Yohannes Yihdego

Modelling of seasonal and long-term trends in lake salinity in southwestern Victoria, Australia

In southwestern Victoria a large number of lakes are scattered across the volcanic plains; many have problems with increasing salinity. To identify the hydrologic components behind this problem, three lakes, Burrumbeet, Linlithgow and Buninjon, were selected for detailed water and salt budget modelling using monthly values of rainfall, evaporation, surface inflow and outflow, and groundwater inflow and outflow (using the new modified difference method developed in this study). On average, rainfall begins to exceed evaporation with the onset of winter rainfall in May, so lake levels rise and lake salinities decline. The modelled lakes have become more saline over the last decade, a time of drought with below average rainfall, and all eventually dried out, their salinities rising to very high levels as they shallowed. Lake Burrumbeet is generally much less saline than Lakes Linlithgow and Buninjon, because it has substantial groundwater outflow, probably due to leakage through one or more volcanic necks. This limits the amount of time the lake water is subject to evaporation, and also allows significant salt export. The other lakes do not leak. The modelling indicates that when the lakes dry out, salt is lost from the lake-beds, probably due to wind deflation of salt crusts and leakage into the underlying groundwater. The removal of salt during drying-out phases resets the salinity of the lakes, limiting their ability to become more saline with time. Drying-out phases may therefore be essential in preventing the increased salinisation of lakes and wetland environments across the volcanic plains.

Modelling water and salt balances in a deep, groundwater-throughflow lake—Lake Purrumbete, southeastern Australia

Abstract Lake Purrumbete is a deep volcanic freshwater lake, cylindrical in shape, located within an extensive basalt plain in southeastern Australia. A modified difference water budget method for lakes, that estimates net groundwater flux through the difference between the level of the lake and the water table, along with the specific yield and area of the aquifer, successfully modelled the lake level fluctuations. The major influences are evaporation and direct precipitation; however groundwater fluxes are significant (~17%). The salt balance modelling shows that the salinity in Lake Purrumbete is largely controlled by groundwater and surface water fluxes. Lake Purrumbete has a greater interaction with the surrounding groundwater system than other nearby volcanic lakes due to its significant depth and the presence of highly permeable basalts along the upgradient shoreline. Its low salinity reflects the substantial salt export in the groundwater and surface outflow, as well as the lake’s relatively small evaporation due to the low surface area/volume ratio. The fluctuations in lake level and salinity for Lake Purrumbete are much less than for other lakes in the region, even during the 1997–2010 drought, due partly to the large, relatively constant groundwater flux. Low salinity, deep water and estimated depth of stratification from this study, which leads to a distinct limnology, implies that the creatures need to adapt to a different habitat to survive in the vicinity of Lake Purrumbete. Thus, Lake Purrumbete will be an important aquatic ecosystem refugium if climate change causes frequent seasonal drying of other lakes in the plain, and it is important to develop management strategies to maintain its water resources and quality. Editor D. Koutsoyiannis Associate editor Not assigned

3-D numerical groundwater flow simulation for geological discontinuities in the Unkheltseg Basin, Mongolia

AbstractGroundwater models which realistically represent the hydrogeology of a complex system, like the Unkheltseg Basin, are critically important to Mongolia. They have flow on benefits to research, governments, management strategies and commercial development within the country. Limited case studies of calibrated 3-D numerical transient simulations in fault-controlled connection between basins, similar to the Unkheltseg Basin, are available in the public domain and the model presented here aims to address this problem. This basin is uniquely geologically controlled and a key water supply resource for future economic development in the Taikh Valley. Commercial exploration projects have produced the high-quality geological and hydrogeological data gathered, necessary for successful model simulation at a basin-wide scale. Using the “DRAIN Package” and “Fracture-Well Package FW4” in MODFLOW-SURFACT, the spatial discretization necessary to fully represent horizontal and vertical flow direction was achieved to effectively constrain recharge and discharge across the fault barrier. This model is an important tool for establishing a long-term monitoring programme in a fault-controlled basin, which predicts regional impacts, both short and long term.

Response to Parker: Rebuttal: ENGE-D-13-00994R2 ‘‘Modelling of lake level under climate change conditions: Lake Purrumbete in southeastern Australia’’ by Yihdego et al. (2015)

Parker (2015) criticises the modelling by Yihdego et al. (2015) of future fluctuations in the level of Lake Purrumbete because we used climate change predictions that include global warming due to anthropogenic carbon dioxide emissions. Parker (2015) analysed Bureau of Meteorology data for weather stations in the area around Lake Purrumbete and states that measured rainfall in this region has been generally stable while exhibiting interannual and multi-decadal oscillations, and recently there has been a small increase. Parker appears to believe that there was no drought in south-eastern Australia in the period 1997–2009, because he criticises Yihdego et al. for claiming that there was a drought induced by global warming and that the drought period started in 2010, when there is clear evidence for abundant rainfall and high lake levels at this time. He goes on to analyse monthly maximum temperatures for the Lake Purrumbete region and states that temperature has been ‘‘reducing rather than increasing from 1864 to 2015’’. Parker then deduces that because temperatures are not warming and there is no consistent rainfall pattern, ‘‘anthropogenic carbon dioxide concentration in the atmosphere (cannot) be the issue’’. Rainfall

Hydro-engineering solution for a sustainable groundwater management at a cross border region: case of Lake Nyasa/Malawi basin, Tanzania

A groundwater resources assessment has been carried out for the Lake Nyasa Basin east Africa, with reference to sub catchments further to the whole basin wise analysis, including quantification of potential yields from the aquifers. Numerical groundwater models, MODFLOW-SURFACT coupled with Visual MODFLOW, were used to assess sub-basin groundwater sustainability (Via “Zone Budget” software package). The model has been calibrated to observed hydraulic head and processed baseflow estimate. It is concluded that the aquifer system is sustained by episodic recharge and the long-term gaining storage which represents the maximum extractable volume. Future predictions of groundwater usage indicate that by 2035 the percentage of annual safe yield extracted will increase to between 11.3 and 103%. Model result suggests that there is a need to revise the current estimate of sustainable yield based on future climate change conditions and projected population growth, which would decrease spring flows substantially and decrease hydraulic head basin-wide. It also suggests that by 2035 some sub catchments will be nearly at or exceeding the annual safe yield leaving no room for socio-economic development, or the need to reduce existing socio-economic demands to meet domestic demands. Besides to improving the natural replenishment capacity through artificial recharge technique, the other option is to increase the percentage of total recharge allocated to the annual safe yield from 10% of the total annual recharge to 20% in these catchments. This provides a basis for management of individual groundwater scheme in sustaining livelihood activities or implications of policies and to develop a plan for potential groundwater extraction scenarios pertaining to water use and allocation.

Validation of a model with climatic and flow scenario analysis: case of Lake Burrumbeet in southeastern Australia.

Forecast evaluation is an important topic that addresses the development of reliable hydrological probabilistic forecasts, mainly through the use of climate uncertainties. Often, validation has no place in hydrology for most of the times, despite the parameters of a model are uncertain. Similarly, the structure of the model can be incorrectly chosen. A calibrated and verified dynamic hydrologic water balance spreadsheet model has been used to assess the effect of climate variability on Lake Burrumbeet, southeastern Australia. The lake level has been verified to lake level, lake volume, lake surface area, surface outflow and lake salinity. The current study aims to increase lake level confidence model prediction through historical validation for the year 2008–2013, under different climatic scenario. Based on the observed climatic condition (2008–2013), it fairly matches with a hybridization of scenarios, being the period interval (2008–2013), corresponds to both dry and wet climatic condition. Besides to the hydrologic stresses uncertainty, uncertainty in the calibrated model is among the major drawbacks involved in making scenario simulations. In line with this, the uncertainty in the calibrated model was tested using sensitivity analysis and showed that errors in the model can largely be attributed to erroneous estimates of evaporation and rainfall, and surface inflow to a lesser. The study demonstrates that several climatic scenarios should be analysed, with a combination of extreme climate, stream flow and climate change instead of one assumed climatic sequence, to improve climate variability prediction in the future. Performing such scenario analysis is a valid exercise to comprehend the uncertainty with the model structure and hydrology, in a meaningful way, without missing those, even considered as less probable, ultimately turned to be crucial for decision making and will definitely increase the confidence of model prediction for management of the water resources.

Mine dewatering and impact assessment in an arid area: Case of Gulf region

Analytical and empirical solution coupled with water balance method were used to predict the ground water inflow to a mine pit excavated below the water table, final pit lake level/recovery and radius of influence, through long-term and time variant simulations. The solution considers the effect of decreased saturated thickness near the pit walls, distributed recharge to the water table and upward flow through the pit bottom. The approach is flexible to accommodate the anisotropy/heterogeneity of the real world. Final pit void water level was assessed through scenarios to know whether it will be consumed by evaporation and a shallow lake will form or not. The optimised radius of influence was estimated which is considered as crucial information in relation to the engineering aspects of mine planning and sustainable development of the mine area. Time-transient inflow over a period of time was estimated using solutions, including analytical element method (AEM). Their primary value is in providing estimates of pit inflow rates to be used in the mine dewatering. Inflow estimation and recovery helps whether there is water to supplement the demand and if there is any recovery issue to be dealt with in relation to surface and groundwater quality/eco-system, environmental evaluations and mitigation. Therefore, this method is good at informing decision makers in assessing the effects of mining operations and developing an appropriate water management strategy.