SOME PROPERTIES OF GUYON’S METHOD FOR DRAIN SPACING DETERMINATION ON THE MARSHY GLEY SOIL

The aim of this work is to show out some properties of Guyon’s method for drain spacing determination in unstaedy state of flow. The analysis has been carried out in the marshy gley soil of the experimental field with drain spacing of 10 m, 20 m and 30 m. The method assumed modeled dynamics of water recharge, which hasn’t been observed during the experimental trial. Therefore, its application is limited.


Materials and Methods
Drain discharge and groundwater table depth were measured at the drainage experimental field Radmilovac, near Belgrade.The drainage treatments I, II and III were set up on the marshy-gley soil type, on the total area of 1.5 ha, with 10 m, 20 m, and 30 m, respectively.The average drain depth was 0.9 m.The mean value of hydraulic conductivity was 0.6 m⋅day -1 , and equivalent drain depth on the treatments I, II and III were d 1 =0.45m d 2 =1.06m d 3 =1.51m.Measured data have been analysed by the equation given by Guyon (Wesseling, ILRI III,IV) ) where: K -soil permeability (m/dan); d -equivalent drain depth (m); t -time of drainage (days); μ -drainage porosity (.); h 0 -maximal groundwater table depth; h t -minimal level of depresion curve at the end of depletion process (m) Using the Guyon's equation (1), it is not possible to calculate drainage spacing as an explicit function L(q,K,h), therefore iteration has been applied.

Results and Discussion
The results of the drain discharge and ground water table depth measurements from the three drainage treatments, selected by the previous analysis (D j u r o v i ć , 1999) for the unsteady state of flow, are shown in Table 1.Chosen data fulfilled the assumed criterion of method such as: groundwater table depth oscillated during the period free of rainfall.The Guyion's method analysis has been based on these measurements.
Estimation of drain spacing on the treatment I is moved toward a higher value (Fig. 1) obtaining the mean value of error even 12.06 m, median of error 10.19 m and standard deviation 11.73.It can be explained that method for drain spacing estimation in unsteady state conditions assumes that the only rainfall and irrigation recharges the groundwater.Regarding that marshy gley soil contents abound in the amount of groundwater, there are some other underground water sources which have been neither measured nor taken into consideration.
Guyion's method explains groundwater depletion by fictive, wider drain spacing.The obtained value of mod of error is zero, which can be related to the phenomenon of very small value of estimation error that appeared most frequently in the series of estimation (Fig. 2).These results are very similar to those obtained by Glover Dumm's method, which was expected, being basically very similar.Yet, Glover'Dumm's method shows better results manifested by lower values of mean and median of estimation error in all three drainage (D j u r o v i ć et al. 2000).Similar explanation could be given for the drain spacing estimation on the drainage treatment II (Fig. 3).The value of mean estimation error is 11 m, median 7.6 m mode 6.9 m and standard deviation 17.42 m.On the drainage treatment III (Fig. 5), the value of mean estimation error and median is 13.9 m, and standard deviation reaches the value of 26.09 m.Histogram of error (Fig. 6) does not show the concentration toward value zero.Water inflow can be considered uncontrolled.Regarding that soil contents abound in the amount of groundwater, there are some effects that the model does not assume.Depletion of groundwater table level in the interval free of rainfall is much slower than it should be without extra ground water inflow.These results are in concordance with those obtained in different regions.(M u r a s h i m a and O g i n o , 1991), where priority was given to the methods of steady flow water assumption.

C o n c l u s i o n
Applicable methods for drain spacing determination in unsteady state of flow assume only water inflow from the rainfall or irrigation.Regarding that marshygley soil contents abound in the amount of water, it means that there are some extra groundwater inflows besides rainfall or irrigation.Guyon's method explains water table depletion by fictive, wider drain spacing, but it is less precise compared with Glover'Dumm's, eventhough they are basically similar.This property of Guyon's method limits its application.The obtained results show weak drain spacing estimation, which leads to the conclusion that this method should not be used in unsteady state of flow in marshy-gley soil.R E F E R E N C E S .
T a b.1.-Measurements in unsteady state of flow No Date h 1 (m)