Es Riddell

A methodology for historical assessment of compliance with environmental water allocations: lessons from the Crocodile (East) River, South Africa

Abstract Environmental flow provisions are a legal obligation under South Africa’s National Water Act (1998) where they are known as the “ecological reserve”, which is now being realized in river operations. This article presents a semi-quantitative method, based on flow–duration curve (FDC) analysis, used to assess the compliance of the Crocodile (East) River with the reserve in an historical context. Using both monthly and daily average flow data, we determine the extent and magnitude of non-compliant flows against environmental water requirements (EWRs) for three periods (1960–1983, 1983–2000, and 2000–2010). The results suggest a high degree of non-compliance, with the reserve increasing with each of these periods (14%, 35%, and 39% of the time), respectively, where effects were most pronounced in the low-flow season. The results also suggest that, whilst the magnitudes of reserve infringements for the latter period are relatively high, there appears to have been some improvement since the implementation of the river’s operating rules. Editor Z.W. Kundzewicz; Guest editor M. Acreman Citation Riddell, E., Pollard, S., Mallory, S., and Sawunyama, T., 2014. A methodology for historical assessment of compliance with environmental water allocations: lessons from the Crocodile (East) River, South Africa. Hydrological Sciences Journal, 59 (3–4), 831–843.

Hydrograph separation using tracers and digital filters to quantify runoff components in a semi-arid mesoscale catchment

1 IHE, Delft, The Netherlands University of KwaZulu‐Natal, Centre for Water Resources Research, Pietermaritzburg, South Africa Section of Water Resources, Delft University of Technology, Delft, The Netherlands UN World Water Assessment Programme (WWAP), UNESCO, Perugia, Italy Umgeni Water Chair of Water Resources Management, University of KwaZulu‐Natal, Pietermaritzburg, South Africa Correspondence Aline Maraci Lopes Saraiva Okello, IHE, Delft, The Netherlands. Email: a.saraiva@un‐ihe.org

Estimating the sediment trap efficiency of intermittently dry reservoirs: lessons from the Kruger National Park, South Africa

The assessment of sediment yield from reservoir siltation requires knowledge of the reservoirs sediment trap efficiency (TE). Widely used approaches for the estimation of the long-term mean TE rely on the ratio of the reservoirs storage capacity (C) to its catchment size (A) or mean annual inflow (I). These approaches have been developed from a limited number of reservoirs (N ≤ 40), most of them located in temperate climate regions. Their general applicability to reservoirs receiving highly variable runoff such as in semi-arid areas has been questioned. Here, we examine the effect of ephemeral inflow on the TE of ten small (≤280 × 103 m3), intermittently dry reservoirs located in the Kruger National Park. Field work was conducted to determine the storage capacity of the reservoir basins. The frequency and magnitude of spillage events was simulated with the daily time step Pitman rainfall-runoff model. Different runoff scenarios were established to cope with uncertainties arising from the lack of runoff records and imperfect input data. Scenarios for the relationship between water and sediment discharge were created based on sediment rating curves. Taking into account uncertainties in hydrological modelling, uncertainties of mean TE estimates, calculated from all scenarios (N = 9), are moderate, ranging from ±6% to ±11% at the 95% confidence level. By comparison, estimating TE from the storage capacity to catchment area (C/A) ratio induces high uncertainty (ranges of 35% to 65%), but this uncertainty can be confined (15% to 33%) when the latter approach is combined with hydrological modelling. Established methods relying on the storage capacity to mean annual inflow (C/I) ratio most probably lead to an overestimation of the TE for the investigated reservoirs. The approach presented here may be used instead to estimate the TE of small, intermittently dry reservoirs in semi-arid climate regions.