Determination of Subsurface Evaporation and Soil Heat Flux: Implementing a Heat Pulse Probe Array for High-Resolution Measurements

Kashifa Rumana1, Markus Tuller2 and Scott B. Jones1

1Department of Plants, Soils and Climate, Utah State University, Logan, UT, 2Department of Soil, Water and Environmental Science, The University of Arizona, Tucson, AZ

Conclusion

Presented at the ASA, CSSA, and SSSA International Annual Meetings, Long Beach, CA, November 02 – 05, 2014.

To evaluate the applicability of new heat pulse probe arrays to accurately determine near-surface soil evaporation rates and to identify advantages and limitations associated with this method.

Fig. 1: Temporal snapshots of temperature and thermal

conductivity in the soil profile starting from the surface

to depths of 6, 12, 15, 21, and 27 mm below the surface.

Theoretical Considerations

1 21,2

1 2

T TG

z z

2,3 1,2LE G G S

Subsurface Evaporation Rate (Heat Balance Method)

Latent heat

Sensible heat Sensible heat storage

(1)

Heitman, Xiao, Horton, Sauer, WRR (2008)

1,2 2,31

2 2 1

j j

j j

z zS C T T

t t

L = Latent Heat of VaporizationE = Subsurface Evaporation RateC = Volumetric Heat Capacity; = /k= Thermal Conductivityk = Thermal Diffusivityt = time; j = time step

Measured soil temperature and thermal properties are used to calculate heat flux, change of sensible heat storage, and latent heat of vaporization.

Experimental Setup

Heat Pulse Probe Array

A thermally insulated, 15-cm tall Plexiglas column packed to a bulk density of 1.8 g/cm3 was used for the laboratory experiment.

The column was saturated from the bottom and then disconnected from the water supply.

The surface was exposed to uniform radiant heat of 450 W/m2 that was monitored with a CG1 Pyrgeometer.

Hexa- and Penta-needle heat pulse probes were deployed to measure sub-surface evaporation.

The HPP arrays were rotated 27.3o from the horizontal plane to make temperature measurements at a millimeter scale.

Top needle of HPP

seen at the surface

Objective

Results The temperature profile was

virtually constant with depth during stage-I evaporation.

The transition between stage-I and stage-II occurred at around 95 hours.

Thermal conductivity exhibited a dramatic transition directly associated with the diminishing water content.

The authors gratefully acknowledge support

from the USDA Cooperative State Research,

Education, and Extension Service supported

by a Special Research Grant # 2008-

34552-19042 and by a USDA NIFA AFRI

Soil Processes Program grant # 2009-65107-

05835.

Acknowledgement

The HPP cannot measure surface evaporation, only subsurface evaporation (Sakai et al. 2011).

Each of the peak rates shown in the inset of Fig. 2 correspond to temperature inflections in Fig. 1.

The heat balance method agrees reasonably well with the water balance method.

Fig. 2: Summation of subsurface evaporation rates

from the HPP for individual layers compared with

load cell measurements. The inset figure shows

diminishing evaporation rates in each layer as the

drying front descends.

Fig. 3: Heat flux density measured at different

soil depths with time and diminishing water

content.

• Heitman, J.L., R. Horton, T.J. Sauer, and T.M. DeSutter (2008a): Sensible Heat Observations

Reveal Soil-Water Evaporation Dynamics, J. Hydrometeor, 9: 165-171.

• Heitman, J.L., X. Xiao, R. Horton, and T.J. Sauer (2008b): Sensible Heat Measurements indicating

depth and magnitude of subsurface soil water evaporation, Water Resour. Res., 44: W00D05.

• Sakai, M., S. B. Jones, and M. Tuller. (2011): Numerical evaluation of subsurface soil water

evaporation derived from sensible heat balance, Water Resour. Res., 47(2).

References

The divergence in heat flux density with

depth is consistent with the expected

heat storage in dry layers and

diminishing heat flux in the drying layer.

1. The HPP array gives real-time estimates of soil thermal properties and heat flux.

2. Subsurface evaporation estimates require post processing and correction of data.

Soil water evaporation is governed by complex interrelations between atmospheric demand and soil properties that affect liquid-vapor transport processes, leading to limited understanding of land-atmospheric interactions.

Near real-time heat pulse-based measurement of subsurface evaporation on a millimeter depth scale will aid in resolving this current knowledge gap.

Introduction

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