Impact of initial water content on infiltration in soils Jorf irrigation district (Errachidia province)
The objective of this work is the study of the behavior of infiltration in two soil samples taken from the two earthen canals Lgdima and Hanabou forming part of the irrigation network of the Jorf irrigated area located in the Province of Errachidia. The infiltration tests were carried out in soil columns subjected to the same water supply regime with a constant water head, for clay fractions added to the samples of 10%, 15%, 20% and 25% and for initial water contents of 0%, 5% and 10%. During all the tests, the time and the position of the wetting front advance in the soil columns were recorded. This made it possible to plot the variation curves of the infiltration rate as a function of time, parameterized as the initial water content for different clay fractions made to the original samples. In all the figures, the variation of the rate of infiltration I (t) as a function of time (t), is better translated by a law of type power expressed by the relation: I (t) = a.t-ß. The coefficients a and ß reflect the effect resulting from the soil texture and the state of its initial water content on the behavior of the infiltration. The different curves obtained showed that the time taken by the wetting front to cross the soil column increases with the increase of the clay fraction for a fixed initial water content, and with the increase in the initial water content for a given fraction of clay. These curves also reveal that the increase in the initial water content is generally followed by a reduction of the infiltration rate which reachs a constant value when the time is prolonged. This reduction is much more pronounced at the beginning of the process. The interpretation of the curves also revealed that the wetting front progresses more rapidly at the beginning of the infiltration process in the sample subjected to a low initial water content than in that which is found in a wetter state. This allows us to say that the reduction of soil water content could result in a significant increase in the amount of water infiltrated especially during the initial phase of infiltration. It is therefore concluded that infiltration losses in an earthen canal can be controlled through the control, in addition to other factors, of two parameters, namely the clay content that makes up the soil of this channel and the initial state of its water content. Optimal values for these parameters should be determined in order to avoid adverse effects, which may be caused by excessive water content and clay content, ie swelling and shrinkage of clays, resulting in excessive seepage losses. We can also conclude that, in practice, the water management mode of an earthen irrigation channel has an impact on its water losses, especially when it is each time dried after a fairly long filling frequency.
Key words: Infiltration, initial water content, clay fraction, wetting front, infiltration losses, swelling and shrinkage