Development of provisional guidelines for the treatment of scale and resolution in assessing streamflow reduction impacts of alien plant infestations and commercial afforestation in water resources modelling studies

[摘要] ENGLISH ABSTRACT:Experiments conducted on afforested catchments in South Africa have shown that alien treescan cause substantial reductions in catchment runoff (Scott et ai, 2000).In recognition of the impact which alien trees can have on the country's water resources,commercial afforestation was declared a stream flow reduction activity (SFRA) in terms ofthe National Water Act (NW A) (No. 36 of 1998), and the Department of Water affairs andforestry launched the Working for Water Programme (WfW) in 1995 with the recovery ofwater resources lost to Invasive alien plants (lAPs) as one of the Programme's objectives.These initiatives have intensified the need to quantify SFR; for example, for licensingpurposes to satisfy the requirements of the NWA and for predicting the effects of lAP clearingby WfW projects. Of interest to water resources practitioners, is the impact of SFR on meanannual runoff (MAR), on low flows and on water resource system, or reservoir, yield.In South Africa two basic methods of streamflow reduction (SFR) estimation have beendeveloped for commercial afforestation and lAPs. These are• free-standing empirical relationships in the form of the CSIR SFR CUNes, used inconjunction with the monthly, calibration-based, Pitman model.• component modules in the physically-based, land-use sensitive ACRU rainfall-runoffcatchment model, run at a daily time step with relatively fine subcatchment delineation.There has been a strong need for an evaluative comparison of the impacts of SFR estimatedvia these two methods. This study aimed to meet this need by using both methods toestimate SFR for a number of commercial afforestation and lAP scenarios in three studysystems, the Berg, Sabie and Mhlatuze, representing different bioclimatic conditions in SouthAfrica, and running the SFR sequences from the two estimation methods through the WaterResources Yield Model to determine the impact of the SFR on yield. The analysisdifferentiated between upland and riparian SFR, and between SFR produced by different treeclasses.Study conclusions included the following points:• Both the ACRU and SHELL models are capable of achieving a reasonable averageseasonal correspondence of high and low flows with the observed averages, though theactual averages produced by the two models can differ substantially.In general, ACRU simulates less SFR than SHELL, and gains in SFR after afforestationor invasion by lAPs may be simulated by ACRU during dry periods. The selection of cropfactors for different plant species has a strong influence on the relative water use of thespecies modelled in ACRU.• The impacts on yield of SFR due to lAPS and afforestation tends to be greater than theimpact on MAR, and impacts tend to be more severe for small subcatchments than forthe total catchment. A simulated reduction in MAR can result in a simulated increase inyield of a given assurance, if the portion of the flow sequence occurring during the criticalperiod is dominated by streamflow gains, and vice versa.Research recommendations centred on improving the availability of reliable fieldmeasurements of parameters and processes required tor the effective modelling of SFR.Based on the results of the study, guidelines were formulated for SFR modelling, focussingon the choice of SFR estimation method and the treatment of various parameters andconsiderations which influence the outcomes of SFR modelling.

[发布机构] Stellenbosch University