Heat Pulse Probe Measurements of Soil Water Evaporation In a Corn Field.

Latent heat fluxes from cropped fields consist of soil water evaporation and plant transpiration. It is difficult to accurately separate evapotranspiration into evaporation and transpiration.  Heat pulse probes have been used to measure bare field subsurface soil water evaporation, however, the applicability of heat pulse probes to measure soil water evaporation in a cropped field is still uncertain. In this study, we applied newly designed (11-needle) heat pulse probes to measure soil water evaporation in a corn field. Soil water evaporation fluxes were measured during a natural drying cycle in the summer of 2009 at the following locations within the corn field: a bare spot, in-row, between-rows, and between rows with roots excluded. In addition to the heat pulse probe measurements, micro-lysimeters were used to measure daily soil water evaporation at each of the locations. The heat pulse probe measurements provided realistic estimates of the soil water evaporation dynamics. A short time after a rainfall event, soil water evaporation was detected at a subsurface depth of 3 mm before shifting downward to deeper soil 3 days after rain at the bare location and between-rows without roots location, 7 days after rain at the between-rows location, and 9 days after rain at the in-row location. The daily soil water evaporation at the bare location was larger than at in-row and between-rows locations. The between-rows location had larger heat-pulse and micro-lysimeter evaporations than the in-row location during the measurement period. Daily soil water evaporation estimates from heat pulse and micro-lysimeter methods differed by less than 0.24 mm, 0.82 mm and 0.60 mm at the bare, in-row and between-rows locations, respectively. Thus, the heat-pulse method is a promising way to accurately measure soil water evaporation in a cropped field.