Field-Measured, Hourly Soil Water Evaporation Stages in Relation to Reference Evapotranspiration Rate and Soil to Air Temperature Ratio

Upon wetting, soil water evaporation (E) is controlled by available energy (Stage 1, or S1), and once water becomes limiting, also by soil hydraulic characteristics (Stage 2, or S2). Determination of the transitioning between stages is important for estimating E. The objectives of this research were to compare changes in E in relation to reference evapotranspiration (ETo), or E ETo(-1), and to the ratio of surface soil temperature (T-s) and air temperature (T-a), or T-s T-a(-1), using its change from < 1 to > 1 as a means to identify stage transitions. Hourly E measurements were made in 17 measurement runs after irrigations ranging from 9 to 52 mm in weighing lysimeters containing clay loam, silt loam, sandy loam, and fine sand at Bushland, TX. On the day of irrigation, E exceeded ETo by 21% on average. The 3-d total E losses from bare soils averaged 47% of irrigation amounts > 30 mm, 72% of irrigation amounts < 30 mm, and nighttime E averaged 28% of total E losses. On the day of irrigation, E/ETo relationships were similar for the four soils, indicating that soil texture had no effect on E/ETo during S1. The T-s T-a(-1) ratio exhibited a consistent response to drying rate changes. For E > ETo, T-s Ta-1 remained <= 0.9, but would be >= 1.0 when E was <= ETo. By using criteria based on T-s T-a(-1), estimates of readily available (REW) and total evaporable water (TEW) were variable but often larger for REW and within 3.5 mm for TEW, when compared with published values.