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:
  1. Detail descriptions of 28 modal profiles located at different sites in the catchments.
  2. Detailed sampling of the different soil horizons of each of these profiles and subjecting them to detailed analyses.
  3. 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.
  4. 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.
  5. 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.
  6. 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:
  1. 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.
  2. 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.
  3. 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.