Effective rainfall, Pe, is usually derived as the difference between inputs (Gross Precipitation, P) minus Losses (Runoff, R, Drainage, Q, and Evaporation, Et). That is, Pe=P-(R+Q+Et). R can be derived by the Soil Water and Conservation >Service of US. It spans from 0 to 25% of P, depending on soil, crops and rainfall intensity, duration. Q spans from 0 to 15% depending on soil type but most important from soil drainage features (macropore voids that drains freely under gravitational forces). Et is quite frequently between 0 and 25% of P but depends on climate, soils and crops. Lets say a monthly rainfall of 75 mm would have Pe=75mm- (Q=10 mm; R=5 mm; Et=20 mm)= 40 mm.

Hope these coments help you to understand effective rainfall.

Kind Regards

Sincerely

Jose Navar, Ph.D

Hi Daniel,

Water in the root zone may be measured by sampling and oven-drying the soil before and after every shower of rain. The increase in soil moisture, plus evapotranspiration loss (ETa) from the time the rain starts until the soil is sampled, is the amount of effective rainfall. After heavy rainfall evapotranspiration can be assumed to be at the potential rate during the short period from cessation of rainfall until the sampling time. This can be taken as 0.4 to 0.8 times the evaporation value of the Class A Pan

ER = M2 - M1 + kp EoER = effective rainfall Eo = Class A Open Pan evaporation value M1 and M2 = moisture status in the effective root zone before and after rain, respectively kp = pan coefficient

The method takes into account the soil and the crop characteristics. The determination is simple and accurate but it may involve errors due to soil variation; the sampling errors may range from 5 to 40 percent. The method is also laborious and time consuming. The use of neutron probes reduces the drudgery of periodic soil sampling, but these are costly methods for routine purposes and also subject to sampling errors

Best Regards.

I am looking for a simple empirical equation between effective and actual rainfall.