**1. Introduction**

A quantitative study of water movement in the vadose zone allows us to identify strategies for water conservation, flood/runo ff and erosion control, and the assessment of aquifer potential contamination due to migration of water-soluble chemicals [1]. Infiltration of irrigation water is one of the most critical processes for successful agricultural activities [2]. It is a key dynamic process to be considered for the design of irrigation systems and optimization, irrigation scheduling, and irrigation managemen<sup>t</sup> [3]. This process assumes much more importance in arid and semi-arid regions where, because of short periods and low amounts of rainfall, water is a scarce resource considered as a limiting factor for agricultural production. In these areas, the chronic water shortage has compelled the decision-makers to look for non-conventional water sources for irrigation. One of these is the treated wastewater (TWW) [4], which also gives the advantage of low cost if compared with other solutions such as seawater. In this context, a continuous monitoring of the TWW and soil parameters is required to guarantee a sufficient level of water and soil quality for efficient plant development. In comparison to freshwater, usually, TWW has a higher content of organic matter and nutrients that, particularly in arid soils, are required for plant growth. However, it contains some elements that can adversely affect soil and plant [5]. Major effects produced by TWW are the physical clogging by suspended solids and the bioclogging facilitated by dissolved organic matter [6,7]. The clogging process typically results in the reduction of soil porosity and potential hydraulic conductivity. Bedbabis et al. [6] reported a decrease in soil infiltration after 4 years of using treated wastewater in irrigation. Similar results were obtained by Alizadeh et al. [8] in Iran as a result of using treated wastewater for irrigation of a cornfield for 2 years where the infiltration rate decreased by 15.6%. Moreover, Tunc and Sahin [7] reported a decrease in soil infiltration after having used treated wastewater in irrigating different crops grown in loamy soil, as a result of decreasing macropores by the suspended materials in the TWW.

On these bases, it is clear that a recommended use of TWW as the irrigation water source requires complete knowledge of its long-term effects on both hydraulic characteristics and the quality level of agricultural soils. Recently, Gharaibeh et al. [9] investigated the long-term impacts of irrigation with TWW on the physicochemical properties of soil through multi-year field trials. Infiltration measurements on three plots subjected to different irrigation durations were involved. They found that irrigation with TWW for a few years affected soil physicochemical properties producing an increase of electrical conductivity and sodium adsorption ratio. On the other hand, a decrease in pH, infiltration and unsaturated hydraulic conductivity due to pore-clogging by surface deposition of suspended materials was also observed. Furthermore, a slight decrease in bulk density and observable increases in aggregation percentage due to a significant increase in the organic matter were also highlighted. However, in this study, the effects of physical and chemical modifications of soil on the soil capability of absorbing water and generating surface runoff were not quantitatively extrapolated.

The objective of this work is to integrate the analysis by Gharaibeh et al. [9] through a quantitative estimate of the expected effects on the infiltration and runoff production processes due to multi-year TWW irrigation. This issue represents an open challenge in the light of optimizing irrigation techniques in arid and semi-arid geographic zones. Field experiments previously conducted on three adjacent field plots characterized by the same clayey soil but subjected to three different irrigation treatments have been used to address this issue. The three irrigation conditions were: non-irrigated (natural conditions) plot, irrigated plot with TWW for two years, and irrigated plot with TWW for five years. Data of infiltration measurements, earlier performed by the Hood infiltrometer, have been used here to estimate the soil hydraulic properties. These quantities have enabled us to calibrate a well-known infiltration model under the ponded conditions representative of the irrigation stage. The adopted approach has provided insights that can be useful in irrigation system designing as well as in optimizing the use of TWW.

#### **2. Materials: Field Experiments**
