Study of Unsaturated Conductivity Dependency on Soil Moisture Suction in Kosovo Clay Loam Soil

1. Study of the Dependency of Unsaturated Conductivity on SoilMoisture Suction in a Typical Clay Loam Soil in the Field of KosovoBesnik Gjongecaj*, Abdullah Nishori **, Demë Abazi, ***

2. Aim of the study In this experimental study we try to quantify: the dependency of unsaturated conductivity on the soil water potential or, on the soil water suction. So, we intend to quantify the function K(unsat)=f(Hm). To fulfill the aim of this study, the internal drainage method is applied, which is considered as the simplest and the most accurate method in the field conditions, namely, in situ.

3. Materials and methods • The study area and the devices • The internal drainage method to quantify the function K(unsat)=f(Hm),was suggested by Richards and Weeks and later by Hillel (1982). This method, when it get carried out in the field, can help to eliminate the possible alteration of soil hydraulics due to the disturbance of structure as well as the doubtful procedure of applying steady-state methods to transient state processes (Hillel, 1982). • To fulfill the aim of this study, the location chosen was Komoran, a very typical soil in a climate area very much representative of the Field of Kosovo. The period of study was 40 days within the period in which the plant requirements for water are in the maximum values. Basically, the method requires frequent and simultaneous measurements of the soil water content and soil water suction and on this base, the respective profiles are supposed to be created. The equipment used to measure the soil water content on the volume basis was the digital electronic device and the device for soil water suction (soil water potential) measurements was tensiometer. • By using these devices, it becomes possible obtaining the instantaneous values of the potential gradients and fluxes operating within the soil profile and hence, also, the values of unsaturated conductivities. The theoretical substance of the method used to quantify the function of the unsaturated conductivity K(Hm) on the soil water suction, Hm, can be illustrated by the following figure and analysis.

4. Fig.1 Change in profile water content, ∆W in the depth zone from z0 (soil surface) to a given depth z1, in the time interval from t1 to t2, during internal drainage in the absence of evaporation.

5. To apply this method, an experimental research was undertaken. A plot of the dimensions 8m x 6m was picked and the devices were located at the middle of it, in order the processes and the measurements not to be affected by the boundaries. Within this plot four tensiometers were installed in the respective depths of 0-15cm, 15-30cm, 30-45cm and 45-60cm. Close to each tensiometer, a digital electronic device for soil water content measurements was placed. Electronic devices were calibrated before use. Water is then ponded on the surface and the plot was irrigated long enough so that the entire profile becomes as wet as it can be. The soil, after this, was covered by a sheet of plastic so as to prevent any water flow across the surface. As the internal drainage was proceeding, the measurements of both, soil water potential and soil water content were made simultaneously throughout the soil profile up to the depth of 60cm. The research lasted over a period of forty days, but 14 days were picked to do the measurements. The period between each day of measurement was kept constant of three days, which means that the entire period of the experiment was 40 days. The photos showing the process of setting the experiment are given as following:

6. Photo 1. Emplacement of tensiomentrs and electronic devices

7. Photo 2. Saturation of the plot

8. Photo 3. Saturation and covering of the plot

9. Photo 4. Placement of tensiometrs

10. Photo 5. Taking records from watch dog

12. 2. Data analysis The method of measuring simultaneously both: the soil water content and the soil water potential, creates the possibility to build up the profiles of soil water content and soil water potential (suction) of the entire profile (space) and over time. In this way, the measurement of soil water potential over the soil depth made possible to calculate the term dHm/dz for each day taken into the consideration. The measurement of soil water content over the time made possible the calculation of the term dθ/dt, so the change of the soil water content over time, which one, multiplied by the change in depth, dz, gives the flux of water flowing from the upper depth to a lower one: q= dz(dθ/dt). Knowing that the water flux is, by definition, a product of the unsaturated conductivity and the soil water potential (suction) gradient, then the unsaturated conductivity can be found as a ratio between the water flux, q, and the soil water potential gradient, dHm/dz, so: K(Hm) = q/dHm/dz.

13. Results and discussion • The results of this experiment, for sake of simplicity and clarity, will be presented and discussed in three sections. • The results on the soil water suction gradient, dHm/dz. • The results on the soil water content over time, -dθ/dt • Calculation of soil water flux , q

14. The results on the soil water suction gradient, dHm/dz. Fig.2 The dependency of soil water potential on the soil depth. The equations given on the right are the best fit between the soil water suction, Hm (y) and the soil depth, z (x). Tab.2 The regression equations found between the soil water potential and the depth in every single days.

15. B. The results on the change of soil water content over time, -dθ/dt The results of measurements done on the change of soil water content over time by using the digital devices are presented in the following graph: Tab. 6 The regression equations belonging the lines in the fig.4 Fig. 4.Soil moisture content as a function of time, in four given soil depths

16. C. Calculation of soil water flux , q The calculation of the soil water flux, q, through the bottom of each depth layer (increment) is done by integrating soil moisture content-time curves fig.4 with respect to depth. First, the slopes -dθ/dt measured and presented in the above table belong to each day of measurement (days: 1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40). Second, these slopes are multiplied by their respective depth increments (layer) to obtain the per layer rate of water contain change, dz(dθ/dt). After this, the flux q through the bottom of each depth is obtained by accumulating the water contain increments of all layers overlying that depth, so q = dz(dθ/dt). The results obtained as it is described already in this procedure are presented in the following tables:

17. Tab.8 Soil moisture flux, q.

18. Calculation of soil water conductivity K(Hm)

19. Regression analysis to quantify the dependency of unsaturated soil water conductivity on the soil water suction, K(Hm) To do the regression analysis the data of two columns in the above table , respectively the column of K and the column of Hm are used. The result of this analysis is presented in the following graph. Fig. 6. Soil water conductivity as a function of soil water suction, K(Hm) The dependency K(Hm) is a power function which was chosen because it provided the highest coefficient of determination. The function is graphed in a way that the water conductivity is presented in a log scale on the y axis. It is very clear that the increase of soil water suction, which means a decrease of soil water content, leads to a decrease of the soil water conductivity as it is foreseen in the equation K = 983.67Hm exp(-1.89). The respective coefficient of determination shows that about 94% of the change of soil water suction is reflected to the change of the magnitude of soil water conductivity as by the equation K = 983.67 Hm exp(-1.89), within the range from 0 to about 400cm of Hm.

20. The procedure shows that the method of measuring the soil water conductivity in an unsaturated soil by internal drainage is an accurate method, based on the fact that the final result taken, so the formulae that expresses the dependency K(Hm), is in accordance with the theoretical work done in this area over years. The dependency of soil water suction on the soil depth, when the evaporation is prevented, is represented by a straight line, whose slope represents the soil water suction gradient. The dependency of soil water content on time for each depth or soil layer has a half logarithmic nature. The slope of the curves gets lower as the soil depth increases, which shows clearly that the soil water content changes happen more abruptly close to the soil surface. The dependency of soil water conductivity in an unsaturated soil on the soil water suction, so the function K(Hm), is a power function, which was chosen because it provided the highest coefficient of determination. The above mentioned function includes measured values between 10cm to about 400cm of soil water suction, which is in fact that range of suction where the most available water to plant roots is found. It means that the applicative value of this study is very much considerable. Conclusions

21. Thank you for your attention