TPC is a 16 000 hectare estate located in Moshi, Tanzania and is currently planted under 8 800 hectares of sugarcane and produces over 60 000 tons of sugar per annum. The influence of volcanic parent material and volcanic ash over TPC, together with the alluvial nature of many of the soils, has imparted a unique combination of soil mineralogical, physical and chemical properties. Furthermore, irrigation with poor quality water has led to sodicity problems on the estate. Understanding the mineralogy and sodicity effects on soil hydraulic properties across the estate can lead to better irrigation management where it is important to prevent the build-up of salts due to over-irrigation. In response to this need, a study was carried out with the aim of characterising the mineralogical, physical and chemical properties in the five management areas of the estate (North, East, West, South and Kahe), in order to determine the relationships between various measured parameters. A total of 70 fields across TPC, as well as four sites outside the estate and two ash layers, were chosen for sampling. Undisturbed soil cores and bulk samples were collected from the A and B horizons from 45 of these fields and the four sites outside. Selected fields were sampled at more than one site to assess field variability, and where cane growth was patchy selected fields were sampled in a patch of poorly growing cane and an adjacent patch of better cane growth. Bulk soil samples were collected from the remaining fields and the two ash layers. Double ring infiltration measurements were carried out on 25 of the selected fields. X-ray diffraction, transmission electron microscopy and aluminium, iron and silica extractions were carried out to determine the mineralogy. Physical and chemical measurements included water retentivity, saturated hydraulic conductivity, bulk density, particle size distribution, organic carbon, pH (H2O), electrical conductivity, water soluble and exchangeable cations (Ca, Mg, K and Na), cation exchange capacity and clay specific surface area. The particle size distribution showed that the soils were mainly loams and sandy loams. Organic carbon values were generally greater in the A horizon compared to the B horizon and varied between 0.4 and 2.5 % in the topsoil and 0.3 and 2.1 % (with the exception of field 11 which had an organic carbon of 4.0 %) in the subsoil. X-ray diffraction patterns of sand and silt fractions were dominated by sanidine while clay patterns were weak and had high backgrounds and very broad peaks, suggesting the presence of poorly ordered material in the clay fraction. The Al and Fe extraction methods and electron micrographs indicated that this poorly ordered material was allophane. However, the dominant clay mineral across the estate was halloysite, in both tubular and spheroidal form, as well as very small (<< 0.5 μm) kaolinite particles. There was also gibbsite in some of the samples analysed. The combination of allophane, halloysite, kaolinite and gibbsite indicated that the primary volcanic minerals have weathered to various degrees across the estate. This is reflected in the alluvial nature of the soils where less weathered material has been periodically deposited onto older, more weathered material over some parts of the estate. The south and west areas had a slightly higher Alo + ½ Feo ratio than the other areas in both the topsoil (1.07 and 0.95, respectively) and the subsoil (1.16 and 1.06, respectively), a possible consequence of less weathered alluvial material that was deposited in these areas. Although the concentration of allophane was low (< 5 %), even in the south and west areas, its presence greatly increased the clay specific surface area (up to 145.94 m2 g-1) and consequently had a significant influence on the soil physical and chemical properties. Water retention across TPC was high, particularly at the lower matric potentials (between 0.13 and 0.45, and 0.09 and 0.24 m3 m-3 at -33 kPa and -1500 kPa, respectively). The high water retention is a result of allophane which gives the soils a high adsorption capacity and a porosity that is dominated by micro-pores. Generally, the south area had the highest water retention at the various measured matric potentials which corresponds to the higher allophane content. Variability in water retentivity across areas and within fields limited further interpretation and correlation with the mineralogical results. Infiltration rate was lowest in the south (60.85 mm hr-1) and highest in the Kahe area (171.20 mm hr-1). The main factor influencing the final infiltration rate was the concentration of sodium in the soil, with higher concentrations causing soil dispersion and blockage of soil pores. Clay dispersion has led to the development of calcareous surface crusts and reduced porosity, thus reducing the infiltration rate. Sodium concentration in the soil is likely to have had a dominating effect over the mineralogical composition of the soil. Poor cane growth in the south and west areas corresponded to higher pH (up to 10.32), electrical conductivity (up to 614 mS m-1), sodium absorption ratio (up to 20.63) and water soluble and exchangeable sodium (up to 53.20 mmolc l-1 and 14.87 cmolc kg-1 soil, respectively) in these areas. The soils are thus more dispersive and the combination of sodicity and allophane has resulted in “fluffy” soils with small particles clogging soil pores and thus surface crusts have formed easily. The combined effect of mineralogy and sodicity in the south is further complicated by the presence of perched water tables. High adsorption capacities and the dominance of micro-pores allow the occurrence of significant capillary rise which brings salts to the soil surface, further exacerbating the sodicity problem. Therefore, over-irrigation should be avoided where soils are prone to sodicity from a combination of irrigation with poor quality water, perched water tables and strong capillary rise action. Fields which are currently experiencing the negative effects of high sodicity, require irrigation with good quality water and adequate sub-surface drainage to ensure the leaching of salts. Further studies with specific focus on the south and west areas would be beneficial in accounting for the variability and in drawing correlations between the mineralogy and sodium content of the soils with the other measured properties. Fields which are prone to increased sodicity through over-irrigation with poor quality water, have strong capillary rise from perched water tables and which require remediation through sub-surface drains can thus be distinguished and the factors influencing sugarcane growth can be more clearly understood. Growth depends on the combination of these soil’s unique mineralogy and sodium content and the influence they have on the infiltration rate, adsorption capacity, micro-porosity and capillary rise from the water table. For future work, water movement modelling to predict saturated and unsaturated flow, as well as in situ measures of unsaturated flow, will lead to further understanding of the soil hydraulic properties and aid in improved irrigation management. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Changes in land use and land cover are a global issue of concern, especially with regard to possible impacts on biophysical processes which affect the hydrological functioning of a system. Tanzania is no exception to this concern. This study, therefore, addresses the implications of land use alterations on the hydrodynamics of the Kilombero River Basin, specifically with regard to the Kilombero Valley’s wetlands and water resources, which have been altered and exploited to a great extent. As its starting point, the study embarked on mapping the current land cover in the Kilombero Basin and the quantification of the historical changes. The study revealed significant changes and, in recent years, increased rates of clearing natural vegetation cover and conversion to agricultural land. The most affected area of the Basin was the Kilombero Valley, a Ramsar Site and formerly extensively inhabited by wildlife, but which now has 62% of its area converted into agricultural and/or human settlements. In line with this observation, the study used two approaches for the impact analysis, a regional scale and a local scale approach. Plant physiology, soil moisture and micro-meteorological measurements were undertaken to quantify the impact of land cover change at local scale. Sensing techniques were then applied to assess the spatial extent of the changes and the basin scale (regional) impact thereof. Investigation of hydrological processes at a local scale placed emphasis on the implications of forest conversion from indigenous Miombo woodland to exotic Teak (Tectona grandis) forests. Field measurements showed the distinctive nature of Teak trees consumptive water use, both in quantity as well as in regard to the seasonal variation as compared to the native Miombo woodland forests. Teak was found to have higher transpiration rates, both during the rainy season (where the rates were approximately 10-fold higher than that of Miombo) and the period immediately after the cessation of rainfall, with consumptive water use rates being four-fold higher than that of Miombo. This contrast in water use was further observed in the measured soil water fluxes which evidenced a large difference in the components of the soil water balance. Less recharge was observed in the Teak forests suggesting significant impacts on the replenishment of groundwater resources in the study area. Assessment of the basin scale impacts of the land cover changes on the evapotranspiration (ET) regime was undertaken using the Surface Energy Balance System (SEBS) remote sensing model. Validation was provided by the Teak field sites and through the monitoring of ET from sugar cane using a Large Aperture Scintillometer (LAS). Results suggest a decrease in ET during the dry season. There is a clear transition of ET that follows the land cover transition from the natural and more adaptable vegetation, to rain-fed dependent crops and bare lands, where minimal ET is observed during the dry season. Similar seasonal leafing, and therefore a similar ET pattern, is observed with the conversion of natural forests to deciduous plantation forests. Irrigated crops, on the other hand, were found to have persistently higher ET throughout the year regardless of rainfall variability. This implies that land cover change in the Kilombera Valley is resulting in higher water use and less recharge in the wet season and a correspondingly lower ET (and possibly lower river flow) in the dry season than would occur under natural conditions. This research provides valuable information relevant to all stakeholders in the Kilombero River Basin (i.e. both smallholders and commercial sugarcane farmers, the forestry industry, Basin Water Authorities etc.). This information will help to inform decision-making around the sustainable management of the water resources in the Kilombero Valley for food security as well as for sustaining livelihoods and ecosystems. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.