The FAO says that effective rainfall is never negative in Paragraph 4.2
http://www.fao.org/docrep/S2022E/s2022e08.htm
Why do I do If I have negative effective rainfall (Pe) in the equation below (I am using the SCS method) ? see equation attached.
I provided the equation for Monthly precipitation. But It is also applicable to Daily precipitation. I get negative daily effective rainfall (Pe) sometimes. So should I nullify Pe then ?
I am not familiar with this publication or equation. The term effective rainfall usually refers to runoff. If I measured 1 inch rainfall, and 0.2 inches was measured from the storm as streamflow, the effective rainfall would be 0.2 Inches. If there is no runoff, the effective rainfall is 0.
No, effective rainfall cant be negative. If due to some data discrepancy you get negative results you can assume effective rainfall depth as 0
The effective rainfall is the rainfall that has runoff. In SCS rainfall- runoff formula if we put a value of zero to runoff it will give a certain value for the rainfall. That means this value must be subtractEd from all rainfall readings to be converted to effective rainfall Readings.
The effective rainfall means after deduction all infiltration, sublimation etc. The portion of rainfall which gives runoff is called effective rainfall, indices
Negative and effective makes it absolutely negative. The rainfall under reference is to that extent negative and therefore not useful.
Effective rainfall cannot be negative and where this may occur reset it to 0. l note that in water balance soil moisture can be zero when calculated during dry episodes but the soil will be having quantifiable moisture deficit - which negative in value. The negative Effective rainfall you are getting from the equation is due to the fact you are calculating for times without rainfall and SF cannot be negative-it can only be zero or positive. Reset your ER to 0.
The term effective rainfall as-used-here is the total amount of rainwater useful for meeting the water need of the crops.
The formula obviously has limitations. The conditions that make the average-monthly effective monthly precipitation negative may be determined and so avoided; or, negative values, if they occur, set to zero.
You may be interested in: Article The Crop Moisture Index: Unnatural Response to Changes in Temperature
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