MEASUREMENT RESULTS

The course of the drained soil profile moistures was expressed graphically for the wet years 1987/2001 and for the dry years 1988/1999. Piezometric pressures courses were evaluated in the wet year 1987 and in the dry year 1988. The piezograph of the dry period of the wet year 1987 (Fig 1) and the piezograph of the wet period of the dry year 1988 (Fig 2) is represented in the enclosed pictures. Both graphs (Figs 1and 2) are depicting the situation for the sloping position HV2.

0

20

40

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80

100

120

10-VIII 15-VIII 20-VIII 25-VIII 30-VIII 4-IX 9-IX

[cm]

No. 7 / 240 cm No. 9 / 120 cm No. 10 / 80 cm No. 16 / 300 cm No. 18 / 190 cm No. 19 / 120 cm

Fig 1: Piezometric pressure [cm] during the dry period of the wet year 1987; Cerhovice HV2.

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100

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20-VII 25-VII 30-VII 4-VIII 9-VIII 14-VIII 19-VIII

[cm]

No. 7 / 240 cm No. 9 / 120 cm No. 10 / 80 cm No. 16 / 300 cm No. 18 / 190 cm No. 19 / 120 cm

Fig 2: Piezometric pressure [cm] during the wet period of the dry year 1988; Cerhovice HV2.

The soil moisture of the drained profile was expressed in percentage of volume for the depths of 0.2, 0.5 and 0.7 m. In the wet year 1987 the soil moisture on the drained land moved in the range of 28 to 50%

of volume, and was slightly decreasing during the vegetation period (with the exception of increases in soil

moisture after intense precipitation towards the end of the month of June). The moisture at the depth of 0.3 m was markedly higher in the level position than in the sloping position. In the level position wetting of the soil profile occurs by water from the stream or by underground water inflow. Drainage continues permanently but underground water contributions evidently exceed the precipitation volume. In the depth of 0.5–0.7 m the soil humidity is, on the contrary, higher in the sloping position. In the slightly sloping position in HV2 it is only the excess infiltrated water from precipitation that is carried away. In the dry year 1988 the moisture near the surface of the soil was markedly lower in the level position HV4 than in the sloping position HV2.

The opposite is true with increasing depth. With increasing depth the moisture range is also reduced, while the difference of the maximum and the minimum moisture content reached 30% at a depth of 0.2 m, this difference was no more than 12% at a depth of 0.7 m. The course of the soil moisture content at depths 0.5 m, 0.7 m and 1.0 m for site HV2 for 1988 is depicted in Fig 3.

6 12 18 24 30 36 42 48 54 60

7.1.1988 29.1. 22.2. 10.3. 24.3. 5.4. 22.4. 19.5. 9.6. 27.6. 14.7. 29.7. 11.8. 4.11. 17.11. 9.12. 27.12.

%

0.7 m 0.5 m 1.0 m

Fig 3: Soil moisture [%] at different depths in the dry year 1988 (January-December); Cerhovice HV2.

In the years 1999 to 2001 soil moisture was also monitored on the retardation drainage in which the damming element – regulator was permanently closed. During closure, the water in the regulation element is dammed by 0.3 m and water flows over the regulator. Drainage occurs also during water retardation, however, the runoff intensity is reduced as a consequence of water retardation. During the summer season, water from the soil profile is extracted by transpiration and partly also by evaporation.

Due to the influence of an extreme drought in the year 2000 caused by a period of almost two months without precipitation, the lowest soil moisture values were measured as early as by the end of June.

While in the year 1999, maximum soil moisture reduction occurred by the end of September, that is as late as by the end of the vegetation period. Water retardation in the drainage system causes a time shift in the onset of the critical drought period.

Pressure changes in the piezometric system (HV2 as well as HV4) were evident to the depth of 1.80 m.

The deepest piezometers (2.4 and 3.0 m) hardly react in a dry year, which means that the water of the deeper layers does not contribute to the creation of drainage runoff. While all the layers are totally saturated, water streaming occurs in the direction to the drains from as far as the depth of 1.8 m. The outlet branches of the piezographs (hydrograms) that usually follow after larger precipitation were analysed from the viewpoint of pressure change in time. However, for the time being it is not possible to evaluate the part of pressure reduction as a consequence of the creation of drainage runoff and of the influence of filtration. While some layers participate actively in “water circulation”, other ones act evidently as insulators. Less permeable layers form impermeable layers from the point of view of streaming.

The monitored situation shows the differences of climatically characteristic years (the wet year 1987 as well as the dry year 1999) and the influence of the position of the drainage system.

Piezometric pressures were put in context with the moisture dynamics of the drained soil profile to the depth of 1.0 m, and with precipitation and drainage runoff. In view of the depth of the soil moisture measurements that was carried out to 1.0 m it was possible to review the given relations to the relevant depth only.

The analysis of the results of piezometric pressure measurement in the individual depths of the drained profile can be compared with the results of theoretical models.

DISCUSSION

The neutron probe type Troxler used for the soil moisture measuring records “moisture” in the spherical space of soil around the probe sensor roughly to the distance equal to the radius 0.2 m from the sensor.

The method is not dependent on the size and quality of the contact area of the sensor with the porous environment (soil) as it is the case with a whole number of other moisture apparatuses. A measuring accuracy for the sensor is currently stated with an error of ± 1.5%, but in our case the potential error also grows because of further inaccuracies, e.g. due to the spatial variability in the physical properties of the soil.

For all the monitoring locations the correction (-0.849 + 0.628n) was used, where n is the deduced value of measurement.

From the point of view of water streaming in the drained soil profile the macro-voids (pedohydatodes), soil permeability and drainage parameters (above all the location depth and the parallel drains range) are fundamentally important. The saturated zone thickness fluctuates due to the occurrence of precipitation and of soil profile drainage. In spite of its upper level being approximately identical with the underground water level, it is necessary to mention that the measured underground water level may be distorted as the probe records the pressure from any depth and place on the periphery of the bore (perforated pipe).

The saturated zone is usually not hundred percent saturated, there always remains a certain aeration share that can be ascribed to the debit of the soil particles forces and also to hysteresis. However, oxygen may be gradually extracted by soil organisms. For these reasons the piezometers suitable for the study of water streaming conditions in a porous environment are those that are sensible tools for recording pressures above all in a non-homogenous layered environment.

The choice of the relatively equal pairs of dry and wet years is based on the identity of the annual precipitation totals. It is a criterion that is acceptable from the hydrological point of view. However, if a markedly different precipitation development occurs in the chosen years (in temporal distribution as well as in intensity), then the chosen pairs may be less homogenous.

CONCLUSIONS

The monitored situation showed evident differences and mutual connections of the course of soil moisture and piezometric pressure in climatically and hydrologically characteristic years (the wet year 1987 and the dry year 1999) for two different positions of the drainage system.

Based on the evaluation of the piezometric system in the dry and the wet period of a wet and a dry year, the soil and underlying layers participating in the creation of drainage runoff were defined. The lowest values of soil moisture content were measured during the extreme drought in the spring of the year 2000. The rate of decrease of soil moisture drops with increasing depth. Water retardation used in the drainage system retains soil moisture in the drained soil profile, which means that the critical drought period is shortened.

The obtained results enable us to describe more precisely the conditions of the drainage runoff origin in a slightly sloping and a level position of the drainage system in the experimental catchment of the Cerhovický stream on the loam-clay soils in the area of the Central Bohemian Hills.

ACKNOWLEDGEMENTS

This work was carried out in the framework of the project “Influence of Melioration Interventions Control on Hydrological Soils Regimes and Hydrological Processes in Landscape” which is a part of the research objectives of Research Institute for Soil and Water Conservation Prague. The authors would like to express their thanks to Mr. P. Hospodka for his carefully and systematically performed measurements in the experimental catchment, and to Mr. J. Hanzlík, a student of the Faculty of Natural Sciences of the Charles University, for his assistance in computer processing of the piezometric measurements.

REFERENCES

Soukup, M., Kulhavý, Z., Pilná, E., Mimrová, K., Eichler, J. (2001) Measures for runoff regulation in an agriculturally exploited catchment. Output of the project NAZV EP 0960006150. VÚMOP Prague methodology, no.26, pp. 51.

Soukup, M., Kulhavý, Z., Hrádek, F., Eichler, J., Pilná, E., Doležal, F., Hospodka, P., Čmelík, M., Mimrová, K., Čížek, V., Kyzlíková, J. (2000) Regulation of surface and ground water runoff in a catchment with regard to water protection. Final report of the project NAZV EP 0960006150. Prague: VÚMOP Prague, pp. 36. + annexes.

Soukup, M., Kulhavý, Z.(2000) Regulation methods of runoff from drainage systems. Methodology, 24, 86 pp.

Švihla, V. (1984) Drainage influence on water management balance in a catchment. The final report of the task C 11-329-208-01-02 VÚZZP Prague, 58 pp. + annexes.

Švihla V. (1991) Drainage systems with hydrological function optimation. Scientific works of VÚMOP Prague, no. 7, 64-72.