Stage level,volume and time‐frequency information content of Lake Tana using stochastic and wavelet analysis methods

Lake Tana is the largest fresh water body situated in the north‐western highlands of Ethiopia. In addition to its ecological services, it serves for local transport, electric power generation, fishing, recreational purposes, and source of dry season irrigation water supply. Evidence shows that the lake has dried at least once at about 15,000–17,000 before present owing to a combination of high evaporation and low precipitation events. Past attempts to understand and simulate historical fluctuation of Lake Tana based on simplistic water balance approach of inflow, outflow, and storage have failed to capture well‐known events of drawdown and rise of the lake that have happened in the last 44 years. This study tested different stochastic methods of lake level and volume simulation for supporting Lake Tana operational planning decision support. Three stochastic methods (perturbations approach, Monte Carlo methods, and wavelet analysis) were employed for lake level and volume simulation, and the results were compared with the stage level measurements. Forty‐four years of daily, monthly, and mean annual lake level data have shown a Gaussian variation with goodness of fit at 0.01 significant levels of the Kolmogorov–Smirnov test. The stochastic simulations predicted the lake stage level of the 1972, 1984, and 2002/2003 historical droughts 99% of the time. The information content (frequency) of fluctuation of Lake Tana for various periods was resolved using Wigner's Time‐Frequency Decomposition method. The wavelet analysis agreed with the perturbations and Monte Carlo simulations resolving the time (1970s, 1980s, and 2000s) in which low frequency and high spectral power fluctuation has occurred. The Monte Carlo method has shown its superiority for risk analysis over perturbation and deterministic method whereas wavelet analysis reconstructed historical record of lake stage level at daily and monthly time scales. Copyright © 2012 John Wiley & Sons, Ltd.     

Revisiting lake sediment budgets: How the calculation of lake lifetime is strongly data and method dependent

Lake sedimentation has a fundamental impact on lake lifetime. In this paper, we show how sensitive calculation of the latter is to the quality of data available and assumptions made during analysis. Based on the collection of a large new dataset, we quantify the sediment masses (1) mobilized on the hillslopes draining towards Lake Tana (Ethiopia), (2) stored in the floodplains, (3) transported into the lake, (4) deposited in the lake and (5) delivered out from the lake so as to establish a sediment budget. In 2012–2013, suspended sediment concentration (SSC) and discharge measurements were made at 13 monitoring stations, including two lake outlets. Altogether, 4635 SSC samples were collected and sediment rating curves that account for land cover conditions and rainfall seasonality were established for the 11 river stations, and mean monthly SSC was calculated for the outlets. Effects of the floodplain on rivers' sediment yield (SY) were investigated using measurements at both sides of the floodplains. SY from ungauged rivers was assessed using a model that includes catchment area and rainfall, whereas bedload and direct sediment input from lake shores were estimated. As a result, the gross annual SY was c. 39.55 (± 0.15) Mt, dominantly from Gilgel Abay and Gumara Rivers. The 2.57 (± 0.17) Mt sediment deposited in floodplains indicate that the floodplains serve as an important sediment sink. Moreover, annually c. 1.09 Mt of sediment leaves the lake through the two outlets. Annual sediment deposition in the lake was c. 36.97 (± 0.22) Mt and organic matter accumulation was 2.15 Mt, with a mean sediment trapping efficiency of 97%. Furthermore, SSC and SY are generally higher at the beginning of the rainy season because soils in cultivated fields are bare and loose due to frequent ploughing and seedbed preparation. Later in the season, increased crop and vegetation cover lead to a decrease in sediment production. Based on the established sediment budget with average rainfall, the lifetime of Lake Tana was estimated as 764 to 1032 years, which is shorter than what was anticipated in earlier studies. The sedimentation rate of Lake Tana (11.7 ± 0.1 kg m^?2 yr^?1) is in line with the sedimentation rates of larger lakes in the world, like Lake Dongting and Lake Kivu. Copyright © 2017 John Wiley & Sons, Ltd.     

A 12-kyr record of microbial branched and isoprenoid tetraether index in Lake Qinghai,northeastern Qinghai-Tibet Plateau: Implications for paleoclimate reconstruction

Branched and Isoprenoid Tetraether (BIT) index was considered as a proxy for terrestrial organic matter input in lake sediments, based on the assumption that branched glycerol dialkyl glycerol tetraethers (bGDGTs) are mainly derived from terrestrial soils. However, mounting evidences have showed that the in situ production of bGDGTs is widespread in lakes, challenging BIT as a reliable terrestrial input proxy. Recently, BIT has been proven to be a reliable proxy for paleohydrology in a small crater lake (Lake Challa) in accordance with a different mechanism. However, the response of BIT to paleohydrology variation may differ for different lakes. In this study, we investigate the variations in the BIT index and the concentrations of its related GDGTs in a 12-ka sediment core from Lake Qinghai, in combination with our previous results for surface sediments. We find that variations in BIT strongly depend on the concentration of crenarchaeol in both surface and ancient sediments of this lake, whereas bGDGT concentration varies much less remarkably. Considering that crenarchaeol production is positively correlated with water depth in Lake Qinghai, water depth may exert negative control on the BIT index in this lake. This case is inconsistent with the positive relationship between BIT and lake levels or rainfall intensity reported for Lake Challa, suggesting that the response of BIT to local paleohydrology is site specific in lacustrine systems. Hence, the application of sedimentary BIT as a paleohydrological proxy in a specific lake requires caution before confirming the environmental controls on BIT in that lake.     

滇池、抚仙湖、阳宗海长期水位变化(1988-2015年)及驱动因子

水位变化影响湖泊水质、水量和生态系统功能,是研究湖泊演变的重要内容,但目前针对滇中高原湖群水位变化特征还少见系统报道.本文选择滇池、抚仙湖、阳宗海3个滇中高原湖泊作为研究对象,基于1988-2015年实测水位数据和Mann-Kendall趋势检验法评估了3个湖泊水位变化特征;运用RClimDex模型获得了流域极端降水指标,结合其他指标构建了基于极端气象因子的湖泊水位驱动力指标体系;采用主成分-多元回归模型,解析了极端降水、蒸发等气象因子对滇中高原湖泊水位变化的贡献.结果表明：①滇池、抚仙湖、阳宗海水位年际波动不突出.滇池的年平均水位总体略呈上升趋势,年均上升0.025 m.阳宗海和抚仙湖水位无明显变化.②滇中高原湖泊流域的极端降水指数年际变化趋势不明显.滇池的蒸发量呈明显减小趋势,年均减小21.05 mm.抚仙湖蒸发量呈明显增加趋势,平均每年增加5.52 mm.阳宗海蒸发量的变化不明显.③气象指标可解释滇池水位变化的49.7%,滇池水位变化受气候变化和人类活动的综合影响;阳宗海和抚仙湖水位变化主要受气象条件控制,蒸发量、综合降水指标和连续降水指标对阳宗海水位变化的解释率高达93.3%;综合降水指标和干旱状况指标可以解释抚仙湖水位变化的64.5%.极端降水指标对解释高原湖泊水位变化具有重要作用.     

Water budget and vertical conductance for Lowry (Sand Hill) Lake in north-central Florida, USA

Water-budget components and the vertical conductance were determined for Lowry (Sand Hill) Lake in north-central Florida, USA. In this type of lake, which interacts with both the surface-water and groundwater systems, the inflow components are precipitation, surface-water inflow, groundwater inflow, and direct runoff (i.e. overland flow), and the outflow components are evaporation, groundwater outflow, and surface-water outflow. In a lake and groundwater system that is typical of many karst lakes in Florida, a large part of the groundwater outflow occurs by means of vertical leakage through an underlying confining unit to a deeper, highly transmissive aquifer called the upper Floridan aquifer. The water-budget component that represents vertical leakage to the upper Floridan aquifer was calculated as a residual using the water-budget equation. For the 13 month period from August 1994 to August 1995, relative to the surface area of the lake, rainfall at Lowry Lake was 1.55 m yr⁻¹, surficial aquifer inflow was 0.79 m yr⁻¹, surface-water inflow was 1.92 m yr⁻¹, and direct runoff was 0.01 m yr⁻¹. Lake evaporation was 1.11 m yr⁻¹, and surface-water outflow was 1.61 m yr⁻¹. The lake stage increased 0.07 m yr⁻¹, and the vertical leakage to the upper Floridan aquifer was 1.48 m yr⁻¹. Surficial aquifer outflow from the lake was negligible. At Lowry Lake, vertical leakage is a major component of the water budget, comprising about 35% of the outflow during the study period. The vertical conductance (K_V/b), a coefficient that represents the average of the vertical conductances of the hydrogeologic units between the bottom of a lake and the top of the upper Floridan aquifer, was determined to be 2.51 × 10⁻⁴ day⁻¹ for Lowry Lake.     

21世纪以来泛北极湖泊水位变化时空特征及原因探讨

在全球气候变暖和极端气候事件增加的背景下,流域水文循环过程受到的影响越来越强烈,导致湖泊水位变化表现出复杂的时空特征。而泛北极地区是地球上湖泊数量与面积分布最为集中的区域之一,该地区湖泊对气候变化响应非常敏感。因此,了解这些湖泊近期水文变化特征十分必要。本研究共搜集了36个泛北极大型湖泊(>500 km²)基于遥感或站点观测的近20年水位数据,分析其时空变化特征。本文使用线性回归模型来估算湖泊水位的变化趋势,进而利用皮尔逊相关分析了其主要水文影响变量和大气环流机制,并运用Mann-Kendall突变检验法探讨了水位突变的原因。结果表明,泛北极湖泊的水位整体上呈现不同程度上升(平均速率为0.013 m/a),有23个(64%)湖泊的水位呈上升趋势;研究湖泊中有10个通过90%统计显著性检验。其中,水位上升速率最大的湖泊是位于哈萨克斯坦的腾吉兹湖,上升速率为0.078 m/a。泛北极湖泊水位的波动主要与径流有关,有19个(53%)湖泊的水位波动与径流的增加更为相关;相比而言,位于亚洲的极地湖泊水位的上升与流域蒸发的降低显著相关,尤其是库苏古尔湖。从区域大气环流影响来看,泛北极湖泊水位变化主要与厄尔尼诺-南方涛动有关,其次是北极涛动和北大西洋涛动。本研究有助于加深对泛北极湖泊近20年水位变化规律及气候影响特征的科学理解。     

乌伦古湖水体矿化度和氟化物浓度的年际变化及模拟

为研究乌伦古湖矿化度和氟化物浓度与各水量因子间的定量关系,本研究以水量平衡和物质平衡为基础,把影响湖水水质的原因分为降雨、蒸发、河流补给、地下水补给以及地下水流出5个水量因子,定量分析湖水矿化度和氟化物浓度与各水量因子之间的关系,并且预测了湖水矿化度和氟化物浓度的动态变化.结果表明,地下水输出流量决定了湖水中矿化度和氟化物浓度,地下水输出流量越大,湖水中的矿化度和氟化物浓度越小.2010年至今乌伦古湖水位的升高导致了地下水输出流量增大,乌伦古湖内的矿化度和氟化物浓度均呈缓慢下降趋势,模型预测新的稳态下湖水中矿化度为1.68 g/L,氟化物浓度为1.70 mg/L,在地下水输出流量不变的情况下达到新的稳态所需时间大约为50 a.     

Estimating daily inflows of large lakes using a water‐balance‐based runoff coefficient scaling approach

Daily river inflow time series are highly valuable for water resources and water environment management of large lakes. However, the availability of continuous inflow data for large lakes is still relatively limited, especially for large lakes situated within humid plain regions with tens or even hundreds of tributaries. In this study, we choose the fifth largest freshwater Lake Chaohu in China as our study area to introduce a new approach to reconstruct historical daily inflows at ungauged subcatchments of large lakes. This approach makes use of water level, lake surface rainfall, evaporation from the lake, and catchment rainfall observations. Rainfall–runoff relationship at a reference catchment was analysed to select rainfall input and estimate run‐off coefficient firstly, and the run‐off coefficient was then transferred to ungauged subcatchments to initially estimate daily inflows. Run‐off coefficient was scaled to adjust daily inflows at ungauged subcatchments according to water balance of the lake. This approach was evaluated using sparsely measured inflows at eight subcatchments of Lake Chaohu and compared with the commonly used drainage area ratio method. Results suggest that the inflow time series reconstructed from this approach consistent well to corresponding observations, with mean R² and Nash–Sutcliffe efficiency values of 0.69 and 0.6, respectively. This approach outperforms drainage area ratio method in terms of mean R² and Nash–Sutcliffe efficiency values. Accuracy of this approach holds well when the number of water‐level station being used decreased from four to one.     

Historical trophic evolutions and their ecological responses from shallow lakes in the middle and lower reaches of the Yangtze River: Case studies on Longgan Lake and Taibai Lake

The evolutions of diatom floras and the total phosphorous (TP) concentrations in the historical period were reconstructed for two lakes, Longgan and Taibai in the middle Yangtze River, based on high resolutional fossil diatom study from two sediment cores and an established regional diatom-TP transfer function. The TP concentration in Longgan Lake changed slightly in the range of 36–62 μg/L and kept its middle trophic level in the past 200 years. The changes of diatom assemblages reflect a macrophyte-dominated history of the lake. During the nineteenth century, the lake TP concentration increased comparatively, corresponding to the increase in abundance of benthic diatoms. The progressive increase of epiphytic diatoms since the onset of the twentieth century indicates the development of aquatic plants, coinciding with the twice drops of water TP level. The TP concentration in Taibai Lake kept a stable status about 50 μg/L before 1953 AD, while diatoms dominated by facultative planktonic Aulacoseira granulata shifted quickly to epiphytic diatom species, indicating a rapid expansion of aquatic vegetation. During 1953–1970 AD, the coverage of aquatic plants decreased greatly inferred by the low abundance of epiphytic diatoms as well as declined planktonic types, and the reconstructed TP concentration shows an obvious rising trend firstly, suggesting the beginning of the lake eutrophication. The lake was in the eutrophic condition after 1970, coinciding with the successive increase of planktonic diatoms. The comparison of the two lakes suggests the internal adjustment and purification function of aquatic plants for nutrients in water. The discrepancy of TP trends in the two lakes after 1960 reflects two different patterns of lake environmental response to human disturbance. Sediments in Taibai Lake clearly recorded the process of lake ecological transformation from the macrophyte-dominated stage to the algae-dominated stage. The limits of TP concentration (68–118 μg/L) in the transitional state can be considered as the critical value between the two stable ecosystems. Further work will be necessary to provide more evidence from the sediments in more eutrophic lakes for the primary inference. The reconstructive TP level and the inference of aquatic plants from fossil diatoms in different lakes, as well as their comparison provide a scientific basis for ecological restoration of eutrophic lakes in research regions.     

Environmental isotope study on hydrodynamics of Lake Naini,Uttar Pradesh,India

Environmental isotopes (δ¹⁸O, δD and ³H) were used to understand the hydrodynamics of Lake Naini in the State of Uttar Pradesh, India. The data was correlated with the in situ physico‐chemical parameters, namely temperature, electrical conductivity and dissolved oxygen. The analysis of the data shows that Lake Naini is a warm monomictic lake [i.e. in a year, the lake is stratified during the summer months (March/April to October/November) and well mixed during the remaining months]. The presence of a centrally submerged ridge inhibits the mixing of deeper waters of the lake's two sub‐basins, and they exhibit differential behaviour. The rates of change of isotopic composition of hypolimnion and epilimnion waters of the lake indicate that the water retention time of the lake is very short, and the two have independent inflow components. A few groundwater inflow points to the lake are inferred along the existing fractures, fault planes and dykes. In addition to poor vertical mixing of the lake due to the temperature‐induced seasonal stratification, the lake also shows poor horizontal mixing at certain locations of the lake. The lake–groundwater system appears to be a flow‐through type. Also, a tritium and water‐balance model was developed to estimate the water retention time of well‐mixed and hydrologically steady state lakes. The model assumes a piston flow of groundwater contributing to the lake. The developed model was verified for (a) Finger Lakes, New York; (b) Lake Neusiedlersee, Austria; and (c) Blue Lake, Australia based on literature data. The predicted water retention times of the lakes were close to those reported or calculated from the hydrological parameters given in the references. On application of this model to Lake Naini, a water retention time of ～2 years and age of groundwater contributing to the lake ～14 years is obtained. Copyright © 2001 John Wiley & Sons, Ltd.     

Modelling lake stage and water balance of Lake Tana,Ethiopia

The level of Lake Tana, Ethiopia, fluctuates annually and seasonally following the patterns of changes in precipitation. In this study, a mass balance approach is used to estimate the hydrological balance of the lake. Water influx from four major rivers, subsurface inflow from the floodplains, precipitation, outflow from the lake constituting river discharge and evapotranspiration from the lake are analysed on monthly and annual bases. Spatial interpolation of precipitation using rain gauge data was conducted using kriging. Outflow from the lake was identified as the evaporation from the lake's surface as well as discharge at the outlet where the Blue Nile commences. Groundwater inflow is estimated using MODular three‐dimensional finite‐difference ground‐water FLOW model software that showed an aligned flow pattern to the river channels. The groundwater outflow is considered negligible based on the secondary sources that confirmed the absence of lake water geochemical mixing outside of the basin. Evaporation is estimated using Penman's, Meyer's and Thornwaite's methods to compare the mass balance and energy balance approaches. Meteorological data, satellite images and temperature perturbation simulations from Global Historical Climate Network of National Oceanographic and Atmospheric Administration are employed for estimation of evaporation input parameters. The difference of the inflow and outflow was taken as storage in depth and compared with the measured water level fluctuations. The study has shown that the monthly and annually calculated lake level replicates the observed values with root mean square error value of 0·17 and 0·15 m, respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Luminescence chronology for late Quaternary lake levels of enclosed Huangqihai lake in East Asian monsoon marginal area in northern China

Lake level fluctuations of closed lakes in monsoonal regions are direct indicators of regional moisture variability related to the intensity of monsoon activities. In the current study, quartz OSL dating was applied to construct the chronology of two sedimentary sequences from the east and west shores of Huangqihai Lake in the East Asian monsoon area, and the history of lake level fluctuations during the late Quaternary was established. In the current study, we present field investigation findings of highstands of >87 m above the modern lake in enclosed Huangqihai Lake in northern China, and also use luminescence dating to establish the timing for the highstands. We conclude that: (1) Lake highstands occurred during both MIS 5 (>87 m above the modern lake) and early Holocene (∼9–8 ka, >54 m above the modern lake), (2) No highstands were observed for MIS 3 and MIS 2 during which loess has accumulated indicative of much drier climate; (3) The evolution of lakes in East Asian monsoon marginal area (e.g. Huangqihai Lake) is linked with the monsoon variation.     

沿江城市中小型闸控通江湖泊总磷变化特征及成因分析——以安庆市石塘湖为例

长江中下游沿江城郊闸控湖泊普遍面临总磷浓度偏高的现象,解析其总磷时空变化特征及影响驱动机制成为精准治理与修复此类湖泊前亟待解决的关键性问题。本文基于安庆市沿江城郊中小型闸控湖泊--石塘湖实测气象降雨、水文、河湖水质等数据,采用多因子相关性分析、变异系数法和主成分分析的方法,研究各指标因子与湖泊总磷浓度时空变化的响应关系。结果表明:(1)湖泊水质呈现丰(5-8月)、平(3-4月和9-10月)、枯(11-次年2月)水期聚类效果显著,但湖泊空间差异不明显;湖泊水质的季节性变化受总磷浓度变化控制,其他理化指标影响较弱。(2)高强度降雨和汛期闸站调度下的水动力变化决定污染物迁移速率,是导致湖泊总磷浓度在丰水期更容易受入湖河流输入影响而达到峰值的主要驱动因子。(3)高强度人类活动导致入湖河流季节性输入是石塘湖总磷上升的决定性因素,从单位土地利用类型产生单位总磷负荷来看,农业用地远大于城镇建设用地,林地和草地充当污染物进入湖泊的预前“汇”。因此,从治理策略和途径来看,可在控制外源输入的同时,适当调节闸站以减弱丰水期水动力强度,减少入湖河流总磷输入影响的同时,发挥湖泊营养盐的滞留净化能力。本文主要从人类活动导致的外源污染及闸站抽排调度角度分析沿江城郊闸控湖泊石塘湖总磷变化、影响因素及驱动机制,可以为此类城郊中小型沿江闸控型湖泊污染治理提供理论参考。     

Phosphorus loading, transport and concentrations in a lake chain: a probabilistic model to compare management options

Phosphorus (P) loading, exports and concentrations of the four lakes of the Yahara chain (Wisconsin, USA) were compared under four load-reduction plans using a model calibrated with 29–33 years of annual data. P mitigation goals must balance reductions in P concentrations in the four lakes and the export from the lake chain to downstream waters. Lake Mendota, the uppermost lake, is most responsive to P load reductions, and benefits diminish downstream. Nonetheless, the greatest reductions in export from the lake chain to downstream waters derive from P load reductions to lakes lower in the chain. The effective grazer Daphnia pulicaria causes large improvements in water quality. Management to maintain populations of D. pulicaria has substantial benefits that augment those from reductions in P loading. Model projections show high variability in water quality and exports under all load-reduction plans. This variability is driven by inter-annual variation in runoff. Thus lake managers and the public should expect ongoing year-to-year variability in water quality, even though P load mitigation will improve water quality on average. Because of high variability from year to year, ongoing monitoring is essential to assess the effects of management of this chain of lakes.     

Analysis of the hydrological response of a tropical terminal lake,Lake Abiyata (Main Ethiopian Rift Valley) to changes in climate and human activities

Integrated dynamic water and chloride balance models with a catchment‐scale hydrological model (PRMS) are used to investigate the response of a terminal tropical lake, Lake Abiyata, to climate variability and water use practices in its catchment. The hydrological model is used to investigate the response of the catchment to different climate and land‐use change scenarios that are incorporated into the lake model. Lake depth–area–volume relationships were established from lake bathymetries. Missing data in the time series were filled using statistical regression techniques. Based on mean monthly data, the lake water balance model produced a good agreement between the simulated and observed levels of Lake Abiyata for the period 1968–83. From 1984 onwards the simulated lake level is overestimated with respect to the observed one, while the chloride concentration is largely underestimated. This discrepancy is attributed to human use of water from the influent rivers or directly from the lake. The simulated lake level and chloride concentration are in better agreement with observed values (r² = 0·96) when human water use for irrigation and salt exploitation are included in the model. A comparison of the simulation with and without human consumption indicates that climate variability controls the interannual fluctuations and that the human water use affects the equilibrium of the system by strongly reducing the lake level. Sensitivity analysis based on a mean climatic year showed that, after prolonged mean climatic conditions, Lake Abiyata reacts more rapidly to an abrupt shift to wetter conditions than to dry conditions. This study shows the significant sensitivity of the level and salinity of the terminal Lake Abiyata to small changes in climate or land use, making it a very good ‘recorder’ of environmental changes that may occur in the catchment at different time scales. Copyright © 2004 John Wiley & Sons, Ltd.     

Analysis of groundwater–lake interaction by distributed temperature sensing in Badain Jaran Desert,Northwest China

The Badain Jaran Desert is the second largest desert of China with a total area of 49 200 km². At least 72 perennial lakes are scattered throughout the desert, sustaining a unique desert–lake ecosystem. Groundwater of various origins was believed to play an essential role in maintaining those desert lakes, but hydrological measurements are lacking due to difficult field conditions. This study applied the distributed temperature sensing technique to continuously measure temperature variations in one of the desert lakes – the Badain Lake – to identify groundwater discharge to the lake based on the temperature differences between groundwater and lake water. Because temperature may be influenced by various unforeseen and temporary factors, it is critical to discern those factors that may affect the temperature such as solar radiation and vertical temperature stratification and to ensure that the temperature variations of the lake water as detected by the distributed temperature sensing are mainly caused by groundwater discharge. A time window was identified during which the groundwater discharge is the dominant factor that determines the temperature pattern of the lake water. The results show that the temperature near the eastern and southeastern lakeshore of the eastern Badain Lake is colder than the average, indicating that this area is the main groundwater discharge zone. Near the northwestern lakeshore adjacent to a sand dune, a weak cold abnormal area was identified, indicating that the sand dune is another recharge source to the lake through channelling the local precipitation toward the lake. The contribution from the sand dune, however, appeared to be less than that from the regional groundwater flow. This study provides the first identification of the temperature abnormal areas, which imply groundwater discharges into desert lakes and contributes to a better understanding of the unique desert–lake ecosystem. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical estimates for the influence of Lake Victoria on flows in the upper White Nile

Abstract

A theoretical model is described for estimating the impacts of changes in Lake Victoria levels on river flows, lake levels and swamp areas in the upper White Nile basin. The basis of the model is to represent the main river channel by a series of interconnected lakes and swamps, whose water balances are described by differential equations relating outflows to levels, areas and the net basin supply at each point. Closed form solutions are obtained for two situations: (a) a long-term change in the mean level of Lake Victoria, and (b) a return to equilibrium levels following an initial disturbance in Lake Victoria levels. A simple model for the net basin supply to Lake Victoria is also used to relate these changes in levels to changes in climate and runoff in the basin. The results illustrate the extreme sensitivity of White Nile flows to changes in Lake Victoria levels and outflows, and in particular to variations in the direct rainfall on the lake surface. Estimates are also presented for the various time scales and time delays which affect the White Nile system.     

The impact of land use change on water resources in sub-Saharan Africa: a modelling study of Lake Malawi

A modelling study to investigate the effects of land use change from natural forest to agricultural land on large-scale catchment runoff in southern Africa is described. The evaporative component of the model considers the catchment to be composed of one of three surface types—forest, agricultural land or water surface. Values of the model parameters for the forest and agricultural lands were obtained from experimental studies carried out in the dry zone of India. Estimates of average monthly potential evaporation, together with measurements of monthly rainfall, were used in the model to predict the monthly levels of Lake Malawi. These were compared with observed levels. From 1896 to 1967 the major fluctuations in lake level, both seasonally and annually, are well described by this model (excepting the period from 1935 to 1945, immediately following the time when there was no outflow from the lake) using a value of 64% for the forest coverage of the catchment. The overall agreement between prediction and observation indicates that variations in rainfall alone, without changes in either evaporative demand or in the hydraulic regime of the lake, are sufficient to explain lake level changes. For the more recent period (1954–1994), model predictions of lake level which take into account a decrease in forest cover of 13% over the period 1967–1990 (consistent with the actual decrease in forest cover for this period) agree well with observations both annually and seasonally. Without this decrease in forest cover, the model predicted that the lake level would have been about 1 m lower than that observed during the southern African drought of 1992. The model, in conjunction with real-time rainfall data obtained from land-based gauges, radar or satellite observations, can be used for real-time water resource management applications such as the operation of barrages regulating the flow from Lake Malawi or for the issuing of flood or drought warnings.     

Volcanic lake systematics I. Physical constraints

 Volcanic lakes have a wide range of characteristics, and we make an attempt to delineate the limiting physical conditions for several lake classes. The ratio between heat input and heat dissipation capacity of a lake constrains the temperature for perfectly mixed steady-state volcanic lakes. Poorly mixed lakes are also conditioned by this ratio, but their temperature structure is also strongly influenced by the hydrodynamics resulting from different mechanisms of heat transfer. The steady-state temperatures of volcanic lakes are largely determined by the magnitude of the volcanic heat influx relative to the surface area of the lake. Small lakes have only a small capacity for heat dissipation and their temperature rises quickly with only small heat inputs; large lakes are buffered against variations in heat input. Both the heat dissipation and meteoric water input into a lake are functions of lake surface area and therefore each lake water temperature demands a certain precipitation rate for mass conservation, independent of lake size. The results of energy/mass-balance modeling shows that under common atmospheric conditions, most steady-state volcanic lakes are unlikely to maintain a temperature in excess of 45–50  °C. Validation of the volcanic lake model was performed using published data from Yugama Lake (Japan) and the Keli Mutu lakes (Indonesia). Also, the model was applied to 24 natural systems to provide a baseline assessment of energy fluxes under the model assumptions so future work on those systems can identify nuances in individual systems that deviate from the simple model conditions. We recommend the model for use in assessing temperature variations and volcanic lake stability in settings with known physical and atmospheric conditions. Application of the energy/mass balance calculations of model lakes provides a genetic classification scheme largely based on physical process parameters. Received: 28 February 1996 / Accepted: 23 November 1996     

Socio-economic drought assessment in Lake Mead,USA, based on a multivariate standardized water-scarcity index

Drought events are increasing worldwide. Socio-economic drought is the least investigated type of drought, and is the only type for which water demand is taken into consideration. In this research, socio-economic drought was studied in Lake Mead, USA, using a multivariate standardized water-scarcity index (MSWSI) over the period 1990–2014, combining two water-scarcity indices based on time series of inflow, outflow, reservoir storage, and water demand. The inflow and outflow were determined from streamgage data, and reservoir storage from lake level data; demand was based on water pumped by the Southern Nevada Water Authority. Missing observations in input streamgage data were filled through regression modeling. The results indicate that Lake Mead has been in socio-economic drought since 2000, with the most severe drought occurring between 2006 and 2012, and the highest intensity drought in April–July 2014. The Lake Mead droughts revealed through the MSWSI are consistent with those reported in US Drought Monitor (USDM) products. The temporal behavior of MSWSI provides an insight into the socio-economic effects of droughts not captured by USDM products.