You can refer following papers for latent heat flux algorithms:
1. S. K. Ambast, Ashok K. Keshari and A. K. Gosain (2002) An operational model for estimating regional evapotranspiration through surface energy partitioning (RESEP). International Journal of Remote Sensing, V. 23, No. 22, 4917-4930.
2. S.K. Ambast, Ashok K. Keshari and A.K. Gosain (2008) Estimating regional evapotranspiration using remote sensing: Application to Sone low level canal system, India. Journal of Irrigation and Drainage Engineering, ASCE, Vol. 134(1), 13-25.
The above two papers utilize satellite remote sensing data for computing latent heat flux.
OAFlux latent and sensible heat fluxes products are constructed from the use of the best possible surface meteorological variables and best possible bulk flux algorithm.
theoretical background on latent heat flux can be found at following link:
http://www.fao.org/docrep/x0490e/x0490e06.htm
which is the well known FAO Paper 56.
In this case you can compute latent heat flux using meteorological variables and crop properties (aerodynamic and surface "bulk" resistance) at metereological station location. if you need spatial dimension (i,e. regional scale, river basin, irrigated district etc) as suggested by Keshari Remote Sensing is the best solution, and you can try to search on internet,in addition to his suggestions, using as key world Surface Energy Balance Algorithm for Land (SEBAL) or SEBS or METRIC.