1. Throughfall Distribution in a Tropical Forest in Northern Thailand
Alan D. Ziegler
& T.W. Giambelluca, M. Nullet
Geography Department, University of Hawaii, Honolulu, HI USA
& Chatchai Tantasarin
Kasetsart University, Bangkok THAILAND
Support: National Science Foundation (USA); National Research Council Thailand

2. THROUGHFALL: fraction of gross rainfall dripping through the canopy
QUESTION 1
wet-canopy evaporation?
Alan D. Ziegler
& T.W. Giambelluca, M. Nullet
Geography Department, University of Hawaii, Honolulu, HI USA
& Chatchai Tantasarin
Kasetsart University, Bangkok THAILAND
Throughfall
Stemflow (small: < 3-5%)
Chappell et al., 2001
Important for water balance estimation

3. PANG KHUM EXPERIMENTAL WATERSHED
STUDY SITE
PANG KHUM EXPERIMENTAL WATERSHED
mixed-use | swidden agriculture
Forest (25%)
NRCT-SPONSORED THAILAND ROADS PROJECT SITE (1995 – 2005)

4. FOREST PLOT
Hill Evergreen Forest: Wendlandia tinctoria, Stryax benzoides, Castanopsis tribuloides
30m
94-ha
1100 – 1800 m RF
Annual rainfall = mm

5. DESIGN: TWO Measurement Methods
4 Spiders
(stationary, 5-cm x 4-m rails, tipping bucket gauges)
QUESTION 2
stationary  mobile
20 Rovers
(mobile, 26-cm standard-type gauges)

6. DESIGN: MEASUREMENT LOCATIONS
Year 2001
108 days | 49 events | 11 Periods
Grid cell (m) 5 10 15 20 25 30 35 4
spider
locations
Grid cell (m)
179
rover
locations
500 m2
102 Trees
20 Evergreen
6 Deciduous
5 trees / grid

7. 2 CORRECTIONS DYNAMIC CALIBRATION DEVICE CALIBRATION
(spiders & rain gauge)
DEVICE CALIBRATION
(vs. RAIN GAUGE)
Intensity-related errors
Splash errors related to design RF

8. THROUGHFALL ESTIMATES
RAINFALL: 49 events | mm
49 event medians
ROVERS
SPIDERS
BASIS
76%
72%
Median of 49 events
Rover-derived TF
Fraction of total RF
79%
81%
mobile stationary
ROVERS > SPIDERS
But…
NOT SIGNIFICANT
Spider-derived TF

9. QUESTION 3: COMPARABLE Range: 70-95% PKEW: ~80%
Prior Hill and Dry Evergreen Forest Studies in Thailand
9 90 1
Tanaka (2003)
4 95 1
30 69 <1
Range: 70-95%
PKEW
DISTURBED FOREST
in the middle FEW LARGE TREES
NON-COMPLEX CANOPY
PKEW: ~80%
QUESTION: Real differences or Differences in techniques?

10. Probability Distribution
Slightly skewed
Range (20-190%)
Values > 100% are real
(“pour” points)
ROVERS
Percent of rainfall (%)
Median = 68%
Data of Dykes (1997),
Hall and Roberts (1990),
Lloyd and Marques (1988)

11. events area Stabilization of TF Estimate all rover data Throughfall
small large
few many
all rover data
Throughfall
Throughfall
Area (m2)
June July Aug Sept Oct
Number of rovers
SUFFICIENT DATA
probably SUFFICIENT DATA

12. Canopy Cover verses Throughfall
No relationship
Although TF distribution is affected by the
3-4 largest trees via pour points and shadows
Non-homogeneous
Somewhat homogeneous
Range = 0 – 4
Range = %
[Interpolation via simple kriging]

13. CONCLUSONS TF Sensitive to RF measurements
e.g., 7% if different rainfall station used
Dynamic Calibration is Crucial
For high-intensity tropical rainfall
Mobile = Stationary measurements
If sufficient area and adequate range of events sampled
Wet Canopy Evaporation on order of 15-20%
Stemflow estimate in progress
TF  f (canopy cover)
Others factors: wind-driven rainfall, topography, role of dominant trees
TF correlation distance < 2 m
(< ½ average crown diameter)
… highlights difficulty in scaling up TF in the catchment

14. Thank
You!
Alan Ziegler

16. Influence of Event Size
Big events have higher TF
Throughfall (mm)
< 60%