For irrigation scheduling, I used Penman Monteith equation (Allen et al, 1998) to estimate Crop evapotranspiration (ETc). However, in calculation of actual ETc, I used the soil water balance method according soil moisture (SM) observed using TDR. SM was measured weekly such that actual ETc was calculated on a weekly basis. There were some rainy days in between some of the TDR measurement periods. How do I calculate actual ETc on days I did not make TDR reading? I need the cumulative ETc during the entire season in order to calculate crop water use efficiency.
Dear E.,
because the soil was wet from rains, I would use potential ETc for such days. The crop would either be wet (and be comparable to a flat wet surface), or transpiring potentially, values that must be very close to each other because the soil has also the same contribution. Reports of a far past that a wet crop surface would evaporate more than potentially have never been experimentally confirmed convincingly. Firstly the crop surface is always wet in patches, secondly a well watered crop has minimal stomatal crop resistance.
Cheers, Kees
Since you have some soil moisture data, you could use that to develop a model (or calibrate an existing crop model). The model can then give you an estimate of the soil moisture on days that you don't have data, as well as an estimate of the actual ET. There are many crop models ranging from very simple model: e.g. CATCHCROP
https://www.researchgate.net/publication/223955133_CATCHCROP_Modelling_Crop_Yield_and_Water_Demand_for_Integrated_Catchment_Assessment_in_Northern_Thailand
to complex models like APSIM
https://www.apsim.info/
Which model you should use depends on the information you have and what you want the model to tell you.
Article CATCHCROP: Modeling crop yield and water demand for integrat...
Dear Danso
If the rainfall was quite sufficient as irrigation, I think you should go with the potential ET for the rainy days when you did not measure soil moisture due to rain.
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