The hydrological effects of fire in South African mountain catchments

Abstract

Stream-flow and storm-flow in four small catchments were analysed by the paired catchment method for a response to fire. Two of the catchments were vegetated with over-mature fynbos (the indigenous scrub vegetation of the southwestern Cape Province, South Africa) , one was afforested to Pinus radiata and the fourth to Eucalyptus fastigata. One of the fynbos catchments was burned in a prescribed fire in the late dry season. The other catchments burned in wildfires. Neither of the fynbos catchments showed a change in storm-flows. Annual total flow increases of around 16% were in line with predictions, being related to the reductions in transpiration and interception. The manner of stream-flow and storm-flow generation appeared to have remained unaltered despite the fire. The two timber plantation catchments experienced large and significant increases in stormflow and sediment yields , while total flow increased by 12% in the pine catchment and decreased marginally in the eucalypt catchment. After fire , storm hydrographs were higher and steeper though their duration was little changed. These fire effects are considered to be due to changes in storm-flow generation consistent with an increased delivery of overland flow to the stream channel. This was caused , in part, by reduced infiltration resulting from water repellency in the soils of the burned catchments. The inherent wettability of a wide range of soil types and textures from beneath timber plantations and other vegetation types over a broad geographic distribution in South Africa was measured by fou r methods. Soils with high repellency ratings , unrelated to fire , are common and are most likely to occur beneath plantations of Eucalyptus and Acacia spp. and indigenous forest. Water repellent soils played a ro le, at two of the three locations, in the generation of overland flow from small plots exposed to simulated rainfall. However, the inherent repellency of the dry soils was extreme , such that fire-induced water repellency was not a factor in the response of the plots. The important role of fire in this experiment was in burning-off of repellency in the surface layer of the soil and in removing ground cover. III PREFACE The experimental work for this thesis was carried out in the experimental catchments of the Jonkershoek Forestry Research Centre near Stellenbosch and the Ntabamhlope research catchments near Estcourt, at field sites near Richmond and Mid-Illovo, Natal, and in the laboratories of the Jonkershoek Forestry Research Centre and the Departments of Agricultural Engineering and Agronomy of the University of Natal , Pietermaritzburg , under the supervision of Professor Roland E. Schulze. These studies represent my own original research and have not previously been submitted in any form for any degree or diploma to any University. Where use has been made of the work of others this is specifically acknowledged in the text.

Wildland fire in ecosystems: effects of fire on soils and water

“…Average annual streamflow discharge increased by about 10 percent to 4.8 inches (120 mm) on a 612 acre (245 ha) watershed in the Cape Region of South Africa following a wildfire that consumed most of the indigenous fynbos (sclerophyllous) shrubs (Scott 1993). This increase was related mostly to the reductions in interception and evapotranspiration losses.…”

“…A burn that was prescribed to reduce the accumulated fuel loads on a 450 acre (180 ha) watershed in the Cape Region of South Africa resulted in a 15 percent increase to 3.2 inches (80 mm) in average annual streamflow discharge (Scott 1993). Most of the fynbos shrubs that vegetated the watershed were not damaged by the prescribed fire.…”

Characterization and evaluation of controls on post-fire streamflow response across western US watersheds

“…Fires in chaparral environments, such as in southern California, exhibited increased flows of up to as much as 2 orders of magnitude, with much of this occurring in the dry season (Coombs and Melack, 2013;Hogue, 2011, 2015;Loáiciga et al, 2001). Fires in other chaparral environments were found to also yield flow increases, such as in South Africa (Lindley et al, 1988;Scott, 1993), Cyprus (Hessling, 1999), and France (Lavabre et al, 1993). Additional increases to post-fire flow regimes were found in temperate, forested catchments as well (Neary et al, 2005;Watson et al, 2001).…”

Assessing the long-term hydrologic response to wildfires in mountainous regions

“…Other studies have documented total runoff increases under post-wildfire conditions (Benavides-Solorio and MacDonald, 2001;Inbar et al, 1998;Lavabre et al, 1993;Robichaud et al, 2000;Scott, 1993). For example, Lavabre et al (1993) used a lumped conceptual hydrological model to evaluate a small Mediterranean basin which experienced a burn covering 85 percent of its surface area in 1990.…”

“…They suggested a 30 percent increase in the annual runoff yield. Scott (1993) showed 25 total streamflow volume increases of 15.3 and 9.4 percent in response to burning in two small mountainous catchments using a paired catchment method. In contrast, Mahat et al (2015) reported no significant change between the modeled streamflow from burned and unburned models.…”