The following summary is based on the paper "Crop evapotranspiration estimation with FAO56: Past and future" by Pereira et al. (2015).
In 1998, the Food and Agriculture Organization of the United Nations (FAO) introduced a paper (FAO56) with guidelines for calculating and computing Evapotranspiration of agricultural land use types using meteorological data and so called crop-coefficients. By combining meteorological data such as net radiation, air temperature and wind speed with a crop specific coefficient the Evapotranspiration can be estimated on a daily or hourly base. Due to the complexity and the great number of factors that influences Evapotranspiration, the FAO56-method is highly standardized. By using meteorological data of poor quality or not-adjusting the crop-coefficient to factors like soil salinity or the age of the crop, errors can be created and the estimated evapotranspiration can deviate from reality. But all in all it is a robust method to estimate evapotranspiration over a wide range of climates and locations.
In 1998, the Food and Agriculture Organization of the United Nations (FAO) introduced a paper (FAO56) with guidelines for calculating and computing Evapotranspiration of agricultural land use types using meteorological data and so called crop-coefficients. By combining meteorological data such as net radiation, air temperature and wind speed with a crop specific coefficient the Evapotranspiration can be estimated on a daily or hourly base. Due to the complexity and the great number of factors that influences Evapotranspiration, the FAO56-method is highly standardized. By using meteorological data of poor quality or not-adjusting the crop-coefficient to factors like soil salinity or the age of the crop, errors can be created and the estimated evapotranspiration can deviate from reality. But all in all it is a robust method to estimate evapotranspiration over a wide range of climates and locations.
For a water
balance model of the tree nursery in Schinznach, the FAO56 method can be certainly
used as a reference and a quality control mechanism for measured and calculated
evapotranspiration values. It should be noted, that for the application of the
FAO56 method in a tree nursery the crop-coefficient can vary due to the space
between the crops, the ground shadowing and the presence of a ground cover or
mulch.
Since there
will be a remote-sensing team in Schinznach there is a possibility of using the
normalized difference vegetation index (NDVI) to estimate the crop coefficients
of the tree nursery. Due to the above-mentioned variability of the
crop-coefficients in a tree nursery it probably will make indeed more sense to
estimate the crop-coefficients by evaluating the different crops on the ground.
Dear kara
AntwortenLöschenThanks for your comment on my blog. So as you recommended i checked your entry about “Estimation of the crop evapotranspiration by using the FAO56-method”. As I understand your blog entry the FAO56 method can be used as an additional method to reassess the Evapotranspirationresults from primary measurements at the soil or crop by calculating the factors irrigation, precipitation, run off and deep water flux by weather- or hydrological- data. As I see you already investigate wherefrom we could get the required data for the task on the spot. In response to your question about my blog entry: What is meant by the term run on? So as I understood the description it is surface water that runs into the perimeter. It appears that we already know two methods to identify the Evapotranspiration.
Hi RastMat
AntwortenLöschenYour entry summarizes a presumably highly complex thematic in a brief and informativ way. Thanks for that.
Since our goal is a statement about the water circulation in average, the FAO56 method seems to be of great use for Schinznach. As we heard at the guided tour at the tree nursery, we can use the meteorolocigal data which they produce from the terrain.
Do you think we can get all the required data togheter for avoiding errors in the calculation? Or would that probably be a too big effort? Record the soil salinity is probably not a big deal but maybe there are other factors that are more difficult to collect?
As we spoke in the tree nursery we are able to say a lot about the overall characteristics of the plants by using remote sensing. We should think about wheither we want to use the evapotranspiration of let's say one or two characteristc plants in the nursery or we focus for example more on the heights of plants and use therefor evapotranspiration factors which already are averaged...
I'm sure we'll find a way!
Keep a stiff upper lip
j.p.
Dear Cornelia
AntwortenLöschenMany thank for your comment on my blog. Previously, I have erroneously published my entry here in RemoteRastmat's blog - sorry for that, RastMat! - but I have now moved my entry into a blog of my own. I hope this avoids confusion.
Best
leukat02
Hi Matthias
AntwortenLöschenThanks for your clear summary I think you got the important points. We should keep in mind your last comments in order to compare our ET-measurements and ET-calculations with remote sensing data.
good luck
Luzi