The Lake Water Balance (or Lake Water Budget) is a quantitative account of the inputs, outputs, and changes in storage within a lake basin over a specific period. It is an application of the principle of Conservation of Mass.
The Lake Water Balance Equation
The fundamental equation governing the water level of a lake is:
Δ S = (P + Rin + Gin) – (E + Rout + Gout + O)
- Δ S: Change in storage (indicated by the rise or fall in lake water level).
- Inputs:
- P (Precipitation): Direct rainfall or snowfall onto the lake surface.
- Rin (Surface Inflow): Water entering via rivers, streams, or overland runoff.
- Gin (Groundwater Inflow): Seepage from the surrounding water table into the lake.
- Outputs:
- E (Evaporation): Water loss from the lake surface to the atmosphere.
- Rout (Surface Outflow): Water leaving through an outlet river or spillway.
- Gout (Groundwater Outflow): Water seeping out through the lake bed into the aquifer.
- O (Other/Anthropogenic): Human extractions for irrigation, industry, or domestic use.
Dynamics of Input and Output
The nature of the lake—whether it is “Open” or “Closed”—dictates its water balance and chemical composition.
Open Lakes (Exorheic)
- Definition: Lakes that have a surface outlet (a river flowing out).
- Water Balance: These lakes are usually freshwater because the outflow carries away dissolved salts. The water level is relatively stable because excess inflow is discharged through the outlet.
- Example: Lake Victoria (Source of the White Nile) or Wular Lake (Jhelum flows through it).
Closed Lakes (Endorheic/Terminal)
- Definition: Lakes with no surface outlet; water leaves primarily through evaporation.
- Water Balance: These lakes are highly sensitive to changes in climate. If evaporation exceeds inflow, the lake shrinks and salinity increases.
- Example: Sambhar Lake (Rajasthan), Dead Sea, and Pangong Tso (Ladakh).
Factors Influencing the Water Budget
- Surface Area to Volume Ratio: Shallow lakes with large surface areas (like Lake Chad) experience much higher evaporation losses compared to deep lakes (like Lake Baikal).
- Climate Variability: In monsoonal regions like India, lakes experience a massive surplus during the rainy season and a “negative balance” during the summer due to high PET (Potential Evapotranspiration).
- Catchment Characteristics: A forested catchment slows down Rin (surface inflow) but increases Gin (groundwater recharge), leading to more sustained water levels.
- Anthropogenic Pressure: Diversion of feeder rivers for irrigation is the primary cause of the collapse of lake ecosystems (e.g., the Aral Sea Crisis).
Lake Hydrology and Salinity
The water balance directly affects the “Residence Time” of water in a lake.
- Short Residence Time: Common in open lakes with high throughput; water remains fresh.
- Long Residence Time: Common in closed lakes; minerals accumulate over thousands of years, leading to high alkalinity or salinity.
Significance for Environmental Management
- Eutrophication Control: Understanding the volume of Rin helps in calculating the “nutrient loading” (Nitrogen/Phosphorus) entering the lake.
- Flood Regulation: Lakes act as “natural sponges” or detention basins. By calculating the storage capacity (Δ S), disaster managers can predict a lake’s ability to buffer downstream flooding.
- Wetland Conservation: Under the Ramsar Convention, maintaining the “Hydrological Character” of a wetland requires keeping the water balance within natural fluctuations to support biodiversity.
Important Trivia for Prelims
- Seiche: A temporary disturbance or oscillation in the water level of a lake, often caused by changes in atmospheric pressure or wind.
- Cryptorheic Drainage: Hidden drainage where a lake appears closed but actually loses water through underground limestone channels (common in Karst regions).
- The Aral Sea Lesson: Once the world’s fourth-largest lake, it shrank to 10% of its size because the Soviet Union diverted its inflowing rivers (Amu Darya and Syr Darya) for cotton irrigation, demonstrating a catastrophic “Negative Water Balance.”