The influence of scale and parent material on hillslope hydrological processes in Kruger National Park.

Abstract

Semi-arid savannas are dynamic and complex systems controlled by fire, nutrient and water availability where water is the major driving force controlling the biological activity in these systems. Knowledge of hydrological processes is therefore, crucial for the prediction of ecosystem changes, given the prospect of climate change and the ever-increasing anthropogenic pressure on resources. There have been numerous advances in hydrological studies especially at the hillslope scale. Many of them have however, been site-specific and mostly comprising of a series of point measurements, thereby making it difficult to link processes at different scales in other sites. Some form of classification system would therefore, enable process integration, which is crucial in understanding and linking processes at various scales. For effective management and water resource planning, knowledge of how a system functions is crucial especially in savannas. It also helps in better prediction of behavior in ungauged systems that are in similar settings since many people rely on savannas for various ecosystem goods and services, the sustainability of livelihoods and biodiversity. Meanwhile, ephemeral systems contribute significantly to how savanna systems work. However, hydrological processes operate at different scales even in these ephemeral systems involving controls such as landscape morphology, geology and climatic conditions in different environments. The effect of scale on hydrological processes is one unresolved issue in hydrological studies and knowledge of how processes vary at different scales is fundamental in understanding scale-dependent processes and those that are not. This also helps in understanding the thresholds and controls at which the processes operate. Hydrological processes were observed and quantified at the hillslope scale in contributing areas to 1st, 2nd and 3rd order ephemeral streams in the Kruger National Park (KNP) landscapes on basalt and granite parent materials. Processes include subsurface lateral and vertical flow, overland flow and potential recharge to groundwater. These were focused on the KNP supersites, which are sites that were especially selected for multidisciplinary research. This would allow the description and measurement of processes and patterns at different scales, rainfall gradients and geological settings within the landscapes. Hillslope hydrological processes are dynamic and very complex but they are nonetheless vital, due to their interactions with surface and groundwater. Hillslope mechanisms of flow and subsurface moisture content are highly variable at both spatial and temporal scales. In order to gain insights into this variability, a number of techniques were used, such as measurement of soil water potential at different depths, geophysical surveys and numerical modelling. From the hydrometry data and modelling exercise, fluxes and water balances were derived in order to derive the hillslope response functions and potential connectivity with the underlying aquifers and/or adjacent streams. Since soils and hydrology have an interactive relationship, hillslope soil type responses were characterized through qualitative hydropedological descriptions. Results showed that hillslope hydrological responses, such as flow mechanisms and connectivity are dependent on scale (incremental stream orders). Hydrological connectivity was highly driven by rainfall intensity on hillslopes associated with high order streams (2nd and 3rd). The major driver of hillslope storage was shown to be ET demand rather than soil depth, in these savanna ecosystems. The initial perception was that deeper soils will have greater storage capacity, but results obtained in this study proved otherwise. This also proved the ability of savanna vegetation to extract large amounts of soil water when available in the soil profile. This study showed that flow mechanisms were mainly influenced by topography and soil hydraulic properties and to a lesser extent the parent material.