ESTIMATION OF SOIL EVAPORATION BY SOIL HYDRAULIC FACTOR FROM BARE DUNE SAND MULCH

Abstract

A method is presented that employs soil moisture measurements and lysimetric reference
evapotranspiration data to estimate a soil hydraulic factor for estimating soil evaporation. Soil evaporation is
a complicated process. The soil evaporation process has two main stages. In this study, soil hydraulic factor
was used to estimate soil evaporation under dune sand mulch. The soil hydraulic factor was derived from
small pan evaporation data and soil evaporation data from dune sand soil columns. Dune sand soil columns
were saturated and gravitational water was drained. The dune sand soil columns were then covered by dry
Tottori dune sand soil of 0 cm, 2 cm and 5 cm thickness. Soil evaporation from these columns was
measured gravimetrically. Potential evaporation was determined from small evaporation pans placed
alongside the dune sand soil columns. Water was applied to three weighing lysimeters (for measuring actual
soil evaporation) in a glasshouse to the field capacity of the dune sand soil. The weighing lysimeters were
also covered by the dune sand soil of 0 cm, 2 cm and 5 cm thickness and that was dried naturally.
Cumulative soil evaporation from dune sand soil columns and cumulative potential evaporation from small
evaporation pans was used to determine the soil hydraulic factor under the three dune sand mulch
treatments. The soil hydraulic factor was then used to estimate soil evaporation under natural drying
conditions in the glasshouse. Estimated cumulative soil evaporation was compared with measured
cumulative soil evaporation. The soil hydraulic factor overestimated cumulative soil evaporation during the
falling rate stage of soil evaporation under 0 cm and 2 cm sand mulch. Adjusting the estimation by
subtracting the difference between the estimated and the measured cumulative soil evaporation at the point
where the square root of cumulative potential evaporation was equal to the soil hydraulic factor from the
estimates moved the values closer to the actual measured. The soil hydraulic factor overestimated
cumulative soil evaporation during the whole drying period under 5 cm dune sand soil mulch. The
adjustment of the estimates did not move the values as close to the actual measured as in the 2 cm mulch.
Using the soil hydraulic factor gave better estimates of cumulative soil evaporation during the constant rate
stage under 0 cm and 2 cm sand mulch than under 5 cm sand mulch.