|Xiao, Xinhua -|
|Horton, Robert -|
|Heitman, J -|
|Ren, Tusheng -|
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 21, 2011
Publication Date: August 9, 2011
Citation: Xiao, X., Horton, R., Sauer, T.J., Heitman, J.L., Ren, T. 2011. Cumulative soil water evaporation as a function of depth and time. Vadose Zone Journal. 10:1016-1022. Interpretive Summary: Evaporation can be the major loss of water from agricultural fields. Understanding whether the source of the evaporated water is the soil surface or the crop plants is necessary to improve crop water management. It is very difficult to measure changes in soil water content with enough accuracy to estimate evaporation. In this study, sensors buried in the soil were monitored continuously to measure the energy (heat) content in very thin soil layers near the surface. These measurements were compared with standard techniques to measure evaporation from bare soil. The results indicate that this new method is very promising and may be able to be developed as a routine approach to measure soil water evaporation. This research is important to scientists and growers studying or applying methods to improve water use efficiency in croplands.
Technical Abstract: Soil water evaporation is an important component of the surface water balance and the surface energy balance. Accurate and dynamic measurements of soil water evaporation enhance the understanding of water and energy partitioning at the land-atmosphere interface. The objective of this study is to measure the cumulative soil water evaporation with time and depth in a bare field. Surface cumulative soil water evaporation was measured by the Bowen ratio method. Subsurface cumulative soil water evaporation was determined with the heat pulse method at fine scale depth increments. Following rainfall, the subsurface cumulative evaporation curves followed a pattern similar to the surface cumulative evaporation curve, with approximately 2-d lag times before evaporation was indicated at the 3 and 9 mm subsurface soil depths, and several days more delay for deeper soil layers. For a 21-d period in 2007, the cumulative evaporation totals at soil depths of 0, 3, 9, 15 and 21 mm were 60, 44, 29, 13 and 8 mm, respectively. For a 16-d period in 2008, the cumulative evaporation totals at soil depths of 0, 3, 9, 15 and 21 mm were 32, 25, 16, 10 and 5 mm, respectively. The cumulative evaporation results from the Bowen ratio and heat pulse methods indicated a consistent dynamic pattern for surface and subsurface water evaporation with both time and depth. These findings suggest that heat pulse probes can accurately measure subsurface soil water evaporation over several rainfall/drying cycles. Thus, heat pulse probes can be used to measure soil water evaporation at weather stations and in areas without fetch large enough for traditional aboveground micrometeorological approaches for measuring evaporation.