temp waves

Soil temperature plays an important role in soil chemical reactions and biological interactions, particularly nutrient and fertilizer transformations, solute transport, gas exchange and the transformation and transport of contaminants (Buchan 2001).

Soil temperature varies in response to exchange processes that take place primarily through the soil surface. These effects are propagated into the soil profile by transport processes and are influenced by such things as the specific heat capacity, thermal conductivity and thermal diffusivity.

Soil temperature can vary greatly throughout the day with increasing and decreasing solar radiation. Soil temperatures also vary greatly with depth from the surface, as well as with differences in soil cover (mulch) and soil water content. The thermal properties of a soil have been found to be indicative of the soil water content. Water is a better thermal conductor than air. The thermal conductivity of soil increases with increasing water contents (Fredlund, 1992).

The apparatus is set initially by digging a hole which exposes a vertical face of the soil in which temperature sensitive thermocouple probes are inserted at varying heights in the soil profile. In this case the thermocouple probes were put into the soil at heights of 1cm, 3cm and 10cm’s, (Figure 1) and temperatures were recorded at 10 minute intervals.

Above the soil surface a Bowen ratio weather station is situated which measures the air temperature, also recorded by a data logger at 10 minute intervals. The data gained by the Bowen ratio weather station can be analysed along with the temperature data in the soil, to see how the soil reacts to climatic factors.

The location that was being monitored, was in a cultivated field which had no vegetation cover. This meant that the soil surface was receiving full solar radiation, with very low albedo and would be subject to high soil temperatures. This prompted the decision to apply a vegetative mulch, as well as a black plastic mulch, as separate treatments to examine the effect that they have on the soil temperature.

The aim of this analysis is to estimate the thermal conductivity of the study soil. Simple overlay plots are used to illustrate the effect of depth on temperature. However to estimate the thermal conductivity of the soil multiple steps must be taken in order to accurately estimate the thermal conductivity of the Black Vertosol. Analyses are performed in the JMP 5.1 statistical package.

The first step is to conform the data to a sigmoidal curve. This allows the natural variations to be averaged out and variables to be ascertained. The data are transformed using the equation below. The Damping depth (Sd) is introduced to account for the dampening effect of depth on temperature. This is seen in the overlay plots of the temperature sensors as a decrease in amplitude with increasing depth.

These curves originally are fitted with mere estimates of variables (Sd, A). For these variables to be accurately calculated a non-linear model is performed. A non-linear model is run for each depth .The figure below is an example of a fitted sigmoidal curve using the non-linear model, notice the estimates for each variable.

The above figure is the non-linear model run for the surface sensor. This initial analysis gives us the estimates for A and t0 to be used for the other depths. The successive models are run with the A and t0 estimates locked so that estimates of sd can be attained.

Due to the close proximity of sensors and assumptions of Specific heat capacity the estimates are found to be very similar. As such the sensors were banded into 3 groups, representing ranges in the profile.

Once Damping depth has been found, thermal conductivity can also be ascertained given the equation below.

Where,

Sd = Damping Depth

Lamda h = thermal conductivity

w = angular freq. of temp oscillation (i.e. 2*pi/P)

Ch = specific heat capacity

Of particular importance to this study is Thermal conductivity. Thermal conductivity is primarily a function of the soils constituents (e.g sand / silt / clay percentage, organic matter) and moisture content. Due to time restrictions a survey of the whole profile was not performed, as such Ch has been assumed to be consistent down the profile (2.1 MJ/m-3/K).Thermal Diffusivity can be calculated once Sd is known based on the equation below.

Of interest to this study was the effect of mulch and surface coverings on soil temperature. Part of the analysis will evaluate the effectiveness of surface coverings in altering the soil temperature profile.

The above figure shows the recordings for the first three days. It is evident that depth has a dramatic effect of temperature, with amplitude decreasing markedly with depth. For this study damping depth is to be calculated and is used to represent the extent of the amplitude reduction. Damping depth is a constant characterizing the decrease in amplitude with an increase in distance from the soil surface. This decreased amplitude describes a soil with very consistent temperature, with minimal diurnal temperature fluctuations. At the surface the damping depth is reached when a soil with a 70% reduction in amplitude is encountered. Below is a summarised table of the Damping depths and associated variables.

Band (cm)	Lamda h (J/m/K)	sd (cm)	Dh	Ch (MJ/m3/K)
0-1	0.75	10	3.6E-07	2.1
2-5	0.62	9	2.95E-07	2.1
5-12	0.53	8.5	2.63E-07	2.1

The above table also states the changes in Thermal conductivity with depth. As the table shows Thermal conducivity decreases with depth. Thermal conducivity is highly responsive to small changes in soil moisture in dry soils. It is perhaps small changes in the moisture content that is influencing the Thermal conducivity. However to be confident of such a statement further investigation into the profiles moisture content would be required.

Of particular interest to this study is the effect of surface coverings to the soil temperature. In this study mulch and back plastic coverings were used to see the difference in temperature effects. All previous analyses were performed disregarding this trial.

The above figure testifies to the effectiveness of mulch in regulating soil temperatures. The mulch effectively reduces the daytime soil temperatures and insulates the soil at night keeping warmer. The efficacy of the black plastic however is not fully appreciated due to time constraints. Had the trial been able to continue it is expected that daytime temperatures would increase and be only marginally effected at night due to a reduced insulative effect compared to the mulch.

Through the experimentation and subsequent analysis, the thermal properties of the Narrabri soil have been described. Due to time constraints the mechanics of the thermal properties could not be properly assessed. Further trials should incorperate a complete soil moisture and bulk density profile to explain the changes in thermal properties through out the profile.

Understanding these properties can improve the accuracy of the decision making process. The trial of the surface coverings has shown that by changing both the albedo and surface insulation soil temperatures can be manipulated. In terms of water conservation the use of mulches has shown to greatly reduce daytime temperatures, and hence evaporation. Mulches have also shown to regulate night time temperatures, which can reduce the incidence of frosts. This is of particular importance to cotton growers in Narrabri, where cool soil temperatures can delay maturation and hence harvest (Brown, 2002).

Due to previously mentioned time constraints some tests could not be performed. As such some assumptions had to be made, of note is the assumption of constant specific heat capacity. For the correct heat capacities to be ascertained bulk density cores would be required for each sensor. This would also improve the interpretation of the final data. Limited data limits the confidence of conclusions. A complete soil moisture profile would also improve the final interpreation and explanation.

Buchan, G.D., (2001) Soil Temperature Regime, in Smith, K.A., and Mullins, C.E. (Eds). Soil and Environmental Analysis: Physical Methods 2nd Ed. 2001. Marcel Dekker. pp, 539-594.

Fredlund, D.G. (1992). Background, Theory, and Research Related to the Use of Thermal Conductivity Sensors for Matric Suction Measurement. Soil Science Society of America. Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice, 249-261.