Wetlands of South Africa: Hydrology and Human Use
South Africa has a relatively dry climate (average 479 mm/y), and consequently wetlands are sparse covering 10-12% of the land surface, but locally extremely important hydrologically, ecologically and as a resource for human use. Given the climate, peatlands occur only where strong and sustained groundwater discharge occurs - either from regional-scale hydrogeological formations or from more localized aquifers such as coastal dunes, etc., and comprise 8-10% of South African wetlands. Elsewhere, the seasonal variation in precipitation typically results in ephemeral wetlands (without peat). In either case the perennial or seasonal availability of fresh-water is a focus of ecological activity and often of human interaction. Human use of wetlands includes water abstraction, grazing and harvesting of materials for building and handicrafts , often done in a sustainable manner. Other activities include totally unsustainable peat extraction and partly sustainable cultivation. Activities adjacent to wetlands including mining, timber plantations and groundwater exploitation for mining, commercial agriculture and urban water needs can also profoundly affect their water supply. Disturbances upstream or within wetlands can cause severe erosion and gullying. From 30 - 50% of wetlands have been lost due to landuse changes in their drainage basins or in the wetland itself. Ecohydrological feedback to even relatively modest disturbance of these systems can elicit a cycle of destructive and ongoing degradation. Wetland management requires a good understanding of the ecohydrological and landscape factors that support wetlands, proactive measures for restoration, and sensitivity to the needs of poverty-stricken users of wetland resources.
Simplified Volume-Area-Depth Method for Estimating Water Storage of Isolated Prairie Wetlands
There are millions of wetlands in shallow depressions on the North American prairies but the quantity of water stored in these depressions remains poorly understood. Hayashi and van der Kamp (2000) used the relationship between volume (V), area (A) and depth (h) to develop an equation for estimating wetland storage. We tested the robustness of their full and simplified V-A-h methods to accurately estimate volume for the range of wetland shapes occurring across the Prairie Pothole Region. These results were contrasted with two commonly implemented V-A regression equations to determine which method estimates volume most accurately. We used detailed topographic data for 27 wetlands in Smith Creek and St. Denis watersheds, Saskatchewan that ranged in surface area and basin shape. The full V-A-h method was found to accurately estimate storage (errors <3%) across wetlands of various shapes, and is therefore suitable for calculating water storage in the variety of wetland surface shapes found in the prairies. Both V-A equations performed poorly, with volume underestimated by an average of 15% and 50% Analysis of the simplified V-A-h method showed that volume errors of <10% can be achieved if the basin and shape coefficients are derived properly. This would involve measuring depth and area twice, with sufficient time between measurements that the natural fluctuations in water storage are reflected. Practically, wetland area and depth should be measured in spring, following snowmelt when water levels are near the peak, and also in late summer prior to water depths dropping below 10 cm. These guidelines for applying the simplified V-A-h method will allow for accurate volume estimations when detailed topographic data are not available. Since the V-A equations were outperformed by the full and simplified V-A-h methods, we conclude that wetland depth and basin morphology should be considered when estimating volume. This will improve storage estimations of natural and human-impacted wetlands in the PPR. Considering more than half of prairie wetlands have been de-water though agricultural drainage, it is important to have accurate methods to estimate storage in order to assess the impact of wetland storage on watershed runoff.
Man-made Influences upon the Water Exchange Driven by Lake Seiches in a Coastal Wetland of the Great Lakes
The water circulation in many shallow coastal wetlands of the Great Lakes is partially driven by the rapid water level fluctuations due to lake seiches. Such seiche-driven flushing of coastal wetlands has an important impact upon the water quality of the wetland and the suitability of habitat for fish breeding. Our research has focused on understanding how the flushing efficiency of wetlands and coastal embayments is related to their physical geometry; in particular how man-made changes to connecting channels have to ability to increase or reduce flushing rates and the residence time of water within these wetlands and coastal embayments. The water level response of many coastal wetlands can be modeled as a forced Helmholtz harmonic resonator, whereby the flushing response of a wetland or coastal embayment depends upon how close the frequency of the seiche is to the resonant frequency of the coastal wetland. We present a comparison between this model and data from a field study of water levels in Frenchman's Bay, a shallow and enclosed embayment located in Lake Ontario; and from previously published observations of coastal wetlands in Lake Superior. In both cases there is good agreement between the observed water level fluctuations and with those predicted by our model. The principle result is that there is a linear increase in the flushing rate of an enclosed coastal wetland with increases in the cross-sectional area of the connecting channel mouth, for cross-sectional areas between 1 and 30 m2. At the moment Frenchman's bay has a channel mouth that has an area of 20 m2 and the flushing time of water in the bay is close to one week. Deepening the channel, so that the area doubles, would halve the hydraulic residence time of the water, and potentially decrease some of the negative impacts of the run-off on this degraded urban wetland. The model we present could support future efforts to design artificial coastal wetlands and to improve existing wetlands water quality, by explaining the influence of the channels geometry upon the hydraulic residence time of water and chemicals within coastal wetlands.
Evaluating the Influence of Hydrologic Variability on Potential CO2 Fluxes From two Perched Basins in the Peace - Athabasca Delta, Alberta
Given the significance and expected amplitude of climate change in northern latitudes, there is a need to better characterize the response and susceptibility of lake sediment and littoral peat carbon stores to changes in climate and hydrology. This study uses both laboratory incubations of littoral peat and lake sediment and paleolimnological records from two ponds in the Peace - Athabasca Delta (PAD) in Alberta to (1) investigate the role that past and present hydrological conditions plays on the amount and lability of stored organic carbon to oxidation and respiration potentials and (2) evaluate potential production of CO2 in light of anticipated future hydroecological conditions. The PAD is a large northern freshwater ecosystem characterized by numerous small perched basins. These basins span a broad hydrological spectrum spatially and temporally due to the relative influence of components comprising their water balances. PAD31 ('Johnny Cabin Pond') is located in the southern more active portion of the Athabasca Delta and has become increasingly influenced by frequent river water inundations since a major upstream change in Athabasca River distributary flow occurred in 1982. This site has consequently experienced a shift from closed- drainage conditions to restricted-drainage conditions. PAD01 ('Devils Gate Pond') in the northern more relict Peace sector of the delta is characterized by predominantly closed-drainage conditions. Laboratory incubations simulating dry, moist and saturated moisture conditions at two (4 and 20°C) temperatures show greater potential CO2 production from each site and substrate under warm, moist conditions and lowest under cool and dry conditions. Potential production of CO2 from PAD31 peat and lake sediments is much greater than those at the infrequently flooded site, PAD01. Substrate type (sediment or peat) and stratigraphy also show differences in potential CO2 production associated with different organic content sources and histories. These results suggest that current hydrological setting and antecedent conditions influence the potential contributions of CO2 from pond-peatland ecosystems. Results from this study will aim to address ongoing concerns over accelerated greenhouse gas contributions from northern wetlands undergoing natural and anthropogenically- induced hydroecological change.
The Effects of Upland Tree Removal on the Ecohydrological Controls on Net CO2 Exchange From a Western Boreal Plain Riparian Peatland
Climate change will have serious implications for sensitive northern ecosystems, such as the forested wetlands throughout the sub-humid Western Boreal Plain (WBP) of Canada. It is expected that the effects here will be among the largest and fastest of any region and will be augmented by landuse change such as forest harvesting. Therefore, quantifying the ecohydrological processes that drive vegetation distributions and patterns of CO2 exchange, and how they may respond to disturbance within the larger catchment, is especially important for this region. It has been well documented in the literature that microtopography drives hydrological and vegetation patterns, and therefore CO2 exchange in wetland systems. However, in sub-humid wetlands, such as those in the WBP, moisture differences (gradients) between microtopographical highs and lows are reduced, and saturated conditions and standing water in topographical lows often lacking, compared to the more humid climates in which other studies have been conducted. Therefore, the gradients in the sub-humid WBP are not large enough to result in vegetation distribution differences but are still large enough to cause differences in the level of productivity within a species, which will be more strongly related to variations in canopy cover. To address the response of these systems to landuse change in their surrounding catchments it is necessary to quantify the resultant changes in these controlling environmental variables in the wetlands to upland disturbance. This study examined the midday (10:00 - 16:00) growing season (April - October) surface cover CO2 relationships with different microtopography (lawn and depression) in a riparian peatland in the WBP, north- central Alberta, Canada for 2 years prior, and 2 years post - harvesting of the surrounding Aspen dominated uplands. A dynamic - closed chamber technique was used to: evaluate the relative contributions of heterotrophic and autotrophic respiration and photosynthesis and assess the relative roles of plant communities, hydrology, and microclimates on CO2 exchange, pre- and post-harvest.
Can we Ecohydrologically Rehabilitate Disturbed Peatlands? From "Wetlands of Mass Decomposition" to "Yes We Can"
The natural carbon storage function of peatland ecosystems can be severely affected by human and natural disturbances such as drainage, peat extraction, drought and wildfire. Cutover peatands, for example, become a large and persistent source of atmospheric CO2 following peat extraction. The recovery (rehabilitation, re- establishment, restoration) of disturbed peatlands to a net carbon sink depends to a large extent on the rate of recovery of the surface peat layer referred to as the acrotelm. The acrotelm serves to stabilize water table variation providing ideal conditions for vegetation re-establishment, particularly peat forming Sphagnum moss. Here we present results from several ecosystem-scale field experiments where we examined the change in hydrophysical properties of peat following peat extraction and subsequent restoration and discuss how this affects peatland-atmosphere CO2. We found that moisture retention properties of a new peat layer at a restored peatland were distinct from near- by natural and naturally regenerated sites. Despite considerable biomass accumulation and increase in peat thickness, the new peat layer differed with respect to its moisture retention properties, an indication that factors other than growth have an impact on the restoration of the returning moss layer. Similarly in an acrotelm transplant experiment we determined that the restored peatland experienced high variability in volumetric moisture content (VMC) in the capitula zone (upper 2 cm) where large diurnal changes in VMC (~30%) were observed, suggesting possible disturbance to the peat matrix structure during the extraction-restoration process. However, soil - water retention analysis and physical peat properties (porosity and bulk density) suggest that no significant differences existed between the natural and restored sites. A simple hydrologic model demonstrated that the new peat layer will become an acrotelm in ~20 years when ~20 cm of peat has accumulated, an approach which may aid in designing a long-term sampling strategy for assessing the long- term effects of restoration of disturbed peatlands on peatland hydrology and ecology. Applications of these findings to a new research collaboration on the effects of wildfire on peatland ecohydrology will be discussed.