How can I do FAO Crop Specific Water balance using Excel?

If you mean that Fao56 ETO you can use this excel file. In addition you can use gridded data for get ET such as CRU data. It has PET data over the world, each 0.5deg(1980-2016). If you mean that the FAO AgroMet Shell you can use DSSAT. The Bureau's evapotranspiration values have been calculated using the Penman-Monteith equation, as recommended by the United Nations Food and Agriculture Organisation in their Irrigation and Drainage paper 56 (FAO56). The FAO56 method is also the approved standard for the U.N. World Meteorological Organization, of which the Bureau is a member agency.

A large number of more or less empirical methods have been developed over the last 50 years by numerous scientists and specialists worldwide to estimate evapotranspiration from different climatic variables. Relationships were often subject to rigorous local calibrations and proved to have limited global validity.

Testing the accuracy of the methods under a new set of conditions is laborious, time-consuming and costly, and yet evapotranspiration data are frequently needed at short notice for project planning or irrigation scheduling design. To meet this need, guidelines were developed and published in the FAO Irrigation and Drainage Paper No. 24 'Crop water requirements'. To accommodate users with different data availability, four methods were presented to calculate the reference crop evapotranspiration (ETo): the Blaney-Criddle, radiation, modified Penman and pan evaporation methods. ETo represents the evapotranspiration rate from a reference surface, not short of water. A large uniform grass field is considered worldwide as the reference surface. The reference crop completely covers the soil, is kept short, well watered and is actively growing under optimal agronomic conditions.

The CRU TS series of data sets (CRU TS = Climatic Research Unit Timeseries) contain monthly timeseries of precipitation, daily maximum and minimum temperatures, cloud cover, and other variables covering Earth's land areas for 1901-2015 (CRU TS4.0 is a recent release). The data set is gridded to 0.5x0.5 degree resolution, based on analysis of over 4000 individual weather station records. Many of the input records have been homogenized, but the data set itself is not strictly homogeneous.