The relationship between soil
water regime and soil profile morphology in the Weatherley catchment,
an afforestation area in the north-eastern Eastern Cape
Report No.1317/1/05
March 2005
EXECUTIVE SUMMARY
South Africa is a water poor country. Procedures for establishing the
water yield of catchments are therefore important, and become
increasingly so, as the demand for water increases. Any expansion of
knowledge in this regard is therefore desirable.
Apart from the surface flow, all outflow from a catchment must have
flowed through the soil at some point. The way in which this process
has occurred in different locations in the catchment over thousands of
years has played a major role in pedogenesis. These results have become
expressed in a discernable way in the soil distribution pattern, in the
morphology of the different soil horizons and profiles, and in their
chemical and physical characteristics. It is the elucidation of this
relationship between hydrology and pedology of the Weatherley catchment
that has been the overall objective of this project. The hypothesis was
that the general elucidation of the relationship between soil profile
morphology and soil hydrology will contribute towards understanding
hillslope hydrological processes and facilitate technology transfer
between catchments. It is significant that the study of the
relationship between hydrology and pedology was formally accentuated
during 2004 by the international launching of a new subject called
hydropedology, by a team of world leaders in hydrology, pedology and
soil physics. They defined hydropedology as ‘The synergistic
integration of classical pedology with soil physics, hydrology and
other related bio- and geo-sciences”.
The 160 ha Weatherley catchment near Maclear was selected in 1995 for a
long-term study aimed at comparing hydrological conditions under
natural grassland with those after afforestation on parts of the
catchment. The study was initiated and executed by the School of
Bioresources Engineering & Environmental Hydrology (SBEEH) of
the University of KwaZulu-Natal in cooperation with North East Cape
Forests and Mondi Forests, with additional funding provided by the
Water Research Commission. The grassland monitoring phase started in
November 1995 and continued over seven rain seasons. The planting of
forest trees on approximately 82 ha, with three selected areas planted
to three different species viz. Eucalyptus n/tens (33 ha), Pinus patula
(23 ha) and Pinus eliottii (26 ha), was done in September 2002.
Hydrological monitoring at the Weatherley catchment has been achieved
by the installation of a wide range of appropriate instruments spread
out over the catchment, including two flow measuring weirs. All results
have been made available for this project. Measurements for this
project commenced during June 2000.
The first objective of this project was to characterize and quantify
the soil water regime and soil profile morphology in the Weatherley
catchment. The soil water regime referred to here is that which
occurred under natural grassland. The objective was reached by the
following actions:
- Detail descriptions of 28
modal profiles located at different sites in the catchments.
- Detailed sampling of the
different soil horizons of each of these profiles and subjecting them
to detailed analyses.
- Digital colour photographs
of each of the modal profiles and horizons and initiating the
development of a computer aided process to quantify the colours in an
objective manner.
- Installing piezometer
pipes at selected profiles and extracting water samples from each of
these at appropriate intervals to study the composition of the soil
water.
- Employing neutron water
meter (NWM) soil water content measurements, taken at a number of
depths at each of the 28 modal profile sites, at approximately weekly
intervals for the six year period July 1997 to June 2003, in a soil
water balance modelling exercise to estimate the soil water content at
each measurement point in each modal profile on a daily basis for the
six year period.
- Making 86 bulk density
determinations, in duplicate, by the core method, supported by
determinations of bulk density using a gamma density probe at all NWM
measurement points, in order to improve NWM calibration lines.
This objective was satisfactorily achieved. For the 28 profiles,
detailed profile descriptions, analytical data, photographs and daily
water regime data for the six years are available. Hydromorphic soils
dominate in the catchment. Of the total area 25 % consists of marshland
with mainly soils of the Katspruit and Kroonstad forms which remain
close to saturation for most of the year. Eleven of the 28 modal
profiles belong to these forms. Other fairly wet soils are those
belonging to the Longlands and Westleigh forms, represented by three
and five modal profiles respectively. Only one of the 28 modal profiles
is freely drained (Hutton form). The remaining eight belong to
the Avalon, Pinedene, Tukulu and Bloemdal forms, all of which
have freely drained characteristics in the upper portion of the profile
overlying horizons with signs of wetness. These soils occur on the hill
slopes surrounding the marsh.
Seven Katspruit, 5 Kroonstad, 4
Westleigh, 3 Longlands, 2 Bloemdal, 2
Pinedene, 2
Tukula, 1 Avalon, 1 Hutton and 1 Oakleaf soil were
identified during field investigations. The chemical data for
the Weatherley soils can be summarized as follows:
Diagnostic
Horizon |
Clay |
S |
CECsoil |
CECclay |
BS |
OC |
N |
pHwater |
PhKcl |
Fe |
Mn |
|
(%) |
(cmolc
kg-1) |
(%) |
(mg
kg-1) |
|
|
(mg
kg -1) |
Neocutanic
B
Yellow-brown |
18.6 |
2.0 |
4.6 |
20.1 |
45.1 |
0.35 |
343 |
5.22 |
4.31 |
9741 |
155.3 |
Apedal
B |
13.6 |
1.2 |
4.4 |
28.0 |
28.3 |
0.22 |
187 |
5.18 |
3.99 |
5152 |
25.5 |
Red
apedal B |
21.4 |
1.6 |
4.5 |
16.3 |
38.5 |
0.35 |
259 |
5.36 |
4.17 |
10338 |
144.6 |
Orthic
A |
14.8 |
3.2 |
6.8 |
27.0 |
46.0 |
0.93 |
645 |
5.33 |
4.36 |
7586 |
46.6 |
Soft
plinthic B |
17.6 |
3.2 |
6.0 |
32.7 |
52.5 |
0.19 |
249 |
5.79 |
4.31 |
6835 |
64.5 |
E |
13.0 |
1.9 |
6.8 |
44.5 |
34.3 |
0.45 |
374 |
5.44 |
4.29 |
7924 |
16.8 |
Unspecified material
with
signs of
wetness |
23.4 |
4.3 |
7.0 |
28.1 |
58.2 |
0.13 |
208 |
5.59 |
4.27 |
8669 |
115.4 |
G |
28.2 |
6.9 |
10.5 |
35.3 |
64.2 |
0.20 |
306 |
6.15 |
4.44 |
9227 |
33.0 |
The second objective was to determine the relationship between soil
water regime and soil profile morphology. This objective was achieved
in the following ways:
A preliminary assumption was made that the degree of saturation (s) at
which anaerobic conditions would be acute enough to cause redox
reactions of sufficient intensity to produce visible signs of redox
morphology was s>0.7. In accordance with this assumption the
daily soil water regime was used to obtain values for the following
parameters for the diagnostic horizons for each modal profile:
ADS>0.7
= the average duration in days year-1 that s was above
0.7 of porosity.
FS>0.7 = the average frequency
of s>0.7 events
year-1.
DS>0.7 = the average duration of
s>0.7 events.
The results from all the profiles were pooled to give average values
for the following diagnostic horizons: G (331 ± 10 days
year-1), unspecified material with signs of
wetness (248 ±
37 days year-1), E (202 ± 45 days year-1),
soft plinthic B
(182 ± 25 days year-1), orthic A (148
± 19 days
year-1) red apedal B (80 ± 58 days
year-1), yellow-brown
apedal B (75 ± 25 days year-1) and
neocutanic B horizons (37
± 24 days year-1).
The third objective was to develop a procedure, in collaboration with
hydrologists, to correlate diagnostic and non-diagnostic soil
characteristics with hydrological response characteristics in the
landscape. The following procedure was followed to achieve this
objective:
- Identification of the
portion of the
hydrograph expected to provide the most information about the soils of
the catchment — necessary because the hydrological response
characteristics of a catchment are reflected in the shape of the
hydrograph. Peaks in the hydrograph are due to surface runoff, and the
baseflow line is determined by flow from the soil water table and the
lower vadose zone. It was therefore decided that the curved portion
between these two would be the most useful for our purposes. It should
reflect the hydrographical response caused mainly by water which had
entered the soil (upper vadose zone) and flowed laterally to the stream
bed at a fairly rapid rate compared to the water flowing laterally at
greater depths.
- Determination of the
amount and rate
of interfiow water released from a small subc-atchment at the end of
the rain season and correlating it with the hydrological
characteristics of the soils in the sub-catchment.
- Identification of
important
preliminary steps in a procedure to be adopted to correlate soil
characteristics and hydrological response characteristics in a
particular catchment.
Results to test the hypothesis presented under (a) were obtained by
comparing typical autumn, rain free period, hydrographs for Weatherley
(160 ha) and Cathedral Peak Catchment VI (68 ha). The total outflow,
presumably mainly from interfiow (i.e. excluding baseflow) from the two
catchments during these periods was 4 362 and 52 093 m3 for Weatherley
and Cathedral Peak VI respectively. This large difference, accentuated
by the differences in sizes of the catchments, is attributed mainly to
two soil-related factors viz. (a) far larger marsh area at Weatherley
(40 ha) compared to Cathedral Peak VI (6 ha); (b) far larger water
storage capacity of the deep, high organic matter content, low bulk
density, high clay content apedal soils on the hillslopes of the
Cathedral Peak catchment. The value of the
‘interfiow” portion of the hydrograph was therefore
demonstrated.
Results of action (b) were obtained for the sub-catchment represented
by the toposequence P201 to P204 from which water flows out over the
sandstone shelf during the rainy season and stops in autumn. The
estimated total flow during a selected late-summer/autumn period in
2001 was 453 m3. The drainable water capacity, i.e. the amount of water
available for interflow, in the soils of the sub-catchment was
estimated to be 1 889 m3. There are many factors which could have
contributed to the poor agreement between the two values. These were
identified. The value of the exercise was in the determination of an
approximate procedure for such a study.
Action (c) resulted in the formulation of the following initial steps
in a procedure to correlate soil characteristics with hydrological
response characteristics in a particular landscape or catchment. First,
construct detailed large-scale topographical, geological and soil
survey maps. Secondly, collect the data that will make it possible to
estimate the total drainable water capacity of the upper vadose zone,
and if possible also the lower vadose zone. Thirdly, obtain reliable ET
data for the vegetation of the catchment; this is an important step
since small errors in this value have a large influence on the water
balance, especially for the “interfiow” portion of
the hydrograph.
Diagnostic horizons can be used to infer hydrological and other soil
properties. Since this study was concerned mainly with soils that have
developed in siliceous parent material, the effect of different parent
materials on Fe supply, and hence soil colour should be researched
further. The same is true for climate — mainly rainfall and
temperature. Similar studies should therefore be carried out under
different climatic conditions. The predictive value of diagnostic
horizon and soil form characterization in relation to hydrology should
also be evaluated by verification against other profiles in other
catchments.
Technology transfer has taken place through four refereed articles,
nine conference presentations and three other publications. Capacity
building took place within the research group, through two students
appointed as co-workers, two Ph.D. students and one Hons. B.Sc. Agric.
student.
The possibilities of digital photograph interpretation holds great
promise and should be investigated further. It is especially the effect
of different cameras and lighting conditions, the relationship between
Munsell and RGB colour notation and meaningful differentiation between
diagnostic red, yellow and grey colours, as well as the interpretation
of soil photographs in the dry and wet states should be investigated
further.
The results of a comparison between the Weatherley catchment and
Cathedral Peak catchment VI showed that the curved portion of the
hydrograph during a rain-free period, and following a high rainfall
period which filled the catchment, was correlated with the soil
distribution pattern and soil characteristics of the catchment. Three
important initial steps — discussed above — were
identified in the procedure to correlate soil characteristics with
hydrological characteristics in a catchment. Future research at
Weatherley should focus on (a) the continuation of measuring and
processing neutron water meter measurements and improving the
relationship between these and other continuous water monitoring
procedures; (b) changes in the water regimes of specific soils caused
by specific tree species; (c) changes in ET caused by afforestation (d)
intensive studies regarding soil redox reactions; and (e) the
formulation of appropriate wetness classes for South African soils.