Simon Lorentz

Identification of runoff generation processes using combined hydrometric, tracer and geophysical methods in a headwater catchment in South Africa / Identification des processus de formation du débit en combinat la méthodes hydrométrique, traceur et géophysiques dans un bassin versant sud-africain

Abstract Classical hydrometric measurements and detailed 2-D electrical resistivity imaging (ERI) surveys were combined with tracer sampling to identify the hydrological processes in a semi-arid headwater catchment in the Eastern Cape Province of South Africa. The analysis of precipitation and runoff events emphasized the strong link between precipitation and runoff formation characteristics. Soil water tension and groundwater level observations demonstrated the development of a perched water table within the soil layer. These results are supported by tracer-based runoff component separations and illustrate the important role of the shallow subsurface component. The ERI investigation permitted further insight into the structure of the subsurface. Finally, the ERI survey, in combination with time domain reflectometry (TDR) measurements, allowed the extrapolation of selective soil water content measurements. To summarize, the application and combination of different field methods led to the development of a conceptual model of the hydrological functioning of this catchment. The dominant role of the subsurface mechanisms was evaluated.

Case Study 3: Modelling the Impacts of Land-use and Climate Change on Hydrological Responses in the Mixed Underdeveloped/Developed Mgeni Catchment, South Africa

The “rainbow nation” tag is often used to depict the colourful natural, social and economic diversity of South Africa. It is also a fitting description of the physical, cultural and developmental diversity which prevails in the Mgeni catchment in the KwaZulu-Natal province and which has left a strong imprint of anthropogenic change on its varied hydrological Systems. The Mgeni is an important catchment in South Africa: While its area of 4079 km2 upstream of Inanda Dam (15 km from its mouth into the Indian Ocean) constitutes a mere 0.33% of South Africa, it supplies water to nearly 15% of the countr’s 41 million inhabitants and to a region which, albeit small, produces around 20% of the country’s gross domestic product.

Transmission of pressure head through the zone of tension saturation in the Lisse effect phenomenon

ABSTRACT The problem of transmission of pressure head through the zone of tension saturation in the Lisse effect (LE), i.e., the rapid response of groundwater level to pressurized pore air in the unsaturated zone, is investigated theoretically and experimentally. From the law of conservation of energy and the continuity equation, a one-dimensional diffusion equation is derived for transmission of pressure head through the zone of tension saturation. The solution to the equation is the pressure head at any point below the upper boundary of the zone of tension saturation and at any time after the compressed pore air pressure is imposed on the boundary. The key parameter, which determines the behaviour of transmission of pressure head, is the newly proposed pressure head diffusivity coefficient. The theoretical results agree with the experimental results, obtained from laboratory column experiments in three physically different soils. Editor Demetris Koutsoyiannis Associate editor Xi Chen

Modelling hydrological processes, crop yields and NPS pollution in a small sub-tropical catchment in South Africa using ACRU-NPS

Abstract The Agricultural Catchment Research Unit—Nitrates, Phosphorous and Sediments (ACRU-NPS) model, which simulates runoff, sediment and nutrient (NO3 and P) production in agricultural catchments, is used to evaluate the impact of farming practices and land-use changes on crop yields, water discharge, sediment and nutrient loads in the 41-km2 Mkabela catchment in the KwaZulu-Natal midlands, South Africa. The ACRU-NPS model was modified with the following objectives: (a) to develop and incorporate algorithms to simulate crop growth and NPS pollution dynamics in process zones and control features; (b) simulate sediment and nutrient production from land segments for various land uses; (c) model crop yield where crop growth is influenced by water and nitrogen stress; and (d) simulate sediment and nutrient fate at controls and buffers in the stream network. Five management practice scenarios were simulated and run with a series of fertilizer management applications. The resulting crop yields, water discharge, sediment and nutrient loadings were analysed. On average, doubling fertilizer application from base rates resulted in the highest sugarcane yield increase of 5 t/ha, zero fertilizer application resulted in the highest sugarcane yield reduction of 11 t/ha, while applying deficit irrigation, with base fertilizer application rates retained, resulted in 16 t/ha sugarcane yield increase. Flood attenuation and retention capacities for the total non-point source (NPS) pollutants for the period 2006–2012 were also investigated in the study area.

Evaluation of agroforestry systems for maize (Zea mays) productivity in South Africa

Maize (Zea mays L.) is the staple food crop grown by most smallholder farmers in South Africa. Decline in soil fertility and expensive chemical fertilisers affect maize production by these farmers. Smallholder farmers cannot afford chemical fertilisers because these are expensive. Agroforestry systems offer cheap alternatives to expensive chemical fertilisers. A field experiment was established in the 2015/16 season at Wartburg. The objective of the study was to evaluate maize yields and productivity in agroforestry systems. The experiment had five treatments: sole (maize; pigeonpea; or Sesbania bispinosa); maize + Sesbania bispinosa; maize + pigeonpea laid out in a randomized complete block design replicated three times. The yield was in order: sole maize > maize + pigeonpea ≥ maize + pigeonpea. The land equivalent ratio (LER) was in the sequence maize + pigeonpea > sole pigeonpea > maize + S. bispinosa > sole S. bispinosa ≥ sole maize. Maize yield might be increased in the following season as a subsequent crop in the same field because of residual nutrients that would have been enhanced and set free for plant uptake during the previous season. Pigeonpea is recommended in agroforestry systems with maize because of its higher LER and combined production of grain for human and livestock consumption and firewood.