7/27/2019 How to Determine Zone Run Times Using
Evapotranspiration Data
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How To Determine Zone Run Times Using Evapotranspiration
Data
The Handbook of Technical Irrigation Information produced by
Hunter Industries providesformulas and other information that can
aid irrigators in arriving at efficient watering schedulesThe
following information is derived from this book!The irrigtaion run
time formula calculates the number of minutes re"uired to apply
enough water
to replace the water lost by evapotranspiration for a specific
crop irrigated with a system at aparticular precipitation rate and
efficiency!
#ollowed by the formula below$
T% &' ( Eto ( )c*************+r ( Ea
,here$
T % Irrigation run time in minutes
Eto % reference evapotransporation rate- in inches
)c % crop coefficient- percent
+r % precipitation rate of the area- in inches per hour
Ea % application efficiency of the system- percent
&' % .onstant for conversion of area- flow- inches per hour
and inches per day into commonunits
E(ample$
Determine the total monthly irrigtaion run time for an athletic
field with an total monthly ETo of /'inches and a crop coefficient
for the warm season turf of '0' The sprinkler precipitation rate is
1' in2hr with an application efficiency of 013
T % &' ( /' ( '0'1' ( 01
T % //4 min of total monthly watering time
Thus if the system is programmed to water on 5' days during a
month- the 6one watering timewould be //4 minutes 7525'th8 of the
total monthly re"uirement
Evapotranspiration Rates
Eto or the evapotranspiration rate is a value given to help
determine the rate at which plants losewater through evaporation
Humidity- temperature and other factors are taken in account
todetermine the Eto rate at a given time These rates vary with the
season and realistically theyvary by the hour 9ut for lawn
irrigation purposes- average Et rates are more than enough
toschedule watering
