**4. Functional assessment of some ground surface parameters**

#### **4.1.** *FIRmax* **in Japanese cedar and Hiba arborvitae plantation forests**

**Table 1** shows the measurement results of surface cover materials, hydraulic conductivity, dry bulk density and *FIRmax*. In all the cases during the experiment, rainfall intensity is in excess of final infiltration rate ('Rainfall intensity' and '*FIR'* in **Table 1**), which suggests that overland flow may occur in Japanese cedar and Hiba arborvitae forests during the intense rainfall events with around 180 mm/h.

**Cover**

> **Type**

**Plot ID**

**Elapsed time** 

**Ground** 

**Dry weight** 

**Dry weight** 

**Dry weight of** 

**Fine** 

**Bulk density** 

**Hydraulic** 

**conductivity** 

**(mm h−1)**

**total cover** 

**fraction** 

**(g cm−3)**

**content** 

**(%)**

**0–5 cm**

**5–10 cm 0–5 cm**

**5–10 cm**

**after thinning** 

**cover** 

**of understory** 

**of litter** 

**ratio (%)**

**vegetation** 

**materials (g** 

**(g m−2)**

**(g m−2)**

**m−2)**

**(year)**

Cedar C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22

1

100

160

950

1110

23

0.38

0.49

680

505

188

142

175

173

Un-thinned

100

0

1150

1150

30

0.74

0.84

210

296

206

196

508

1

80

50

420

470

4

0.60

0.78

1370

330

179

173

538

Un-thinned

90

0

880

880

32

0.77

0.81

1

2

192

169

301

1

90

0

360

360

20

0.59

0.69

560

717

206

187

373

http://dx.doi.org/10.5772/intechopen.70575

2

90

80

1670

1750

26

0.85

0.995

100

155

177

166

389

3

80

110

1780

1890

32

0.49

0.73

740

673

189

156

238

Un-thinned

60

290

680

970

21

0.83

0.90

340

330

153

114

146

1

100

0

250

250

14

0.44

0.58

1120

169

149

137

282

Over 5

40

180

1160

1340

27

0.55

0.57

1110

1651

180

174

537

Un-thinned

30

10

460

470

4

0.85

0.95

1300

1799

183

173

324

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils...

1

70

370

540

910

34

0.81

0.79

840

1218

194

183

488

1

80

20

630

650

38

0.56

0.64

1020

1195

170

162

449

Un-thinned

100

190

1710

1900

22

0.80

0.83

700

697

178

171

317

Over 5

90

290

1200

1490

12

0.86

0.81

1170

1239

169

146

227

Un-thinned

80

70

320

390

33

0.80

0.79

810

565

190

128

142

1

70

200

580

780

19

0.77

0.89

710

584

178

158

270

3

70

270

1270

1540

30

0.66

0.95

430

202

179

152

209

2

90

190

1170

1360

19

0.60

0.75

1650

593

186

180

562

Over 5

70

70

500

570

26

0.47

0.58

1120

620

186

171

359

Un-thinned

80

30

490

520

8

0.72

0.78

1060

429

170

141

212

1

100

70

220

290

18

0.51

0.69

1420

559

171

147

241

**Soil**

**Experiment**

**Rainfall** 

*FIR*

*FIRmax*

**intensity** 

**(mm h−1)**

**(mm h−1)**

**(mm h−1)**

*FIRmax* ('*FIRmax'* in **Table 1**) was obtained by applying the Eq. (2) to the rainfall intensity and final infiltration rate obtained in this experiment. *FIRmax*, as shown in **Table 1**, distributes in


re-dried in an oven at 70°C for 48 h to determine the dry weight. Photos were taken directly above the plot, and the floor cover percentage was estimated by calculating the percentage of

Generally, soil properties not only change for each type of soil, vary depending on the location in the same type of soil. It is desirable to make laboratory test (e.g. pF test, permeability

We collected soil samples after the experiments to estimate soil properties. To investigate soil properties affecting final infiltration rate, particle size, hydraulic conductivity and bulk density were estimated. Sampling and test were conducted by the following methods. After collecting surface cover materials, collection of undistributed sample soils was made using

The reason for examining the physical properties of the surface layer (up to 10 cm from the surface) was because the surface layer was found to be a major influencing factor on infiltration rate. Undistributed soil samples were taken to a depth of 0–5 and 5–10 cm using 100 cc

lected at each layer to overcome the difficulties caused by inhomogeneity of soil properties.

Saturated hydraulic conductivity was measured by a permeability test after capillary rise for over 48 h. Determination of permeability was carried out using a constant head permeability test, but a falling head permeability test was used for the lower permeability materials. Then, to find the dry bulk density, the soil was oven dried at 105°C for 24 h and the weight of oven-

Particle size distribution was determined by means of the sieving method and by using a particle size analyzer (SALD-3100; Shimadzu Corp., Kyoto, Japan) for fine fractions. We observed the content of particles finer than 0.063 mm, especially clay and silt fractions, in this

**Table 1** shows the measurement results of surface cover materials, hydraulic conductivity, dry bulk density and *FIRmax*. In all the cases during the experiment, rainfall intensity is in excess of final infiltration rate ('Rainfall intensity' and '*FIR'* in **Table 1**), which suggests that overland flow may occur in Japanese cedar and Hiba arborvitae forests during the intense

*FIRmax* ('*FIRmax'* in **Table 1**) was obtained by applying the Eq. (2) to the rainfall intensity and final infiltration rate obtained in this experiment. *FIRmax*, as shown in **Table 1**, distributes in

The average size of these three samples was taken to be the representative value.

**4. Functional assessment of some ground surface parameters**

**4.1.** *FIRmax* **in Japanese cedar and Hiba arborvitae plantation forests**

and 4 cm in height) to measure particle size.

and 5.1 cm in height). Three samples were col-

forest floor that is covered with either litter or understories based on image analysis.

test) using a small sample of soil *in situ* to obtain the characteristic value.

**3.5. Measurement of soil properties**

172 Hydrology of Artificial and Controlled Experiments

400 cc core sampler (cross section area of 100 cm<sup>2</sup>

core sampler (cross section area of 19.6 cm<sup>2</sup>

dried soil was measured.

rainfall events with around 180 mm/h.

experiment.

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils... http://dx.doi.org/10.5772/intechopen.70575 173


**Table 1.** Result of *in situ* artificial rainfall experiments using an oscillating nozzle rainfall simulator. a range from 141.9 to 562.3 mm/h in the Japanese cedar forest and from 93.3 to 641.0 mm/h in the Hiba arborvitae forests. **Figure 3** shows frequency distribution of *FIRmax* for every 100 mm. A peak is observed in the *FIRmax* distribution in Hiba arborvitae forests around 200–300 mm/h, but tends to distribute equally in the range of 100–700 mm/h in both forests.

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils...

http://dx.doi.org/10.5772/intechopen.70575

It is well-known that infiltration rate is affected by the amount of surface cover materials [

4, 17]. Also reported is an increasing trend of surface cover materials and understories with increasing time elapsed after thinning [40]. However, there was apparent variability among data, and no correlation was found between the elapsed year after thinning and the amount of surface cover materials for both types of forest. In fact, we could not clarify the correlation, because of only a few numbers of data for surface cover materials corresponding to elapsed year after thinning. We also looked at the separate correlation of understories and litter mate

rials with elapsed year, but no correlation was found among these three groups. The effect of thinning on surface cover materials was also examined in both types of forest. We compared the effects on volume of surface cover materials between un-thinned plots and post-thinned sites with a variety of elapsed years. As a result, there was no noticeable difference in vol

ume of surface cover materials between un-thinned and post-thinned sites in Japanese cedar forests, while a marked decrease in cover materials was observed in post-thinned plots in Hiba arborvitae forests. **Figure 4** shows the box-and-whisker plots of *FIRmax* in un-thinned and post-thinned sites. Despite a decrease in surface cover materials due to thinning, *FIRmax* increases in both types of forest. This result challenges the widely accepted notion that there is correlation between surface cover materials and *FIRmax* in Japanese cedar and Hiba arbor

Previous studies have shown that the effect of understories and leaf litters on infiltration rate is significant, which suggests that surface cover materials reduce raindrop impact on sur

vitae forests, and no correlation was observed especially in Hiba arborvitae forests.

face soil, and therefore the formation of surface crusts and HOF is restricted (e.g. [

**Figure 3.** Frequency distribution of *FIRmax*. (A) Japanese cedar and (B) Hiba arborvitae.

1,

175





9, 12, 15]).

Low *FIRmax* (<100 mm/h) was observed at only one site in Hiba arborvitae forests.

a range from 141.9 to 562.3 mm/h in the Japanese cedar forest and from 93.3 to 641.0 mm/h in the Hiba arborvitae forests. **Figure 3** shows frequency distribution of *FIRmax* for every 100 mm. A peak is observed in the *FIRmax* distribution in Hiba arborvitae forests around 200–300 mm/h, but tends to distribute equally in the range of 100–700 mm/h in both forests. Low *FIRmax* (<100 mm/h) was observed at only one site in Hiba arborvitae forests.

It is well-known that infiltration rate is affected by the amount of surface cover materials [1, 4, 17]. Also reported is an increasing trend of surface cover materials and understories with increasing time elapsed after thinning [40]. However, there was apparent variability among data, and no correlation was found between the elapsed year after thinning and the amount of surface cover materials for both types of forest. In fact, we could not clarify the correlation, because of only a few numbers of data for surface cover materials corresponding to elapsed year after thinning. We also looked at the separate correlation of understories and litter materials with elapsed year, but no correlation was found among these three groups. The effect of thinning on surface cover materials was also examined in both types of forest. We compared the effects on volume of surface cover materials between un-thinned plots and post-thinned sites with a variety of elapsed years. As a result, there was no noticeable difference in volume of surface cover materials between un-thinned and post-thinned sites in Japanese cedar forests, while a marked decrease in cover materials was observed in post-thinned plots in Hiba arborvitae forests. **Figure 4** shows the box-and-whisker plots of *FIRmax* in un-thinned and post-thinned sites. Despite a decrease in surface cover materials due to thinning, *FIRmax* increases in both types of forest. This result challenges the widely accepted notion that there is correlation between surface cover materials and *FIRmax* in Japanese cedar and Hiba arborvitae forests, and no correlation was observed especially in Hiba arborvitae forests.

Previous studies have shown that the effect of understories and leaf litters on infiltration rate is significant, which suggests that surface cover materials reduce raindrop impact on surface soil, and therefore the formation of surface crusts and HOF is restricted (e.g. [9, 12, 15]).

**Figure 3.** Frequency distribution of *FIRmax*. (A) Japanese cedar and (B) Hiba arborvitae.

**Cover**

> **Type**

**Plot ID**

**Elapsed time** 

**Ground** 

**Dry weight** 

**Dry weight** 

**Dry weight of** 

**Fine** 

**Bulk density** 

**Hydraulic** 

**conductivity** 

**(mm h−1)**

**total cover** 

**fraction** 

**(g cm−3)**

**content** 

**(%)**

**0–5 cm**

**5–10 cm 0–5 cm**

**5–10 cm**

**after thinning** 

**cover** 

**of understory** 

**of litter** 

**ratio (%)**

**vegetation** 

**materials (g** 

**(g m−2)**

**(g m−2)**

**m−2)**

**(year)**

Hiba

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16

> **Table 1.**

2

90

10

580 Result of *in situ* artificial rainfall experiments using an oscillating nozzle rainfall simulator.

590

23

0.42

0.53

740

818

177

151

225

1

100

0

1180

1180

17

0.55

0.74

1290

485

187

178

411

Un-thinned

60

310

470

780

25

0.72

0.91

1030

312

178

135

149

1

70

0

1200

1200

24

0.37

0.45

2170

625

146

138

347

Un-thinned

70

400

1910

2310

20

0.67

0.84

1840

284

177

156

194

3

80

40

450

490

34

0.51

0.58

1050

684

183

163

270

Over 5

100

70

1240

1310

28

0.54

0.85

770

533

192

188

641

Un-thinned

80

70

1570

1640

20

0.40

0.45

950

944

193

171

278

1

100

0

420

420

36

0.50

0.73

1290

403

192

187

623

Un-thinned

100

900

800

1700

30

0.44

0.67

530

310

166

128

150

Over 5

90

50

930

980

23

0.53

0.81

610

192

176

151

242

1

90

0

570

570

23

0.56

0.90

460

489

179

146

217

1

80

10

860

870

26

0.50

0.59

1630

782

177

165

372

2

60

0

470

470

19

0.51

0.69

1140

674

193

153

214

Un-thinned

60

10

1020

1030

34

0.42

0.61

1360

858

185

90

93

3

100

60

580

640

18

0.54

0.71

1340

581

230

164

204

174 Hydrology of Artificial and Controlled Experiments

**Soil**

**Experiment**

**Rainfall** 

*FIR*

*FIRmax*

**intensity** 

**(mm h−1)**

**(mm h−1)**

**(mm h−1)**

**Figure 4.** Box-and-whisker plots of *FIRmax* in un-thinned and post-thinned sites. (A) is Japanese cedar and (B) is Hiba arborvitae. The box represents 50% of the data between the 25th and the 75th percentile, the line represents the median and whiskers the minimum and maximum.

Previous study pointed out that relationship surface cover ratio and final infiltration have a positive correlation. **Figure 5** shows the relationship surface cover ratio and final infiltration, but in my case, there is no correlation. Some researchers adapt a linear approximation to the relationship between the two sides. Although statistical analysis was carried out and significance was observed, despite the low correlation coefficient, there was a close relationship between the two, and by using this relationship. It is expected that it will lead to appropriate management guidelines considering maintenance functions. I do not think about denying statistical analysis, but it is unknown whether it is easy to reach such consideration easily.

Surface cover materials were measured using image analysis. A high concentration (>60%) of cover materials was seen both in Japanese cedar and in Hiba arborvitae forests, but no correlation was observed between *FIRmax* and surface cover materials. **Figure 6** shows relation between surface cover materials and *FIRmax* in Japanese cedar and Hiba arborvitae forests. For comparison purposes, also included in the figure are the data of *C. obtusa* plantations [9]. **Figure 6** clearly shows that *FIRmax* in Japanese cedar and Hiba arborvitae forests is generally higher than that in *C. obtusa* plantations. Japanese cedar and Hiba arborvitae forests, especially at sites with a low concentration of surface cover materials (<1000 g/m<sup>2</sup> ), had a twofold to threefold greater *FIRmax* than that in *C. obtusa* plantations. This means that Japanese cedar and Hiba arborvitae forests may limit the occurrence of overland flow and soil erosion compared to *C. obtusa* plantations. Thus, Japanese cedar and Hiba arborvitae forests may provide a more effective protection of soil surface. The results of our experiment support previous studies and conclusions presented by Ogura and Takahashi [46] and Ogura and Kodani [18]. The regression line in **Figure 8** might show a positive correlation between the amount of surface cover materials and *FIRmax*. A similar finding was also reported by Kato et al. [8]. As shown in **Figure 6** (Japanese cedar: *r* = 0.173, *p* = 0.443; Hiba arborvitae: *r* = 0.024, *p* = 0.929), however, no such correlation was recognized between the amount of surface cover materials and *FIRmax*. Both Japanese cedar and Hiba arborvitae forests gave a relatively high *FIRmax* values (>100 mm/h) for any amount of surface cover materials. We also took the separate correlation of understories and litter materials with *FIRmax*, but no correlation was found among these three groups.

**4.2. Surface cover materials in Japanese cedar and Hiba arborvitae plantation forests**

) induced the lowering of the frequency.

**4.3. Soil properties in Japanese cedar and Hiba arborvitae plantation forests**

cover materials per area of 500 m<sup>2</sup>

frequency of 500–1000 g/m<sup>2</sup>

1500–2000 g/m<sup>2</sup>

from Kato et al. [12]).

Measurement of surface cover materials ('Understory vegetation' and 'Litter materials' in **Table 1**) shows that understories comprised approximately 10% of surface cover materials and the remaining 90% of litter materials, which signifies that litter materials are a major component in Japanese cedar and Hiba arborvitae forests. The frequency distribution of surface

**Figure 5.** Relationships between the total surface cover and the infiltration rate at the (A) Japanese cedar and the (B) Hiba arborvitae sites (the solid square and open triangle indicate the desert grassland site and shrubland site, respectively. Steppe grassland (*y* = 0.92*x* + 5.74, *r*<sup>2</sup> = 0.59, *p* = 0.029) and desert grassland (*y* = −7.67*x* + 220.4, *r*<sup>2</sup> = 0.98, *p* = 0.042) In Mongolia, results by Gutierrez and Hernandez [41] (*y* = 0.96*x* + 25.9), Loch [11] (*y* = 0.96*x* + 17.0), Loch [42] (*y* = 0.68*x* + 7.74), Kato et al. [12], Frot and van Wesemael [43], Gao et al. [44], Hiraoka et al. [15] and Li et al. [45]. Modified

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils...

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177

A high degree of variability in hydraulic conductivity was found, but high values (100– 2000 mm/h) were obtained in the 0–5 cm and 5–10 cm layers in both types of forest (except for

(no figure is presented) shows that the highest peak is at the

,

in both types of forest, and an increase in weight (1000–1500 g/m<sup>2</sup>

Previous study pointed out that relationship surface cover ratio and final infiltration have a positive correlation. **Figure 5** shows the relationship surface cover ratio and final infiltration, but in my case, there is no correlation. Some researchers adapt a linear approximation to the relationship between the two sides. Although statistical analysis was carried out and significance was observed, despite the low correlation coefficient, there was a close relationship between the two, and by using this relationship. It is expected that it will lead to appropriate management guidelines considering maintenance functions. I do not think about denying statistical analysis, but it is unknown whether it is easy to reach such consideration easily.

**Figure 4.** Box-and-whisker plots of *FIRmax* in un-thinned and post-thinned sites. (A) is Japanese cedar and (B) is Hiba arborvitae. The box represents 50% of the data between the 25th and the 75th percentile, the line represents the median

Surface cover materials were measured using image analysis. A high concentration (>60%) of cover materials was seen both in Japanese cedar and in Hiba arborvitae forests, but no correlation was observed between *FIRmax* and surface cover materials. **Figure 6** shows relation between surface cover materials and *FIRmax* in Japanese cedar and Hiba arborvitae forests. For comparison purposes, also included in the figure are the data of *C. obtusa* plantations [9]. **Figure 6** clearly shows that *FIRmax* in Japanese cedar and Hiba arborvitae forests is generally higher than that in *C. obtusa* plantations. Japanese cedar and Hiba arborvitae forests, espe-

to threefold greater *FIRmax* than that in *C. obtusa* plantations. This means that Japanese cedar and Hiba arborvitae forests may limit the occurrence of overland flow and soil erosion compared to *C. obtusa* plantations. Thus, Japanese cedar and Hiba arborvitae forests may provide a more effective protection of soil surface. The results of our experiment support previous studies and conclusions presented by Ogura and Takahashi [46] and Ogura and Kodani [18]. The regression line in **Figure 8** might show a positive correlation between the amount of surface cover materials and *FIRmax*. A similar finding was also reported by Kato et al. [8]. As shown in **Figure 6** (Japanese cedar: *r* = 0.173, *p* = 0.443; Hiba arborvitae: *r* = 0.024, *p* = 0.929), however, no such correlation was recognized between the amount of surface cover materials and *FIRmax*. Both Japanese cedar and Hiba arborvitae forests gave a relatively high *FIRmax* values (>100 mm/h) for any amount of surface cover materials. We also took the separate correlation of understories and litter materials with *FIRmax*, but no correlation was found among

), had a twofold

cially at sites with a low concentration of surface cover materials (<1000 g/m<sup>2</sup>

these three groups.

and whiskers the minimum and maximum.

176 Hydrology of Artificial and Controlled Experiments

**Figure 5.** Relationships between the total surface cover and the infiltration rate at the (A) Japanese cedar and the (B) Hiba arborvitae sites (the solid square and open triangle indicate the desert grassland site and shrubland site, respectively. Steppe grassland (*y* = 0.92*x* + 5.74, *r*<sup>2</sup> = 0.59, *p* = 0.029) and desert grassland (*y* = −7.67*x* + 220.4, *r*<sup>2</sup> = 0.98, *p* = 0.042) In Mongolia, results by Gutierrez and Hernandez [41] (*y* = 0.96*x* + 25.9), Loch [11] (*y* = 0.96*x* + 17.0), Loch [42] (*y* = 0.68*x* + 7.74), Kato et al. [12], Frot and van Wesemael [43], Gao et al. [44], Hiraoka et al. [15] and Li et al. [45]. Modified from Kato et al. [12]).

#### **4.2. Surface cover materials in Japanese cedar and Hiba arborvitae plantation forests**

Measurement of surface cover materials ('Understory vegetation' and 'Litter materials' in **Table 1**) shows that understories comprised approximately 10% of surface cover materials and the remaining 90% of litter materials, which signifies that litter materials are a major component in Japanese cedar and Hiba arborvitae forests. The frequency distribution of surface cover materials per area of 500 m<sup>2</sup> (no figure is presented) shows that the highest peak is at the frequency of 500–1000 g/m<sup>2</sup> in both types of forest, and an increase in weight (1000–1500 g/m<sup>2</sup> , 1500–2000 g/m<sup>2</sup> ) induced the lowering of the frequency.

#### **4.3. Soil properties in Japanese cedar and Hiba arborvitae plantation forests**

A high degree of variability in hydraulic conductivity was found, but high values (100– 2000 mm/h) were obtained in the 0–5 cm and 5–10 cm layers in both types of forest (except for

[47] reported that crust was formed when the fine fraction content in the sand pyroclastic flow deposits exceeded 35%. A soil profile at our experimental sites appears to be a brown forest soil. Therefore, the use of a theory developed by Yokoi et al. [47] will be constrained. If the theory is to be applicable, it seems difficult to form crust in Japanese cedar and Hiba arborvitae forests as a whole because the fine fraction content does not generally exceed 35% in these types of forest. In fact, no crust could be visually observed after the experiment, and from the final infiltration rate obtained from the experiment, it also seemed unlikely that the formation of crust occurred. **Figure 7** is the relation between fine fraction content and *FIRmax*, but we could not find significant correlation (Japanese cedar: *r* = 0.033, *p* = 0.883; Hiba arborvitae: *r* = 0.013, *p* = 0.154). **Figure 8** shows the relation between fine fraction content and hydraulic

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils...

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179

**Figure 9** shows relation between hydraulic conductivity and *FIRmax* in the 0–5 cm surface layer, which revealed no correlation in either type of forests (Japanese cedar: *r* = 0.244, *p* = 0.274; Hiba arborvitae: *r* = 0.101, *p* = 0.710). Similarly, no correlation was detected between hydraulic conductivity and *FIRmax* for soil at 5–10 cm depth. It also became clear that *FIRmax* is definitely lower than hydraulic conductivity at most sites in Japanese cedar forest (19 out of 22 sites) and all sites in Hiba arborvitae forest (16 sites). It is postulated that the hydraulic gradient in the surface soil layer was estimated to be about one and that *FIRmax* had the similar value to hydraulic conductivity, which needs careful consideration. The 2–3 cm layer within 1 h after the sprinkling experiment was moistened, but the dry soil underlay in the deeper soil layers. The presence of macropores in soils might lead to the preferentially infiltration of sprinkled water. With fingering infiltration (unstable infiltration), the preferential water was infiltrated into the soils that are exceptionally conductive. The effect of entrapped air, hyphae respiration and bacteria in unsaturated soil also needs to be examined in future studies. There may be an underlying mechanism to limit infiltration of water into soil profile, which caused

conductivity, in which no correlation could be established.

lower *FIRmax* compared to the permeability test for fully saturated soil.

**Figure 7.** Relation between fine fraction content and *FIRmax*. (A) is Japanese cedar and (B) is Hiba arborvitae.

**Figure 6.** Relation between ground cover materials and *FIRmax*. (A) is Japanese cedar and (B) is Hiba arborvitae and results by Hiraoka et al. [9] (*FIRmax* = 0.14 × (dry weight of total cover) − 15.65).

C19). The same trend was commonly seen in other forests as reported in previous studies [3, 21]. Hiba arborvitae forests had relatively higher hydraulic conductivity than Japanese cedar forests. It was also found that hydraulic conductivity measured at depth of 5–10 cm was comparatively lower than those at depth of 0–5 cm depth.

Dry bulk density ('bulk density' in **Table 1**) was as follows: 0.38–0.86 g/cm<sup>3</sup> in Japanese cedar forests and 0.37–0.72 g/cm<sup>3</sup> in Hiba arborvitae forests at depth of 0–5 cm; 0.49–0.995 g/cm<sup>3</sup> in Japanese cedar forests and 0.49–0.91 g/cm<sup>3</sup> in Hiba arborvitae forests at depth of 5–10 cm. Although there was considerable variability, we found that bulk density increased with depth and was higher in Hiba arborvitae forests compared to Japanese cedar forests. According to Miyata et al. [3], the average bulk density was 0.75 g/cm3 in Hiba arborvitae forests, which was slightly lower than those obtained in our experiment (Japanese cedar forests: 0.67 g/cm3; Hiba arborvitae forests: 0.51 g/cm3). It was presumed that a decrease of porosity induced by an increasing bulk density results in a decrease of hydraulic conductivity with increasing depth. We, however, could not find significant correlation between hydraulic conductivity and bulk density. Therefore, there is another unknown factor for the decrease of hydraulic conductivity.

Fine fraction content ('fine fraction content' in **Table 1**) was 4–38% (average 22%) in Japanese cedar forests and 17–36% (average 25%) in Hiba arborvitae forests, but most of the data for both types of forest were below 35% except maximum values. The maximum values for these forests do not vary significantly as much as the minimum values. The minimum value was 4% for Japanese cedar forests and 17% for Hiba arborvitae forests, respectively. The result obtained in Japanese cedar forests exhibits a larger variability compared to Hiba arborvitae forests.

The high fine fraction (clay + silt) content may increase the effect of clogging under the impacts of raindrops, which can reduce the infiltration rate and hydraulic conductivity [3]. Yokoi et al. [47] reported that crust was formed when the fine fraction content in the sand pyroclastic flow deposits exceeded 35%. A soil profile at our experimental sites appears to be a brown forest soil. Therefore, the use of a theory developed by Yokoi et al. [47] will be constrained. If the theory is to be applicable, it seems difficult to form crust in Japanese cedar and Hiba arborvitae forests as a whole because the fine fraction content does not generally exceed 35% in these types of forest. In fact, no crust could be visually observed after the experiment, and from the final infiltration rate obtained from the experiment, it also seemed unlikely that the formation of crust occurred. **Figure 7** is the relation between fine fraction content and *FIRmax*, but we could not find significant correlation (Japanese cedar: *r* = 0.033, *p* = 0.883; Hiba arborvitae: *r* = 0.013, *p* = 0.154). **Figure 8** shows the relation between fine fraction content and hydraulic conductivity, in which no correlation could be established.

**Figure 9** shows relation between hydraulic conductivity and *FIRmax* in the 0–5 cm surface layer, which revealed no correlation in either type of forests (Japanese cedar: *r* = 0.244, *p* = 0.274; Hiba arborvitae: *r* = 0.101, *p* = 0.710). Similarly, no correlation was detected between hydraulic conductivity and *FIRmax* for soil at 5–10 cm depth. It also became clear that *FIRmax* is definitely lower than hydraulic conductivity at most sites in Japanese cedar forest (19 out of 22 sites) and all sites in Hiba arborvitae forest (16 sites). It is postulated that the hydraulic gradient in the surface soil layer was estimated to be about one and that *FIRmax* had the similar value to hydraulic conductivity, which needs careful consideration. The 2–3 cm layer within 1 h after the sprinkling experiment was moistened, but the dry soil underlay in the deeper soil layers. The presence of macropores in soils might lead to the preferentially infiltration of sprinkled water. With fingering infiltration (unstable infiltration), the preferential water was infiltrated into the soils that are exceptionally conductive. The effect of entrapped air, hyphae respiration and bacteria in unsaturated soil also needs to be examined in future studies. There may be an underlying mechanism to limit infiltration of water into soil profile, which caused lower *FIRmax* compared to the permeability test for fully saturated soil.

C19). The same trend was commonly seen in other forests as reported in previous studies [3, 21]. Hiba arborvitae forests had relatively higher hydraulic conductivity than Japanese cedar forests. It was also found that hydraulic conductivity measured at depth of 5–10 cm was com-

**Figure 6.** Relation between ground cover materials and *FIRmax*. (A) is Japanese cedar and (B) is Hiba arborvitae and

Although there was considerable variability, we found that bulk density increased with depth and was higher in Hiba arborvitae forests compared to Japanese cedar forests. According to Miyata et al. [3], the average bulk density was 0.75 g/cm3 in Hiba arborvitae forests, which was slightly lower than those obtained in our experiment (Japanese cedar forests: 0.67 g/cm3; Hiba arborvitae forests: 0.51 g/cm3). It was presumed that a decrease of porosity induced by an increasing bulk density results in a decrease of hydraulic conductivity with increasing depth. We, however, could not find significant correlation between hydraulic conductivity and bulk density. Therefore, there is another unknown factor for the decrease of hydraulic

Fine fraction content ('fine fraction content' in **Table 1**) was 4–38% (average 22%) in Japanese cedar forests and 17–36% (average 25%) in Hiba arborvitae forests, but most of the data for both types of forest were below 35% except maximum values. The maximum values for these forests do not vary significantly as much as the minimum values. The minimum value was 4% for Japanese cedar forests and 17% for Hiba arborvitae forests, respectively. The result obtained in Japanese cedar forests exhibits a larger variability compared to Hiba arborvitae

The high fine fraction (clay + silt) content may increase the effect of clogging under the impacts of raindrops, which can reduce the infiltration rate and hydraulic conductivity [3]. Yokoi et al.

in Hiba arborvitae forests at depth of 0–5 cm; 0.49–0.995 g/cm<sup>3</sup>

in Hiba arborvitae forests at depth of 5–10 cm.

in Japanese cedar

paratively lower than those at depth of 0–5 cm depth.

results by Hiraoka et al. [9] (*FIRmax* = 0.14 × (dry weight of total cover) − 15.65).

in Japanese cedar forests and 0.49–0.91 g/cm<sup>3</sup>

forests and 0.37–0.72 g/cm<sup>3</sup>

178 Hydrology of Artificial and Controlled Experiments

conductivity.

forests.

Dry bulk density ('bulk density' in **Table 1**) was as follows: 0.38–0.86 g/cm<sup>3</sup>

**Figure 7.** Relation between fine fraction content and *FIRmax*. (A) is Japanese cedar and (B) is Hiba arborvitae.

The above results prove that the change in *FIRmax* in either type of forest cannot be explained by surface cover materials, hydraulic conductivity or fine fraction content. In the present research, we could not clarify the influencing factors to *FIRmax* in Japanese cedar and Hiba arborvitae forests. However, we found that *FIRmax* increased after thinning, and this might

Relation between Infiltration Rate, Cover Materials and Hydraulic Conductivity of Forest Soils...

http://dx.doi.org/10.5772/intechopen.70575

181

Ishikawa Prefecture is currently collecting 'Ishikawa Forest Environmental Tax', and implementing intensive thinning in forest degradation area throughout the prefecture to improve the public function of forests, such as watershed conservation and preventing landslide disaster. Since fiscal 2017, Ishikawa Prefecture has implemented additional efforts to promote the

Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan

[1] Tsujimura M, Onda Y, Harada D. The role of Horton overland flow in rainfall-runoff process in an unchanneled catchment covered by unmanaged Hinoki plantation. Journal of Japan Society of Hydrology & Water Resources. 2006;**19**:17-24 (in Japanese with English

[2] Gomi T, Sidle RC, Miyata S, Kosugi K, Onda Y. Dynamic runoff connectivity of overland flow on steep forested hillslopes: Scale effects and runoff transfer. Water Resources

[3] Miyata S, Onda Y, Gomi T, Mizugaki S, Asai H, Hirano T, Fukuyama T, Kosugi K, Sidle RC, Terajima T, Hiramatsu S. Factors affecting generation of Hortonian overland flow in forested hillslopes: Analysis of observation results at three sites with different geology and rainfall characteristics. Journal of the Japanese Forest Society. 2009;**91**:398-407 (in

[4] Onda Y. An Overview of the Effects of Forest Devastation on Soil and Water Loss. Tokyo:

[5] Murai H, Iwasaki Y. Studies on function of water and soil conservation based on forest land (I) influence of difference in forest condition upon water run-off, infiltration and soil erosion. Bulletin of the Government Forest Experiment Station. 1975;**274**:23-84 (in

Address all correspondence to: komatsu.yoshitaka.ws@alumni.tsukuba.ac.jp

Research. 2008;**44**:W08411. DOI: 10.1029/2007WR005894

Japan: Iwanami Shoten; 2008. p. 245 (in Japanese)

Japanese with English summary)

Japanese with English summary)

be attributed to Ishikawa Forest Environmental Tax effect.

use of timber.

**Author details**

Yoshitaka Komatsu

**References**

summary)

**Figure 8.** Relation between fine fraction content and hydraulic conductivity (*ko*). (A) is Japanese cedar and (B) is Hiba arborvitae.

In summary, surface cover materials, fine fraction content, and hydraulic conductivity had no correlation with *FIRmax* in either type of the forests examined in this study. Both Japanese cedar and Hiba arborvitae forests gave relatively high *FIRmax* values (>100 mm/h), which is higher than that of the entire *C. obtusa* plantations. These forests, especially at sites with a low concentration of surface cover materials (<1000 g/m<sup>2</sup> ), had a twofold to threefold greater *FIRmax* than that in C. obtusa plantations. Thus, Japanese cedar and Hiba arborvitae forests may provide a more effective protection of the soil surface. Based on fine fraction content, visual observation, and final infiltration rate, it seemed unlikely that the formation of crust occurs in both types of forest. In both types of forest, *FIRmax* is exceptionally lower than hydraulic conductivity at the soil surface. Fingering might occur during infiltration due to entrapped air and hyphae respiration.

**Figure 9.** Relation between hydraulic conductivity (*ko*) and *FIRmax*. (A) is Japanese cedar and (B) is Hiba arborvitae.

The above results prove that the change in *FIRmax* in either type of forest cannot be explained by surface cover materials, hydraulic conductivity or fine fraction content. In the present research, we could not clarify the influencing factors to *FIRmax* in Japanese cedar and Hiba arborvitae forests. However, we found that *FIRmax* increased after thinning, and this might be attributed to Ishikawa Forest Environmental Tax effect.

Ishikawa Prefecture is currently collecting 'Ishikawa Forest Environmental Tax', and implementing intensive thinning in forest degradation area throughout the prefecture to improve the public function of forests, such as watershed conservation and preventing landslide disaster. Since fiscal 2017, Ishikawa Prefecture has implemented additional efforts to promote the use of timber.
