Investigating the restoration of Lake Urmia using a numerical modelling approach

We assessed the effectiveness of multiple hydrologic restoration scenarios for Lake Urmia, as well as the variation of its salinity regime under no intervention, using a 2D shallow water model. Tested scenarios, proposed by Urmia Lake Restoration Program Committee, include: Preservation of current lake status (no intervention), complete closing of Shahid Kalantari causeway, dyke construction in the southern part of Lake Urmia, water transfer from Zarrinehrood River to Siminehrood River and reduction of agricultural water consumption by best agricultural practices. Results indicate that neither the closure of the causeway nor the construction of the southern dyke would significantly improve lake conditions when compared to preservation of current lake status. The water transfer alternative doesn’t seem to have any effect on the current lake conditions either. However, the reduction on water diversions by improving agricultural practices in the lake's basin leads to a partial restoration of the lake in terms of water level, surface area and volume. If current conditions persist, salinity in the northern part of Lake Urmia will reach supersaturation levels (340?g/L), generating further salt deposits.     

Investigation Anthropogenic Impacts and Climate Factors on Drying up of Urmia Lake using Water Budget and Drought Analysis

The water level of Urmia Lake, the largest inland lake in Iran with maximum water surface area of about 6000 km², has been shrinking for the last two decades. Although a number of study have been performed to determine drought condition and coastline changes of Urmia Lake, there has not been a detailed study to distinguish anthropogenic effects from climate impacts on the drying of Urmia Lake. In this study, water budget of Urmia Lake and the intensity of drought in the basin were analyzed in the period from 1985 to 2010 and a new hypothesis is proposed to quantify anthropogenic and climate impacts in reducing the volume of Urmia Lake. The results of this study indicate that human impacts on the Lake and its basin are more important than climate factors. Though previous studies assumed that ground water output from Urmia Lake is negligible, the results of this study show the presence of significant groundwater seepage from Urmia Lake. Major changes in the variables that reduced the water level of Urmia Lake were observed since 1998. Anthropogenic impacts and climate factors have roughly 80% and 20% effects on the drying up of Urmia Lake, respectively. Hence, the first step to recover Urmia Lake could be the revision of management surface water, operation of dams and groundwater resources. The second step could be the review and classification of agricultural products grown in the region in terms of water consumption and teach local people the best practice methods for irrigation.     

Lake Turkana,major Omo River developments,associated hydrological cycle change and consequent lake physical and ecological change

This study aimed to explore Lake Turkana's ecological reliance on hydrology and to determine the hydrological changes and consequences arising from the major hydropower and irrigation developments in the lake's basin. The major developments on Ethiopia's Omo River are especially significant as this river provides over 80% of the lake's annual freshwater influx and associated nutrients. The cascade of hydropower dams permanently dampens the natural hydrological cycles and lake level variability. The driving force of the flood influx to the lake is curtailed and the pattern of lake currents will adjust. Ultimately 80% of the river inflow to the lake will be regulated. Large volumes of water are required to initially fill the hydropower dam reservoirs. During 2015–16 when the huge Gibe III reservoir was filled, Lake Turkana's water level declined 2?m.The study has shown that large-scale irrigation schemes in the Lower Omo can potentially abstract 50% of the Omo River water, and that this would cause the lake level to shrink permanently to the detriment of the lake ecology. Possible lake level drops of over 15?m are demonstrated. The basin's natural capital is being replaced by large-scale plantation developments. The hydrological changes are drastic and the ecological consequences on Lake Turkana have not been fully understood. Without serious mitigation measures, Lake Turkana is a potential African Aral Sea disaster in the making, emulating what has happened to other great lakes such as Lake Chad.     

Using satellite data to extract volume–area–elevation relationships for Urmia Lake,Iran

Urmia Lake in the northwest of Iran is the second largest hyper-saline lake worldwide. During the past two decades, a significant water level decline has occurred in the lake. The existing estimations for the lake water balance are widely variable because the lake bathymetry is unknown. The main focus of this study is to extract the volume–area–elevation (V–A–L) characteristics of Urmia Lake utilizing remote sensing data and analytical models. V–A–L equations of the lake were determined using radar altimetry data and their concurrent satellite-derived surface data. Next, two approximate models, a power model (PM) and a truncated pyramid model (TPM), were parameterized for Urmia Lake and checked for accuracy. Results revealed that in comparison with the satellite-derived reference volume–elevation equation, the PM slightly over-predicts the volume of Urmia Lake while the TPM under-estimates the lake storage. Variations of the lake area and volume between 1965 and 2011 were examined using the developed V–A–L equations. Results indicated that the lake area and volume have declined from the historical maximum values by 2200 km² and 33 km³, respectively. To restore Urmia Lake to a level to maintain ecological benefits, 13.2 km³ of water is required. This study demonstrates the use of remote sensing data of different types to derive V–A–L equations of lakes. Substituting satellite-derived V–A–L equations for common empirical formulas leads to more accurate estimations of a lake water balance, which in turn, provides insight to water managers for properly assessing and allocating water resources to downstream ecosystems.     

Environmental management in Urmia Lake: thresholds approach

Urmia Lake is a vivid example of negligence in providing the environmental water requirement rooted in prolonged droughts and excessive upstream water withdrawal. The aim of this study is to propose a reclamation strategy for the lake by focusing on estimation of environmental requirement thresholds under various drought severity conditions. On the average, the lake will remain in a moderate hydrological drought condition if the business-as-usual scenario prevails. The shares of natural drought, water withdrawals and water resources development projects (i.e. dams) in recent lake inflow reduction are estimated at 45%, 40% and 15%, respectively; thus, the aggregated contribution of human-induced factors has a more significant impact on the current lake condition than that of natural drought, and a firm and consistent approach to reduce water consumption by the agricultural sector is required for lake reclamation.     

Effects of Anthropogenic Influences on the Trophic State,Land Uses and Aquatic Vegetation in a Shallow Mediterranean Lake: Implications for Restoration

Lake Pamvotis, NW Greece is a shallow Mediterranean eutrophic lake that has changed drastically over the past 50 years. Strong effects, resulted mainly from anthropogenic causes, in the hydrological regime are shown for this area using long term hydrological data and a GIS system for extracting land cover/use changes. A set of aerial imagery acquired in 1945 through 2002 were used to monitor and assess the spatial and temporal changes in land cover/use, focused mainly on the lake’s surface area and its surrounding ecosystem (Natura 2000 area). The significance of the changes in land cover/use distribution within Pamvotis wetland is further discussed depicting the role of the anthropogenic influence on the fragile ecosystem that resulted in the shrinkage of lake’s habitats extent. The purpose of this analysis was to examine the long-term changes on macrophyte community composition, species occurrence and relative abundance with water quality and water level changes over the past century, using historical data, aerial photos and GIS techniques. The results showed that for the last 25 years annual water level fluctuation ranged from 70 to 159 cm. Water level starts decreasing in mid June and increasing again gradually from November until March–April. Intra annual water level fluctuation seems to be affected by land use for agricultural purpose through intensive irrigation and the summer drought as well. A dramatic decline of the submerged vegetation is apparent mainly attributed to anthropogenic pressures. Regarding the land cover/use changes, the most notable and significant alterations are concerning the urban development around the lake, the disappearance of wet meadows and the extension of reed beds. Finally it seems that water budget data as well as the response of the key eutrophication parameters are affected from both hydrological alterations and point/non-point pollution sources.     

Analysis of Effective Environmental Flow Release Strategies for Lake Urmia Restoration

Saline lakes have diminished considerably due to large-scale irrigation projects throughout the world. Environmental flow (EF) release from upstream reservoirs could help conserve and restore these lakes. However, experiences from regions lacking environmental legislation or with insufficient water resources management show that, despite EF allocation, farmers tend to use all available water for agriculture. In this study, we employed a new method for designing environmental flow release strategies to restore desiccated terminal lakes in arid and semi-arid regions with intensive cultivation within the catchment. The novelty of the method is that it takes into account farmers’ water use behavior and the natural flow regime in upstream systems to design an optimum monthly EF release strategy for reservoirs. We applied the method to the water resource system of Lake Urmia, once the largest saline lake in the Middle East and now one of the most endangered saline lakes in the world. The analysis showed that the EF released is exploited by lowland farmers before reaching Lake Urmia and that inflow to the lake from some rivers has decreased by up to 80%. We propose a new EF release strategy that requires a considerable change in practice whereby water is released in the shortest possible time (according to reservoir outlet capacity) during the period of lowest irrigation demand in winter. Restoring the lake to minimum ecological level would require 2.4–3.4 km³ EF allocation by different methods of release based on the recent condition (2002–2011) of the lake.     

Hydrological Characterization of an Ice-Contact Lake: Miage Lake (Monte Bianco,Italy)

The Miage Lake is an ice-contact lake located on a side of the Miage glacier (Monte Bianco Massif, Alps), the third Italy’s largest glacier, and is characterized by rapid drawdown episodes that have occurred with varying frequency in the lake’s history. The emptying episode of September 2004 gave the possibility of performing many activities, up to the filling phases which occurred during spring and summer 2005. In situ tests and measurements and laboratory tests lead to a reliable reconstruction of the path of water recharge and draining and to the development of a numerical model of unsaturated-saturated groundwater flow to evaluate natural losses through the lake bed sediments. Results show that material at the bed of the lake has a low permeability and can be responsible for water losses of a few litres per second, very far from the discharge calculated during the rapid drawdown episodes. These results gave the experimental and numerical evidences to support the hypothesis that identifies sudden and temporary failure in the ice floor as the only possible cause of the rapid draining events historically occurred in Miage Lake. In the monitoring period the hydrological balance has also been assessed to evaluate the factors affecting the main recharge phase of the year. Calving was found to be the most important factor contributing to recharge, followed by direct melting and precipitation while water losses occurring through the lake bed sediments and evapotranspiration are much smaller even if not negligible.     

Deriving and Evaluating Bathymetry Maps and Stage Curves for Shallow Lakes Using Remote Sensing Data

Urmia Lake as a most vital water bodies in Iran, has been shrinking since the late twentieth century and its area has dramatically decreased. To develop and apply any plans to survive the lake, qualitative and quantitative analysis and any modeling, deriving physical information such as volume, area and their changes are very crucial. The objectives of this study were therefore, intended firstly, to study the bathymetry of Urmia Lake with a more satisfactory approach using Landsat- LDCM satellite image and in situ measurement data. The polynomial model was developed to predict the water depth in Urmia Lake area. This model was developed with the input series of reflectance values from blue, green, red and NIR bands in the Landsat- LDCM satellite imagery for Urmia Lake taken on 12 April 2013 of the sampling sites from actual depth measured were taken on the same date. Also, using a large archive of Landsat imagery (TM, ETM+ and LDCM), a counter of equivalent elevation were established for deriving the bathymetry of desiccated areas by mapping the edges of the lake and finally assembled with bathymetry derived from polynomial model. In-situ depth measurements were used to evaluate resultant derived bathymetric map. This comparison shows reasonable agreement between the Landsat-derived depths and those measured in the field with RMSE of 0.27 cm and R² = 0.91. The maximum and mean depths measured were 4.9 and 11 m respectively. The maximum depth measured was located at the upper part of the lake. As well as, developed multi-regression equation used for deriving another bathymetry map using Landsat- LDCM satellite image taken on Sep. 2015 for salt deposition monitoring. Results indicates that about 64 cm salt deposition has occurred during the last two years. Secondly, to make stage curves of lake, multi-temporal changes of water body have been derived from Landsat, MODIS and AVHRR satellite images sets since 1972. In this regard, the area of Urmia Lake at different level was estimated base on object oriented and pixel base classification using 78 satellite images. Finally, stage curve (volume- area- level relations) was derived from bathymetry map.     

Satellite based lake bed elevation model of Lake Urmia using time series of Landsat imagery

This study presents a lake bed elevation model of Lake Urmia. In the course of model generation, a time series of the extent of the lake surface was derived from 129 satellite images with different acquisition dates based on the Landsat sensors Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), and Operational Land Imager (OLI). Due to the rapid shrinking of the lake during the last two decades, lake surface areas ranging from 890 km² to 6125 km² could be covered. The water edge of the various lake extents was then linked to the observed water level on the day of the satellite image acquisition. The resulting contour lines, covering water levels between 1270.04 m and 1278.42 m a.s.l. and thus representing the lakebed morphology in its shallow parts, were merged with existing data (deeper parts) and interpolated to generate a lake bed elevation model. Finally, Lake Urmia’s Level-Area-Volume relationships were derived from the lake bed elevation model and compared to bathymetric data previously published.     

Modeling the effects of human influences on water quality and quantity in the Zarrineh River Basin,Iran

This study aims to analyze the environmental and economic impacts of land use change and water abstraction in the Zarrineh River basin, which is Lake Urmia’s main inflow source in the north-west of Iran. The Soil & Water Assessment Tool (SWAT) is used to model water quantity and quality in the basin. Environmental Kuznets Curves (EKC) are used to assess the relationship between environmental degradation and the development of the agricultural sector. Two scenarios were employed to identify the effects of dam construction and land use change on stream flow, evapotranspiration, groundwater recharge and nitrate loads in the area. The SWAT model showed a satisfactory to very good performance for monthly stream flow at most of the gauges in calibration and validation periods as well as a reasonable performance for nitrate loads. The estimated EKC indicates that the environmental degradation in the inverted U-shape that has passed its peak and the rate of degradation has decreased. Based on the scenarios, the dam has a strong impact on nitrate loads in the basin and water inflow into Lake Urmia. Results verify that human influences have had negative impacts on the Zarrineh River basin. In particular, the extensive development of the agricultural sector has not only drastically reduced the inflow of water to Lake Urmia accelerating the drying up of the lake, but it has also increased nitrate loads. Therefore, it can be recommended to take counter measures within the catchment area to improve the ecological status of Lake Urmia.     

Evaluation of the Bankruptcy Approach for Water Resources Allocation Conflict Resolution at Basin Scale,Iran’s Lake Urmia Experience

Water conflicts appear when there are insufficient and less available water resources than water demands claimed by different agents. In this study, a new bankruptcy approach is investigated to resolve water conflicts in the Zarrinehroud River Basin, the largest and most important sub-basin of Lake Urmia’s Basin in the northwest of Iran. The new bankruptcy method is compared with the proportional rule (PRO) and another alternative based on the cessation of irrigated agriculture in the region proposed to supply and save environmentally in danger Lake Urmia. Four scenarios consisting of the current situation, optimistic, average and pessimistic scenarios regarding the future of water resources of the basin and agricultural developments were considered. According to the results, both bankruptcy rules helped Lake Urmia to receive more water, but neither could utterly overcome the water shortage of the Lake, so can be used as supplementary actions alongside other solutions. The cessation of irrigated agriculture throughout the basin overcame the average annual shortage of Lake Urmia in the first and second scenarios equal to 137 and 148 million cubic meters respectively. It showed disability to fully supply the lake in the third and fourth scenarios. These three methods must be combined with a social-economic policy like the purchase of decreased water allocations to farmers to be socially acceptable.     

Environmental impacts of desalination on the ecology of Lake Urmia

Lake Urmia, the second largest hypersaline lake by area in the world, has fluctuated in salinity over time, but in recent years, it has reached a maximum of 340 g/L. The lake is the main habitat for the endemic Iranian brine shrimp, Artemia urmiana, and is a protected aquatic environment. Efforts have been made by the Iranian government to enhance the diversity of its wildlife. One approach has been to look for a method to reduce the salt content of the lake. We investigate the feasibility of this by first considering the water chemistry of Lake Urmia and then the various technologies used to extract salt from marine and brackish waters. Average concentrations of Na, Mg, K, Ca, Cl, SO₄, and HCO₃ were 125 g/L, 11.3 g/L, 2.63 g/L, 0.55 g/L, 216 g/L, 22.4 g/L, and 1.38 g/L, respectively, and cations and anions were balanced, However, Lake Urmia waters have a ‘very high’ salinity hazard and a high sodium adsorption ratio (SAR). Moreover, the saturation index (SI) for each of the major cations was greater than zero, indicating that the water in Lake Urmia is supersaturated, and precipitation is likely. The extraction of available salts from the lake for the use in petrochemical industries is economically feasible. However, technologies based on removing salts by distillation or reverse osmosis and then using this fresh water to dilute lake salinity are problematic. A better strategy would be better to allow more fresh water to reach the lake rather than creating fresh water through reverse osmosis and distillations processes. While concerns have been raised about the salinity tolerance of A.urmiana, it has successfully tolerated various salinity ranges from 166 to 340 g/L, and so the species is not threatened, unless the lake desiccates. Because the lake is saturated with salts, it seems unlikely that salinity could increase much higher.     

Lake Urmia restoration success story: A natural trend or a planned remedy?

Lake Urmia is the second-largest hypersaline lake in the world. There has been a drastic water level drop of 7.2 m from 1995 to 2016. Beginning in October 2013, the Lake Urmia Restoration Plan (LURP) launched a 10-year program. An increase in water level and a relative improvement in Lake Urmia condition has been observed since 2017. It is an undecided and controversial issue whether the recent positive trend of Lake Urmia has been due to the LURP activities or it is a natural contribution of climate factors variations. To shed some light on this issue, we examine three other lakes, adjacent to the Lake Urmia basin, with similar rainfall variability to investigate their status during the same period. Van (Turkey), Mosul, and Tharthar (both in Iraq), are evaluated as well as Lake Urmia. Three decades of remotely sensed data including precipitation ($P$), water level ($\mathit{WL}$), and lake extent ($A$) were considered for the mentioned lakes. A significant correlation was observed between standardized $\mathit{WL}-P$, and $A-P$ over the long-term period, especially for the recent three years ($R$² = 0.63–0.87). We show that the cumulative precipitation in the antecedent months played a major role in the improvement of these lakes' situation but with different time lags (up to 6 months for Van and Mosul lakes and up to 36 months for Lake Urmia and Tharthar lake). These findings could inform the planners of LURP to adopt strategies for achieving a sustainable state of Lake Urmia based on a more realistic outlook.     

Evaluation of dike-type causeway impacts on the flow and salinity regimes in Urmia Lake,Iran

Urmia Lake, located in a closed basin in north-west Iran, is the largest lake (5000–6000 km²) in the Middle East. It is very saline with total dissolved salts reaching 200 g/l compared with a normal seawater salinity of about 35 g/l. The construction of a causeway, which was initiated in 1979 but then abandoned until the early 2000s, is near completion and will provide road access between the western and eastern provinces. The causeway has an opening 1.25 km long and divides Urmia Lake into a northern and southern basin and restricts water exchange. The flow and salinity regimes are affected by the presence of this new causeway, and there are concerns over the well being of the Artemia population. This study investigates the effects of the construction of the causeway on flow and salinity regimes, considers remedial actions, and examines the effects of climatic variability on salinity and flow. Flow and salinity regimes were numerically simulated by using a commercially available two and three-dimensional (2D and 3D) MIKE model. The validity of the numerical model was assessed through sensitivity analysis of the model and comparing the simulated results against field measurements; the 3D model provided the higher correlation between simulated and actual data. Wind input was the main climatic and hydrologic factor influencing flow regime while river discharge, evaporation and rainfall were the key parameters affecting salinity distribution in the lake models. The 3D model was subsequently used to predict lake conditions in typical dry, wet and normal climates, to examine the environmental impacts from the new causeway, and to evaluate possible improvements that some remedial measures may provide.     

The use of NDVI and its Derivatives for Monitoring Lake Victoria’s Water Level and Drought Conditions

Normalized Difference Vegetation Index (NDVI), which is a measure of vegetation vigour, and lake water levels respond variably to precipitation and its deficiency. For a given lake catchment, NDVI may have the ability to depict localized natural variability in water levels in response to weather patterns. This information may be used to decipher natural from unnatural variations of a given lake’s surface. This study evaluates the potential of using NDVI and its associated derivatives (VCI (vegetation condition index), SVI (standardised vegetation index), AINDVI (annually integrated NDVI), green vegetation function (F _g ), and NDVIA (NDVI anomaly)) to depict Lake Victoria’s water levels. Thirty years of monthly mean water levels and a portion of the Global Inventory Modelling and Mapping Studies (GIMMS) AVHRR (Advanced Very High Resolution Radiometer) NDVI datasets were used. Their aggregate data structures and temporal co-variabilities were analysed using GIS/spatial analysis tools. Locally, NDVI was found to be more sensitive to drought (i.e., responded more strongly to reduced precipitation) than to water levels. It showed a good ability to depict water levels one-month in advance, especially in moderate to low precipitation years. SVI and SWL (standardized water levels) used in association with AINDVI and AMWLA (annual mean water levels anomaly) readily identified high precipitation years, which are also when NDVI has a low ability to depict water levels. NDVI also appears to be able to highlight unnatural variations in water levels. We propose an iterative approach for the better use of NDVI, which may be useful in developing an early warning mechanisms for the management of lake Victoria and other Lakes with similar characteristics.     

Lake-Level Change and Water Balance Analysis at Lake Qinghai,West China during Recent Decades

Lake Qinghai, the largest saline lake with an area of 4,260 km² (2000) and average depth of 21 m (1985) in West China, has experienced severe decline in water level in recent decades. This study aimed to investigate water balance of the lake and identify the causes for the decline in lake level. There was a 3.35-m decline in water level with an average decreasing rate of 8.0 cm year⁻¹ between 1959 and 2000. The lake water balance showed that mean annual precipitation between 1959 and 2000 over the lake was 357 ± 10 mm, evaporation was 924 ± 10 mm, surface runoff water inflow was 348 ± 21 mm, groundwater inflow was 138 mm ± 9 and the change in lake level was −80 ± 31 mm. The variation of lake level was highly positively correlated to surface runoff and precipitation and negatively to evaporation, the correlation coefficients were 0.89, 0.81 and −0.66, respectively. Water consumption by human activities accounts for 1% of the evaporation loss of the lake, implying that water consumption by human activities has little effect on lake level decline. Most dramatic decline in lake level occurred in the warm and dry years, and moderate decline in the cold and dry years, and relatively slight decline in the warm and wet years, therefore, the trend of cold/warm and dry climate in recent decades may be the main reasons for the decline in lake level.     

Water Balance Study and Irrigation Strategies for Sustainable Management of a Tropical Ethiopian Lake: A Case Study of Lake Alemaya

Lake Alemaya in the Ethiopian Highlands has historically provided the surrounding area with water for domestic use, irrigation, and livestock and has served as a local fishery tank. Increasing irrigation and domestic water use, change in the local climate and changes in the surrounding land cover are believed to be the causes of Lake Alemaya’s demise. Expansion of major irrigated crops in particular chat (Catha Edulis), potato and vegetables and non-judicious use of irrigation water in the Lake Alemaya watershed led to presumption that irrigation is partly responsible for the withdrawal of large quantity of water from the lake. Thus, water balance study of Lake Alemaya was carried out under presumed scenarios in order to study the possible trends and fluctuations of the lake water level in response to proposed scenarios. Further, it is essential to study the irrigation performance for developing optimal irrigation schedules in the study area to make the best use of available water for long term sustainability of the water resources of Lake Alemaya. It was identified that expansion of the irrigated area in general and chat cultivation in particular in the study area have been the key to sustainable management of lake water, hence its expansion during the past 37 years (1965–2002) was studied through interpretation of satellite data. Subsequently, performance evaluation of the small-scale irrigation practices for major irrigated crops was carried out. Optimal irrigation schedules for different crop seasons were also developed for these irrigated crops using CROPWAT software. It was found that chat area increased from 190 ha in 1996 to nearly 330 ha in 2002. Further, it was observed that 43% surface area of the lake has reduced within a span of 37 years. Overall, maximum irrigation intensity of chat, potato and vegetables is observed during the first irrigation season of the crop calendar. Particularly, in case of chat, irrigation performance indicators such as Relative Water Supply (RWS), Relative Irrigation Supply (RIS), Depleted Fraction (DF) and Overall Consumed Ratio (OCR) values indicated poor performance of irrigation practices. From the analysis, it was found that the application of a fixed irrigation depth and fixed irrigation interval combinations of (25 mm—25 day), (20 mm—20 day), or (20 mm—25 day) are recommended for chat in the study area. Optimal irrigation schedules were decided on the basis of combination of irrigation interval and depth that results in low loss of irrigation water with reasonable yield reduction. Thus, determination of appropriate water management strategy can ensure proper utilization of the available water resources and improve the water application efficiency of the small-scale irrigation practices around Lake Alemaya, Ethiopia.     

Prediction of Water Level using Monthly Lagged Data in Lake Urmia,Iran

Water Resources Management - Prediction of water level fluctuations in lakes is a necessary task in hydrological and limnological studies. Lake Urmia, a hyper-saline lake in the North Western part...     

Simulation of Recent and Projected Total Phosphorus Trends in Lake Ontario

Recent trends (1965 through 1978) of total phosphorus were analyzed with a time-variable, nutrient budget model of Lake Ontario. Future conditions were also simulated to estimate the effect of anticipated control measures on the lake's water quality.The analysis suggests that recent improvements in Lake Ontario's total phosphorus concentration are attributable to point source reductions due to detergent limitations and waste treatment by the Province of Ontario and the State of New York. Projections show that present point source controls will maintain the lake at the upper level of mesotrophy (15-20 μgP/L) to the year 2000, whereas the absence of controls would result in eutrophy (～30 μgP/L). Further indications are that some diffuse source reduction may be required if oligotrophy (<10 μgP/L) is the ultimate goal and that Lake Ontario's fate is closely related to that of Lake Erie.An attempt is made to assess the effect of model uncertainty and phosphorus availability on the projections. In general, the inclusion of uncertainty indicates that more stringent load reductions will be needed to meet water quality objectives with greater than 50% certainty. Inclusion of availability tends to improve prospects for lake restoration and to enhance point as opposed to diffuse source controls.