Agriculture Reference
In-Depth Information
Box 4.5 How to Measure Rainfall
Rainfall is measured using a rain gauge such as that shown in figure B4.5.1. The
gaugeshouldbeinstalledatastandardheight(therimshouldbe30cmabovethe
groundinAustralia),becausewindturbulencecausestheamountofraincollected
to decrease with the height of the gauge. The gauge should not be overshadowed by
vegetation or buildings, nor too exposed to wind. Read the gauge daily at a standard
time,usually9am.Rainismeasuredinmillimetersorinches(forconversion,see
appendix1),and1mmofrainisequivalentto1Lperm
2
of ground surface.
Rainfall rate or intensity is measured with a recording gauge that usually
hastwosmallbucketsofcapacityequivalentto0.1to0.2mmofrain,whichtip
alternately. An electrical switch sends a pulse to a data logger each time a bucket
tips, so rainfall intensity can be measured very accurately in millimeters per minute.
SomeadvisersinAustraliaandCaliforniacalculate“efective”rainfallas
afraction(65%-80%)ofmeasuredrainfall.heconceptofefectiverainfall
allows for surface runoff, which is obviously rain that does not infiltrate the soil.
However, it should not be used to correct for rainfall intercepted by the canopy
andsubsequentlyevaporated,becausetheenergyusedinevaporatingthiswateris
not available to drive transpiration from the leaves. Thus, although the intercepted
water does not reach the soil, the vines' transpiration loss is reduced and the
balance between actual evapotranspiration and rainfall does not change.
Winegrowersshouldusetheirknowledgeofrainfallintensityandvineyard
conditions(soilsurfacecondition,mulching,andslope)todecidewhetherto
correct for effective rainfall.
Figure B4.5.1
A standard rain gauge.
(White,2003)
