238
Nuria Melia, Juan BelIot and V. Ramon Vallejo
17.2 Gravitational Water
Gravitational water percolating through the soil was collected at the sites and
by the methods described in Section 16.5. Briefly, zero-tension lysimeters
were used at Prades and throughflow collectors at Montseny. Detailed descriptions of these studies can be found in Gazquez (1995) and Poch (1992)
for Prades and in Avila et al. (1995) for Montseny.
At both sites much higher water volumes were collected at the deepest
sampled depth than under the forest floor, revealing the importance of deep
subsurface flow in these highly permeable soils. At Prades, the mean water
volume per sample was 25 L m- 2 below the forest floor, 16 L m- 2 at 10-cm
depth, and 50 L m- 2 at 30-cm depth, although the coefficient of variation was
higher than 100% at all depths. Similarly, at Montseny, the mean water volume per sample was 0.8 L below the forest floor and 30 L at 160 cm (Avila
et al. 1995); an intermediate sampler at 50 cm never collected water. This
suggests that the thro\lghflow trench collected basically the horizontal water
flow at the soil-bedrock contact, as opposed to the vertical flow collected
with the zero-tension lysimeters at Prades.
17.2.1 General Characteristics and Variations Within the Soil Profile
Gravitational water in both holm oak forests showed chemical changes along
the soil prome, as also found in other studies (Ranger et al. 1993; Tokuchi
et al. 1993; Menendez et al. 1995). In general, water collected below the forest
floor presented higher volume-weighted ion concentrations than at lower
depths, although the extent of the changes with depth varied according to the
site and the ion being considered (Fig. 17.1). At Prades, the soil water pH was
nearly neutral below the forest floor, with a volume-weighted mean of 6.63,
which differed significantly from the lower, slightly acidic values found in the
mineral layers (5.96 at 10 cm and 6.04 at 30 cm). Lower values were observed
at Montseny, where the mildly acidic mean pH of 6.28 below the forest floor
decreased in the deep subsurface flow to a pH of 5.53 (Avila et al. 1995).
Calcium was the dominant cation at all depths at both sites, representing
about 50% of the total cationic charge in soil water collected below the forest
floor (Fig. 17.1). Among the strong anions, dominance alternated between
CI- and sol- at Prades, whereas sol- dominated at Montseny. However,
weak anions determined as alkalinity made the largest contribution to anion
charge in throughflow below the forest floor at Montseny (Fig. 17.1 b), which
had a volume-weighted mean alkalinity of 481 j.1eq L- 1 (29% of the total
cation charge). This high alkalinity was provided mainly by dissolved organic anions. There was a substantial charge imbalance (Fig. 17.1 b), resulting
from an anion deficit of 20% of the total cation charge. This deficit arose
probably (1) from organic anions that were not weak enough to protonate
during the alkalinity titration, and (2) from part of the cations that could be
Nuria Melia, Juan BelIot and V. Ramon Vallejo
17.2 Gravitational Water
Gravitational water percolating through the soil was collected at the sites and
by the methods described in Section 16.5. Briefly, zero-tension lysimeters
were used at Prades and throughflow collectors at Montseny. Detailed descriptions of these studies can be found in Gazquez (1995) and Poch (1992)
for Prades and in Avila et al. (1995) for Montseny.
At both sites much higher water volumes were collected at the deepest
sampled depth than under the forest floor, revealing the importance of deep
subsurface flow in these highly permeable soils. At Prades, the mean water
volume per sample was 25 L m- 2 below the forest floor, 16 L m- 2 at 10-cm
depth, and 50 L m- 2 at 30-cm depth, although the coefficient of variation was
higher than 100% at all depths. Similarly, at Montseny, the mean water volume per sample was 0.8 L below the forest floor and 30 L at 160 cm (Avila
et al. 1995); an intermediate sampler at 50 cm never collected water. This
suggests that the thro\lghflow trench collected basically the horizontal water
flow at the soil-bedrock contact, as opposed to the vertical flow collected
with the zero-tension lysimeters at Prades.
17.2.1 General Characteristics and Variations Within the Soil Profile
Gravitational water in both holm oak forests showed chemical changes along
the soil prome, as also found in other studies (Ranger et al. 1993; Tokuchi
et al. 1993; Menendez et al. 1995). In general, water collected below the forest
floor presented higher volume-weighted ion concentrations than at lower
depths, although the extent of the changes with depth varied according to the
site and the ion being considered (Fig. 17.1). At Prades, the soil water pH was
nearly neutral below the forest floor, with a volume-weighted mean of 6.63,
which differed significantly from the lower, slightly acidic values found in the
mineral layers (5.96 at 10 cm and 6.04 at 30 cm). Lower values were observed
at Montseny, where the mildly acidic mean pH of 6.28 below the forest floor
decreased in the deep subsurface flow to a pH of 5.53 (Avila et al. 1995).
Calcium was the dominant cation at all depths at both sites, representing
about 50% of the total cationic charge in soil water collected below the forest
floor (Fig. 17.1). Among the strong anions, dominance alternated between
CI- and sol- at Prades, whereas sol- dominated at Montseny. However,
weak anions determined as alkalinity made the largest contribution to anion
charge in throughflow below the forest floor at Montseny (Fig. 17.1 b), which
had a volume-weighted mean alkalinity of 481 j.1eq L- 1 (29% of the total
cation charge). This high alkalinity was provided mainly by dissolved organic anions. There was a substantial charge imbalance (Fig. 17.1 b), resulting
from an anion deficit of 20% of the total cation charge. This deficit arose
probably (1) from organic anions that were not weak enough to protonate
during the alkalinity titration, and (2) from part of the cations that could be