The infiltration capacity of soils is a key component of the hydrological cycle that can control the non-sustainable rates of runoff and erosion (Cerdà, 1997,1999). In this way, research focused on the soil hydrological properties will bring knowledge on how to control the high erosion rates (Cammeraat et al., 2010).
Saturated hydraulic conductivity, ks, is the most determining physical parameter in terms of quantifying the components of the global water balance as it interferes in all those processes which are related with water and solute movement and transport through the soil. ks values are required for an adequate modelling of the infiltration and runoff generation processes. However, it is a variable with high variability when it comes to agricultural soils due to different soil managements and the fact that the soil is not a continuous media (Polo et al., 2003).
Single ring infiltrometer
Small diameter rings can also be inserted to a depth of 0.03 m for the infiltrometer runs (Lassabatere et al., 2006). According to Bagarello et al. (2014a), a relatively small diameter of the ring can be chosen to detect more clearly potential effects of soil disturbance due to water. Several runs can be carried out by pouring water at a small height above soil surface, i.e. at a height, hw, of 0.03 m (low, L, runs). The energy is dissipated with the hand fingers, in an attempt to minimize soil disturbance due to water application, as commonly suggested (Reynolds, 1993). To ensure flow verticality and prevent
wind effects, a transparent pipe was used. A height of 0.6 m can be chosen in order to perform a run characterized by a gravitational potential energy, Ep, comparable to the Ek value of a simulated rainfall. Each water application generally contributes to alter the soil surface and the total energy of the applied water was found to be an appropriate predictor of the expected changes in Ks for another sandy-loam soil (Bagarello et al., 2014a). Also, models of transient soil surface sealing and infiltration establish that, for a given soil, the final saturated conductivity depends on the cumulative energy of the applied water (e.g., Brakensiek and Rawls, 1983; Shainberg and Singer, 1988; Mualem et al., 1990; King and Bjorneberg, 2012).
Collision of raindrops on a bare soil surface results in mechanical changes of the exposed soil, expressed in terms of compaction, particle detachment, and splash. Therefore, an H run characterized by a gravitational potential energy equal to the kinetic energy of the simulated rainfall can be expected to be the best choice for a comparison between different methods. For a given water volume, the time needed for the water to infiltrate is logged, and the mean cumulative infiltration time of the applied volumes can be calculated for both the L, Δt L (s), and the H, Δt H (s), runs. These calculations can be made since, for a given amount of applied water, run duration is inversely related to infiltration rate (Alagna et al., 2016a).
The pressure head on the porous disk of the device, with a diameter of 45 mm, can be set at –5 mm and no more than 2 mm of contact material. At each sampling point, the infiltration process continues until the reservoir of the device, which has a capacity of 135 mL, emptied. The final soil water content can be determined by using a knife to collect a small amount of the wetted soil under the disk a few seconds after the end of the experiment. The recommended method by Decagon Device Inc. (2014) can be used to calculate the soil hydraulic conductivity corresponding to a pressure head of –5 mm.
The source is small, which implies the possibility of performing the mini-disc run exactly on the area previously subjected to the ponding infiltration process. The run is expected not to alter, or alter only minimally, the infiltration surface (Alagna et al., 2016b). This means that the technique should be appropriate to reveal intrinsic differences in hydrodynamic parameters of different sampling points. The effect of almost all pores on flow of water is taken into account since a high-pressure head (close to zero) is established on the porous disk. In other terms, the measured conductivity with this device
can be considered very close to Ks (Alagna et al., 2016b). Finally, the MDI appears usable for estimating infiltration rates of soil crusts (Li et al., 2005); which can be of interest when the soil surface crusts.