*4.3.2. FDR diviner for the other sampling sites*

**4.3. Soil moisture analysis**

66 Soil Moisture

sitivity of each one.

provides a R<sup>2</sup>

*4.3.1. FDR and TDR devices at the Modesto Ángel station*

tions at this specific time. In the case of the FDR\_Dec, R<sup>2</sup>

with RMSE, MBE and RE values of 0.18 cm<sup>3</sup>

was obtained for it with an R<sup>2</sup>

Soil moisture instruments tested report changes in time or frequency related indirectly to the dielectric permittivity to the volumetric water contents. Results are presented per site and per type of technique, and this means that TDR were analysed with Campbell sensors (TDR\_Campbell), FDR using the Decagon sensors (FDR\_Dec) and FDR using Diviner 2000 (FDR\_Div). A specific analysis for the field conditions under the different operating sensors was not necessary since there were the same conditions at the sites (Modesto Ángel with three methods and the other two sites). Individual calibrations per depth offer equations that improve the sensor performance. Then all together were compared in order to know the sen-

In the Modesto Ángel station, the three devices to measure soil moisture were installed: TDR\_ CS, FDR\_Dec and FDR\_Div. TDR\_CS calibration process includes a first analysis using data collected from the datalogger with the default equations of the device. Both lineal and quadratic equations were tested founding that the lineal equation offered better results than the quadratic one with RE of 0.22 and 0.41, respectively. Secondly, the calibration using the gravimetric data measured at the field was done in terms of volumetric water moisture, θgv. The gravimetric measurements were 10 samples per site from 2012 to 2013 years, at 2.4, 5, 10, 20 30 cm depth. Six more gravimetric samples per site were measured during 2014–2015 to confirm the reliability of the calibration for a different weather, soil and vegetation conditions. The TDR\_CS

respectively. The major deviation was observed at the 2.5 and 5 cm, this is because the place where the sensors were installed was not disturbed but the place where the sample was taken, even if it was close to the area of the station, was more susceptible to the surface soil condi-

cm−<sup>3</sup>

FDR\_Div, results showed that the first 30 cm have the same texture; thus, a calibration equation

FDR\_Div was the best device, but it is important to mention that only three depths were tested at 10, 20 and 30 cm. Between TDR\_CS and FDR\_Dec, it is quite difficult to analyse since results are not conclusive, but TDR\_CS could be expected to provide a more consistent value. This is because Decagon devices demonstrated to be more sensible to the weather (see temperature and precipitation results) and to soil conditions at the time of the sampling. However, the RE is major for TDR\_CS. Despite the previous results, the calibration equations were applied to 400 daily θs records with a latency of 20 min for the 2012–2015 period for both TDR\_CS and

As one can observe, there is a good agreement between TDR\_CS and FDR\_Dec, following a similar pattern taking into account the accuracy of each device. However, there is a major response of the Decagon sensor when rain is presented, being evident an increase in some

of 0.86, RMSE 0.086 cm<sup>3</sup>

FDR\_Dec. **Figure 3** plots the behaviour of the devices at a 10 cm depth for 2013–2014.

, 0.407 cm<sup>3</sup>

cm−<sup>3</sup>

cm−<sup>3</sup>

, MBE 0.079 cm<sup>3</sup>

cm−<sup>3</sup>

, 0.107 cm<sup>3</sup>

once calibrated the readings was 0.90

cm−<sup>3</sup>

cm−<sup>3</sup>

and 0.083, respectively. For

and 0.12,

and RE 0.069.

of 0.96, with RMSE, MBE and RE values of 0.101 cm<sup>3</sup>

The Diviner 2000 allows monitoring different areas once the accessed pipe in each site was installed. Thus, eight aguadas were monitored; Carlos A Madrazo was analysed at the La Ceiba site only. More than 30 readings were registered in the period of 2012–2015, as well as several gravimetric analyses were performed a less two per year. As soil moisture is function of the texture along the profile, this implicates different water aggregation and, in consequence, a different behaviour. For that, along the profile, one could have more than one calibration equations in order to represent what actually happened to soil moisture in the profile. For each site, a graph was developed as shown in **Figure 4** for the Modesto Ángel site with 120 cm depth. For this place, three equations were established:

$$\text{(a) } 10 \text{--} 30 \text{ cm:}\\y = 425.6 \,\text{x}^{-0.997};\\R^2 = 0.93\tag{5}$$

group shows a very poor correlation, there is a tendency towards the PWP, and in some cases, it seems that water was not available from the 40–80 cm depth. For these specific cases, FDR\_Div values correspond to late March to mid-June of 2015 where a drought also took place. In addition, the drop between these depths was evident in the gravimetric measurements, although it was not in such an accentuated way. This could be interpreted as a combined effect of soil properties in these strata, the horizontal development of vegetation root systems and the dielectrical methods characteristics in that range of soil moisture. However,

Correlation between TDR and FDR Soil Moisture Measurements at Different Scales to Establish…

http://dx.doi.org/10.5772/intechopen.81477

69

One interesting factor resulted from the analysis at the sites of Flores Magón and Ley de Fomento was that both FDR\_Div and the gravimetric carried out the same day with less than 30 min of difference had not been similitude within the first 50 cm depth (**Figure 5**). In these cases, it was assumed that the disturbing at the moment of sampling in the gravimetric method was significant to obtain different values. Also, in these sites, the aguadas were extended and the types of soil were clayish in Ley de Fomento, and frank and clay in Flores

Although one knows from [35] that the approximated number of the aguadas in the CBR was 1353 and the majority was in the North with 868 and 485 in the South, with an approximate density of 1.87 aguadas per hectare. One does not know the rate of increment in the number of aguadas, in particular for the artificial ones since they are the only source of water during the dry season. Also, the area of the aguadas is variable from some small as one observed in La Ceiba at Carlos A Madrazo to many hectares such as Flores Magón. In terms of estimation of the water availability, it can be considered that the aguadas are generically of the same size; thus, the contribution area (basin) of each aguada can be calculated within the CBR as the relation between the total area of 724,000 ha and the total aguadas (1353), so here are 535 ha

To estimate the water availability based on the real soil moisture during the year, it is necessary to review the distribution in the horizontal line at the different profile. Thus still, there is work to do. Until now, the work done has established the variation along the profile finding some constant values after the 50 cm depth that could be related to the water table that keep water in some aguadas during the year although at its minimum value, in particular those less

One already knows the average rainfall, and the evapotranspiration could also be estimated moreover if one consider that practically all the water infiltrates (probably 92.7%), which results in little runoff (7.3%) that is concentrated in natural aguadas. However, these data are not enough since soil moisture needs to be considered as the water storage capacity that can be removed by evapotranspiration is the function of the type of vegetation and the depth of the root zone. But, in the study area, root depths are quite smaller growing horizontally rather than vertically. This demands a major study in the horizontal line in order to compute reliable water balances.

this requires a further research.

**4.4. Water availability**

of contribution per aguada.

impacted by humans.

Magón, conditions that change the water content in the soil.

$$\text{(b) } 40 \text{--} 80 \text{ cm:} \newline y = \text{ } \text{37.33} \newline x^{0.123} \newline \text{;} R^2 = 0.02 \newline \tag{6}$$

$$(\text{c) } 90 \text{--} 110 \text{ cm}; y = 136.8 \text{ x}^{0.308}; R^2 = 0.35\tag{7}$$

The equation proposals agree with the findings of [43] who defined two equations according to the texture: one group for fields with heavier soils where clay content was >40% and other group with coarser textured fields with clay content <40%. Looking at Eq. (6) for the second

**Figure 4.** The texture and field capacity (CC) and permanent wilt point (PWP) through the profile of the Modesto Ángel site.

**Figure 5.** Soil moisture profile for Ley the Fomento and Flores Magón sites.

group shows a very poor correlation, there is a tendency towards the PWP, and in some cases, it seems that water was not available from the 40–80 cm depth. For these specific cases, FDR\_Div values correspond to late March to mid-June of 2015 where a drought also took place. In addition, the drop between these depths was evident in the gravimetric measurements, although it was not in such an accentuated way. This could be interpreted as a combined effect of soil properties in these strata, the horizontal development of vegetation root systems and the dielectrical methods characteristics in that range of soil moisture. However, this requires a further research.

One interesting factor resulted from the analysis at the sites of Flores Magón and Ley de Fomento was that both FDR\_Div and the gravimetric carried out the same day with less than 30 min of difference had not been similitude within the first 50 cm depth (**Figure 5**). In these cases, it was assumed that the disturbing at the moment of sampling in the gravimetric method was significant to obtain different values. Also, in these sites, the aguadas were extended and the types of soil were clayish in Ley de Fomento, and frank and clay in Flores Magón, conditions that change the water content in the soil.
