Hydrology in the mountains

By Faith Jumbi, Doctoral Student, University of the Western Cape Tweet

In South Africa, the Middle East and some parts of the Andes and the Rocky Mountains lie some of the most critically important mountain regions in terms of water supply - they feed rivers that compensate, to some extent, for the water deficits experienced in populated semi-arid areas.

With water resources being depleted worldwide, it is imperative to manage them more sustainably to enhance security in future water supply. One part of achieving this goal is to improve our understanding of the dominant hydrological processes and fluxes in these catchments.

For my PhD I am studying the hydrology of the Kromme River catchment in the Eastern Cape. It is bounded by the Suuranys and Tsitsikamma Mountains, part of the Cape Fold region of South Africa.

A palmiet wetland site (L) and a black wattle-invaded site in the Kromme Catchment.

The Kromme is unique in nature: the landscape is dominated by very steep mountains that lead to high-velocity flows during floods, typically associated with erosion and rocky bedded rivers, and yet it supports large valley-bottom wetlands in its narrow floodplain. The Kromme is also regionally important for water supply. It sustains the livelihoods of people and the ecosystems within the catchment and the neighbouring city of Port Elizabeth.

The Kromme River catchment showing the major dams and sites where hydrological monitoring is being done.

Land use in the Kromme includes intensive fruit, livestock and vegetable farming. Natural cover has been replaced by intensive agriculture in much of the floodplain and most of the riparian zones have been invaded by alien plants, primarily Acacia mearnsii or black wattle. This has resulted in drainage of valley-bottom wetlands.

Land use/land cover types and hydrological response

Mountainous catchments are sensitive to land uses and land cover types, which have the potential to reduce the available water resources.

A Google Earth image showing some of the land cover types in the Kromme River Catchment: a) grass, b) wetland dominated by palmiet (Prionium serratum) and c) black wattle (Acacia mearnsii, a non-native tree species) invasion in the floodplain. The area with the palmiet wetland shown in b) is recovering after having been cleared of black wattle trees by Working for Water (Source: CNES/AIRBUS, 2017)

In the Kromme, the conversion of wetlands to agricultural fields is perceived to increase the catchment’s responsiveness to floods. Invasion by black wattle may also influence the hydrology of the catchment by decreasing groundwater levels, reducing average streamflow as well as base flow in periods of low flow.

It is therefore important to examine how different land cover types will influence average water yield, surface and groundwater flows in this catchment, as well as the spatial scale at which land use and land cover types influence hydrological responses.

Faith Jumbi is a PhD student at the University of the Western Cape (UWC) being supervised by Professor Dominic Mazvimavi and Dr Julia Glenday (postdoctoral researcher at the SAEON Fynbos Node). Her research contributes to a larger collaborative project (Living Lands, SAEON, UWC and CSIR) to use hydrologic modelling and participatory processes to assess catchment management options for the western Algoa region, funded by the Water Research Commission.

Hydrological models for use by catchment communities

My research aims to improve the understanding of dominant hydrological processes and mechanisms of water flow in the Kromme as an example of a semi-arid mountainous catchment. More specifically, it aims to understand the hydrological behaviour in the valley bottoms as well as the surrounding mountain tributaries, and how these are affected by land use and land cover types. I will be doing this through both physical characterisations, by measuring streamflow and groundwater fluctuations in different parts of the catchment, and hydrological modelling.

Through research facilitation by SAEON and Living Lands, and funding from the Water Research Commission of South Africa, a network of monitoring sites covering the various land cover types at different scales has been established in the catchment. Data being collected will include rainfall, soil, groundwater levels, soil moisture and stream flow. These data will be used to characterise the dominant processes in the river catchment as well as to calibrate and validate the hydrological model that will be used for scenario analysis.

The results of this research will enhance understanding of the dynamic hydrological functioning of mountain catchments in semi-arid settings and how different land cover affects these processes. From this, a hydrological model will be built to predict the impacts of various management options on water availability, in the hope that the information will be useful to people living in the Kromme catchment as they manage the land and water.

The study will also contribute towards the understanding of how different processes can be represented in hydrological models and how this influences the predicted outputs.

Further reading

• Mander, M., Blignaut J., Van Niekerk M., Cowling, R., Horan, M., Knoesen D., Mills, A., Powell, M and Schulze, R. 2010. Baviaanskloof-Tsitsikamma Payment for Ecosystem Services: A Feasibility Study. Synthesis Report. Report submitted by Future Works: Making Environments Work for People (www.futureworks.co.za) for SANBI, CAPE, Working for Water, UNDP
• Viviroli, D., Weingartner, R., 2004. The hydrological significance of mountains: from regional to global scale. Hydrol. Earth Syst. Sci. Discuss. 8, 1017–1030.
• Rebelo, A.J., 2012. An ecological and hydrological evaluation of the effects of restoration on ecosystem services in the Kromme River System, South Africa. Stellenbosch: Stellenbosch University.