Calculation of evaporation rates during the transition from energy-limiting to soil-limiting phases using albedo data

A method is presented that utilizes albedo measurements to partition the fraction of the soil surface area exhibiting energy-limiting (potential) evaporation and the fraction exhibiting soil-limiting evaporation to calculate actual evaporation rates during the transition phase (energy limiting to soil limiting). Since albedo is proportional to the surface water content, the change in albedo from day to day is indicative of the fraction of the soil surface that is dry and evaporating at the soil-limiting rate. By denoting the partitioning factor as β and by using a square root of time relation with a coefficient C for the soil- limiting phase the evaporation rate Ec for day n after the soil was wet is (Ec)n=βnEp+C∑i=1n(βi‐1‐βi)(n‐i+1)−1/2 where Ep is the energy-limiting rate. The Priestley-Taylor formula was used to calculate Ep. Calculated evaporation rates were compared with lysimetrically determined rates. It was concluded that this method was reliable for calculating evaporation rates during the transition phase of soil drying.

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