Measurements of infiltration were carried out in situ in July 2013. Infiltration runs were conducted at the upland and lowland landscape positions of each of the four transects in the ITE (n = 15 runs at each of eight locations) spanning four consecutive rain-free days. Transects thus served as blocks in which two sampling locations were established per transect. Before runs, ≤10 minidisk infiltrometers (METER Environment, Pullman, USA) were set up in a 2 × 2 m area near the center line of transects. Spatial clustering was necessary to minimize trampling of experimental plots and because infiltrometers were fitted with pressure transducers that fed data to a CR23X data logger (Campbell Scientific, Logan, USA). During runs, infiltrometers were held at each of five pressure potentials (−0.5, −1.5, −2.5, −3.5, and − 5.5 hPa) for approximately 30 min, while water was released into the soil (n = 600 infiltration measurements in total); water level was recorded at 3-s intervals. Infiltration rates were computed from smoothed data when steady state was achieved (77% of measurements after data cleaning). Data were smoothed using the Savitzky-Golay method with 3-min windows (n = 40 values per window). Infiltration rates were subsequently corrected to 20°C by accounting for temperature effects on the viscosity and density of water.

Soil cores were used for characterizing water retention. Two cores (8 cm diameter × 5 cm height) were collected from each of the eight sampling locations and returned to the laboratory intact. A HYPROP system (METER Environment, Pullman, USA) was used to measure volumetric water content (θv) at pressure potentials spanning −3 to −1000 hPa, while a WP4C system (METER Environment, Pullman, USA) was used for pressure potentials spanning −1000 to −10,000 hPa. Data were then fitted with smoothing splines such that θv could be determined at equivalent pressure potentials across the full measured range.

The maximum diameter of pores (d) holding water at a given pressure potential (h) was determined using the Young-Laplace equation (37), which can be expressed asd=4γ cos αρgh(1)where γ is the surface tension of water (0.0728 J m−2 at 20°C), α is the apparent contact angle between water and the pore (assumed to be zero), ρ is the density of water (1000 kg m−3), g is the gravitational constant (9.81 m s−2), and h is the pressure potential. This relationship assumes that pores are circular in cross section. Given values of h in hPa, equivalent diameters in micrometers were calculated from Eq. 1.

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