Location: Soil and Water Management ResearchTitle: Field-measured, hourly soil water evaporation stages in relation to reference evapotranspiration rate and soil to air temperature ratio
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2015
Publication Date: 7/2/2015
Citation: Tolk, J.A., Evett, S.R., Schwartz, R.C. 2015. Field-measured, hourly soil water evaporation stages in relation to reference evapotranspiration rate and soil air temperature ratio. Vadose Zone Journal. doi:10.2136/vzj2014.07.0079.
Interpretive Summary: Upon wetting, soil water evaporation (E) is driven by available energy (Stage 1, or S1), and once water becomes limiting, also soil hydraulic characteristics (Stage 2, or S2). Determination of the transition between stages is important for estimating E losses. The objectives of this research were to compare changes in E in relation to reference evapotranspiration (ETo) and to the change in the ratio of soil temperature (Ts) and air temperature (Ts/Ta) from less than 1 to greater than 1, the latter as a means to determine the stage transition. Hourly E measurements were made in weighing lysimeters containing clay loam, silt loam, sandy loam, and fine sand at Bushland, TX. Irrigations ranged from 9 to 52 mm. On the day of irrigation, E exceeded ETo by an average of 21 percent, and E greater than ETo often continued through the night. The transition from S1 to S2 could occur abruptly the following day. The 3-day total E losses from bare soils averaged 48 percent of irrigation amounts greater than 25 mm, and nighttime E 28 percent of total E losses. Soil hydraulic properties had no significant effect on the E:ETo relationship on the day of irrigation, but afterwards they were significantly related only within a soil textural class. The Ts/Ta ratio exhibited a consistent response to drying rate changes. When E was greater than ETo, Ts/Ta remained less than or equal to 0.9. Depending on the time of day that E became greater than ETo, Ts/Ta would approach 1.0 and, when daytime E rates became and remained small, Ts/Ta became consistently greater than 1.0. This suggests that the ratio could provide a useful tool for establishing the change from S1 to S2 evaporation.
Technical Abstract: Soil water evaporation takes critical water supplies away from crops, especially in areas where both rainfall and irrigation water are limited. This study measured bare soil water evaporation from clay loam, silt loam, sandy loam, and fine sand soils. It found that on average almost half of the irrigation water put on the soils would evaporate in three days. Evaporation even occurred at night. This highlights the need to use soil covers or farming practices which help prevent water waste through evaporation.