Vincent Kakembo

Diffusivity Models and Greenhouse Gases Fluxes from a Forest, Pasture, Grassland and Corn Field in Northern Hokkaido, Japan

Information on the most influential factors determining gas flux from soils is needed in predictive models for greenhouse gases emissions. We conducted an intensive soil and air sampling along a 2000 m transect extending from a forest, pasture, grassland and corn field in Shizunai, Hokkaido (Japan), measured CO2, CH4, N2O and NO fluxes and calculated soil bulk density (ρ(subscript b)), air-filled porosity (f(subscript a)) and total porosity (Φ). Using diffusivity models based on either f(subscript a) alone or on a combination of f(subscript a) and Φ, we predicted two pore space indices: the relative gas diffusion coefficient (D(subscript s)/D(subscript o)) and the pore tortuosity factor (τ). The relationships between pore space indices (D(subscript s)/D(subscript o) andτ) and CO2, CH4, N2O and NO fluxes were also studied. Results showed that the grassland had the highest ρ(subscript b)while f(subscript a) and Φ were the highest in the forest. CO2, CH4, N2O and NO fluxes were the highest in the grassland while N2O dominated in the corn field. Few correlations existed between f(subscript a), Φ, ρ(subscript b) and gases fluxes while all models predicted that D(subscript s)/D(subscript o) and t significantly correlated with CO2 and CH4 with correlation coefficient (τ) ranging from 0.20 to 0.80. Overall, diffusivity models based on f(subscript a) alone gave higher D(subscript s)/D(subscript o), lower τ, and higher R^2 and better explained the relationship between pore space indices (D(subscript s)/D(subscript o) and τ) and gases fluxes. Inclusion of D(subscript s)/D(subscript o) andτin predictive models will improve our understanding of the dynamics of greenhouse gas fluxes from soils. D(subscript s)/D(subscript o) andτcan be easily obtained by measurements of soil air and water and existing diffusivity models.

The use of high resolution digital camera imagery to characterize the distribution of Pteronia incana invader species in Ngqushwa (formerly Peddie) District, Eastern Cape, South Africa

High resolution infrared digital camera imagery was used to determine the extent of the invasion of hill slopes by Pteronia incana, an unpalatable dwarf shrub. On the basis of a priori field surveys, the imagery was classified into different degrees of the invasion and other surface cover types using Idrisi32 GIS. The imagery was noted to have limited constraints, as evidenced by the high level of classification accuracy. A range of vegetation indices was examined to identify one that best characterized the spatial distribution of the shrub and its degree of invasion. Distinct spectral separability of the invader from other surface cover types was achieved by means of the perpendicular vegetation index (PVI), as opposed to the ratio based vegetation indices (NDVI, SAVI, and MSAVI). Depending on the local geographical characteristics, the PVI could be particularly suited for the identification of perennial shrubs with characteristics similar to P. incana, which are usually interspersed with considerable bare zones.

Trends in Soil Erosion and Woody Shrub Encroachment in Ngqushwa District, Eastern Cape Province, South Africa

Woody shrub encroachment severely impacts on the hydrological and erosion response of rangelands and abandoned cultivated lands. These processes have been widely investigated at various spatial scales, using mostly field experimentation. The present study used remote sensing to investigate spatial and temporal patterns of soil erosion and encroachment by a woody shrub species, Pteroniaincana, in a catchment in Ngqushwa district, Eastern Cape Province, South Africa between 1998 and 2008. The extreme categories of soil erosion and shrub encroachment were mapped with higher accuracy than the intermediate ones, particularly where lower spatial resolution data were used. The results showed that soil erosion in the worst category increased simultaneously with dense woody shrub encroachment on the hill slopes. This trend is related to the spatial patterning of woody shrub vegetation that increases bare soil patches—leading to runoff connectivity and concentration of overland flow. The major changes in soil erosion and shrub encroachment analysed during the 10-year period took place in the 5–9° slope category and on the concave slope form. Multi-temporal analyses, based on remote sensing, can extend our understanding of the dynamics of soil erosion and woody shrub encroachment. They may help benchmark the processes and assist in upscaling field studies.

Use of sediment source fingerprinting to assess the role of subsurface erosion in the supply of fine sediment in a degraded catchment in the Eastern Cape, South Africa.

Sediment source fingerprinting has been successfully deployed to provide information on the surface and subsurface sources of sediment in many catchments around the world. However, there is still scope to re-examine some of the major assumptions of the technique with reference to the number of fingerprint properties used in the model, the number of model iterations and the potential uncertainties of using more than one sediment core collected from the same floodplain sink. We investigated the role of subsurface erosion in the supply of fine sediment to two sediment cores collected from a floodplain in a small degraded catchment in the Eastern Cape, South Africa. The results showed that increasing the number of individual fingerprint properties in the composite signature did not improve the model goodness-of-fit. This is still a much debated issue in sediment source fingerprinting. To test the goodness-of-fit further, the number of model repeat iterations was increased from 5000 to 30,000. However, this did not reduce uncertainty ranges in modelled source proportions nor improve the model goodness-of-fit. The estimated sediment source contributions were not consistent with the available published data on erosion processes in the study catchment. The temporal pattern of sediment source contributions predicted for the two sediment cores was very different despite the cores being collected in close proximity from the same floodplain. This highlights some of the potential limitations associated with using floodplain cores to reconstruct catchment erosion processes and associated sediment source contributions. For the source tracing approach in general, the findings here suggest the need for further investigations into uncertainties related to the number of fingerprint properties included in un-mixing models. The findings support the current widespread use of ≤5000 model repeat iterations for estimating the key sources of sediment samples.

THE USE OF LABORATORY SPECTROSCOPY TO ESTABLISH PTERONIA INCANA SPECTRAL TRENDS AND SEPARATION FROM BARE SURFACES AND GREEN VEGETATION

ABSTRACT Spectral trends of Pteronia incana, an unpalatable invader shrub, and its separation from bare surfaces and green vegetation are examined using a high resolution spectrometer. As a technique that provides reliable reflectance values, direct spectroscopy was used to measure spectral reflectance in the field and laboratory at 0.45µm to 0.88µm wavelengths. The reflectance of P. incana leaves and branches of varying proportions was compared using laboratory spectroscopy and the effect of increasing leaf percentage in branch samples was determined by performing regression analyses at 0.55µm, 0.68µm and 0.88µm. The Students t-test was used to establish whether the difference between the mean reflectance of the respective surfaces was statistically significant. Results showed a strong correlation between average spectral measurements and increasing percentage of leaves. Whereas separability between the respective surfaces was achieved in the near infrared band, a consistent spectral trend showing a clear distinction between the surfaces was achieved using the first order derivative.

Historical rainfall variability in selected rainfall stations in Eastern Cape, South Africa

There is evidence of climate shifts in Africa shown by changing rainfall patterns, temperatures and increased fire incidences. As such, the annual rainfall variability for 41 years (1970–2010) for nine stations at Amakhala reserve, Grahamstown, Bathurst, Port Alfred, Uitenhage and Port Elizabeth in Eastern Cape, South Africa was studied through trend and time series analysis. In order to identify the trends for extreme rainfall events, the daily rainfall index (DI), highest daily rainfall and frequency of dry and wet spells for the stations were also analysed. Pearsons product moment correlation test was used to compute relationships between measured parameters over the years. Results showed a declining trend in annual rainfall over time at Grahamstown (r = − 0.59), Uitenhage and Bathurst (r = − 0.32), while Amakhala, Port Alfred and Port Elizabeth remained unchanged. Most of the rainfall declines occurred in the 1980s and 1990s sub-periods, with both the DI and daily rainfall subclasses above 10 mm showing similar declines. The frequency of dry days decreased with time at Port Alfred and Uitenhage, while the length of dry spells increased at Bathurst (r = +0.41). Reports from the literature suggest that the 1970s–1990s rainfall variations were due to the El Niño southern oscillation cycle and sea surface temperatures over the Atlantic and Indian Oceans, which gave drier conditions during El Niño and wetter than normal conditions during La Niño events.

Exploring ground-based methods for the validation of remotely sensed evapotranspiration

Accurate estimates of evapotranspiration (ET) are essential when calculating catchment-scale water balances. An open top chamber (OTC), a ground-based method for measuring actual ET, was used to validate modeled ET from earth observation MODIS and agrometeorological data. We studied and compared the modeled ET (ETMODIS of two adjacent, quaternary catchments from the Bushmans River primary catchment, namely P10A and Q91C, representing light and heavy stocking density, respectively. Within each catchment, a relatively homogenous 1 km2 pixel of grassland was selected for intensive study. Mean annual ETMODIS results from 2009 to 2011 showed that P10A (light) was higher by 14 mm than Q91C (heavy). No significant difference was observed between hourly ETMODIS and hourly ETOTC. A good relationship was observed between daily ETMODIS and ETOTC (r2 = 0.7065, p value < 0.01, df = 8) for each 1 km2 pixel during the 2010/11 growing season. This confirmed that the OTC is a useful instrument to determine actual ET over grassland and short shrubland, and may be used in model validation.

Soil carbon dioxide effluxes from different vegetation environments in semi-arid Eastern Cape, South Africa

Though important, CO2 effluxes from non-agricultural ecosystems are poorly characterised. Given that thicket vegetation is prevalent in Eastern Cape, South Africa, we monitored soil CO2 effluxes from an intact thicket, degraded thicket (DT) and grassland (G) over 10 months, as affected by temperature, moisture and penetration resistance (PR). High soil moisture (16%) reduced PR (4 kg cm−2), which raised winter effluxes (1.2 µmol m−2 s−1), while low moisture (2%) resulted in hard dry soil (14 kg cm−2) that suppressed spring effluxes (0.2 µmol m−2 s−1). There was good interaction between PR and moisture (r = −0.53), with seasonal effluxes increasing with increasing moisture (r = 0.9, p = 0.0001) and decreasing PR (r = −0.66, p = 0.02). Temperature effects were significant under unlimited moisture supply. Thus high summer temperature (40 °C) gave lower effluxes in DT and G (<1 µmol m−2 s−1) due to limited moisture (<10%), whereas high autumn temperature (48 °C) and good moisture (16%) accelerated CO2 emissions (averaging 2 µmol m−2 s−1) from all covers. Although semi-arid ecosystems are limited by low moisture and sandy soils, they contribute to CO2 emissions under high moisture and increasing temperature. Keywords: Eastern Cape, grassland, soil carbon dioxide effluxes, soil organic carbon, thicket

A Temporal Analysis of Elephant-Induced Thicket Degradation in Addo Elephant National Park, Eastern Cape, South Africa

ABSTRACT Elephant-induced thicket degradation in the Addo Elephant National Park (AENP), Eastern Cape, South Africa, was assessed during 1973 and 2010 using multitemporal satellite imagery. Changes in the thicket condition, in relation to the AENP expansion, were analyzed using the Normalized Difference Vegetation Index, postclassification, and landscape metrics. The change detection of land-cover classes was analyzed by postclassification. Landscape-spatial metrics were used in order to gain an understanding of vegetation-fragmentation trends. Temporal changes in vegetation gradients in relation to water points and thicket conditions within the botanical reserves were also assessed. The thicket condition was noted to have deteriorated, as the AENP had expanded. An expansion of degraded vegetation away from the water points was identified during the study period. The thicket condition in botanical reserve 1 had fluctuated, whereas it remained constant in reserves 2 and 3. Landscape spatial-metric analyses revealed evidence of increased vegetation fragmentation, as new areas of the AENP had been opened for elephant activity. A progressive decline in intact thicket and an increase in degraded thicket were observed. Remote-sensing techniques can assist with thicket-clump restoration by applying “target monitoring” for the timeous identification of potential-degradation hotspots.

Comparison of pixel and sub-pixel based techniques to separate Pteronia incana invaded areas using multi-temporal high resolution imagery

Remote Sensing using high resolution imagery (HRI) is fast becoming an important tool in detailed land-cover mapping and analysis of plant species invasion. In this study, we sought to test the separability of Pteronia incana invader species by pixel content aggregation and pixel content de-convolution using multi-temporal infrared HRI. An invaded area in Eastern Cape, South Africa was flown in 2001, 2004 and 2006 and HRI of 1x1m resolution captured using a DCS 420 colour infrared camera. The images were separated into bands, geo-rectified and radiometrically corrected using Idrisi Kilimanjaro GIS. Value files were extracted from the bands in order to compare spectral values for P. incana, green vegetation and bare surfaces using the pixel based Perpendicular Vegetation Index (PVI), while Constrained Linear Spectral Unmixing (CLSU) surface endmembers were used to generate sub-pixel land surface image fractions. Spectroscopy was used to validate spectral trends identified from HRI. The PVI successfully separated the multi-temporal imagery surfaces and was consistent with the unmixed surface image fractions from CLSU. Separability between the respective surfaces was also achieved using reflectance measurements.