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Soil moisture (kg water/kg dry soil)/soil moisture at saturation

Figure 8.2 Water retention curves for the three different layers 0-10, 10-20, 20-40 cm of soil sampled at Pallanzeno. The offset and the slope give the air entry potential and the exponent b of power functions (8.1) respectively

Soil moisture (kg water/kg dry soil)/soil moisture at saturation

Figure 8.2 Water retention curves for the three different layers 0-10, 10-20, 20-40 cm of soil sampled at Pallanzeno. The offset and the slope give the air entry potential and the exponent b of power functions (8.1) respectively

Daily soil moisture and water loss at the MAP station of Pallanzeno (Italy)

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Daily soil moisture and water loss at the MAP station of Pallanzeno (Italy)

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Figure 8.3 The trend of the daily mean of the soil water content 9 of the whole period of measurement is shown. The data of the probes at 5 cm (circle), 15 cm (triangle), 25 cm (diamond) and 40 cm (square) are reported. The vertical bars are the daily cumulative precipitation values. The thick solid line represents the cumulative evaporation

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Days of the year (91 = first of April) 1999

Figure 8.3 The trend of the daily mean of the soil water content 9 of the whole period of measurement is shown. The data of the probes at 5 cm (circle), 15 cm (triangle), 25 cm (diamond) and 40 cm (square) are reported. The vertical bars are the daily cumulative precipitation values. The thick solid line represents the cumulative evaporation volumetric water content value: 17.7% (not evident in Figure 8.3 because of the average effect). August was quite rainy, and the soil moisture remained almost constant; a new minimum was reached on September 17. On 19-21 of September (MAP-IOP-02), a total precipitation of 225.4 mm produced the maximum soil moisture value measured: 56.0% (probe at 5 cm; not evident in Figure 8.3 because of the average effect); this value represents the saturation of the soil investigated. October showed a new decreasing trend in the soil water content until a new precipitation event (76.4 mm) on October 21 (MAP-IOP-O8). Afterwards, the soil moisture maintained high values at all depths until the end of the measurement period.

A diurnal cycle is present in the TDR soil moisture experimental data at all depths (Menziani etal. 2000). This cycle does not represent a cycle in the soil water content, but it results from the daily cycle oftemperature. Temperature affects the TDR data in two different ways: dielectric constant of the soil (Roth etal. 1990; Pepin et al. 1995) and the behaviour of the electronics of the instrument. Nevertheless, the amplitude of this daily cycle is lower than the instrumental accuracy (±2%) and therefore does not affect our ability to use the data.

8.3 TDR TECHNIQUE

The soil water content measurements performed at Pallanzeno utilised the TDR technique. TDR was originally developed by the telecommunications industry to localise breaks, short circuits and the presence of water in buried coaxial cable. With TDR, for example, a break is located by applying a fast-rise electrical pulse at the free end of the cable and then measuring the time it takes for a signal to travel to and reflect back from the point of disruption. Topp et al. (1980) altered this technique by applying the electrical pulse to probes inserted into the earth. The pulse shape and the transit time along the probes depend on the properties of the soil, on the probe length and on the type of termination where the pulse is reflected. The reflection depends on the equivalent load impedance of the circuit; in TDR measurements, the lines are usually open and this produces a reflected pulse in phase with the incoming pulse. The velocity of the pulse, propagating down the probe, reflected at the end and running back to its source is:

where v is the velocity of the electromagnetic wave in the transmission lines embedded in the medium; c is the velocity of the electromagnetic wave in the void; er and /zr are the relative dielectric permittivity and the relative magnetic permeability, respectively.

Because of the large difference between the dielectric constant of water and the other constituents of the soil (e.g. air, mineral particles), the speed of a voltage pulse in parallel transmission buried lines is essentially dependent on the volumetric water content of the soil (Topp et al. 1980). Because virtually all soils lack ferromagnetic materials, /zr can be assumed equal to the unit. Therefore, inserting in the soil a probe of known length, the apparent c ■ At\2

where At is the time required to the signal to reach the end of the wave-guide (travel time) and L is the wave-guide length.

Ka is so termed because the imaginary part of the complex permittivity is negligible with respect to the real part at the usual frequencies 10 MHz-1 GHz (Dirksen and Dasberg 1993) used for TDR soil moisture measurements; thus, Ka essentially represents the real part. The value of the apparent dielectric constant, given by Equation (8.3) measuring the travel time (At) along the wave-guide of length L, is a sort of mean value in the volume around the probe. The travel time can be calculated by analysing the TDR trace. The TDR trace depends on both the type of the wave-guide and on the dielectric (soil) under investigation. In the TDR trace, the start time, at the beginning of the wave-guide, and the reflection time, at the end of the wave-guide, have to be exactly identified. Usually, the start time corresponds to the maximum of the derivative of the incoming pulse TDR trace. The Soilmoisture buriable probes present a V dip feature at the beginning of the trace; the bottom of the V corresponds to the start time of the pulse in the wave-guide. The reflection time can be estimated analysing the TDR trace using the method of the tangent lines (see an example in Figure 8.4). This method permits finding the point of reflection of the electromagnetic pulse travelling down the wave-guide (Menziani et al. 1996). Soil water content is then calculated from the apparent dielectric constant by means of empirical relationships (Topp etal. 1980; Ledieu etal. 1986; Roth etal. 1990; D'Urso 1992; Heimovaara 1993; Heimovaara and de Water 1993) or experimentally determined Look-Up Tables (LUTs, Soilmoisture TRASE system I technical manual 2000). They give similar results but LUTs have a wider volumetric water content range of application. The greatest differences between the Topp equation in percentage (8.4) and the Soilmoisture LUT are found for values of 0 > 40% (Menziani et al. 2000).

Pallanzeno (Italy)-TDR test at the installation time

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Pallanzeno (Italy)-TDR test at the installation time

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1 1 II 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 TRASE: Ka = 23.7 ==> q = 37.7%

i i i i 1 \ \j i

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Af = 3.25 ns ==> Ka = 23.8 ==> q = 38.8%

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/

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i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i 11 \

i i i i I i i i i

Figure 8.4 Analysis of a TDR trace obtained by a buriable probe installed at the hydrological station of Pallanzeno. The pulse travel time At is the difference between the reflection time at the end of the probe and the start time at the beginning of the wave-guide. The reflection time is identified by means of the tangent lines method; the start time is identified by the V dip. Ka is obtained by Equation (8.3) and 0 is obtained by the Topp Equation (8.4). The results of the TRASE system (using LUT data) are also reported in the figure dielectric constant Ka is:

0 = (-530 + 292 • Ka - 5.5 • K2a + 0.043 • K¡)/\02

In this work, the Soilmoisture LUT for the buriable probe was used.

8.4 THE WATER MASS BALANCE

The water vapour flux at the soil-air interface is usually estimated, on the basis of meteorological data, using the surface energy balance method (Bowen ratio) or measuring the turbulent transport (eddy correlation method). Here, the water mass balance at the soil surface is obtained, under simple assumptions, by means of soil moisture measurements at different depths. A simple formula to compute the amount of water added or withdrawn in a given volume of soil, during a certain period, is presented.

The soil water content 9 and its flux are related by the conservation equation (Hillel 1980a). Considering a volume element inside a soil column of unitary cross section and depth Hc, d0 d®7

where is the flux of 9 along the vertical direction z (pointing downward), and the last term is the horizontal divergence of the horizontal flux component.

Integrating the above Equation (8.5) with respect to z and t, the following expression is obtained.

- LH(t) + $z(Hc,t') ■ dt + / divh^h dz' dt' Jo Jo Jo f '

LH(t) is defined as

0 0

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