used to estimate the next irrigation after the plant has germinated and the the roots have grown several inches. The irrigations needed to get stand establishment depend on keeping the soil in the seed zone moist enough for germination. The soil surface dries based on the potential evapotranspiration rate and the soil hydraulic parameters. Soil hydraulic parameters being the maximum water holding capacity of the soil, above which all excess water drains or overflows
Below you will find steps to retrieve climate data and how to calculate Evapotranspiration using an Excel spreadsheet to schedule irrigations for a particular field
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