*2.1. Experimental Site*

The field experiments used here and previously described by Gharaibeh et al. [9] were conducted at Jordan University of Science & Technology (JUST) (32◦2757.4" N latitude, 35◦5754.4" E longitude), 70 km north of Amman, Jordan. The experiments were performed on three adjacent field plots (each of 0.8 ha) characterized by the same soil texture. The soil is classified as fine and mixed with clayey soil texture (clay 48%, silt 37%, and sand 15%) and thermic Typic Calcixerert characterized by 15% CaCO3 content. The three plots were subjected to three different TWW irrigation patterns. The first plot was not-irrigated (rain-fed) and is used here as the no-TWW benchmark. The second plot was irrigated with TWW for two years, while the third plot was irrigated with TWW for five years. The TWW used in the experiments was supplied from a wastewater treatment plant located in the JUST campus, which uses rotating biological contactors. More details about water characteristics and irrigation strategies can be found in Gharaibeh et al. [9]. In the following sections, these treatments are referred to as 0 YR, 2 YR, and 5 YR, respectively. The main chemical soil characteristics of the three plots are presented in Table 1.

**Table 1.** Selected chemical properties for the soil of the three plots: pH, electrical conductivity (EC), organic matter (OM), and cation-exchange capacity (CEC).


#### *2.2. Measurements of Hydraulic Properties*

The Hood infiltrometer (IL-2700, Umwelt-Gerate-Technik GmbH, Muncheberg, Germany) was used for in-situ infiltration measurements following Schwarzel and Punzel [10]. This device was chosen because it maintains the soil surface and pore system undisturbed. Furthermore, it allows us to perform infiltration measurements at di fferent water tensions. The measurements were realized in 2013 on an undisturbed soil surface. Each experiment was performed at least three days after the last rain or irrigation event. Five measurements (replicates) per each site (treatment) were conducted on randomly distributed locations. Infiltration tests were carried out by applying on the soil surface tensions ranging from 0 mm to the value of the soil bubbling point with steps of 20 mm. For each tension value, the infiltration rate was allowed to reach steady conditions for approximately 8 min beforethetensionlevelwaschangedtothenextlevel.

Following Gharaibeh et al. [9], steady-state infiltration can be described using Wooding model [11]:

$$\mathbf{q}\_{\mathfrak{s}}(\Psi) = \mathbf{K}(\Psi)(1 + \frac{4}{\pi \mathbf{r} \,\alpha}) \tag{1}$$

where qs(Ψ) is the steady infiltration rate at the fixed tension Ψ, K( Ψ) is the unsaturated hydraulic conductivity, r is the infiltrometer radius and α is the sorptive number. Substituting the exponential model of Gardner [12]:

$$\mathbf{K}(\Psi) = \mathbf{K}\_{\mathbf{s}} \mathbf{e}^{\alpha \Psi} \tag{2}$$

and applying the natural logarithm to both sides, Equation (1) becomes:

$$\ln\left[\mathbf{q}\_{\text{s}}(\Psi)\right] = \alpha \Psi + \ln[\mathbb{K}\_{\text{s}}(1 + \frac{4}{\pi \text{r}\alpha})] \tag{3}$$

where Ks is the saturated hydraulic conductivity. Equation (3) highlights a linear relationship between ln[qs(Ψ)] and Ψ with α representing the slope. Linear regression can be therefore used on experimental pairs of ln[qs(Ψ)] and Ψ for estimating α. The estimate of Ks can be then obtained by Equation (1) for Ψ = 0:

$$\mathbf{K}\_{\mathbf{s}} = \frac{\mathbf{q}\_{\mathbf{s}}(0)}{1 + \left(4/\pi \mathbf{r}\alpha\right)}\tag{4}$$

Figure 1 shows an example of results based on the experiments performed on the three plots and described by Gharaibeh et al. [9]. Specifically, Figure 1a shows the instantaneous infiltration curves obtained during the first replicate of the infiltration experiment in each of the three plots subjected to di fferent irrigation treatments (0YR, 2YR, and 5YR). Figure 1b represents cumulative infiltration curves averaged over the five replicates performed in each plot. As can be seen, the cumulative infiltration at the end of the 90 min experiments for the 0YR, 2YR, and 5YR treatments were 140 mm, 68 mm, and 59 mm, respectively, showing a significant reduction of soil infiltration capacity when TWW is applied. This reduction is likely due to the high load of the suspended solids present in the treated wastewater [9,13].

**Figure 1.** (**a**) Instantaneous infiltration rates associated with the first replicate; (**b**) mean cumulative infiltration curves obtained on the three plots with different earlier irrigation treatments (0YR, 2YR, and 5YR) [9].

α Through the procedure described in Section 2.2, Gharaibeh et al. [9] obtained the saturated hydraulic conductivity and sorptive number averaged over five replicates of infiltration measurements realized on each plot/treatment (Table 2). Based on these results Ks is not significantly different among the three considered treatments even though a slightly decreasing trend can be detected with increasing the number of TWW irrigation years. This result suggests that the usage of TWW in irrigation does not alter morphology and connectivity of the largest pores which mainly influences the saturated hydraulic conductivity. However, the estimate of the sorptive number shows a relevant difference in the three plots with values significantly increased for TWW irrigated sites where, as a consequence, the unsaturated hydraulic conductivity expressed through the Gardner [12] model had lower values (see Figure 2). Considering that the sorptive number parameter indicates the relative magnitudes of gravity and capillarity forces during unsaturated flow [14], this outcome suggests a significant reduction of fine pores, that drain water at suction levels < 0 cm, with respect to the total porosity. This evidence was justified by Gharaibeh et al. [9] also with the presence in TWW of both high loads of organic material and suspended solids that tend to settle in the finer soil pore spaces where the flow velocity is lower. Furthermore, the application for long periods of wastewater determined a reduction and disconnection of soil micro- and mesopores leading to a significant drop in hydraulic conductivity of unsaturated soils.


**Table 2.** Saturated hydraulic conductivity, Ks, and sorptive number, α, estimated through the procedure described in Section 2.2 and averaged on five replicates of the infiltration tests performed on each plot [9].

**Figure 2.** Unsaturated hydraulic conductivity, K, as a function of tension, Ψ, according to the Gardner (1958) model estimated for the three experimental plots with different irrigation treatments (0YR, 2YR, and 5YR).
